Design method of gravel layer PRC prestressed pipe pile deep foundation pit supporting system

Abstract

The invention relates to the technical field of foundation pit supporting design, and particularly discloses a design method of a gravel sand layer PRC prestressed pipe pile deep foundation pit supporting system, which comprises the PRC prestressed pipe piles which are arranged at intervals along the side of a foundation pit to form row piles by optimizing the structure of the supporting system, two prestressed anchor cables are arranged at the upper part of the foundation pit, and the PRC prestressed pipe piles are arranged at intervals along the side of the foundation pit. Comprising a first anchor cable locked on a top beam and a second anchor cable locked on a waist beam, the top beam is connected to the top of the PRC prestressed pipe pile, the waist beam is arranged on the upper portion of the middle of the PRC prestressed pipe pile, numerical simulation analysis is carried out, the feasibility of the construction technology is analyzed through data simulation, and the construction efficiency of the PRC prestressed pipe pile is improved. A support system structure and a construction process scheme are preliminarily determined, a monitoring system is arranged to monitor and control in real time, the safety of the construction process is ensured, and the support system structure and the process are verified, perfected and confirmed through the monitoring system.

Description

Technical Field

[0001] The invention relates to the technical field of foundation pit support construction, and in particular to a design method for a gravel-sand layer PRC prestressed pipe pile deep foundation pit support system. Background Art

[0002] With the development of economy and the acceleration of urbanization, the scale of underground space development is getting larger and larger, the depth of foundation pit is getting deeper and deeper, and the safety requirements of foundation pit engineering are getting higher and higher. For different underground soil layers, what form of foundation pit support structure should be adopted to ensure that the support structure is safe and reliable, economically reasonable, technically feasible, and convenient for construction is the goal pursued by foundation pit support design.

[0003] Different support types can adapt to different soil layers. The existing deep foundation pit support technologies mainly include underground continuous wall support technology, steel sheet pile support technology, soil nail support technology, anchor rod or internal support technology, etc. However, some support technologies cannot adapt to deep foundation pits with an excavation depth of more than 6 meters. Some support methods have requirements for the geological conditions of the foundation pit. Some support technologies have problems that affect the construction space and cannot meet the requirements of construction quality, construction efficiency and construction progress. Especially in cases adjacent to other excavation pits, and in cases where there are large-scale existing structures around the excavation pit, better requirements are placed on deep foundation pit support technology.

[0004] At present, PRC prestressed pipe pile anchor support is also used for support, but this support method currently only stays at the level of basic theoretical research and laboratory specimens. When using PRC prestressed pipe pile deep foundation pit support technology, the following problems are faced: First, under the geological conditions of gravel layer, what kind of structural support method has relative advantages and can ensure the safety of deep foundation pit excavation; Secondly, since the PRC prestressed pipe pile anchor support method is currently only at the theoretical research and laboratory level, when using the PRC prestressed pipe pile anchor support system, how to initially ensure the safety of the support system is a very important issue; Third, due to the lack of internal force and deformation data of PRC prestressed pipe pile anchor support in actual soil layers, it is difficult to grasp the changing development law of the support structure during deep foundation pit excavation. Therefore, how to ensure that the prestressed anchor reaches the designed prestress value in the underground soil layer is the key to the success of the support structure, including how to ensure that the PRC prestressed pipe pile passes through the medium-dense gravel sand layer smoothly on the one hand, and how to ensure that the prestressed anchor reaches the designed prestress value under the condition of tight construction period and cost saving on the other hand; In addition, when the PRC prestressed pipe pile anchor support method is used for soil anchoring, the anchor transfers the tension to the foundation pit rock and soil layer, and transfers the soil load to the deep rock and soil layer through the anchor, and the tension it can withstand is limited.

[0005] Therefore, under the geological conditions of gravel layers, how to design the safe use of the PRC prestressed pipe pile deep foundation pit support system and formulate a PRC prestressed pipe pile deep foundation pit support system structure that meets the requirements so that the support system can meet the support requirements during deep foundation pit excavation and ensure complete foundation pit excavation is a major technical issue faced in the application of the PRC prestressed pipe pile deep foundation pit support system. Summary of the invention

[0006] At least one of the purposes of the present invention is to provide a design method for a PRC prestressed pipe pile deep foundation pit support system in a gravel layer, aiming at the problem of how to apply the PRC prestressed pipe pile deep foundation pit support system to the deep foundation pit support process under geological conditions in a gravel layer and ensure the safety of deep foundation pit support excavation. The design method preliminarily determines the feasibility of the support system structure and the construction process by optimizing the support system structure, performing numerical simulation analysis and trial construction, and ensures the safety of the construction process by arranging a monitoring system for real-time monitoring and control, and verifies and improves the support system structure and process through the monitoring system.

[0007] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention includes the following aspects.

[0008] A design method for a gravel-sand layer PRC prestressed pipe pile deep foundation pit support system includes the following steps: Step A: Geological exploration to determine the geological composition under the geological conditions of the gravel layer; Step B: According to geological conditions, a gravel-sand layer PRC prestressed pipe pile deep foundation pit support system is designed, including PRC prestressed pipe piles arranged at intervals along the side of the foundation pit to form a row of piles, and two prestressed anchor cables are arranged at the upper part of the foundation pit, including a first anchor cable locked on a crown beam and a second anchor cable locked on a waist beam, the crown beam is connected to the top of the PRC prestressed pipe pile, and the waist beam is arranged at the upper middle part of the PRC prestressed pipe pile; Step C: Numerical simulation analysis to determine whether the foundation pit support form meets the requirements; Step D: Formal construction, the PRC prestressed pipe pile deep foundation pit support system in step B is constructed using the construction process, and a monitoring system is arranged for monitoring; Step E: Analyze the monitoring data and determine whether the monitoring data meets the quality standards and requirements, and analyze the differences between the monitoring data and the numerical simulation analysis results; Step F: When the monitoring results in step E are determined to meet the quality standards and requirements, a determined gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction process are formed; when the monitoring results in step E are determined to fail to meet the quality standards and requirements, the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction process are adjusted.

[0009] Through the deep foundation pit support system structure set in step B, two rows of prestressed anchor cables are arranged on the prestressed pipe piles, and cement slurry is injected to form a prestressed anchor structure. The support system fixes one end (anchor section) of the tension structure rod in the stable stratum through cement slurry, and the other end is connected to the PRC prestressed pipe pile to withstand the thrust applied to the structure due to soil pressure, water pressure, etc., thereby utilizing the anchoring force of the stratum to maintain the stability of the structure (or rock and soil layer) and protect the safety of the foundation pit support; The design method of the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system is adopted, and a numerical simulation analysis is carried out on the designed PRC prestressed pipe pile deep foundation pit support system, so as to preliminarily determine that the foundation pit support system can meet the support requirements, and arrange a monitoring system for real-time monitoring during the formal construction process to avoid conditions that affect the safety of foundation pit excavation construction. The construction process can be adjusted and remedial measures can be taken at any time according to the monitoring data, and whether the support system and construction process meet the safety construction requirements is analyzed according to the monitoring data results, so as to determine the feasibility of the support system and construction process or make adjustments.

[0010] The gravel-sand layer PRC prestressed pipe pile deep foundation pit support system can ensure the safety of deep foundation pit support excavation construction. At the same time, by monitoring the support system, the internal force and deformation data of the support structure in the actual soil layer can be fully understood, thereby providing a basis and reference data for further optimizing the structure and construction process of the PRC prestressed pipe pile deep foundation pit support system.

[0011] Preferably, water-stop mixing piles are further arranged on the outer side of the PRC prestressed pipe piles, and two adjacent water-stop mixing piles are arranged to intersect with each other, thereby forming a water-stop mixing pile curtain.

[0012] A double row of piles support structure is formed by setting PRC prestressed pipe piles and water-stop mixing piles. The PRC prestressed pipe piles arranged along the four sides of the deep foundation pit form an inner row of piles structure, and water-stop mixing piles are arranged outside the PRC prestressed pipe piles along the four sides of the deep foundation pit to form an outer row of piles structure. The double row of piles support structure can resist the problem of large soil pressure under deep foundation pit working conditions. The water-stop mixing piles of the outer pile row structure are arranged in an intersecting manner. On the one hand, they can form a water-stop mixing pile curtain to prevent groundwater in the deep foundation pit from leaking into the deep foundation pit and maintain a good construction environment. On the other hand, they can also support the foundation pit soil and form a double support structure with the PRC prestressed pipe piles. It can provide strong support capabilities and solve the problem of high soil pressure and easy water leakage in the deep foundation pit construction environment.

[0013] Preferably, in step B, when designing the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system, it also includes setting a reinforcement structure between the PRC prestressed pipe piles, including first anchoring an anchor steel pipe in the soil between the PRC prestressed pipe piles, placing the anchor end of the anchor steel pipe in the water-stop mixing pile curtain, and then pressing reinforcing bars between the waist beam and the PRC prestressed pipe piles, and then connecting a steel mesh to the anchor steel pipe, the reinforcing bars including transverse bars, vertical bars and diagonal bars.

[0014] When designing the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system, this step is adopted to reinforce the PRC prestressed pipe piles, anchor steel pipes are anchored between the PRC prestressed pipe piles, and reinforcing ribs are arranged between the inner side of the PRC prestressed pipe piles and the water-stop mixing pile curtain, and a steel mesh is connected to the anchor steel pipe. Two rows of anchor cables squeeze and support the reinforcing ribs through the outer row of piles structure, which can improve the support effect of the support system on the foundation pit wall. At the same time, by spraying concrete on the steel mesh, the reinforcing ribs, the steel mesh and the outer row of piles structure form a whole, forming an overall closed surface support structure, which can not only improve the overall support capacity, but also prevent debris from falling from the inner wall of the foundation pit and affecting the normal excavation of the foundation pit.

[0015] Preferably, in step D, when the PRC prestressed pipe pile deep foundation pit support system is used for support and excavation, the following construction steps are adopted: Step D1: construct PRC prestressed pipe piles along the sides of the foundation pit; Step D2: constructing water-stop mixing piles outside the PRC prestressed pipe piles, wherein the water-stop mixing piles are arranged to intersect and form a water-stop mixing pile curtain; Step D3: excavating topsoil; Step D4: Construction of top slope and slope protection; Step D5: construct the cap beam and the first anchor cable, the first anchor cable is anchored on the top of the PRC prestressed pipe pile through the cap beam; Step D6: excavating the foundation pit in layers and spraying the support system concrete of the excavated soil layers; Step D7: constructing a waist beam and a second anchor cable, wherein the second anchor cable is fixed to the PRC prestressed pipe pile through the waist beam, and the waist beam is arranged at the upper middle part of the PRC prestressed pipe pile; Step D8: excavating the foundation pit in layers to the bottom of the pit and spraying concrete; Step D9: Construction of basement structure; Through construction steps D1 and D2, the PRC prestressed pipe piles and the water-stop mixing piles form a double row pile support structure, the PRC prestressed pipe piles arranged along the four sides of the deep foundation pit form an inner row pile structure, and the water-stop mixing piles are arranged outside the PRC prestressed pipe piles along the four sides of the deep foundation pit to form an outer row pile structure. The double row pile support structure can resist the problem of large soil pressure under the deep foundation pit working condition; Through step D5 and step D7, the crown beam and the first anchor cable and the waist beam and the second anchor cable are constructed, the crown beam and the waist beam are connected to the PRC prestressed pipe pile, the crown beam is connected to the top of the PRC prestressed pipe pile, the waist beam is arranged at a position slightly above the middle of the PRC prestressed pipe pile, the first anchor cable is anchored on the crown beam, the second anchor cable is anchored on the waist beam, one section of the prestressed anchor cable is anchored in the stratum, and the other end is connected and anchored to the PRC prestressed pipe pile through the crown beam and the waist beam, so that the inner pile row structure forms support around the deep foundation pit as a whole through the two rows of anchor cables, and the upper soil and the lower soil can be supported and reinforced at the same time to withstand the thrust applied to the structure due to soil pressure, water pressure, etc., so as to utilize the anchoring force of the stratum to maintain the stability of the structure (or rock and soil layer) and protect the safety of the foundation pit support.

