PRC prestressed pipe pile anchor cable deep foundation pit supporting system and construction method thereof

Abstract

The invention relates to the technical field of foundation pit supporting construction, and particularly discloses a PRC prestressed pipe pile anchor cable deep foundation pit supporting system and a construction method thereof.The supporting system comprises PRC prestressed pipe piles which are arranged along the side of a foundation pit at intervals to form row piles, water stop stirring piles are arranged on the outer sides of the pipe piles, and every two adjacent stirring piles are arranged in a crossed mode; two pre-stressed anchor cables are arranged on the upper portion of the foundation pit and comprise the first anchor cable locked on the top beam and the second anchor cable locked on the waist beam, the top beam is connected to the top of the PRC pre-stressed pipe pile, the waist beam is arranged on the upper portion of the middle of the PRC pipe pile, and the first anchor cable and the second anchor cable are arranged on the upper portion of the middle of the PRC pipe pile. The supporting system solves the problem that an existing supporting structure cannot completely resist the pressure of soil around the foundation pit, meanwhile, underground water in the soil body on the outer side of the foundation pit can be prevented from seeping into the foundation pit, large supporting anchoring force is formed on the upper portion and the lower portion of the foundation pit through the supporting system, and the good supporting effect is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of foundation pit support construction, and in particular to a PRC prestressed pipe pile anchor cable deep foundation pit support system and a construction method thereof. Background Art

[0002] With the development of the economy and the acceleration of urbanization, the scale of underground space development is getting larger and larger, the depth of foundation pits is getting deeper and deeper, and the safety requirements of foundation pit projects are getting higher and higher. Different foundation pit support structures can be used for different underground soil layers and foundation pit conditions. Different support forms can adapt to the characteristics of different soil layers.

[0003] Existing foundation pit support forms include different support forms such as support pile support, soil nail wall support, beam support method support, steel support method support, anchor support method support, etc. For support pile support, the existing support pile structure has the problem of poor bending resistance, especially in the deep foundation pit support environment, the support piles cannot completely resist the earth pressure, and there are gaps between the support piles, there are problems of local soil scattering and insufficient support in the deep foundation pit, and it is impossible to form an overall structure to support the deep foundation pit; and soil nail wall support mainly implants prestressed steel bars in the foundation pit soil. Although it can solve the problem of surface support and has a low construction cost, it solves the soil pressure problem and can only be used for foundation pit construction occasions with strong self-stabilization ability; and for beam support method support, although it is simple to construct and has a low cost, it can be used for underground water-containing areas with soft soil. It is suitable for foundation pit excavation with high volume, but on the one hand, its supporting capacity is limited, and on the other hand, there are problems such as inconvenience in foundation pit excavation and low excavation construction efficiency; the steel support method also has problems such as inconvenience in foundation pit excavation and low excavation construction efficiency, and the support process is complicated and the cost is relatively high, and it cannot effectively control the seepage and leakage problems in areas with high underground water content; when the anchor support method is used to support the foundation pit, columns and piles are first driven along the edge of the foundation pit, retaining plates are supported on the inner side of the columns and piles, and the upper ends of the columns and piles are tightened with tie rods for support, but groundwater will accumulate on the rear side of the retaining plates of this support structure, resulting in seepage and leakage problems, and the tie rods on the upper ends of the columns and piles can only anchor and support the upper part of the foundation pit, and cannot achieve anchoring within the overall range of the columns and piles. It can only be used in foundation pit excavation construction occasions with a small depth (usually less than 5 meters).

[0004] From the above, we can see that in the process of deep foundation pit excavation, no matter which of the above foundation pit support structures is used, there are problems that need to be solved, such as large soil pressure, groundwater seepage affecting normal construction, and excavation construction efficiency. In addition, when two adjacent foundation pits are excavated at the same time, but the excavation construction units are different, how to ensure the support safety of the connection parts of the two foundation pits is a common problem encountered in the simultaneous development and application of multiple plots. Therefore, in the process of deep foundation pit excavation, what kind of structural form of foundation pit support structure to use and how to construct the adopted foundation pit support structure to ensure that the support structure is safe, reliable, economically reasonable, technically feasible, and convenient to construct are technical issues that need to be considered in the design and construction of foundation pit support structure. Summary of the invention

[0005] At least one of the purposes of the present invention is to provide a PRC prestressed pipe pile anchor cable deep foundation pit support system and construction method for the problem that conventional or single support structures cannot completely resist soil pressure and are prone to water leakage during the excavation construction of deep foundation pits rich in groundwater. The PRC prestressed pipe pile anchor cable deep foundation pit support system adopts a double pile column support structure, which can solve the problem that the support structure cannot completely resist the soil pressure around the foundation pit under the existing deep foundation pit construction environment, and can form a water-stop mixing pile curtain to prevent groundwater in the soil outside the foundation pit from seeping into the foundation pit. At the same time, by arranging two rows of prestressed anchor cables on the PRC prestressed pipe piles, the PRC prestressed pipe piles form a large supporting anchoring force on the upper and lower parts of the foundation pit, having a better support effect, and at the same time can recycle the steel strands in the anchor cables to reduce construction costs.

[0006] In order to achieve the above objectives, the technical solutions adopted by the present invention include the following aspects.

[0007] A PRC prestressed pipe pile anchor cable deep foundation pit support system comprises PRC prestressed pipe piles arranged at intervals along the side of the foundation pit to form a row of piles, water-stop mixing piles are arranged on the outside of the PRC prestressed pipe piles, and two adjacent water-stop mixing piles are arranged to intersect each other to form a water-stop mixing pile curtain, and two prestressed anchor cables are arranged on the upper part of the foundation pit, comprising 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, and the anchor cable is a detachable core prestressed anchor cable.

[0008] The PRC prestressed pipe pile anchor cable deep foundation pit support system of this scheme forms a double row pile support structure 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 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 deep foundation pit working conditions; By arranging two rows of prestressed anchor cables and injecting cement slurry to form a prestressed anchor cable structure, the support system fixes one end (anchor section) of the tension structure anchor cable in the stable stratum by injecting cement slurry, and the other end is connected to the structure on the PRC prestressed pipe pile to withstand the thrust applied to the structure due to soil pressure, water pressure, etc. This method has more advantages than the anchor support method. The anchor support mainly transmits the tension to the rock and soil layer of the foundation pit, and transmits the soil load to the deep rock and soil layer through the anchor rod. The tension it withstands is limited, while the prestressed anchor cable can withstand greater tension and is very suitable for large-tonnage anchoring projects in deep foundation pits. The prestressed anchor cable is 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 safety of foundation pit excavation support and the safety of the surrounding environment.

[0009] The deep foundation pit described in this technical solution refers to a foundation pit with an excavation depth of 6 meters or more. The support system and support system construction method in this technical solution are suitable for deep foundation pit working conditions with an excavation depth of 6 meters or more.

[0010] 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.

[0011] Preferably, a reinforcement structure is also provided between the PRC prestressed pipe piles, including an anchoring steel pipe anchored in the soil between the PRC prestressed pipe piles, the anchoring end of the anchoring steel pipe ending in the water-stop mixing pile curtain, the reinforcement structure also includes reinforcing ribs pressed between the waist beam and the PRC prestressed pipe piles, and a steel mesh connected to the anchoring steel pipe, the reinforcing ribs including transverse ribs, vertical ribs and diagonal ribs.

[0012] After reinforcing bars are set between the inner side of the PRC prestressed pipe piles and the water-stop mixing pile curtain, two rows of prestressed anchor cables squeeze and support the reinforcing bars through the outer row of piles structure, which can improve the support effect of the support system on the wall of the foundation pit. At the same time, by spraying concrete on the steel mesh, the reinforcing bars, 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.

[0013] Preferably, the PRC prestressed pipe pile anchor cable deep foundation pit support system is applied to the construction conditions of adjacent foundation pit excavation, and 3-5 rotary jet piles are constructed at the overlap of the water-stop mixing pile curtains of the two foundation pits.