[0016] Preferably, when constructing PRC prestressed pipe piles around the sides of the foundation pit in step D1, a trial pile construction is first carried out, and the formal construction is carried out after obtaining the construction process parameters. The construction parameters include the pile driving speed, the static pressure during the pile driving process, the final pressure when reaching the predetermined elevation, the pile length and the quality standard after the pile is completed. The steps of the formal construction include: Step D101: measuring and laying out the positions of the PRC prestressed pipe piles to be constructed; Step D102: Weld the pile tip and hoist and center it. Before hoisting and centering, control the lifting point and verticality. The deviation between the lifting point center and the pile position should be within 10mm, and the verticality deviation between the upper end and the lower end should be ≤0.3%; Step D103: Press the pile to a predetermined elevation and control the pressure, and monitor and control the verticality of the pile body during the pressing process to ensure that the verticality deviation between the upper end and the lower end of the pile body is ≤0.3%; Step D104: Move the pile to the next pile position for construction.

[0017] By testing the PRC prestressed pipe piles, it can be fully guaranteed that the PRC prestressed pipe piles are in the designed pile position to avoid problems such as offset or dislocation. Test piles can be carried out before formal construction to obtain construction process parameters, and the hanging points and verticality of the supporting pipe piles can be controlled during the formal construction process to fully ensure the construction quality of the PRC prestressed pipe piles.

[0018] Preferably, before constructing the PRC prestressed pipe piles, monitoring sensors are first installed on the PRC prestressed pipe piles, including cutting grooves on the PRC prestressed pipe piles for installing densely distributed strain sensing optical cables, so that the monitoring process is based on the pile body analysis principle of optical fiber strain to measure the bending moment, disturbance and shear force of the PRC prestressed pipe piles, and the pile body analysis principle of optical fiber strain includes the support pile axial force calculation process, and the support pile deflection, bending moment and shear force calculation process.

[0019] By monitoring the bending moment, disturbance and shear force of PRC prestressed pipe piles, the monitoring frequency is high and real-time monitoring can be carried out around the clock. The strain sensing optical cable is corrosion-resistant, anti-electromagnetic interference, and has good long-term stability. It is very suitable for the construction environment in the foundation pit that is humid and highly corrosive. Through the monitoring of the strain sensing optical cable, not only can the bending moment, disturbance and shear force be monitored, but also the settlement displacement, horizontal displacement, strain, temperature, water level, soil pressure, inclination and vibration and other multi-variable sensing can be realized. The data monitored by other monitoring subsystems can be compared and verified with each other to avoid the multi-interference of a single factor. Comprehensive detection improves the accuracy of detection and reduces the false alarm rate. In addition, the use of optical fiber sensor monitoring uses optical signals, does not require power supply, has low cost, is easy to carry out system integration, and component network monitoring. It also has the advantages of high accuracy, precise positioning, and easy to troubleshoot abnormal situations. The fiber optic strain-based pile body analysis principle is used to measure the bending moment, disturbance and shear force of the support piles, which can further ensure the accuracy of the monitored support pile deflection, bending moment and shear force, ensure the monitoring accuracy, further ensure the correctness of subsequent data analysis and analysis results, and improve the reference value of data analysis.

[0020] In the step D, when the monitoring system is used to support the PRC prestressed pipe pile deep foundation pit support system, the prestressed anchor cable is obliquely anchored in the gravel and sand geological layer outside the foundation pit, and a monitoring system including a first monitoring subsystem, a second monitoring subsystem, a third monitoring subsystem, a fourth monitoring subsystem and a fifth monitoring subsystem is arranged on the support system structure and outside the foundation pit, wherein: The first monitoring subsystem is used to monitor the vertical displacement and settlement displacement of the top of the retaining wall, including multiple monitoring points WY1~WY14 arranged at the slope position, and no less than 3 settlement reference points. The settlement reference points are set in the area outside the foundation pit more than 30 meters away from the basement contour line, and a closed loop is formed between the settlement reference points. The first monitoring subsystem is also used to monitor the horizontal displacement of PRC prestressed pipe piles. The monitoring points are set at the top of the PRC prestressed pipe piles, and no less than 3 horizontal displacement reference points are arranged.

[0021] The second monitoring subsystem is used to monitor the surrounding adjacent soil and affiliated settlement, displacement, including multiple monitoring points W1 ~ W11 arranged outside the foundation pit and within a range of 12-30 meters from the basement contour line; The third monitoring subsystem is used to monitor the deep displacement of the foundation pit, including a plurality of monitoring points CX1 to CX4 arranged on the PRC prestressed pipe piles; The fourth monitoring subsystem for monitoring groundwater, including a plurality of monitoring points arranged outside the water-stop mixing pile curtain SW1 ~ SW5; The fifth monitoring subsystem is used to monitor the stress of anchor cables and the stress conditions of PRC prestressed pipe piles, and to assist in monitoring the parameters monitored by other monitoring subsystems, including multiple monitoring points YL1~YL5 arranged on anchor cables at different positions, and densely distributed strain sensing optical cables arranged on multiple PRC prestressed pipe piles, which monitor various parameters of foundation pit support through optical fiber sensors.

[0022] By arranging a monitoring system on the structure of the support system, including burying strain and stress gauges on the PRC prefabricated pipe piles and prestressed anchor cables in typical support sections, the stress changes of the PRC prestressed pipe piles and prestressed anchor cables during excavation can be monitored. Then, by combining the monitored stress conditions with numerical simulation analysis, the influence of construction excavation on the support structure system, especially the stress change law of the prestressed anchor cables, can be obtained, thereby confirming the reliability of the support structure system; By arranging a monitoring system on the support system and outside the foundation pit, it is possible to effectively monitor the deformation of the foundation pit support structure, changes in anchor cable stress, rise and fall of the water level outside the pit, and the structural safety of underground pipelines and buildings (structures) involved in the surrounding area of ​​the foundation pit. By monitoring the soil and settlement around the support structure and the excavated foundation pit, the real-time excavation safety of the foundation pit can be guaranteed.

[0023] Preferably, each monitoring point is equipped with a monitoring instrument, and the following steps are used to install the monitoring instrument: Step D1´: installing monitoring sensors on the PRC prestressed pipe piles, which are used to monitor the deep horizontal displacement inside the PRC prestressed pipe piles, the bending moment of the supporting piles, the displacement of the supporting piles and the shear force of the supporting piles; Step D2´: Install vertical displacement monitoring points on the top of the retaining wall. Vertical displacement monitoring includes leveling benchmark points, monitoring points and building settlement observation points. Excavate foundation trenches to the original soil at the leveling benchmark points and monitoring points, and make foundations and observation piers on the upper part. The leveling benchmark points will bury the marker stones in the stable original soil layer outside the foundation pit at a distance of more than 30 meters from the basement contour line, or embed the markers on the exposed bedrock; Step D3´: Install the embedded inclinometer tube in the PRC prestressed pipe pile, and use the inclinometer and the matching inclinometer tube for monitoring, including the embedded inclinometer tube installation in the pile, the drilling and installation of the inclinometer hole, and the measurement and observation; Step D4´: When the anchor cable is tensioned and locked, an anchor cable dynamometer is installed at the anchor head to measure the anchor cable stress, and the monitoring data of the anchor cable dynamometer is transmitted to the monitoring station by communication. The frequency value is read by the computer, and the anchor cable stress value is obtained to achieve the purpose of monitoring the internal force of the anchor cable. At the same time, an alarm value is set, and a warning is proposed based on the stress value obtained by monitoring and calculation. By installing the anchor cable dynamometer, the law of the strength growth of the anchor cable grouting body in the gravel sand layer can be mastered; Step D5´: Drill holes at water level monitoring points and install water level pipes to monitor the water level of the foundation pit, including installation of measuring points and measurement and observation, and transmitting the measured and observed data to the monitoring station.

[0024] The steps D1´ to D5´ are not in any particular order. The monitoring parameter value obtained in each step is set with an alarm value, and a warning is proposed for each monitoring parameter value. When the monitoring parameter value is greater than the corresponding alarm value, an alarm reminder is activated; By setting monitoring points at different parts of the foundation pit and different parts of its supporting structure, and installing monitoring devices at the corresponding monitoring points, and adopting the installation method and measurement monitoring method of steps D1´~D5´, the accuracy of the data can be guaranteed, and the data can be transmitted or manually input into the computer server of the monitoring station for subsequent analysis of the foundation pit status and the supporting structure status.

[0025] Preferably, in steps D5 and D7, when constructing the first and second anchor cables, anchor cable trial anchoring construction is first carried out. The design method of this scheme adopts the two-line anchor cable support form of the first and second anchor cables. Due to the geological conditions of the gravel layer, in order to determine the ultimate bearing capacity and safety factor of the anchor cables in the stratum, reveal the various influencing factors and their influence degree that affect the anchoring force of the anchor cables under the stratum conditions, test the construction process of the pre-set anchor cable engineering and verify the design parameters to ensure the safety, economy and rationality of the anchoring engineering, and carry out prestressed anchor cable trial anchoring construction before formal construction. The anchor cable hole is formed by casing follow-up mechanical hole forming method. The hole diameter is 120-180mm and the length is 18-25m. 3+1 bundles or 4+1 bundles of two types of steel stranded wire detachable core prestressed anchor cables are used. The anchor cable grouting adopts P.0 42.5 silicate mixing, water-cement ratio of 0.40-0.45, cement-sand ratio of 0.50-1.00, use removable core mechanical recyclable anchor cable, one of the designed steel strands is unlocked steel strand, the diameter of the steel strand is between φ12 and φ18, the standard value of the axial internal force of the anchor cable is 250kN, 380kN, 450kN, the locking value is 180kN, 200kN, 250kN, before the trial anchoring construction, first detect and determine the soil distribution and physical and mechanical properties of each soil layer in the construction area, find out and avoid the surrounding underground pipelines, structures and facilities, and carry out anchor cable pull-out test when the slurry strength of the anchor cable anchoring section reaches 20MPa or reaches 80% of the design strength grade. When drilling, the drill rod and casing are synchronously rotated and drilled section by section. The construction content of the two-line anchor cable support includes the following steps: Step D571: determine the anchor hole position, locate the anchor hole, and process and manufacture the anchor cable; Step D572: Positioning the drilling rig, during which the anchoring position and anchoring angle are determined; Step D573: drilling, after the drilling is completed, checking the hole depth and installing the anchor cable; Step D574: grouting, including primary grouting and secondary grouting; Step D575: Curing after grouting, tensioning and locking after curing.

[0026] By adopting the trial anchoring construction of the above two anchor cables, the construction process of the prestressed anchor cable can be retrieved and the design parameters can be checked. After the trial anchoring, the anchor cable pull-out test can be carried out to determine whether the soil conditions of the construction site meet the predetermined design requirements, and to determine the ultimate bearing capacity and safety factor of the anchor cable in the stratum, as well as the factors affecting the anchoring force of the anchor cable under the geological conditions and their degree of influence, so as to ensure that the two anchor cables in all parts of the construction foundation pit meet the anchoring requirements, thereby ensuring the safety of the foundation pit support; through the trial anchoring and anchor cable pull-out test, combined with the numerical simulation analysis of step C in the design method, it can be further ensured that the anchoring project of the two anchor cables in this design scheme is carried out safely, economically and reasonably; Through two grouting, the second grouting time is determined according to the grouting process test or after the strength of the anchor body reaches 5MPa. When the strength of the anchor body is greater than 20MPa, the anchor cable is tensioned and locked. After the anchor cable is tensioned, the free section is grouted. The grouting material is the same as the anchoring segment. When grouting the free section, avoid the grouting slurry from entering the stress-bearing bars in the free section to ensure the free expansion and contraction of the stress-bearing bars in the free section. During the anchor cable construction, pull-out test and creep test are carried out to ensure the quality of the anchor cable construction.

[0027] Preferably, before the formal construction of the two anchor cables, on-site anchor pulling and recyclability tests of the prestressed anchor cables are carried out to verify the recyclability of the prestressed anchor cables. When the basement structure is constructed after the excavation is completed, the support replacement and anchor cable recovery are carried out. The support replacement plate includes two plain concrete force transmission belts located on the outer sides of the negative second floor floor and the negative first floor floor, respectively. The thickness of the two plain concrete force transmission belts is between 350 and 600 mm. The support replacement construction is interspersed with the construction of the basement second floor structure. After each section of the underground second floor structure is completed, the support replacement construction of the section is carried out, including the following construction steps: Step D111: construction preparation; Step D112: Construction of the negative second floor slab and the first force transmission belt; Step D113: Basement structure construction, including the construction of the exterior wall and the basement floor, as well as the waterproofing and protective layer construction of the exterior wall of the basement floor; Step D114: backfilling of sandy soil and construction of the second force transmission belt; Step D115: dismantling and recovering the second anchor cable; Step D116: backfilling the sandy soil to the bottom of the crown beam; Step D117: Remove and recover the first anchor cable.

[0028] By adopting the above method, the PRC prestressed pipe piles are supported and fixed by backfilling sandy soil and plain concrete force transfer belts. One side of the backfilling sandy soil and plain concrete force transfer belt relies on the two-layer bottom plate structure of the basement structure for force transfer and fixation, thereby replacing the prestressed anchor cable support structure to ensure the safety of the basement structure and support. At the same time, the prestressed anchor cables are removed and recycled, which can be reused. After the prestressed anchor cables are removed, the surrounding construction is avoided from being affected, resulting in the surrounding construction excavation process. The damage to the prestressed anchor cables affects the support safety of the foundation pit, forming a safety hazard.

[0029] Preferably, when shotcrete construction is involved in the process of support and excavation construction using a support system, concrete with an early strength agent added is used, and an early strength test is carried out before shotcrete is sprayed. Multiple groups of concrete with different water-cement ratios and whether or not an early strength agent and a water reducing agent are added are used to test their strength changes in different time periods, and it is determined to use concrete with 0.5% FCM-10 nano early strength agent added.

[0030] After the foundation pit is excavated, the supporting structure will have a certain displacement. If the displacement is too large or develops too quickly, it will cause serious consequences. On the one hand, the displacement development can be stopped by strengthening the supporting system structure while excavating. On the other hand, by increasing the strength grade of concrete and adding early strength agent, the concrete can increase its strength as soon as possible, which also plays a key role in stopping the displacement development and giving full play to the "horizontal support" of the cushion layer as soon as possible. During the test, the factors affecting the strength growth of the grouting body of the PRC prestressed pile anchor system were collected, the key factors were found, and the reasonable technical parameters for the mix ratio were determined. Preferably, the design method of the gravel layer PRC prestressed pipe pile deep foundation pit support system of this scheme also includes analyzing and calculating the construction costs of different support systems to obtain the economic efficiency of the PRC prestressed pipe pile anchor cable system.

[0031] Preferably, the design method of the gravel layer PRC prestressed pipe pile deep foundation pit support system of the present scheme also includes using numerical simulation to simulate the changes in the displacement, bending moment, internal support axial force, settlement displacement around the foundation pit, etc. of the PRC prestressed pipe pile under different working conditions.

[0032] In summary, due to the adoption of the above technical solution, the present invention has at least the following beneficial effects: 1. By adopting the design method of the PRC prestressed pipe pile deep foundation pit support system of the present application, a support system structure and construction process method are initially formulated, and then numerical simulation analysis and trial construction are carried out, and formal implementation is carried out and monitored and verified by the monitoring system, a perfect and feasible PRC prestressed pipe pile support system structure and construction process can be obtained; in addition, by comparing the on-site measured data and analysis of the monitoring system, the effectiveness of the support system structure and construction process method can be verified, and the construction process and quality control standards of PRC prestressed pipe piles and prestressed anchor cables by the static pressure method of gravel and sand layers are summarized, providing a reasonable and reliable basis for promotion and application; 2. In the actual construction process, as the foundation pit is excavated, the on-site excavation conditions change. The monitoring system can monitor all monitoring data at different stages of the foundation pit excavation process, thereby completing the actual measurement of the displacement, deformation and internal force changes of the support structure under different working conditions, and analyzing and comparing the measured values ​​with the numerical simulation results to obtain the force law of the prestressed pipe pile anchor cable foundation pit support system, which can ensure that the structural design parameters and process parameters of the support system are more reasonable; 3. This design method studies the influence of soil stress release on the deformation of the pipe pile anchor support system through numerical simulation and field testing, so as to propose a reasonable excavation process for this type of support structure, and lay a theoretical and technical foundation for preventing excessive deformation of the pipe pile anchor support system and rationalizing the selection of design parameters during the excavation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the planar structure of the gravel layer PRC prestressed pipe pile foundation pit support system of the present invention.

[0034] Figure 2 The present invention is a process flow chart of the design method of the gravel layer PRC prestressed pipe pile foundation pit support system.

[0035] Figure 3 It is a schematic diagram of the elevation structure of the gravel layer PRC prestressed pipe pile foundation pit support system of the present invention.

[0036] Figure 4 It is a schematic diagram of the relative position arrangement of the prestressed pipe piles and the water-stop mixing piles of the PRC prestressed pipe pile deep foundation pit support system in the present invention.

[0037] Figure 5 It is a structural schematic diagram of the gravel layer PRC prestressed pipe pile foundation pit support system reinforcement structure in the present invention.

[0038] Figure 6 This is the construction flow chart of PRC prestressed pipe piles in the gravel layer PRC prestressed pipe pile foundation pit support system.

[0039] Figure 7This is the construction flow chart of water-stop mixing piles in the gravel layer PRC prestressed pipe pile foundation pit support system.

[0040] Figure 8 This is a schematic diagram of the locations of monitoring points in the slope area of ​​the monitoring system.

[0041] Fig. 9 This is a schematic diagram of the monitoring point locations of the second monitoring subsystem in the monitoring system.

[0042] Fig.10 This is a schematic diagram of the monitoring point locations of the third monitoring subsystem in the monitoring system.

[0043] Fig.11 This is a schematic diagram of the monitoring point locations of the fourth monitoring subsystem in the monitoring system.

[0044] Fig.12 Schematic diagram of the locations of monitoring points on the anchor cables in the monitoring system.

[0045] Fig.13 This is a schematic diagram of the arrangement structure of the PRC prestressed pipe piles and the corresponding prestressed anchor cables arranged in the monitoring system in Example 2.

[0046] Fig.14 Schematic diagram of horizontal displacement of 135# pile and 160# pile at different buried depths in PRC prestressed pipe pile foundation pit support.

[0047] Fig.15 Schematic diagram of the bending moment of 135# pile and 160# pile at different buried depths in PRC prestressed pipe pile foundation pit support.

[0048] Fig.16 Schematic diagram of shear force of 135# pile and 160# pile at different buried depths in PRC prestressed pipe pile foundation pit support.

[0049] Figure 17-19 Schematic diagram of vertical settlement at the top of the retaining wall in PRC prestressed pipe pile foundation pit support.

[0050] Figure 20-21 This is a schematic diagram of the maximum cumulative settlement of surrounding roads and pipelines in the PRC prestressed pipe pile foundation pit support.

[0051] Fig. 22 It shows the internal force of the anchor cables at monitoring points YL1-YL3 in the PRC prestressed pipe pile foundation pit support monitoring system.

[0052] Fig.23 This is a schematic diagram of the groundwater level drop at monitoring points SW1 to SW5 in the PRC prestressed pipe pile foundation pit support monitoring system.

[0053] Fig.24This is a schematic diagram of the cumulative displacement values ​​of deep horizontal displacements in the supporting piles at monitoring points CX1 to CX4 in the PRC prestressed pipe pile foundation pit support monitoring system.

[0054] Fig.25 It is a schematic diagram of the sequence of foundation pit excavation in the construction method of the PRC prestressed pipe pile deep foundation pit support system of the present invention.

[0055] Fig.26 Structural schematic diagram of removing the second anchor cable after completing the filling of plain concrete strip between the basement negative first floor slab and PRC prestressed pipe piles.

[0056] Fig. 27 Schematic diagram of the structure after removing the first anchor cable after backfilling sandy soil to the bottom elevation of the crown beam.