[0014] During the excavation and construction of two adjacent foundation pits, synchronous support is usually carried out through coordination. However, due to different construction plans and progress, there are often problems of weak support and water seepage protection at the two adjacent foundation pits. Jet jet piles are constructed at the overlap of the water-stop mixing pile curtains of the two adjacent foundation pits to form a support structure of three rows of piles. This can enhance the compressive strength of the support at the adjacent foundation pits, ensure the safety of foundation pit excavation and support, and avoid water seepage, settlement or collapse at the connection parts.

[0015] Correspondingly, the present application also provides a PRC prestressed pipe pile anchor cable deep foundation pit construction method. When the PRC prestressed pipe pile anchor cable deep foundation pit support system as described above is used for foundation pit support construction, the construction process includes the following steps: Step A: construct PRC prestressed pipe piles along the sides of the foundation pit; Step B: 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 C: excavating the topsoil; Step D: Construction of top slope and slope protection; Step E: 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 F: excavate the foundation pit in layers and spray the support system concrete of the excavated soil layers; Step G: 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 H: excavate the foundation pit in layers to the bottom of the pit and spray concrete; Step I: construct the basement structure, and carry out the replacement of supports and recovery of prestressed anchor cables; Before the construction of two prestressed anchor cables in step E and step G, an on-site anchor pulling test and a recoverable 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 belts located on the outside of the basement floor and the basement negative first floor respectively. The thickness of the two filled plain concrete belts is between 350 and 600 mm. The support replacement construction is interspersed with the basement structure construction. 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 I1: construction preparation; Step I2: Construction of basement floor and first filling plain concrete strip; Step I3: Basement structure construction, including basement exterior wall and basement negative 1 floor slab construction, as well as negative 2 floor exterior wall waterproofing and protective layer construction; Step I4: backfilling sandy soil and constructing the second plain concrete belt; Step I5: dismantling and recovering the second anchor cable; Step I6: backfilling the sandy soil to the bottom of the crown beam; Step I7: dismantling and recovering the first anchor cable; Through construction steps A and B, 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 E and step G, 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] The deep foundation pit described in this technical solution refers to a foundation pit with an excavation depth of 6 meters or more. The support system and support system construction method in this technical solution are suitable for deep foundation pit working conditions with an excavation depth of 6 meters or more.

[0017] Through the method of intersecting the water-stop mixing piles in step B, a water-stop mixing pile curtain can be formed to prevent groundwater in the deep foundation pit from leaking into the deep foundation pit and maintain a good construction environment. At the same time, the foundation pit soil can be supported, forming a double support structure with the PRC prestressed pipe piles, which can provide strong support capabilities and solve the problem of high soil pressure and easy water leakage in the deep foundation pit construction environment.

[0018] When constructing PRC prestressed pipe piles according to step A, the following steps are specifically included: Step A1: firstly conduct a trial pile construction to obtain construction process parameters, including pile driving speed, pile driving pressure, pile length and quality standard after pile construction, wherein the pile driving pressure includes the static pressure during the pile driving process and the final pressure when the predetermined elevation is reached; Step A2: Measure and lay out the positions of the PRC prestressed pipe piles to be constructed; Step A3: 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 and lower ends should be ≤0.3%; Step A4: Press the pile to the predetermined elevation and control the pressure. During the pressing process, monitor and control the verticality of the pile body to ensure that the verticality deviation between the upper and lower ends of the pile body is ≤0.3%; Step A5: Move the pile to the next pile position for construction.

[0019] The above construction steps are used to construct PRC prestressed pipe piles, which can fully ensure that the PRC prestressed pipe piles are at the designed pile position, avoiding problems such as offset or dislocation. The construction process parameters are obtained through trial piles, and the construction process parameters obtained from the trial piles are used during the construction of the PRC prestressed pipe piles. The hanging points and verticality of the supporting pipe piles are controlled to ensure the construction quality of the PRC prestressed pipe piles.

[0020] Preferably, in step B, water-stop mixing piles are constructed according to construction parameters, the construction parameters including: pile diameter D600-D1000, spacing between two adjacent piles 0.4-0.7m, pile length 11-15m, cement consumption per meter 110-150 kg / m, material water-cement ratio 0.5-0.6, sinking speed 0.8m / min≤v1≤1.2m / min, lifting speed 0.6m / min≤v2≤1m / min, shotcrete flow rate 55-75L / min; the specific steps of using the determined construction parameters and constructing water-stop mixing piles with a four-mixing and four-spraying process include the following: Step B1: Survey and lay out the pile positions using a total station; Step B2: Pile foundation alignment, leveling, and mud preparation; Step B3: Shotcrete mixing and sinking to the designed pile bottom elevation, and prepare cement slurry and pour it into the aggregate hopper before shotcrete mixing; Step B4: The shotcrete mixing is lifted to 30-50cm above the designed pile top elevation; Step B5: Repeat the spraying and stirring to sink to the designed pile bottom elevation, and 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 B6: Repeat the spraying and mixing to raise the pile to 30-50cm above the designed pile top elevation; Step B7: Pull out the mixer shaft above the ground; Step B8: Cleaning construction tools, including aggregate hopper, mortar pump, shotcrete pipe, and mixing head; Step B9: The mixer moves to the next pile position for construction.

[0021] The water-stop mixing pile construction is carried out according to the above-mentioned construction parameters and construction steps. The four-mixing and four-spraying refers to two sinking and pulling-out processes, and four mixing and spraying are completed during the sinking and pulling-out processes. This construction method can ensure the quality of the water-stop mixing piles, form a dense structure of water-stop mixing piles, and ensure that the water-stop mixing piles form a water-stop mixing pile curtain, which can effectively prevent the foundation pit groundwater from infiltrating into the inner pile structure and the foundation pit, and smoothly realize excavation, and can also form a strong resistance to the outer soil pressure, thereby improving the support capacity.

[0022] Preferably, in step D, a soil nail support structure is used when constructing the top slope and slope protection structure. Since it is a deep foundation pit excavation with an excavation depth of more than 6m, the slope is cut according to the slope gradient calculated by soil stability theory. When protecting the slope, the slope is repaired first, and then soil nails are placed in the holes, and steel mesh is tied and hung, and finally the concrete surface is sprayed.

[0023] Preferably, in steps E and G, when constructing the first anchor cable 7 and the second anchor cable 9, a trial anchoring of the anchor cables is first performed, and the construction parameters of the trial anchoring of the anchor cables include: a hole diameter of 120 to 180 mm, a length of 18 to 25 m, and a 3+1 bundle or 4+1 bundle of two types of steel strands with detachable core prestressed anchor cables, the diameter of the steel strands is between φ12 and φ18, the standard value of the axial internal force of the anchor cables is 250 kN, 380 kN, 450 kN, and the locking value is 180 kN, 200 kN, 250 kN. 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 and structures and facilities are identified and avoided. When the slurry strength of the anchor cable anchoring section reaches 20 MPa or reaches 80% of the design strength grade, the anchor cable pull-out test is carried out.

[0024] Through the above-mentioned trial anchoring and the anchor pull-out test after the trial anchoring, it can be determined whether the soil conditions at the construction site meet the predetermined design requirements, and the ultimate bearing capacity and safety factor of the anchor in the stratum can be determined, as well as the factors affecting the anchoring force of the anchor under the geological conditions and the degree of their influence, so as to ensure that the two anchors in all parts of the construction pit meet the anchoring requirements, thereby ensuring the safety of the foundation pit support.

[0025] Preferably, during the anchor cable construction in step E and step G, the anchor cable uses a mechanical casing and adopts the pipe drilling construction. During the drilling, the drill rod and the casing are synchronously rotated and drilled section by section. The anchor cable construction specifically includes the following steps: Step E1: determine the anchor hole position, locate the anchor hole, and process and manufacture the anchor cable; Step E2: Positioning the drilling rig, during which the anchoring position and anchoring angle are determined; Step E3: Drilling, after the drilling is completed, check the hole depth and install the anchor cable; Step E4: grouting, including primary grouting and secondary grouting; Step E5: Curing after grouting, tensioning and locking after curing.