[0057] Markings in the figure: 1- foundation pit, 2- PRC prestressed pipe piles, 3- water-stop mixing piles, 4- water-stop mixing pile curtain, 5- prestressed anchor cable, 6- crown beam, 7- first anchor cable, 8- waist beam, 9- second anchor cable, 10- reinforcement structure, 11- anchor steel pipe, 12- reinforcing ribs, 13- transverse ribs, 14- vertical ribs, 15- oblique ribs, 16- gravel clay soil layer, 17- fully weathered granite soil layer, 18- jet grouting piles, 19- sprayed concrete layer, 20- steel mesh, 21- soil nails, 22- pipes, 23- drainage ditch, 24- foundation pit guardrail, 25- intercepting ditch, 26- plain fill soil layer, 27- silty clay layer, 28- gravel sand layer, 29- slope part, 30- first zone, 31- second zone, 32- dividing line, 33- excavation port, 34- basement contour line, 35- adjacent projects, 36- dividing line, 36- monitoring station, 37- backfill sandy soil, 38- basement exterior wall, 39- basement floor slab, 40 basement -1 floor slab, 41- filled plain concrete belt, 42- monitoring station. DETAILED DESCRIPTION

[0058] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments to make the purpose, technical solutions and advantages of the present invention more clearly understood. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Example 1

[0059] This embodiment shows the design method of the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system, taking a certain project under implementation as an example. The project covers an area of ​​about 10,030 square meters, has two basements, and the foundation pit excavation depth is about 8.45-8.95m. Figure 1As shown, the upper part is the north side, the lower part is the south side, the left side is the west side, and the right side is the east side. The west side of the foundation pit is the adjacent project 35, which is planned to be excavated together with the foundation pit of a project constructed at the same time to the bottom of the pit. The perimeters of the north, east and south sides of the foundation pit of this project are about 275 meters, and the foundation pit area is about 8990 square meters. The deep foundation pit is divided into multiple sections along the perimeter direction of the deep foundation pit. In this embodiment, it includes AB section, BC section, CD section, DE section, and EF section. Each section of the deep foundation pit is a continuous terrain. The AB section is a straight line section on the south side of the project, the BC section is a straight line section that expands from the south side of the project to the outside of the foundation pit, and B is a turning point. The CD section is an arc section at the south corner. The DE section is a straight line section on the east side, and it turns 90 degrees at E. The EF section is a straight line section on the north side. The FA section is the boundary 36 of the adjacent project 35. The foundation pit of this project is excavated simultaneously with the adjacent project 35, and the FA section is not supported. According to the surrounding environmental conditions of each section of the foundation pit and the depth of the foundation pit, the safety level of the ABCDE section on the south and east sides of the foundation pit is level 1, and the safety level of the EF section on the north side is level 2. The gravel-sand layer PRC prestressed pipe pile deep foundation pit support system of this embodiment is designed to have a service life of 12 months. From the inside to the outside of the foundation pit 1, there are the basement contour line 34, PRC prestressed pipe piles 2, water-stop mixing pile curtain 4, slope part 29, and foundation pit guardrail 24.

[0060] like Figure 2 As shown in the figure, the design method of the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system includes the following steps: Step A: Geological exploration, determine the geological composition under the geological conditions of the gravel layer, and consult relevant information to analyze other deep foundation pit support structure forms and application conditions, and analyze the application conditions of PRC prestressed pipe piles and other pipe piles; Step B: According to the geological conditions of the gravel layer, design the gravel sand layer PRC prestressed pipe pile deep foundation pit support system. The design method of this embodiment adopts the following method: Figure 3 The support system structure shown in the figure is arranged in a way that a PRC prestressed pipe pile row structure is formed along the side of the foundation pit, and two prestressed anchor cables 5 are arranged at the upper part of the foundation pit, including a first anchor cable 7 locked on a crown beam 6 and a second anchor cable 9 locked on a waist beam 8. The crown beam 6 is connected to the top of the PRC prestressed pipe pile 2, and the waist beam 8 is arranged at the upper middle part of the PRC prestressed pipe pile 2. Two anchor cables are arranged under the geological conditions of the gravel layer, and the anchor end of the first anchor cable 7 is fixed to the bottom of the foundation pit. The anchoring end of the second anchor cable 9 is deeply fixed in the gravel clay layer 16, and the anchoring end of the second anchor cable 9 is deeply fixed in the fully weathered granite soil layer 17, which can ensure that the prestressed anchor cable reaches the designed prestressed value in the underground soil layer. At the same time, the two prestressed anchor cables 5 can effectively support the foundation pit 1 through the PRC prestressed pipe piles 2, and can withstand the thrust of the soil pressure, water pressure, etc. applied to the structure under the condition of the deep foundation pit, so as to use the anchoring force of the stratum to maintain the stability of the structure (or rock and soil layer) and protect the safety of the foundation pit support; Step C: Numerical simulation analysis to determine whether the foundation pit support form meets the requirements, using MIDAS, PLAXIS or FLAC3D and other analysis and calculation software, including analysis of the change rules of PRC prestressed pipe pile displacement, bending moment, internal support axial force, and settlement displacement around the foundation pit under different working conditions; Step D: Formal construction, determine the construction process steps according to the PRC prestressed pipe pile deep foundation pit support system in step B and carry out formal construction, and arrange a monitoring system for monitoring; Step E: Collect and analyze monitoring data, and determine whether the monitoring data meets the quality standards and requirements, and analyze the differences between the monitoring data and the numerical simulation analysis results; Step F: When the monitoring results in step E are determined to meet the quality standards and requirements, a determined gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction process are formed; when the monitoring results in step E are determined to fail to meet the quality standards and requirements, the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction process are adjusted.

[0061] The gravel-sand layer PRC prestressed pipe pile deep foundation pit support system of this design scheme is adopted, and the corresponding construction technology is used for construction, which can ensure the stability of the support system structure, avoid the problem of slope landslide or settlement, and ensure the safety of deep foundation pit excavation construction. At the same time, a monitoring system is set up to monitor the support system in real time, which can monitor the internal force and deformation changes of the support system structure in the actual soil layer in real time, so as to set real-time alarms and handle them in time. At the same time, the monitoring system can provide a basis and reference data for further optimizing the PRC prestressed pipe pile deep foundation pit support system structure and construction technology; In the technical solution of this design method, PRC prestressed pipe pile anchor cables are used instead of PRC prestressed pipe pile anchor cables for support. Prestressed anchor cables can withstand greater tension and are very suitable for large-tonnage anchoring projects in deep foundation pits. Prestressed anchor cables are anchored on the PRC prestressed pipe pile row structure and transmitted to the foundation pit soil through the pile row structure, thereby generating additional stress in the foundation pit rock and soil, which can fully ensure the stability of foundation pit excavation and the safety of the surrounding environment.

[0062] In step A, multiple exploration points are selected for geological exploration, with 8 to 20 exploration points, which are appropriately adjusted according to the size of the project. The geological exploration report of this project shows that the geology of this project from the surface to the bottom is as follows: a plain fill layer 26 with a thickness of 2 to 2.4 meters, a silty clay layer 27 with a thickness of 1.8 to 2.2 meters, a gravel sand layer 28 with a thickness of 2.5 to 2.8 meters, a gravel clay layer 16 with a thickness of 10 to 12 meters, a fully weathered granite layer 17 with a thickness of 5 to 6.5 meters, a strongly weathered granite layer with a thickness of 7.5 to 8.5, a moderately weathered granite layer with a thickness of 2.8 to 3.3, and slightly weathered granite below. The initial groundwater level is buried at a depth of 3.10 to 5.30 meters, and the stable water level is buried at a depth of 3.70~6.10m, average 5.19m, elevation -0.38~1.86m, average 0.55m. Based on regional experience, the annual change of groundwater level is about 0.50~1.50m. According to the address exploration, the cross-section diagram of the project status is drawn to prepare for the subsequent anchoring of prestressed anchor cables, and the construction process of PRC prestressed pipe piles is determined in combination with the surrounding environment of the project. There are residential communities around this project, and the static pressure pile driver construction method is adopted. The anchoring end of the first anchor cable 7 is at least anchored into the gravel clay layer 16, and the anchoring end of the second anchor cable 9 is at least anchored into the fully weathered granite soil layer 17. The fixed sections of the two prestressed anchor cables are locked on the PRC prestressed pipe piles through the anchoring structure.

[0063] In order to further optimize the support system structure and ensure the safe excavation of the deep foundation pit of this project, in step B, according to the geological conditions of the gravel layer, the support system also includes setting water-stop mixing piles 3 on the outside of the PRC prestressed pipe piles 2, such as Figure 1 and Figure 3 As shown, two adjacent water-stop mixing piles 3 are arranged to intersect each other, thereby forming a water-stop mixing pile curtain 4, and rotary jet piles 18 are arranged at the junction of two adjacent projects. The water-stop mixing pile curtain 4 can prevent groundwater in the deep foundation pit soil from leaking into the deep foundation pit, maintain a good construction environment, and also support the foundation pit soil. The water-stop mixing piles 3 and the PRC prestressed pipe piles 2 form a double support structure, which can provide strong support capacity and solve the problem of large soil pressure and easy water leakage in the deep foundation pit construction environment. The PRC prestressed pipe piles 2 arranged on the four sides of the deep foundation pit form an inner row of piles structure, and the water-stop mixing piles 3 are arranged on the outer side 2 of the PRC prestressed pipe piles along the four sides of the deep foundation pit to form an outer row of piles structure. Combined with the two rows of prestressed anchor cables 5 arranged on the PRC prestressed pipe piles, the structural form of double row of piles + double row of prestressed anchor cables can improve the applicability and safety reserve capacity of the support system of this scheme, so that it can meet the deep foundation pit excavation support under various construction conditions and various geological conditions, and is particularly suitable for gravel layer geological conditions.

[0064] As one preferred embodiment, when designing the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system structure, it also includes setting a reinforcement structure 10 between the PRC prestressed pipe piles 2, such as Figure 5 As shown, the reinforcement structure 10 is arranged at the upper and lower parts of the waist beam 8, below the crown beam 6, including first anchoring an anchor steel pipe 11 in the soil between the PRC prestressed pipe piles 2, inserting the anchor end of the anchor steel pipe 11 into the water-stop mixing pile curtain 4, and then pressing the reinforcing ribs 12 between the waist beam 8 and the PRC prestressed pipe piles 2, and then connecting the steel mesh 20 to the anchor steel pipe 11, the reinforcing ribs 12 include transverse ribs 13, vertical ribs 14 and oblique ribs 15, the transverse ribs 13, vertical ribs 14 and oblique ribs 15 are interconnected, when spraying concrete between the PRC prestressed pipe piles 2, including spraying concrete on the reinforcement structure 10, forming an overall closed surface support structure, which can not only improve the overall support capacity, but also prevent debris from falling from the inner wall of the foundation pit, thereby ensuring normal excavation of the foundation pit.

[0065] The PRC prestressed pipe pile deep foundation pit support system also includes a sprayed concrete layer 19 arranged at a foundation pit slope position 29, and the thickness of the sprayed concrete layer 19 is between 50 and 80 mm. When constructing the sprayed concrete layer 19 at the slope position 29 of the foundation pit, soil nails 21 are first driven in, and then a steel mesh 20 is hung on the soil nails 21 for pouring. A foundation pit guardrail 24 is arranged on the outside of the foundation pit 1, and a drainage ditch 25 is arranged on the outside of the guardrail to discharge the surface water outside the foundation pit. At the same time, a pipe 22 is arranged under the ground outside the foundation pit, including a water supply pipe and a rainwater pipe. When the foundation pit 1 is excavated to the bottom of the basement floor cushion layer, in order to avoid the influence of precipitation or accumulation of water in the foundation pit on the foundation pit 1, a drainage ditch 23 is also arranged near the side of the foundation pit 1 near the prestressed pipe pile 2 for drainage, so as to avoid rainwater or precipitation from soaking the PRC prestressed pipe pile anchoring structure, and then softening and collapsing, affecting the safety of the support system structure.

[0066] As one preferred implementation, in step D, when the PRC prestressed pipe pile deep foundation pit support system is used for support and excavation, Figure 1 , Figure 3 and Figure 4 As shown, the following construction steps are adopted: Step D1: construct PRC prestressed pipe piles 2 along the sides of the foundation pit 1; Step D2: construct water-stop mixing piles 3 outside the PRC prestressed pipe piles 2, and the water-stop mixing piles 3 are arranged to intersect and construct to form a water-stop mixing pile curtain 4, which forms a double-row pile support structure with the PRC prestressed pipe piles 2 constructed in step D1, which can resist the problem of large soil pressure under deep foundation pit conditions; Step D3: excavating the topsoil of the foundation pit 1; Step D4: construct the top slope section 29 and perform slope protection; Step D5: construct the cap beam 6 and the first anchor cable 7, the first anchor cable 7 is anchored on the top of the PRC prestressed pipe pile 2 through the cap beam 6, one end of the first anchor cable 7 is anchored in the stratum, and the other end is connected and anchored to the PRC prestressed pipe pile 2 through the cap beam 6; Step D6: excavating the foundation pit 1 in layers, and spraying the support system concrete of the excavated soil layers; Step D7: construct the waist beam 8 and the second anchor cable 9. The second anchor cable 9 is fixed to the PRC prestressed pipe pile 2 through the waist beam 8. The waist beam 8 is arranged in the middle and upper part of the PRC prestressed pipe pile 2. One end of the second anchor cable 9 is anchored in the stratum, and the other end is connected and anchored to the PRC prestressed pipe pile through the waist beam 8. Combined with step D5, the two rows of prestressed anchor cables 5 form a support around the deep foundation pit as a whole, and can simultaneously support and reinforce the upper soil and the lower soil to withstand the thrust applied to the structure due to soil pressure, water pressure, etc., so as to use the anchoring force of the stratum to maintain the stability of the structure (or rock and soil layer) and protect the safety of the foundation pit support; Step D8: excavating the foundation pit in layers to the bottom of the pit and spraying concrete; Step D9: Construction of basement structure; When excavating the foundation pit, first divide the foundation pit into two parts according to the different functional areas of the plot, such as Fig.25 As shown, it includes a first area 30 and a second area 31. At the same time, multiple dividing lines 32 are set along the horizontal and vertical directions of the deep foundation pit to divide the deep foundation pit into multiple strip blocks. The direction of the arrow in the figure is the excavation direction, and the excavated soil is transported out from the excavation port 33 to carry out the construction of the foundation pit basement structure.

[0067] When constructing PRC prestressed pipe piles 2, first conduct a trial pile construction. The trial pile construction can test the feasibility and effect of the construction scheme adopted under the geological conditions on the one hand, and on the other hand, determine the construction process flow, operation points and process parameters of PRC prestressed pipe piles, and provide guidance and specifications for the overall construction of prestressed pipe piles. The construction process determined by the inspection includes pile driving speed, pile driving pressure, pile length and pile quality after completion, etc. It also includes obtaining the pile driving speed, static pressure during pile driving and reaching the predetermined design elevation final pressure, and through the trial pile inspection, whether the predetermined design requirements can be met under the soil layer conditions, such as Figure 6 As shown, the construction method and process of formally constructing the PRC prestressed pipe pile 2 in step D1 are as follows: Step D11: Lay out and locate, and check the benchmark points, base axis lines and leveling points of the construction site; clearly mark and number the calibrated benchmark points and protect them properly; use theodolite and steel tape measure, etc., to measure the edge of the pile position area using the coordinate method; measure and locate all piles in the construction area, and the measured pile position deviation shall not exceed 20mm. Make clear and reliable pile position marks. At the same time, keep measurement records for review; Step D12: Pile tip welding, ① Perform pile tip welding before lifting the pile, avoid holding the pile tip in a vertical state after feeding the pile and then welding, first weld the upper part of the pile tip in a horizontal state of the pile body, then rotate the pile body 180° axially, and then weld the remaining part, and the weld is continuously closed; ② During welding, the surface of the pile body is kept clean, first brush it with an iron brush, and the flatness of the pile tip and the pile body is less than 0.5mm; ③ Manual arc welding is used for welding layer by layer, with no less than 2 layers, and avoid welding defects such as slag inclusions and pores in the weld; ④ After the joint is welded, cool it naturally for more than 5 minutes before pressing the pile, and avoid cooling it with water or pressing it immediately after welding; Step D13: Lifting the pile, using the pile driver hoisting equipment to lift the prefabricated pile in a one-point lifting manner, slightly increasing the angle (reducing the overhang length) during lifting, slowly lifting it to the side of the pile driver and then starting to vertically lift the hook, steadily sending the pile to the pile clamp of the pile driver, and lifting the pipe pile at a relatively slow speed to avoid the hook lifting speed being too fast, causing the pile to slide quickly toward the support of the crane and cause the pile to collide and break; Step D14: Pile insertion. Check the pile position twice with the rectangular coordinate method according to the set control points. After checking, start to put down the first pile. Align the center point of the first pile with the pile position (within 10mm) and start to pile. When the pile is pressed into the soil for 1m, use two theodolites or plumb bobs at 90° to adjust the verticality of the first pile (the theodolite is usually set up 30m away from the pile driver). When observing, the verticality deviation between the upper and lower ends should be ≤0.3%. Then start to pile. Observe while pressing. If the deviation is exceeded, make adjustments to avoid cracking of the pile body. At the same time, control the verticality of the pile to ≤0.3%. Step D15: pile delivery and elevation control. When delivering piles, the press machine continuously operates and sinks piles at the maximum pressure and full load state to avoid intervals. When the pile delivery rod delivers piles, the level meter is set up more than 20m away from the pile driver to measure and record the ground elevation data, and the pile delivery depth is calculated according to the predetermined design elevation, and a conspicuous mark is marked on the pile delivery rod. When the pile is pressed to about 1m away from the design elevation, the press machine adopts a slow pressurization method to measure and observe the pile delivery situation. When the observation reaches the predetermined design elevation, the pressurization is stopped. When the pile top elevation is finally checked, the deviation is ensured not to exceed 50mm; Step D16: Shifting. After the pile is pressed to the predetermined design elevation, the motor is turned off, and then the pile driver is moved to a new pile position, and the above steps are repeated to construct the next pile.

[0068] When constructing the water-stop mixing pile in step D2, the four-mixing and four-spraying process is adopted, and the water-stop mixing pile is constructed according to the construction parameters. The construction parameters include: the pile diameter is between 600 and 1000 mm, such as Figure 4As shown, two adjacent piles are intersected and the spacing is between 0.4 and 0.7m, the pile length is between 11 and 15m, the cement consumption per meter is between 110 and 150 kg / m, the material water-cement ratio is between 0.5 and 0.6, the sinking speed is 0.8m / min≤v1≤1.2 m / min, the lifting speed is 0.6 m / min≤v2≤1 m / min, the shotcrete flow rate is 55 to 75L / min, and the construction process is referred to Figure 7 The specific steps include: Step D21: Survey and set out, use a total station to lay out the pile positions, check the benchmark points, base axis lines and leveling points of the construction site, make obvious marks and numbering for the calibrated benchmark points and protect them, use theodolite and steel tape measure, etc., use the coordinate method to measure the edge lines of the pile position area, measure and locate all the piles in the construction area, make obvious and reliable pile position marks, and record the measurements for verification; Step D22: After the pile foundation is aligned, leveled, and equipped with mud, the mixer is assembled and moved to the pile position by the walking system and positioned. After the pile driver is positioned and centered, it is stable and round, so that the center of the drill rod and the pile position are on the same plumb line, and the positioning deviation of the mixing head is ≤3cm. While the mixer is pre-stirring and sinking, the cement slurry is mixed according to the predetermined mix ratio. The amount of cement per shaft of the mixing pile is determined according to the test pile, and the amount of cement per shaft of the empty mixing section is tentatively set at 30kg / m, which is specifically determined in the early stage of construction based on the actual construction statistics of 30 to 50 piles; Step D23: After the positioning is completed, start the mixing motor, wait until the mixer head speed is normal and the cooling water circulation of the deep mixer is normal, loosen the crane wire rope, and start the mortar pump when the drill bit reaches the ground, so that the mixer mixes and cuts the soil along the guide frame and sinks to the designed pile bottom elevation and controls the sinking speed within 1.2m / min according to the construction parameters, and prepare cement slurry and pour it into the aggregate hopper before spraying and mixing; Step D24: After the mixer sinks to the pre-designed depth, lift the mixing head and continuously press the cement slurry from the central tube of the mixer into the foundation, spraying and stirring at the same time, and lift the mixer at the lifting speed of the construction parameters (within 1m / min). When the deep mixer sprays and lifts to 0.3-0.5m above the designed pile top elevation, the first grouting lifting and stirring is completed; Step D25: To mix the soft soil and cement slurry evenly, repeat the spraying and mixing until the pile bottom elevation is designed, and then lift the mixer out of the ground. During the entire mixing process, keep the mortar pump turned on to continuously press the cement slurry into the foundation. Check the piles 2 to 3 meters below the pile top every 5 minutes, and spray more cement slurry according to the lack of mortar. Step D26: Repeat the spraying and mixing to raise the pile to 30-50 cm above the designed pile top elevation; Step D27: Pull out the mixer shaft to above the ground; Step D28: Clean the construction tools, including the aggregate hopper, mortar pump, shotcrete pipe, and mixing head. Pour clean water into the aggregate hopper, start the mortar pump, clean the residual cement slurry in all pipes until they are clean, and clean the soft soil adhering to the mixing head. Step D29: After the deep mixer is lifted out of the ground, turn off the motor, then move the pile driver to a new pile position and repeat the above steps to construct the next pile.

[0069] Before construction, test piles are constructed first, and the construction is guided by the test pile results. The test pile construction also adopts four-mixing and four-spraying, and the test piles are carried out according to the three sets of parameter construction parameters of the water-stop mixing pile test piles. Since the construction parameters of the water-stop mixing piles are determined based on a large number of experiments in the early stage, the test piles are mainly determined based on the actual geological conditions. Therefore, only some of the parameters are changed, and most parameters remain consistent.

[0070] According to the predetermined design requirements, the test results of the pile test include the coring test of the water-stopping mixing pile. The 28-day pile strength is not less than 1.0MPa, the cement is evenly mixed within the pile body, and the verticality deviation of the pile body is not more than 1 / 100. The specific construction parameters are determined by the test pile results. According to the test pile results, the formal construction process adopts the four-mixing and four-spraying construction process. The construction parameters are implemented as follows: pile diameter D800mm, spacing 0.55m, pile length 13m, cement consumption per meter 110kg / m, water-cement ratio 0.55, mud specific gravity 1.776, single cement consumption 1430, sinking speed not more than 1.2m / min, but it cannot be too low to affect the construction efficiency, at least 0.8m / min, take 1.2m / min, lifting rate not more than 1m / min, and it cannot be too low to affect the construction efficiency, at least 0.6m / min, take the upper limit of 1m / min for construction.

[0071] By designing the water-stop mixing pile construction process steps of steps D21 to D29, the construction quality of the water-stop mixing piles can be improved, the water stopping of deep foundation pit support can be effectively achieved, the leakage of groundwater into the foundation pit can be avoided, and excavation can be smoothly achieved.