[0026] When constructing anchor cables, the construction method of following pipe drilling is adopted, which can prevent the hole wall from collapsing or quicksand from filling the borehole during the drilling process, and grouting operations are carried out in time after the hole is formed to avoid hole collapse; through two grouting operations, the second grouting time is determined according to the grouting process test or after the strength of the anchor body reaches 5MPa, and 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 anchor cable construction.

[0027] When constructing the PRC prestressed pipe pile anchor cable deep foundation pit support system, the PRC prestressed pipe pile anchor cable deep foundation pit construction method simultaneously excavates the foundation pit. The foundation pit excavation includes excavating the first layer of earthwork to 0.1m below the bottom elevation of the crown beam before the crown beam and the first anchor cable in the construction step E. After the slope protection, the crown beam and the first prestressed anchor cable are constructed, the overall excavation is continued to 0.5m below the bottom elevation of the waist beam, and then the waist beam is constructed and the second prestressed anchor cable is installed. After the construction of the step G waist beam and the second anchor cable, the construction step H is carried out to excavate the foundation pit in layers to the bottom of the pit. When the foundation pit is excavated, the foundation pit is divided into two parts according to different functional areas of the plot, including the first area and the second area. At the same time, multiple dividing lines are set along the horizontal and vertical directions of the deep foundation pit to divide the deep foundation pit into multiple strip blocks. The construction steps of the foundation pit excavation include: Step I: Excavate the first layer of earthwork, from the original ground to 0.1m below the top beam. The layer height L1 is set between 2 and 2.5m. The layer height L1 includes the height L11 of the slope protection and the height L12 of the top beam. When excavating, start the excavation along one corner of the foundation pit and then remove the interval. After the excavation, carry out the slope protection, top beam and the first anchor cable construction; Step II: excavate the second and third layers of earthwork. The second and third layers of earthwork are the earthwork between the crown beam and the waist beam. The layer height L2 is set between 3 and 3.5m, and the layer bottom is between 5 and 6m from the original ground. Spray concrete and construct the second zone’s excavation port. After excavation, construct the waist beam and the second anchor cable. The waist beam and anchor cable at the excavation port are not constructed for the time being. Step III: excavate the fourth and fifth layers of earthwork in the first zone. After the construction of the waist beam and the second anchor cable, excavate the earthwork in the first zone to the base first. Set the layer height L3 of this layer to between 3.2 and 4 m. After the excavation is completed, carry out the pile foundation construction of the existing working surface. Step IV: Excavate the fourth and fifth layers of earth in the second zone. After the construction of the engineering piles on the existing working surface is completed, excavate the remaining earth in the second zone and adjust the position of the excavation port for the construction of the waist beams and engineering piles in the unfinished area.

[0028] The above excavation steps can not only ensure the safety of foundation pit excavation construction, but also improve the construction efficiency of the deep foundation pit support system, and transport the excavated soil out of the foundation pit in a timely and orderly manner to ensure the on-site construction environment.

[0029] Preferably, in step G, during the construction of the waist beam and the second anchor cable, waterproof measures are used to control the water and sand gushing in the drilling and grouting process, including the following water and sand gushing prevention construction contents: G1: After the hole is completed, an expansion water-stop sand bag is added at the hole mouth. Since the expansion water-stop yarn has the characteristic of rapid expansion when it comes into contact with water, but it has no strength and can be arbitrarily shaped, it can be made into an expansion water-stop sand bag after being sealed with non-woven fabric; G2: The casing and drill pipe are drilled synchronously and the wall protection and hole forming process is adopted. This construction method can control the slag discharge, and can squeeze and compact the surrounding soil during the drilling process, reduce the disturbance of water and soil outside the casing, and effectively reduce the accumulation of pore water pressure; G3: When water and sand gushing occurs, fix the steel plate with expansion bolts at the hole position and seal it with quick-drying cement. The steel plate should be 3 to 5 meters thick.

[0030] Since the total height of the first floor L1 and the second floor L2 is between 5 and 6 meters, and is located in a relatively thick sand layer, there are construction problems such as water and sand gushing during the construction of the second anchor cable. The use of anti-water and sand gushing construction content can avoid the collapse of holes caused by water and sand gushing, ensure the safety of excavation construction and foundation pit support, and effectively avoid construction safety risks.

[0031] In summary, due to the adoption of the above technical solution, the present invention has at least the following beneficial effects: 1. The PRC prestressed pipe pile anchor cable foundation pit support system of this scheme adopts a double-row pile structure, which is safe and reliable, has stable support, and is technically feasible. Moreover, this support system is convenient for excavation and later construction, and can provide a large enough construction space for the excavation of the foundation pit and the construction of the underground structure. It has the characteristics of convenient construction and high construction efficiency. It has a good effect on the displacement control of the foundation pit slope, can resist the soil pressure during the deep foundation pit excavation process, and can also prevent the problem of groundwater leakage; 2. In the support system, after reinforcing bars are set between the inner side of the PRC prestressed pipe pile and the water-stop mixing pile curtain, two rows of anchor cables squeeze and support the reinforcing bars through the outer pile row 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 bars, steel mesh and the outer pile row structure form a whole, forming an overall closed surface support structure, which can not only improve the overall support capacity, but also avoid the problem of debris falling from the inner wall of the foundation pit affecting the normal excavation of the foundation pit; 3. The PRC prestressed pipe pile anchor cable deep foundation pit support system construction method of the present application adopts waterproof measures to control the water and sand gushing in the drilling and grouting process, thereby solving the problem that the prestressed pipe pile is prone to water and sand gushing in the construction of thick and dense sand layers. In addition, by setting the first anchor cable and the second anchor cable and grouting in batches, the strength of the prestressed anchor cable grouting body is improved, thereby solving the problem that the prestressed anchor cable can only support and anchor part of the soil in the deep foundation pit support structure; 4. Through the above-mentioned test anchoring and the pull-out test of the anchor cable after the test anchoring, it can be determined whether the soil conditions of the construction site meet the predetermined design requirements, and 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; 5. Through the recycling of prestressed anchor cables, the PRC prestressed pipe piles are supported and fixed by backfilling sandy soil and filling plain concrete belts. One side of the backfilled 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 recycled prestressed anchor cables can be removed for reuse. Moreover, after the prestressed anchor cables are removed, the surrounding construction can be 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1It is a schematic diagram of the elevation structure of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0033] Figure 2 It is a schematic plan view of the structure of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0034] Figure 3 It is a schematic flow chart of the construction method of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0035] Figure 4 The present invention is a construction flow chart of PRC prestressed pipe piles in the PRC prestressed pipe pile anchor cable deep foundation pit support system.

[0036] Figure 5 The present invention is a construction flow chart of water-stop mixing piles in the PRC prestressed pipe pile anchor cable deep foundation pit support system.

[0037] Figure 6 The present invention is a construction flow chart of prestressed anchor cables in the PRC prestressed pipe pile anchor cable deep foundation pit support system.

[0038] Figure 7 It is a structural schematic diagram of the reinforcement structure in the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0039] Figure 8 It is a schematic diagram of the relative position arrangement of prestressed pipe piles and water-stop mixing piles in the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0040] Fig. 9 It is a schematic plan view of the first layer of earthwork excavation during foundation pit excavation in the construction method of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0041] Fig.10 It is a schematic plan view of the second and third layers of earthwork excavation during foundation pit excavation in the construction method of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0042] Fig.11 It is a schematic plan view of earth excavation in the first zone during foundation pit excavation in the construction method of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0043] Fig.12 It is a schematic plan view of earth excavation in the second zone during foundation pit excavation in the construction method of the PRC prestressed pipe pile anchor cable deep foundation pit support system of the present invention.

[0044] Fig.13 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.

[0045] Fig.14 Schematic diagram of the structure after removing the first anchor cable after backfilling sandy soil to the bottom elevation of the crown beam.

[0046] Markings in the figure: 1- foundation pit, 1A- adjacent foundation pit, 2- PRC prestressed pipe pile, 3- water-stop mixing pile, 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 pile, 19-sprayed concrete layer, 20-reinforcement mesh, 21-soil nails, 22-pipes, 23-drainage ditch, 24-guardrail, 25-intercepting ditch, 26-plain fill layer, 27-silty clay layer, 28-gravel and sand layer, 29-slope portion, 30-first zone, 31-second zone, 32-dividing line, 33-excavation outlet, 34-backfill sandy soil, 35-basement exterior wall, 36-basement floor slab, 37-basement negative first floor floor slab, 38-filled plain concrete belt. DETAILED DESCRIPTION

[0047] 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

[0048] Figure 1 and Figure 2 The present invention shows an exemplary embodiment of a PRC prestressed pipe pile anchor cable deep foundation pit support system, which includes PRC prestressed pipe piles 2 arranged at intervals along the side of a foundation pit 1, water-stop mixing piles 3 are arranged on the outer side of the PRC prestressed pipe piles 2, and two adjacent water-stop mixing piles 3 are arranged to intersect each other to form a water-stop mixing pile curtain 4, and two prestressed anchor cables 5 are arranged on 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. As a preferred embodiment, a reinforcement structure 10 is further provided between the PRC prestressed pipe piles 2. Figure 7As shown, it includes an anchoring steel pipe 11 anchored in the soil between the PRC prestressed pipe piles 2, the anchoring end of the anchoring steel pipe 11 is in the water-stop mixing pile curtain 4, and must not penetrate the water-stop mixing pile curtain 4. The anchoring steel pipe 11 adopts a φ48-3 steel pipe with a length of 0.8 to 1.2 meters. This embodiment uses a 1-meter-long anchoring steel pipe 11. The reinforcement structure 10 also includes a reinforcing rib 12 pressed between the waist beam 8 and the PRC prestressed pipe pile 2. The reinforcing rib 12 includes a transverse rib 13, a vertical rib 14 and a diagonal rib 15. The diameter of the reinforcing rib 12 is φ14-φ2 0, a vertical reinforcement 14 is arranged between each PRC prestressed pipe pile 2, the interval between the transverse reinforcements 13 is between 0.9 and 1.5 meters, the interval between the transverse reinforcements 13 between the waist beam 8 and the crown beam 6 is between 0.9 and 1.4 meters, and the oblique reinforcements are arranged in a 45-degree direction and are between 0.9 and 1.4 meters apart. In this embodiment, φ16 reinforcement bars are used. The reinforcement structure 10 also includes a steel mesh 20 connected to the anchor steel pipe 11. The steel mesh 20 is a mesh structure woven with steel bars with a diameter of φ6 or φ8 and a steel bar interval of 120 to 180 mm.

[0049] The foundation pit project in which the support system is applied is 98.5 meters long, 83.5 meters wide and about 8.45 meters deep. The support system includes 2 PRC prestressed pipe piles and two recyclable prestressed anchor cable structures. The first anchor cable 7 is arranged at an elevation of about 3.5 meters above the absolute elevation. The anchor cable consists of 4 bundles of steel strands with a diameter of 15.2 mm. The length of the steel strand is L=18 meters, the spacing is 1.6 meters, the axial horizontal angle is 25°, the free section is 6 meters, the standard value of the axial internal force of the anchor cable is 250KN, and the locking value is 180KN. The second anchor cable 9 is at an elevation of about 0 meters above the absolute elevation. The anchor cable consists of 5 bundles of steel strands with a diameter of 15.2 mm. The steel strand is L=18 meters, the spacing is 1.6 meters, the axial horizontal angle is 25°, the free section is 5 meters, the standard value of the axial internal force is 380KN, and the locking value is 250KN.

[0050] In view of the geological conditions that the construction site contains a large area of ​​medium-dense gravel and sand layers, from the ground downward, there are plain fill layer 26, silty clay layer 27, gravel and sand layer 28, gravel clay layer 16 and fully weathered granite layer 17. PRC prestressed pipe piles plus two recyclable prestressed anchor cables are used as the support system. The anchoring end of the first anchor cable 7 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 layer 17, ensuring that the recyclable prestressed anchor cable reaches the designed prestress value in the underground soil layer.

[0051] For another working condition where two adjacent foundation pits are excavated, such as Figure 2As shown in the figure, an adjacent foundation pit 1A which is under excavation construction next to the foundation pit 1 has a discontinuous problem in the construction of the water-stopping structure at the two adjacent foundation pits, which is prone to cold joints, unstable support and water seepage in the foundation pit, increasing the risk of foundation pit collapse. 3 to 5 rotary jet piles 18 are constructed at the overlap of the water-stopping mixing pile curtains of the two adjacent foundation pits, thereby forming a support structure of three rows of piles, which can enhance the compressive strength of the support at the adjacent foundation pits, ensure the safety of foundation pit excavation and support, and avoid water seepage, settlement or collapse at the connection parts.

[0052] The PRC prestressed pipe pile anchor cable deep foundation pit support system also includes a sprayed concrete layer 19 arranged at the 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 of the foundation pit, first drive the soil nails 21, and then hang the steel mesh 20 on the soil nails 21 for pouring. A 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.

[0053] The construction of water-stop piles at the joints of the foundation pit may be discontinuous, which may easily cause cold joints and increase the risk of water seepage in the foundation pit. The excavation and construction of the foundation pit may be asynchronous and uncoordinated, increasing the risk of foundation pit collapse. Example 2

[0054] This embodiment provides a PRC prestressed pipe pile anchor cable deep foundation pit construction method. When the PRC prestressed pipe pile anchor cable deep foundation pit support system in Example 1 is used for foundation pit support construction, combined with Figure 1 and Figure 2 ,refer to Figure 3 The construction flow chart of the PRC prestressed pipe pile anchor cable deep foundation pit support system shown in the figure is as follows: Step A: construct PRC prestressed pipe piles 2 along the sides of the foundation pit 1; Step B: constructing water-stop mixing piles 3 outside the PRC prestressed pipe piles 2, wherein the water-stop mixing piles 3 are arranged to intersect and form a water-stop mixing pile curtain 4; Step C: excavating the topsoil; Step D: construct the slope-cutting part 29 and the slope protection; Step E: constructing the cap beam 6 and the first anchor cable 7, wherein the first anchor cable 7 is anchored on the top of the PRC prestressed pipe pile 2 through the cap beam 6; Step F: excavating the foundation pit 1 in layers, and spraying the support system concrete of the excavated soil layers; Step G: 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 upper middle part of the PRC prestressed pipe pile 2. The first anchor cable 7 and the second anchor cable 9 are both prestressed anchor cables. Step H: excavating the foundation pit 1 layer by layer to the bottom of the pit and spraying concrete; Step I: construct the basement structure, and carry out the replacement of supports and the recovery of the prestressed anchor cable 5; Before the construction of two prestressed anchor cables 5 in step E and step G, an on-site anchor pulling test and a recoverable test of the prestressed anchor cables 5 are carried out to verify the recoverability of the prestressed anchor cables. The prestressed anchor cables in the prior art are permanently retained in the geological layer, which not only affects the surrounding construction, but also causes the surrounding construction excavation process to affect the support safety of the foundation pit due to the damage of the prestressed anchor cables, forming a safety hazard. In order to solve this problem, when the basement structure is constructed after the excavation is completed, the support is replaced and the prestressed anchor cables 5 are recovered, such as Fig.13 and Fig.14 As shown, it includes two filled plain concrete strips 38 located on the outside of the basement floor 36 and the basement negative first floor 37, respectively. The thickness of the two filled plain concrete strips 38 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 I1: construction preparation; Step I2: Construction of basement floor 36 and first filling plain concrete belt 38; Step I3: basement structure construction, including construction of basement exterior wall 35 and basement negative first floor floor 37, as well as construction of waterproofing and protective layer of negative second floor exterior wall; Step I4: backfilling sandy soil 34 and constructing the second filling plain concrete belt 38; Step I5: dismantling and recovering the second anchor cable 9; Step I6: backfilling the sandy soil to the bottom of the crown beam 6; Step I7: dismantle and recover the first anchor cable 7.