[0072] As one of the preferred implementations, in steps D5 and D7, when constructing the first anchor cable 7 and the second anchor cable 9, a trial anchoring construction of the anchor cable is first carried out. The design method of this scheme adopts the two-line anchor cable support form of the first anchor cable 7 and the second anchor cable 9. Due to the geological conditions of the gravel layer, in order to determine the ultimate bearing capacity and safety factor of the anchor cable in the stratum, reveal the various influencing factors and their influence degree that affect the anchoring force of the anchor cable under the stratum conditions, test the construction process of the pre-set anchor cable project and check the design parameters to ensure the safety, economy and rationality of the anchoring project, and carry out a trial anchoring construction of the prestressed anchor cable before the formal construction. The anchor cable hole is formed by a casing follow-up mechanical hole forming method. The hole diameter is 120-180mm and the length is 18-25m. 3+1 bundles or 4+1 bundles of two types of steel stranded wire detachable core prestressed anchor cables are used. The anchor cable grouting adopts P.0 42.5 silicate mixing, water-cement ratio of 0.40 ~ 0.45, ash-sand ratio of 0.50 ~ 1.00, using detachable core mechanical recyclable anchor cable, one of the designed steel strands is unlocked steel strand, so that the prestressed anchor cable can be removed and recovered after the completion of the foundation pit basement structure construction. The diameter of the steel strand is between φ12 and φ18, the standard value of the axial internal force of the anchor cable is 250kN, 380kN, 450kN, and the locking value is 180kN, 200kN, 250kN. Before the trial anchoring construction, the soil layer distribution and the physical and mechanical properties of each soil layer in the construction area are first detected and determined, and the surrounding underground pipelines, structures and facilities are identified and avoided. The slurry strength of the anchor cable anchoring section reaches 20MPa Or when the design strength grade reaches 80%, the anchor cable pull-out test is carried out. When drilling, the drill rod and the casing are rotated synchronously section by section to prevent the hole wall from collapsing or the flowing sand from filling the hole during the drilling process. Grouting is carried out in time after the hole is formed to avoid hole collapse. The construction content of the two-way prestressed anchor cable support includes the following steps: Step D571: determine the anchor hole position, locate the anchor hole, and process and manufacture the anchor cable; Step D572: Positioning the drilling rig, during which the anchoring position and anchoring angle are determined; Step D573: drilling, after the drilling is completed, checking the hole depth and installing the anchor cable; Step D574: Grouting, including primary grouting and secondary grouting. The secondary grouting time is determined according to the grouting process test or after the strength of the primary grouting anchor body reaches 5MPa. When the strength of the anchor body is greater than 20MPa, the anchor cable is tensioned and locked. After the anchor cable is tensioned, the free section is grouted. The grouting material is the same as the anchoring section. When grouting the free section, avoid the grouting slurry from entering the stress-bearing bars in the free section to ensure the free expansion and contraction of the stress-bearing bars in the free section. During the construction of the anchor cable, pull-out test and creep test are carried out to ensure the construction quality of the anchor cable. Step D575: Curing after grouting, tensioning and locking after curing.

[0073] After the trial anchoring, the anchor cable pull-out test is carried out to determine whether the construction site with gravel geological conditions meets the predetermined design requirements, verify the construction process and check the design parameters, and determine the ultimate bearing capacity and safety factor of the anchor cable in the stratum, as well as the factors affecting the anchoring force of the anchor cable under the geological conditions and their degree of influence, so as to ensure that the two anchor cables in all parts of the construction foundation pit meet the anchoring requirements, thereby ensuring the safety of the foundation pit support; through the trial anchoring and anchor cable pull-out test, combined with the numerical simulation analysis of step C in the design method, it can further ensure that the anchoring project of the two anchor cables in this design scheme is carried out safely, economically and reasonably; As one of the preferred implementation methods, before the formal construction of the two prestressed anchor cables, an on-site anchor pulling test and a recyclable test of the prestressed anchor cables are carried out to verify the recyclability of the prestressed anchor cables. When the basement structure is constructed after the excavation is completed, the support is replaced and the prestressed anchor cables are recovered. Fig.26 and Fig. 27 As shown, the replacement support plate includes two filled plain concrete strips 41 respectively located outside the basement floor 39 and the basement negative first floor 40. The thickness of the two filled plain concrete strips 41 is between 350 and 600 mm. The replacement support construction is interspersed with the basement structure construction. After each section of the underground second floor structure is completed, the replacement support construction of the section is carried out, including the following construction steps: Step D111: construction preparation; Step D112: Construction of basement floor 39 and first filling plain concrete belt 41; Step D113: basement structure construction, including construction of basement exterior wall 38 and basement negative first floor floor 40, as well as construction of waterproofing and protective layer of negative second floor exterior wall; Step D114: backfilling the sandy soil 37 and constructing the second filling plain concrete belt 41; Step D115: dismantling and recovering the second anchor cable 9; Step D116: backfilling the sandy soil to the bottom of the crown beam 6; Step D117: dismantle and recover the first anchor cable 7.

[0074] By adopting the above method, the PRC prestressed pipe piles are supported and fixed by backfilling sandy soil and filling plain concrete belts. One side of the backfill sandy soil and the filling plain concrete belt relies on the two-layer bottom plate structure of the basement structure for force transmission and fixation, thereby replacing the prestressed anchor cable support structure to ensure the safety of the basement structure and support. At the same time, the prestressed anchor cables are removed and recycled, which can be reused. After the prestressed anchor cables are removed, the surrounding construction is avoided from being affected, resulting in the surrounding construction excavation process. The damage to the prestressed anchor cables affects the support safety of the foundation pit, forming a safety hazard.

[0075] As one of the preferred implementations, when shotcrete construction is involved in the process of support and excavation construction using a support system, concrete with an early strength agent added is used, and an early strength test is carried out before shotcrete is sprayed. Multiple groups of concrete with different water-cement ratios and whether or not an early strength agent or a water reducing agent is added are used to test their strength changes in different time periods, and it is determined to use concrete with 0.5% FCM-10 nano early strength agent added.

[0076] After the foundation pit is excavated, the supporting structure will have a certain displacement. If the displacement is too large or develops too quickly, it will cause serious consequences. On the one hand, the displacement development can be stopped by strengthening the supporting system structure while excavating. On the other hand, by increasing the strength grade of concrete and adding early strength agent, the concrete can increase its strength as soon as possible, which also plays a key role in stopping the displacement development and giving full play to the "horizontal support" of the cushion layer as soon as possible. The early strength test comparative analysis includes multiple groups of different concretes, and tests their strength at 3.5 days, 7 days and 11 days, and calculates the growth rate. The specific test and comparative results are analyzed as follows: From the analysis of the above test results, we can see that adding 0.5% FCM-10 nano early strength agent has the advantages of significantly increasing the early cement strength, partially improving the workability of cement slurry, and being suitable for pumping construction of slurry with a small water-cement ratio. However, adding 2% Csp-11 water reducer has no obvious increase in the early cement strength or even a decrease, and increases the amount of cement, resulting in increased costs. There is also the problem of cement and water segregation affecting construction quality. Therefore, concrete with 0.5% FCM-10 nano early strength agent is used.

[0077] During the test, the factors influencing the strength growth of the grouting body of the PRC prestressed pipe pile anchor system were collected, the key factors were found out, and the reasonable technical parameters for application were determined. Example 2

[0078] This embodiment shows an implementation method for arranging a monitoring system for a deep foundation pit support system structure of a PRC prestressed pipe pile and a formal construction process of the foundation pit excavation in a gravel layer. The monitoring system is used to monitor the stress of the support system structure and the settlement of the foundation pit. The internal force and deformation data of the PRC prestressed pipe pile anchor cable support method in the actual soil layer are grasped through the monitoring system, so as to grasp the change and development law of the support structure during the deep foundation pit excavation process, ensure that the prestressed anchor cable reaches the designed prestress value in the underground soil layer, and ensure that the PRC prestressed pipe piles pass through the medium-dense gravel sand layer smoothly while the changes in bending moment, stress, etc. meet the requirements.