[0055] Through step E of this implementation, the crown beam and the first anchor cable are constructed, and the waist beam and the second anchor cable are constructed in step G, and the two anchor cables are anchored in the stratum by concrete pouring to form a PRC prestressed pipe pile anchor cable structure. One end (anchoring section) of the two anchor cables is fixed in the stable stratum by concrete pouring, and the other end is connected to the PRC prestressed pipe pile to withstand the thrust applied to the support system due to soil pressure, water pressure, etc., so as to utilize the anchoring force of the stratum to maintain the stability of the foundation pit and protect the safety of the foundation pit support; The construction process of constructing PRC prestressed pipe piles first and then water-stop mixing piles can effectively avoid the influence of PRC prestressed pipe piles on water-stop mixing piles, so that the water-stop mixing piles constructed later can fully reinforce the deep foundation pit foundation, and rely on PRC prestressed pipe piles to improve the support strength of the deep foundation pit and ensure the support effect. The double-row pile structure has a higher bearing capacity, and the ductility and bending resistance of PRC prestressed pipe piles are used to further improve the support capacity of the support system; The support is replaced and the prestressed anchor cable is recovered in step I, and the PRC prestressed pipe piles are supported and fixed by backfilling sandy soil and filling plain concrete belts. One side of the backfilling 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 cable is removed and recovered, which can be reused and avoid affecting the surrounding construction after the prestressed anchor cable is removed, resulting in the damage of the prestressed anchor cable during the excavation of the surrounding construction, which affects the support safety of the foundation pit and creates a safety hazard.

[0056] When constructing PRC prestressed pipe piles in step A, the construction process is as follows: Figure 4 Before construction, test pile construction is carried out. PRC prestressed pipe piles include construction preparation, measuring and laying out the pile position, putting the press machine in place, welding the pile tip, controlling the lifting point and verticality, then centering the pile, driving the pile to the designed elevation and controlling the pressure (monitoring and controlling the verticality of the pile body at any time during the pile driving process), moving the pile driver to the next pile position for construction, and recording and checking relevant pile inspection information.

[0057] The trial pile construction can test the feasibility and effect of the construction scheme adopted under the geological conditions on the one hand, and determine the construction process flow, operation points and process parameters of PRC prestressed pipe piles on the other hand, provide guidance and specification for the overall construction of prestressed pipe piles, and test and determine the construction process including 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 tests whether the predetermined design requirements can be met under the soil conditions through trial piles.

[0058] The construction method and process of prestressed pipe pile construction are as follows: Line laying and positioning: Review 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 to measure the edge of the pile 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; Pile tip welding: ① Perform pile tip welding before lifting the pile, and avoid holding the pile tip in a vertical state before welding after feeding the pile. When the pile body is lying horizontally, first weld the upper part of the pile tip, and then rotate the pile body 180° along the axial direction, and then weld the remaining part, and the weld is continuously closed; ② During welding, the surface of the pile body should be kept clean, and it should be cleaned with an iron brush first. The flatness of the pile tip and the pile body should be less than 0.5mm; ③ Manual arc welding should be used for welding layer by layer, with no less than 2 layers, and welding defects such as slag inclusion and pores should be avoided 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; Pile lifting: Use the pile driver's lifting equipment to lift the prefabricated pile in a one-point lifting manner. When lifting, slightly increase the angle (reduce the outward extension length), slowly lift it to the side of the pile driver, and then start vertically lifting the hook. Smoothly send the pile to the pile clamp of the pile driver. Lift the pipe pile at a slower speed to avoid the hook lifting speed being too fast, causing the pile to slide quickly toward the crane's support and cause the pile to collide and break. Pile driving: 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 driving the pile. When the pile is driven 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 driving the pile. Observe while driving. If the deviation is exceeded, make adjustments to avoid cracking of the pile body. At the same time, control the verticality of the driven pile to ≤0.3%; Pile delivery and elevation control: When delivering piles, the press runs continuously at full load with maximum pressure to sink piles, avoiding intervals. When delivering piles, the level is set up more than 20m away from the pile driver to measure and record 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 uses 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; Shifting: After the pile is pressed to the predetermined design elevation, turn off the motor, then move the pile driver to a new pile position and repeat the above steps to construct the next pile.

[0059] After completing the construction of the PRC prestressed pipe piles in step A, spraying concrete between the PRC prestressed pipe piles includes the following construction steps: Step A1: Laying out the lines, determining the excavation boundary of the foundation pit according to the location of the foundation pit, and marking the excavation line. The excavation line is marked with wooden stakes and white lime; Step A2: Excavate the working surface. The excavation of the working surface is carried out in sections and layers, and the surface is sprayed after excavation (each section and / or each layer). The surface is sprayed with water for more than 24 hours before excavating the next layer of soil; Step A 3: Hanging the mesh. After the working surface is dug out, the slope is repaired and the mesh is hung. The mesh is made of Φ6mm first-grade steel bars, and the reinforcing bars are made of Φ16 steel bars. The middle part is pressed into the anchor cable waist beam. Figure 2 As shown in the figure, the steel mesh is made in batches in advance, the steel bars are kept straight and evenly spaced, the upper slope is fixed with mesh soil nails, and the steel mesh is directly driven in, and the lower part uses φ48 steel pipes anchored 1m into the soil between piles to hang steel mesh, the surface reinforcement is connected by double-sided welding, and the lap length is ≥50d (d is the diameter of the steel bar), the reinforcement steel bar is pressed against the steel mesh and welded to the anchor head, and the reinforcement bar in the steel mesh is firmly connected to the fixed segment bar; Step A 4: spraying concrete. The strength of the sprayed concrete is C20. The thickness of each support section is 60mm. It is pneumatically transported by an artificial mixing air compressor and sprayed mechanically. During spraying, the nozzle is perpendicular to the sprayed surface. The distance between the nozzle and the sprayed surface is maintained between 0.8 and 1.2m under normal wind pressure to minimize rebound and maximize density. The thickness of a single spray is between 30 and 50mm, and the overall average thickness of the spray is 80mm (based on no exposed reinforcement). The spray distance and water-cement ratio are adjusted according to the rebound situation and soil conditions. After the concrete is finally set for 2h, it is sprinkled with water for maintenance.

[0060] When constructing water-stop mixing piles in step B, the four-mixing and four-spraying process is adopted, and the water-stop mixing piles are constructed according to the construction parameters. The construction parameters include: the pile diameter is between 600 and 1000 mm, such as Figure 8 As 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 5 The specific steps include: Step B1: Survey and lay out the pile positions using a total station; Step B2: Pile foundation alignment, leveling, and mud preparation; Step B3: Shotcrete mixing and sinking to the designed pile bottom elevation, and prepare cement slurry and pour it into the aggregate hopper before shotcrete mixing; Step B4: The shotcrete mixing is lifted to 30-50cm above the designed pile top elevation; Step B5: Repeat the spraying and stirring to sink to the designed pile bottom elevation, and 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 B6: Repeat the spraying and mixing to raise the pile to 30-50cm above the designed pile top elevation; Step B7: Pull out the mixer shaft above the ground; Step B8: Cleaning construction tools, including aggregate hopper, mortar pump, shotcrete pipe, and mixing head; Step B9: The mixer moves to the next pile position for construction.

[0061] 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 groups of parameters in the water-stop mixing pile construction parameters. 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.

[0062] 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.

[0063] Adopting steps B1 to B9 to construct water-stop mixing piles can improve the construction quality of water-stop mixing piles, effectively realize water-stopping of deep foundation pit support, avoid groundwater leakage into the foundation pit, and smoothly realize excavation.

[0064] The construction method and process of water-stop mixing pile construction are as follows: Measurement and positioning: Review the benchmark points, base axis lines and leveling points of the construction site, clearly mark and number the calibrated benchmark points and protect them, use theodolite and steel tape measure, etc., adopt the coordinate method to measure the edge lines of the pile position area, measure and locate all piles in the construction area, and make clear and reliable pile position marks, and record the measurements for review; Positioning and centering: After the mixer is assembled, it will use the walking system to climb to the pile position and position it. After the pile driver is in position and centered, it will be stable and straight, 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; Preparation of cement slurry: While the mixer is pre-stirring and sinking, mix the cement slurry according to the pre-determined 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 in the empty mixing section is tentatively set at 30kg / m, which will be determined in the early stage of construction based on the actual statistical situation of 30 to 50 piles; Shotcrete sinking: After the emplacement 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 control the sinking speed within 1.2m / min according to the construction parameters; Shotcrete, mixing, and lifting: After the mixer sinks to the pre-designed depth, lift the mixing head and continuously press the cement slurry from the center tube of the mixer into the foundation. While spraying and mixing, lift the mixer at the lifting speed of the construction parameters (within 1m / min). When the deep mixer is sprayed and lifted to 0.5m above the designed pile top elevation, the first grouting lifting and mixing is completed; Repeated mixing: After the deep mixer is lifted to the predetermined elevation, in order to mix the soft soil and cement slurry evenly, the mixer is sunk into the soil again while rotating. After reaching the designed reinforcement depth, the mixer is lifted out of the ground. During the entire mixing process, the mortar pump is kept turned on to continuously press the cement slurry into the foundation. Cleaning equipment and pipelines: Pour clean water into the aggregate hopper, start the mortar pump, clean the residual cement slurry in all pipelines until they are clean, and clean the soft soil adhering to the mixing head; Shifting: After the deep mixer is lifted out of the ground, turn off the motor, then move the pile driver to the new pile position and repeat the above steps to construct the next pile.

[0065] For the top slope and slope protection of step D, soil nail support structure is adopted. Due to the deep foundation pit excavation, the excavation depth is more than 6m. The slope is cut according to the slope calculated by soil stability theory. When protecting the slope, the slope is repaired first, and then the soil nails are placed in holes, and the steel mesh is tied and hung. Finally, the concrete surface is sprayed. The construction method and process of slope protection are as follows: Slope repair: The slope repair is carried out by combining mechanical and manual methods. On the basis of rough layout, the upper and lower edge lines of the slope are marked with lime lines. Mechanical slope repair is first used to reserve a certain thickness, and then precise layout is carried out. Manual repair is performed to the determined slope gradient. During mechanical slope repair, the slope gradient and flatness of the slope surface are promptly inspected by measuring means to ensure that it meets the predetermined design requirements. Drilling and inserting soil nails: Under normal circumstances where the foundation pit slope is temporarily self-supporting, before drilling, the hole position is determined by marking the ruler according to the pre-determined design requirements. After the hole position is determined, the hole is quickly drilled at the pre-set angle, and the soil nails are inserted according to the pre-determined design requirements, and the insertion length and sinking angle are controlled; Tie and hang steel mesh 20: Each node of the mesh is tied with Φ0.5 low-carbon wire, the steel bars are stretched and straightened in advance, and the connection between each section of steel bars is welded. After the steel mesh is made, the connecting bars and soil nails are spot welded to fix it; Shotcrete surface: ① The strength of shotcrete is C20, and the thickness of each support section is 80mm; ② Use pneumatic conveying with an artificial mixing air compressor and mechanical concrete spraying; ③ The working wind pressure of the sprayer should be moderate; ④ During spraying, the nozzle is perpendicular to the sprayed surface, and the nozzle is kept at a distance of 0.8 to 1.2m from the sprayed surface under normal wind pressure to reduce rebound and improve density; ⑤ The thickness of one spraying should be kept between 60-100mm, and the spraying distance and water-cement ratio should be adjusted according to the rebound situation and soil conditions; ⑥ Regular watering and maintenance; When the foundation pit is excavated in layers in step F and the support system concrete of the excavated soil layer is sprayed, and when the foundation pit is excavated in layers to the bottom of the pit and concrete is sprayed in step H, the construction method and process of shotcrete are carried out in this way.

[0066] In order 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, the construction process of the anchor cable is tested and the performance parameters are verified. When the anchor cable construction is carried out in the steps E and G, the anchor cable trial anchoring is first constructed to ensure the safety and economy of the anchoring project. The construction parameters of the anchor cable trial anchoring include: the hole diameter is 150mm, the length is 20m, and two types of steel strand detachable core prestressed anchor cables, 3+1 bundles of φ15.2 and 4+1 bundles of φ15.2, are used. The standard values ​​of the axial internal force of the anchor cable are 250kN, 380kN, and 450kN respectively, and the locking values ​​are 180kN and 250kN. Before construction, the soil distribution and physical and mechanical properties of each soil layer in the construction area should be determined by detection, and surrounding underground pipelines, structures and other facilities should be identified and avoided. When the slurry strength of the anchor cable anchoring section reaches 20MPa or reaches 80% of the design strength grade, the anchor cable pull-out test can be carried out to determine whether the site soil conditions meet the predetermined design requirements.

[0067] In the above steps E and G, the anchor cable is constructed by using a mechanical casing and following the pipe drilling. When drilling, the drill rod and the casing are synchronously rotated and drilled section by section. The anchor cable is constructed according to Figure 6 The prestressed anchor cable construction process flow chart includes the following steps: Step EG1: determine the anchor hole position, locate the anchor hole, process and manufacture the anchor cable and conduct quality inspection; Step EG2: Drilling rig positioning: during positioning, determine the anchoring position and anchoring angle; Step EG3: Drilling, after drilling is completed, check the hole depth and install the anchor cable; Step EG4: grouting, including primary grouting and secondary grouting; Step EG5: Curing after grouting, tensioning and locking after curing.

[0068] The anchor cable construction adopts the construction method of following the pipe drilling, which can prevent the hole wall from collapsing or the flowing sand from filling the borehole during the drilling process, and carry out grouting operations in time after the hole is formed to avoid hole collapse.

[0069] The construction methods and processes for the first anchor cable construction and the second anchor cable construction are as follows: Drilling: First, find out the situation and specific location of underground pipelines within the construction scope to avoid damaging underground pipelines. According to the anchor hole location points and construction drawings, measure and place the anchor hole positions and the positions of the horizontal and vertical beams of the frame, brush the slope lines, and number and mark the holes. Prestressed anchor cables are drilled mechanically with casing. When drilling, test drill first and adjust the drilling rig at any time to ensure the accuracy of the hole inclination. When installing the drill rod, install it section by section from the beginning. Before installing the power head, first remove the pulley block and thread the wire rope. When lowering the anchor cable, the anchor cable will place the grouting pipe on the middle of the anchor body and follow the anchor cable to the bottom of the hole; Production and installation of anchor cable steel strand: The anchor cable body adopts non-bonded steel strand with undamaged surface of PE casing. Cut and cut anchor cables. The length of anchor cable is the sum of designed length and reserved length outside the hole (generally about 1.5 meters). The reserved length is used for anchor cable tensioning and locking. After laying the steel strand straight, set a positioning bracket every 2 meters, put the steel strand into the groove of the positioning bracket in turn, and then use wire to tighten the anchor cable steel strand and the bracket. At the bottom of the anchor section, use wire to bundle the steel strand together and then fix it in the guide cap. The secondary grouting pipe is made of PVC pipe. After the anchor cable is tied, first pass the PVC pipe from the middle hole of the positioning bracket from the free section to the bottom along the axis of the anchor cable (keep a 20cm distance from the outlet to the bottom of the cable or the stop grouting ring / bag of the anchor section). After passing through, in the PVC within one-third of the bottom of the anchor cable A hole with a diameter of Φ5 is punched on the pipe, which is used as the grouting hole for secondary grouting. The hole is then sealed with adhesive tape to prevent cement slurry from entering the secondary grouting pipe through the hole during primary grouting. Finally, the secondary grouting pipe is tied together with the steel strand to prevent it from falling off during anchoring. When anchoring, the loose soil particles and accumulated dust in the hole are blown out with an air compressor, and then the grouting pipe is placed on top of the middle of the anchor body and put into the hole together with the anchor cable. It is manually pushed into the hole slowly and evenly. After the anchor cable body reaches the bottom of the hole, the exposed part is left with a length of 1.0~1.5m; Grouting: The primary grouting pipe is placed in the borehole together with the anchor cable. The inner end of the grouting pipe is 500~1000mm away from the bottom of the hole. The grouting hole and the end of the secondary high-pressure grouting pipe are sealed to prevent the slurry from entering the secondary high-pressure grouting pipe during the primary grouting. The anchoring section of the anchor cable adopts the secondary grouting method. The primary grouting uses cement mortar with a grouting pressure of 0.4~0.6MPa, a water-cement ratio of 0.40-0.45, a ash-sand ratio of 0.5~1.0, a cement grade of 42.5, and a grouting body strength of ≥25MPa. After the initial setting of the primary grouting, the secondary high-pressure splitting grouting is carried out: the secondary high-pressure splitting grouting uses pure cement slurry, the grouting pressure is controlled above 2.5MPa, the water-cement ratio is 0.50-0.55, and the pressure is stabilized for two minutes. The time of secondary grouting is determined according to the grouting process test or after the strength of the anchor body reaches 5MPa after the first grouting. 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.