[0079] The gravel-sand layer PRC prestressed pipe pile deep foundation pit support monitoring system of this embodiment includes a first monitoring subsystem, a second monitoring subsystem, a third monitoring subsystem, a fourth monitoring subsystem and a fifth monitoring subsystem. Figure 1 and Figure 8 to Figure 12 As shown, the monitoring points of each monitoring subsystem are arranged as follows: The first monitoring subsystem is used to monitor the vertical displacement and settlement displacement of the top of the retaining wall, such as Figure 8 As shown, it includes a plurality of monitoring points WY1 to WY14 arranged at the slope position 29 (slope top), and monitoring points WY1, WY2, WY3, WY4, WY5, and WY6 are arranged at the segmented positions A, B, C, D, E, and F, respectively. At the same time, a monitoring point WY7 is arranged in the middle of the BC segment, a monitoring point WY8 is arranged in the middle of the CD segment, monitoring points WY9, WY10, and WY11 are arranged between the DE segments, and monitoring points WY12, WY13, and WY14 are arranged between the EF segments. The distance between two adjacent monitoring points does not exceed 20 meters, and they can be appropriately densely arranged, but cannot exceed the maximum range value. The first monitoring subsystem also includes not less than 3 (5 in this embodiment) settlement reference points, a closed loop is formed between the settlement reference points, and they are arranged in an area more than 30 meters outside the foundation pit from the basement contour line. The slope position 29 (slope top) is also called the top of the retaining wall; The first monitoring subsystem is also used to monitor the horizontal displacement of the PRC prestressed pipe pile 2. The monitoring point is set at the top of the supporting pipe pile, that is, the top of the crown beam 6, to monitor the horizontal displacement of the PRC prestressed pipe pile 2. In the same way, no less than 3 horizontal displacement reference points are set, and the building settlement observation points are arranged for monitoring. The foundation trench is excavated to the original soil at the location of the leveling reference point and the monitoring point, and the foundation and observation pier are made on the upper part. The leveling reference point buries the mark stone outside the foundation pit in a stable original soil layer more than 30 meters away from the basement contour line, or the mark is embedded in the exposed bedrock. This monitoring method is used to simultaneously monitor the horizontal position of the PRC prestressed pipe pile, which can be compared with the supporting pipe pile data monitored by other monitoring subsystems to remove data with large errors; The second monitoring subsystem is used to monitor the settlement and displacement of the surrounding soil and its accessories. In this embodiment, it is arranged outside the foundation pit and near the center line of the surrounding roads 12-30 meters away from the basement contour line 34. It mainly monitors the settlement and displacement of roads and pipelines, including rainwater pipelines, sewage pipelines, and electrical pipeline corridors. Fig. 9As shown, it includes W1, W2, W3, W4, and W5 arranged at the segmented parts A, B, C, D, and E. At the same time, since the BC segment is adjacent to the existing residential area, a monitoring point W6 is added between the BC segments. W6 is located on the side of the foundation pit guardrail 24. The monitoring point W6 is added to monitor the settlement safety of the soil and position near the adjacent residential area. The monitoring point W7 is arranged in the middle of the CD segment, and the monitoring points W8, W9, and W10 are arranged between the DE segments. The monitoring point W11 is arranged outside the segmented part E. In addition, since the distance from the outside of the EF segment to the road is more than 30 meters, and there are no related auxiliary facilities and structures, the monitoring points of the second monitoring subsystem are not arranged; The third monitoring subsystem is used to monitor the horizontal displacement of the deep foundation pit. Multiple monitoring points are arranged on the PRC prestressed pipe piles in different sections of the deep foundation pit, such as Fig.10 As shown, monitoring point CX1 is arranged at position B, monitoring point CX2 is arranged in the middle of section CD, monitoring point CX3 is arranged in the middle of section DE, and monitoring point CX4 is arranged in the middle of section EF. Inclinometer and matching inclinometer are used for monitoring by installing an inclinometer pre-buried pipe in the PRC prestressed pipe pile 2; The fourth monitoring subsystem is used to monitor groundwater, such as Fig.11 As shown, it includes multiple monitoring points SW1 to SW5 arranged outside the water-stop mixing pile curtain 4, which are arranged according to the height of the terrain. The monitoring points at the higher terrain are arranged between 8 and 20 meters outside the water-stop mixing pile curtain 4, and the monitoring points at the lower terrain are arranged between 2 and 5 meters outside the water-stop mixing pile curtain 4. In this embodiment, the monitoring point SW1 is arranged in the middle of the AB section, the monitoring point SW2 is arranged in the middle of the CD section, the monitoring point SW3 is arranged in the E section, the monitoring point SW4 is arranged in the F section, and the monitoring point SW5 is arranged in the middle of the EF section. The monitoring points are arranged according to the height of the terrain. By drilling holes and installing water level pipes at the water level monitoring points, the changes in the groundwater level during the excavation of the foundation pit can be monitored; The fifth monitoring subsystem is used to monitor the stress of the anchor cable and the stress of the PRC prestressed piles, and to assist in monitoring the parameters monitored by the other monitoring subsystems. Multiple monitoring points are arranged on the anchor cables at different sections of the deep foundation pit, such as Fig.12 As shown, monitoring point YL1 is arranged on the anchor cable at position B, monitoring point YL2 is arranged on the anchor cable at position C, monitoring point YL3 is arranged on the anchor cable at position D, monitoring point YL4 is arranged in the middle of section DE, and monitoring point YL5 is arranged in the middle of section EF. Each monitoring point is a monitoring section. Two dynamometers are set on each monitoring section. The strength growth law of the anchor cable grouting body in the gravel sand layer is monitored by the dynamometer. In this embodiment, a total of 9 PRC prestressed pipe piles and corresponding prestressed anchor cables are monitored. Fig.13As shown, including the 334# pile at the B side of segment A, the 310# pile at the B side of segment B, the 285# pile between segments BC, the 261# pile at the D side of segment C, the 235# pile between segments CD, and the 210# pile, 185# pile, 160# pile, and 135# pile at segment DE, monitoring sensors are installed on 9 PRC prestressed pipe piles 2, and grooves are cut and installed on the foundation pit excavation surface and the facing soil surface of each PRC prestressed pipe pile to be monitored. The monitoring sensor adopts a densely distributed strain sensing optical cable to monitor the deep horizontal displacement of the PRC prestressed pipe pile, the bending moment of the support pile, the displacement of the support pile, and the shear force of the support pile. The dynamometer and the monitoring sensor are connected through a lead line and transmit the data communication to the monitoring station 42 via the main optical cable.

[0080] An electrical box is arranged at the head of each PRC prestressed pipe pile. All the leads are placed in the electrical box and protected for later connection with the main optical cable. The densely distributed strain sensing optical cable is based on the pile body analysis principle of optical fiber strain and is used to measure the bending moment, disturbance and shear force of the support pile.

[0081] The monitoring readings of each monitoring point are transmitted to the computer server of the monitoring station through communication, and it is judged whether the monitored data meets the quality requirements, and the monitored data is compared and analyzed with the numerical simulation data, which can not only judge the accuracy of the numerical simulation analysis results, but also verify that the designed gravel-sand layer PRC prestressed pipe pile deep foundation pit support system meets the use requirements, thereby determining the formation of a certain gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction technology.

[0082] Taking the monitoring parameters of PRC prestressed pipe piles as an example, the monitoring situation of the monitoring system on the PRC prestressed pipe pile deep foundation pit support system in gravel and sand layer is explained.

[0083] like Fig.14 As shown in the figure, the horizontal displacement of 135# pile and 160# pile at different buried depths is shown, and the horizontal displacement of other supporting pipe piles is no longer shown. According to the displacement change, it can be seen that the bending strain distribution and deep horizontal displacement distribution of each supporting pile are relatively consistent, and all of them have obvious deformation at about 4~8m. The maximum horizontal displacement occurs at the middle excavation position of the pile, and the maximum horizontal displacement is about 11.62mm; the average maximum horizontal displacement is about 8mm. The strain of the pile body changes continuously, and there is a small deformation transfer at the bottom of the pile. During the entire foundation pit excavation, the deformation distribution of each pile is relatively consistent, and all of them have obvious deformation at about 4~8m. The maximum horizontal displacement occurs at the middle excavation position of the pile. Among them, the 185# pile has the largest deformation, and the maximum horizontal displacement is about 11.62mm; the 285# pile has the smallest deformation, and the maximum horizontal displacement is about 3.0mm. The strain of all piles changes continuously, and there is a small deformation transfer at the bottom of the pile. The warning value of the horizontal displacement of the supporting pipe pile is 15mm, and the alarm value is 20mm.

[0084] like Fig.15 As shown, the bending moments of 135# and 160# piles at different buried depths are displayed, and the bending moments of the other supporting pipe piles are no longer displayed. It can be seen that during the entire foundation pit excavation period, the bending moment distribution patterns of the piles are slightly different. Among them, 160#, 185#, 210#, and 310# piles are located in the middle of the northeast and southwest sides of the foundation pit, and the bending moment distribution is relatively consistent, with the maximum bending moment between 4-10m, and there are some bending moment changes at the bottom of the piles. 235#, 261#, and 285# piles are located at the arc-shaped corner on the south side of the foundation pit, and the bending moment distribution is relatively consistent, with large bending moment changes within 2m of the pile bottom. The bending moment warning value of the supporting pipe piles is 300KN.m, and the alarm value is 350KN.m.

[0085] like Fig.16 As shown in the figure, the shear force of 135# and 160# piles at different buried depths is displayed, and the shear force of the other supporting pipe piles is no longer displayed. It can be seen that during the entire foundation pit excavation, the shear force distribution of each pile is slightly different. Among them, 160#, 185#, 210#, and 310# piles are located in the middle of the northeast and southwest sides of the foundation pit, and the shear force distribution is relatively consistent, with the largest shear force between 2-6m, and there are some shear force changes at the bottom of the pile. 235#, 261#, and 285# piles are located at the arc corner on the south side of the foundation pit, and the shear force distribution is relatively consistent, with large shear force changes within 2-3m of the pile bottom. The shear force warning value of the supporting pipe pile is 150KN, and the alarm value is 180KN.

[0086] like Figure 17-19 As shown, after 120 days of cumulative monitoring, the vertical settlement of the top of the retaining wall was 3.6mm~7.9mm, the early warning value was 10mm, and the alarm value was 20mm, and none of them exceeded the warning value.

[0087] like Figure 20-21 As shown, after 118 days of cumulative monitoring, the maximum cumulative settlement of surrounding roads and pipelines was 13.5mm, the early warning value was 20mm, and the alarm value was 30mm, and none of them exceeded the warning value.

[0088] like Fig. 22 As shown, the internal force of the anchor cables at monitoring points YL1-YL3 for a total of 120 days is displayed. The other monitoring points are not displayed. It can be seen that the maximum change rate of the internal force pressure change of the anchor cables is 1.2KN / d, the early warning value is 1.5KN / d, and the alarm value is 2KN / d. None of them exceeds the warning value.

[0089] like Fig.23As shown, the cumulative decline of monitoring points SW1~SW5 over the cumulative monitoring period of 120 days is displayed. The maximum cumulative decline of groundwater level is 1220mm, the early warning value is 1500mm, and the alarm value is 2000mm. None of the warning values ​​have been exceeded.

[0090] like Fig.24 As shown, the cumulative displacement values ​​of deep horizontal displacement in the support pipe piles at the four monitoring points CX1 to CX4 are displayed. The maximum value is 28.4 mm, and no warning value is exceeded. The early warning value is 35 mm, and the alarm value is 50 mm, and no warning value is exceeded.

[0091] A further preferred implementation method is that when conducting early warning judgment analysis and processing of monitoring points, ① the stage deformation rate and cumulative deformation of the foundation pit are compared with the control standard. If the stage deformation rate or the cumulative deformation value is less than the warning value, it is a normal state; if the stage deformation rate or the cumulative deformation value is greater than the warning value but less than the alarm value, it is a warning state; if the stage deformation rate or the cumulative deformation value is greater than the alarm value but less than the control value, it is an alarm state; if the stage deformation rate or the cumulative deformation value is greater than the control value, it is a control state; ② if the data shows that the warning standard is reached, the inspection information should be combined to comprehensively analyze the construction progress, construction measures, support and enclosure structure stability, and surrounding environment stability status, and make a comprehensive judgment; ③ when the analysis confirms that there is an abnormal situation, measures should be taken to deal with it, including adding support structures, adjusting support status, and adding auxiliary support.

[0092] When analyzing and processing early warning judgments at monitoring points, multiple stages including early warning values, alarm values, and control values ​​are set, and the stage monitoring data of the foundation pit is divided into states, so as to distinguish and process the monitoring system of the foundation pit support and focus on monitoring. By dividing the foundation pit deformation rate and cumulative deformation of the foundation pit support monitoring system into different states, when a monitoring subsystem or a support structure is in an abnormal state, the series of monitoring data used to monitor the object may all be concentrated in a certain state, which is convenient for control and processing.

[0093] The design method of the gravel layer PRC prestressed pipe pile deep foundation pit support system of this scheme also includes analyzing and calculating the construction costs of different support systems to obtain the economic efficiency of the PRC prestressed pipe pile anchor cable system. For example, compared with the internal support of reinforced concrete cast-in-place piles, it saves 15 days of construction time and saves about 203 yuan per linear meter. Through comparative analysis of the economic efficiency of the support system, the superiority and economy of the gravel layer PRC prestressed pipe pile deep foundation pit support system and construction method determined by the design method of this technical scheme are obtained.

[0094] The above is only a detailed description of the specific implementation of the present invention, rather than a limitation of the present invention. Various substitutions, modifications and improvements made by those skilled in the relevant art without departing from the principle and scope of the present invention should be included in the protection scope of the present invention.

Claims

1. A design method for a gravel-sand layer PRC prestressed pipe pile deep foundation pit support system, characterized in that: The design method includes the following steps: Step A: Geological exploration to determine the geological composition under the geological conditions of the gravel layer; Step B: designing a gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to geological conditions, comprising PRC prestressed pipe piles (2) arranged at intervals along the side of the foundation pit (1) to form a row of piles, two prestressed anchor cables (5) are arranged at the upper part of the foundation pit (1), comprising a first anchor cable (7) locked on a crown beam (6) and a second anchor cable (9) locked on a waist beam (8), the crown beam (6) is connected to the top of the PRC prestressed pipe piles (2), and the waist beam (8) is arranged at the upper middle part of the PRC prestressed pipe piles (2); Step C: Numerical simulation analysis to determine whether the foundation pit support form meets the requirements; Step D: Formal construction: According to the PRC prestressed pipe pile deep foundation pit support system in step B, the construction process is determined and formal construction is carried out, and a monitoring system is arranged for monitoring; Step E: Analyze the monitoring data and determine whether the monitoring data meets the quality standards and requirements, and analyze the differences between the monitoring data and the numerical simulation analysis results; Step F: When the monitoring results in step E are determined to meet the quality standards and requirements, a determined gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction process are formed; when the monitoring results in step E are determined to fail to meet the quality standards and requirements, the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system and construction process are adjusted.

2. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 1 is characterized in that: In the step B, when designing the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system, a row of water-stop mixing piles (3) is further arranged outside the PRC prestressed pipe piles (2), and two adjacent water-stop mixing piles (3) are arranged to intersect each other, thereby forming a water-stop mixing pile curtain (4).

3. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 2 is characterized in that: In step B, when designing a gravel-sand layer PRC prestressed pipe pile deep foundation pit support system, it also includes designing and arranging a reinforcement structure (10) between the PRC prestressed pipe piles (2), including first anchoring an anchor steel pipe (11) in the soil between the PRC prestressed pipe piles (2), anchoring the anchor end of the anchor steel pipe (11) in the water-stop mixing pile curtain (4), and then pressing reinforcing ribs (12) between the waist beam (8) and the PRC prestressed pipe piles (2), and then connecting a steel mesh (20) to the anchor steel pipe (11), wherein the reinforcing ribs (12) include transverse ribs (13), vertical ribs (14) and diagonal ribs (15).

4. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 2 is characterized in that: In step D, when the PRC prestressed pipe pile deep foundation pit support system is used for support and excavation, the following construction steps are adopted: Step D1: construct PRC prestressed pipe piles (2) along the sides of the foundation pit; Step D2: constructing water-stop mixing piles (3) outside the PRC prestressed pipe piles (2), wherein the water-stop mixing piles (3) are arranged to intersect with each other to form a water-stop mixing pile curtain (4); Step D3: excavating topsoil; Step D4: construct the slope-cutting area (29) and perform slope protection; Step D5: constructing a crown beam (6) and a first anchor cable (7), wherein the first anchor cable (7) is anchored to the top of the PRC prestressed pipe pile (2) through the crown beam (6); Step D6: excavating the foundation pit (1) in layers, and spraying the support system concrete of the excavated soil layers; Step D7: constructing a waist beam (8) and a second anchor cable (9), wherein the second anchor cable (9) is fixed to the PRC prestressed pipe pile (2) through the waist beam (8), and the waist beam (8) is arranged at a middle upper portion of the PRC prestressed pipe pile (2); Step D8: excavating the foundation pit (1) in layers to the bottom of the pit and spraying concrete; Step D9: Construction of basement structure.

5. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 4 is characterized in that: When constructing the PRC prestressed pipe piles (2) along the sides of the foundation pit (1) in step D1, a trial pile construction is first carried out, and formal construction is carried out after obtaining construction process parameters. The construction parameters include the pile driving speed, the static pressure during the pile driving process, the final pressure when reaching the predetermined elevation, the pile length and the quality standard after the pile is completed. The formal construction steps include: Step D101: measuring and laying out the positions of the PRC prestressed pipe piles to be constructed; Step D102: Weld the pile tip and hoist and center it. Before hoisting and centering, control the lifting point and verticality. The deviation between the lifting point center and the pile position should be within 10mm, and the verticality deviation between the upper end and the lower end should be ≤0.3%; Step D103: Press the pile to a predetermined elevation and control the pressure, and monitor and control the verticality of the pile body during the pressing process to ensure that the verticality deviation between the upper end and the lower end of the pile body is ≤0.3%; Step D104: Move the pile to the next pile position for construction.

6. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 5 is characterized in that: In step D, when the monitoring system is used to support the PRC prestressed pipe pile deep foundation pit support system, the prestressed anchor cable (5) is obliquely anchored in the gravel and sand geological layer outside the foundation pit, and a monitoring system including a first monitoring subsystem, a second monitoring subsystem, a third monitoring subsystem, a fourth monitoring subsystem and a fifth monitoring subsystem is arranged on the support system structure and outside the foundation pit, wherein: The first monitoring subsystem is used to monitor the vertical displacement and settlement displacement of the top of the retaining wall, including a plurality of monitoring points WY1-WY14 arranged at the sloped portion (29), and no less than 3 settlement reference points, the settlement reference points are arranged in an area more than 30 meters away from the basement contour line (34) outside the foundation pit, and a closed loop is formed between the settlement reference points. The first monitoring subsystem is also used to monitor the horizontal displacement of the PRC prestressed pipe pile (2), the monitoring points are arranged at the top of the PRC prestressed pipe pile (2), and no less than 3 horizontal displacement reference points are arranged; The second monitoring subsystem is used to monitor the settlement and displacement of the surrounding adjacent soil and its accessories, including a plurality of monitoring points W1 to W11 arranged outside the foundation pit and within a range of 12 to 30 meters from the basement contour line (34); The third monitoring subsystem is used to monitor the deep displacement of the foundation pit, including a plurality of monitoring points CX1 to CX4 arranged on the PRC prestressed pipe pile (2); The fourth monitoring subsystem for monitoring groundwater, including a plurality of monitoring points SW1 ~ SW5 arranged outside the water-stop mixing pile curtain (4); The fifth monitoring subsystem is used to monitor the stress of the anchor cables and the stress conditions of the PRC prestressed pipe piles, and to assist in monitoring the parameters monitored by the other monitoring subsystems, including a plurality of monitoring points YL1 to YL5 arranged on the prestressed anchor cables (5) at different locations, and densely distributed strain sensing optical cables arranged on a plurality of PRC prestressed pipe piles (2), to monitor various parameters of the foundation pit support through optical fiber sensors.

7. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 6 is characterized in that: Each monitoring point is equipped with monitoring instruments, which are installed using the following steps: Step D1´: installing monitoring sensors on the PRC prestressed pipe piles, which are used to monitor the deep horizontal displacement inside the PRC prestressed pipe piles, the bending moment of the supporting piles, the displacement of the supporting piles and the shear force of the supporting piles; Step D2´: Install vertical displacement monitoring points on the top of the retaining wall. Vertical displacement monitoring includes leveling benchmark points, monitoring points and building settlement observation points. Excavate foundation trenches to the original soil at the leveling benchmark points and monitoring points, and make foundations and observation piers on the upper part. The leveling benchmark points will bury the marker stones in the stable original soil layer outside the foundation pit at a distance of more than 30 meters from the basement contour line, or embed the markers on the exposed bedrock; Step D3´: Install the embedded inclinometer tube in the PRC prestressed pipe pile, and use the inclinometer and the matching inclinometer tube for monitoring, including the embedded inclinometer tube installation in the pile, the drilling and installation of the inclinometer hole, and the measurement and observation; Step D4´: When the anchor cable is tensioned and locked, an anchor cable dynamometer is installed at the anchor head to measure the anchor cable stress, and the monitoring data of the anchor cable dynamometer is transmitted to the monitoring station by communication. The frequency value is read by the computer, and the anchor cable stress value is obtained to achieve the purpose of monitoring the internal force of the anchor cable. At the same time, an alarm value is set, and a warning is proposed based on the stress value obtained by monitoring and calculation. By installing the anchor cable dynamometer, the law of the strength growth of the anchor cable grouting body in the gravel sand layer can be mastered; Step D5´: drilling holes at water level monitoring points to install water level pipes for monitoring the water level of the foundation pit, including installation of measuring points and measurement and observation, and transmitting the measured and observed data to the monitoring station; The steps D1' to D5' are in no particular order, and the monitoring parameter value obtained in each step is set with an alarm value, and whether to issue an early warning is proposed for each monitoring parameter value. When the monitoring parameter value is greater than its corresponding alarm value, an alarm reminder is activated.

8. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 5 is characterized in that: In steps D5 and D7, when constructing the first anchor cable (7) and the second anchor cable (9), a trial anchoring construction is first carried out. The anchor cable holes are formed by a casing-following mechanical hole forming method. The hole diameter is 120 to 180 mm and the length is 18 to 25 m. Two types of steel strands, 3+1 bundles or 4+1 bundles, are used. The prestressed anchor cable with a detachable core is used. One of the steel strands is designed to be an unlocking steel strand. The diameter of the steel strand is between φ12 and φ18. The standard values ​​of the axial internal force of the anchor cable are 250 kN, 380 kN, and 450 kN. The locking values ​​are 180 kN, 200 kN, and 250 kN. The construction content of the two anchor cable supports includes the following steps: Step D571: determine the anchor hole position, locate the anchor hole, and process and manufacture the anchor cable; Step D572: Positioning the drilling rig, during which the anchoring position and anchoring angle are determined; Step D573: drilling, after the drilling is completed, checking the hole depth and installing the anchor cable; Step D574: grouting, including primary grouting and secondary grouting; Step D575: Curing after grouting, tensioning and locking after curing.

9. The design method of gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to claim 5 is characterized in that: Before the formal construction of the two prestressed anchor cables (5), an on-site anchor test and a recyclability test of the prestressed anchor cables are carried out. When the basement structure is constructed after the excavation is completed, the support replacement and prestressed anchor cable recovery are carried out, including two filled plain concrete strips (41) respectively located on the outside of the basement floor (39) and the basement negative first floor floor (40). The thickness of the two filled plain concrete strips (41) is between 350 and 600 mm. The support replacement construction and the basement structure construction are interspersed. After each section of the underground second floor structure is completed, the support replacement construction of the section is carried out, including the following construction steps: Step D111: construction preparation; Step D112: Construction of the basement floor (39) and the first filling plain concrete strip (41); Step D113: basement structure construction, including the construction of the basement exterior wall (38) and the basement negative first floor floor slab (40), as well as the construction of the waterproofing and protective layer of the negative second floor exterior wall; Step D114: backfilling the sandy soil (37) and constructing the second filling plain concrete belt (41); Step D115: dismantling and recovering the second anchor cable (9); Step D116: backfilling the sandy soil to the bottom of the crown beam (6); Step D117: dismantling and recovering the first anchor cable (7).

10. The design method of the gravel-sand layer PRC prestressed pipe pile deep foundation pit support system according to any one of claims 2 to 9, characterized in that: When shotcrete construction is involved in the process of support and excavation using a support system, concrete with an early strength agent added is used, and an early strength test is carried out before shotcrete is shotcrete. Multiple groups of concrete with different water-cement ratios and with or without the addition of early strength agents and water reducing agents are used to test their strength changes in different time periods, and it is determined to use concrete with 0.5% FCM-10 nano early strength agent added.

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