[0070] In step E, when constructing the crown beam and the first anchor cable, the first anchor cable is constructed according to the above technical scheme, and the process steps for constructing the crown beam include: measuring and setting out, chiseling out the floating slurry on the top of the enclosure, tying the crown beam steel bars, supporting the crown beam formwork, pouring the crown beam concrete, removing the formwork and curing the concrete. After the crown beam construction is completed, the anchor cable is tensioned and locked on the crown beam.

[0071] The crown beam adopts 800×700mm reinforced concrete structure. Before constructing the crown beam, first excavate the earth in the foundation pit to the bottom of the crown beam cushion layer. After measuring and laying out, mark the elevation position of the bottom of the crown beam, and then make the crown beam. First, tie the crown beam steel bars. If the crown beam is constructed at the construction joint, it is necessary to straighten the steel bars at the joint and reserve waist beam suspension bars. After supporting the formwork, pour the concrete. The crown beam adopts C30 concrete and is poured in three layers. It is poured in sections and batches in the length direction. The joint surface of the new and old concrete at the joint is roughened, and the joint position is left on the crown beam. Within 12 hours after pouring, the concrete is covered and moisturized. The concrete strength reaches 1.2N / mm 2 Before installation, avoid stepping on or installing formwork and brackets. After the crown beam reaches the predetermined design strength, tension and lock the prestressed anchor cables on the crown beam.

[0072] After the construction of the crown beam and the first anchor cable, continue to excavate the foundation pit in layers and spray concrete. In the process of constructing the waist beam and the second anchor cable in step G, first complete the construction of the steel strands, complete the anchor cable grouting, then install the waist beam, and temporarily protect the slope. Fifteen days after the grouting is completed (the cement strength reaches more than 90%), the second anchor cable is tensioned and locked on the waist beam. The specific construction process of the anchor cable construction (including the tunnel anchor cable and the second anchor cable) is as follows: ① Level the pressure plate before tensioning ② The bearing plate includes the crown beam and the waist beam. The bearing plate, the pad and the anchor are installed and kept perpendicular to the axis of the anchor cable. When installing the jack, the axis of the jack and the axis of the anchor cable are kept coincident; ② Before installation, the jack and the high-pressure oil pump are calibrated, and tensioning is performed according to the calibrated data to ensure that the high-pressure oil pump is normal and the oil pipe is unobstructed to avoid tensioning failure; ③ Pre-tensioning, pre-tensioning the anchor cable twice before formal tensioning, and the tensioning force is 0.1~0.2 of the design load times to ensure that all parts of the anchor cable are in close contact and the cable body is straight, and the hidden deformation of the cable body is eliminated. When tensioning is carried out after straightening, the starting point of the deformation is recorded from this state as the zero point; ④ Formal tensioning, the tensioning stress is controlled within the range of 0.65~0.75 times the standard value of the anchor cable strength; ⑤ The tensioning load is applied step by step. The last tensioning will increase the load to 1.1 times the design load, and the load is held for observation for 10~15min. When the prestress of the tie rod does not decay significantly, the anchor cable tie rod can be locked. The locking prestress should be 75% of the design axial tension. After the anchor cable is locked, if there is obvious prestress loss, compensatory tensioning is carried out, and the prestressed steel strand is anchored when the tensioning control stress is in a stable state.

[0073] The PRC prestressed pipe pile anchor cable deep foundation pit construction method also includes foundation pit excavation, including the crown beam 6 and the first anchor cable 7 in the construction step E, first excavating the first layer of earthwork to 0.1m below the crown beam bottom elevation, after constructing the slope protection, the crown beam 6 and the first prestressed anchor cable, continuing the overall excavation to 0.5m below the waist beam bottom elevation, then constructing the waist beam 8, and installing the second prestressed anchor cable, after completing the construction of step G waist beam 8 and the second anchor cable 9, the construction step H is carried out to excavate the foundation pit in layers to the bottom of the pit. When the foundation pit is excavated, the foundation pit is first divided into two parts according to different functional areas of the plot, such as Fig. 9 As shown, it includes a first area 30 and a second area 31, and a plurality of dividing lines 32 are arranged along the transverse and longitudinal directions of the deep foundation pit to divide the deep foundation pit into a plurality of strip blocks. The arrow direction in the figure is the excavation direction. The construction steps of the foundation pit excavation include: Step Ⅰ: Excavate the first layer of soil, combined with Figure 1 and Fig. 9As shown, excavation is carried out from the original ground to 0.1m below the crown beam, and the layer height L1 is set between 2 and 2.5m. The layer height L1 includes the height of the slope protection and the height of the crown beam. During excavation, the interval section is excavated along the left side of the first area 30 of the foundation pit (the northwest corner of the geographical location) and then the interval section is excavated. The excavation port 33 is set on the right side of the second area 31. After the excavation, the slope protection, the crown beam 6 and the first anchor cable 7 are constructed; Step II: Excavate the second and third layers of earthwork, such as Figure 1 and Fig.10 As shown, the second and third layers of earthwork are earthwork between the crown beam 6 and the waist beam 8, the layer height L2 is set between 3 and 3.5 m, the layer bottom is between 5 and 6 m from the original ground, and concrete is sprayed and the excavation opening 33 of the second area 31 is constructed. After excavation, the waist beam 8 and the second anchor cable 9 are constructed, and the waist beam 8 and the second anchor cable 9 at the position of the excavation opening 33 are not constructed for the time being; Step III: excavate the fourth and fifth layers of earthwork in the first area 30, such as Fig.11 As shown, after the construction of the waist beam 8 and the second anchor cable 9, the earthwork of the first area 30 is excavated to the base, and the height L3 of the earthwork layer is set to be between 3.2 and 4 meters. After the excavation is completed, the pile foundation construction of the existing working surface is carried out, and the first area 30 is excavated first to facilitate the pile foundation construction of this area; Step IV: excavate the fourth and fifth layers of earthwork in the second area 31, such as Fig.12 As shown, after the construction of the engineering piles of the existing working surface is completed, the remaining earthwork of the second area 31 is excavated, and the position of the excavation port is adjusted to the right side of the second area 31 (the southeast corner of the geographical location) for the construction of the waist beams and piles of the unfinished parts.

[0074] According to the different functional uses of the construction site or the size of the foundation pit, the foundation pit can be divided into two or three large areas. In steps II, III, and IV, the foundation pit can be excavated first, so as to construct the support structure to ensure the safety of the foundation pit excavation. At the same time, through the zoning method, the excavation outlet can be reasonably arranged to facilitate the transportation of the excavated soil, and the excavation method of block separation can be adopted for the excavation site to avoid excessive local stress or settlement of the foundation pit.

[0075] As one of the preferred implementations, during the construction of the waist beam 8 and the second anchor cable 9, since the total height of the first layer L1 and the second layer L2 is between 5 and 6 meters and is located in a thicker sand layer, during the construction of the second anchor cable 9, waterproof measures are used to control the water and sand gushing in the drilling and grouting process, including the following water and sand gushing prevention construction contents: g1: After the hole is completed, an expansion water-stop sand bag is added at the hole mouth. Since the expansion water-stop yarn has the characteristic of rapid expansion when it comes into contact with water, but it has no strength and can be arbitrarily shaped, it can be made into an expansion water-stop sand bag after being sealed with non-woven fabric; g2: The casing and drill pipe are drilled synchronously and the wall protection and hole forming process is adopted. This construction method can control the slag discharge, and can squeeze and compact the surrounding soil during the drilling process, reduce the disturbance of water and soil outside the casing, and effectively reduce the accumulation of pore water pressure; g3: When water and sand gushing occurs, fix the steel plate with expansion bolts at the hole position and seal it with quick-drying cement. The steel plate should be 3 to 5 meters thick.

[0076] The above-mentioned water and sand prevention construction contents can avoid the hole collapse caused by water and sand gushing, ensure the safety of excavation construction and foundation pit support, and effectively avoid construction safety risks.

[0077] In order to ensure the safety of the foundation pit of this embodiment and the adjacent foundation pit excavation parts during the excavation process, rotary jet grouting piles are constructed at the overlap of the two foundation pit water-stop mixing pile curtains, and three rotary jet grouting piles are set at each overlap, including the following construction steps: Step L1: Positioning and laying out. When laying out the holes, the holes are arranged according to the pre-determined design positions; Step L2: The drilling rig performs the construction in sequence according to the arranged hole positions; Step L3: The final hole depth is to the designed height, the hole diameter is 800mm, and the hole does not collapse; Step L4: Position the high-pressure jet grouting machine in front of the completed hole, check whether the whole system works normally, pass water through the nozzle on the ground, check whether there is any leakage and pressure condition, and whether the construction conditions are met; Step L5: Check whether the high-pressure pump and mud pump are normal at the same time; Step L6: Use tape to seal the water nozzle and slurry nozzle of the nozzle, lower the nozzle to the bottom of the hole and reach the designed depth, start the mud pump at the bottom, and lower the pipe with water or slurry; Step L7: Operation sequence of the whole system: 1) Start the high-pressure pump → 2) Wait for the nozzle to rotate several times → 3) After inserting the nozzle to the bottom of the hole, lift the nozzle upward at 15mm / min, 8-15 rpm, and 25Mpa slurry pressure → 4) To the pre-treatment height, turn off the mud pump, and lift the nozzle to the ground → 5) Inject the return slurry into the hole → 6) Clean the bottom plate hole and move the machine to the next hole → 7) Repeat steps 1) to 8) → 9) Clean the equipment; Step L8: The jet grouting pile is constructed by double-tube method, the pile diameter is not less than φ800mm, the cement dosage is not less than 250kg / m; the cement is P.O42.5 silicate cement, the water-cement ratio is 1.0-1.2, the lifting rate is not more than 15mm / min, the injection pressure is not less than 25MPa, and the pile depth is the same as the water-stop mixing pile construction of this technical solution.

[0078] The construction method and process include: ① drilling, the hole position deviation is less than 5cm, the hole slope is not more than 0.5%, the hole depth is not less than the designed depth, and the hole diameter is 130mm; ② the wall-solidifying mud is determined by experiments to achieve: the mud does not harden and solidify in the hole, the high spray boom can be smoothly inserted into the bottom of the hole within 7-10 days, the mud basically does not flow or condense in the pores, and the PH value 8-10, specific gravity 1.05-1.1, sand content <8%; ③ When the shotcrete pipe is inserted into the bottom depth of the borehole, prepare the cement slurry according to the designed mix ratio, and operate as follows: rotate the shotcrete pipe in situ at the designed speed → input cement slurry and water according to the double-tube shotcrete method → ​​lift the spray rod to make the shotcrete nozzle reach the pile top elevation → after the pump pressure and wind pressure rise to the designed specified values, return slurry from the hole mouth (the specific gravity of the return slurry must be greater than 1.2) → rotate the spray pipe at the designed speed and perform the shotcrete operation from bottom to top; ④ Cement slurry is filtered, and two filtering devices are set according to the nozzle diameter; a filter screen is set between the cement slurry mixing tank and the mud pump suction mixing tank, and a filter screen is set at the tail of the mud suction pipe; ⑤ During the high-pressure spraying operation, the specific gravity of the cement slurry inlet and return slurry should be tested frequently. When the error between the slurry specific gravity and the specified water-cement ratio (1.0-1.2) exceeds 0.1, the shotcrete operation should be suspended immediately, the water-cement ratio of the new liquid should be readjusted, and then the shotcrete operation should be resumed quickly; ⑥ The cement slurry should be used as soon as it is prepared, and should be stirred continuously during the high-pressure spraying operation (the stirring volume should be 1.0m3 at a time); ⑦ After the high-pressure spraying operation is completed, fresh slurry should be continuously added to the hole until the slurry surface in the hole no longer sinks.

[0079] 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 PRC prestressed pipe pile anchor cable deep foundation pit support system, characterized in that: The PRC prestressed pipe pile anchor cable deep foundation pit support system comprises PRC prestressed pipe piles (2) arranged at intervals along the side of a foundation pit (1) to form a row of piles, water-stop mixing piles (3) are arranged on the outside of 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), and two prestressed anchor cables (5) are arranged on the upper part of the foundation pit (1), 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).

2. The PRC prestressed pipe pile anchor cable deep foundation pit support system according to claim 1 is characterized in that: A reinforcement structure (10) is also provided between the PRC prestressed pipe piles (2), comprising an anchoring steel pipe (11) anchored into the soil between the PRC prestressed pipe piles (2), the anchoring end of the anchoring steel pipe (11) being located within the water-stop mixing pile curtain (4), the reinforcement structure (10) further comprising reinforcing ribs (12) pressed between the waist beam (8) and the PRC prestressed pipe piles (2), and a steel mesh (20) connected to the anchoring steel pipe (11), the reinforcing ribs (12) comprising transverse ribs (13), vertical ribs (14) and diagonal ribs (15).

3. The PRC prestressed pipe pile anchor cable deep foundation pit support system according to claim 1 or 2, characterized in that: The PRC prestressed pipe pile anchor cable deep foundation pit support system is applied to the construction condition of excavating adjacent foundation pits (1A), and 3-5 rotary jet grouting piles (18) are constructed at the overlap of the water-stop mixing pile curtains (4) of the two foundation pits.

4. A PRC prestressed pipe pile anchor cable deep foundation pit construction method, when the PRC prestressed pipe pile anchor cable deep foundation pit support system as described in any one of claims 1 to 3 is used for foundation pit support construction, the construction process comprises the following steps: Step A: constructing PRC prestressed pipe piles (2) along the sides of the foundation pit (1); Step B: 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 C: excavating the topsoil; Step D: Construction of top slope and slope protection; Step E: 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 F: excavating the foundation pit in layers (1) and spraying the support system concrete of the excavated soil layers; Step G: 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 H: excavate the foundation pit in layers to the bottom of the pit and spray concrete; Step I: construct the basement structure, and carry out the replacement of supports and recovery of prestressed anchor cables (5); Before the construction of the two prestressed anchor cables (5) in step E and step G, an on-site anchor test and a recyclability test of the prestressed anchor cables (5) are carried out. When the basement structure is constructed after the excavation is completed, the support replacement and the prestressed anchor cables (5) are recovered, including two filled plain concrete strips (38) respectively located on the outside of the basement floor (36) and the basement negative first floor floor (37). The thickness of the two filled plain concrete strips (38) is between 350 and 600 mm. The support replacement construction is interspersed with the basement structure construction. 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 I1: construction preparation; Step I2: Construction of the basement floor (36) and the first filling plain concrete strip (38); Step I3: basement structure construction, including the construction of the basement exterior wall (35) and the basement negative first floor floor slab (37), as well as the construction of the waterproofing and protective layer of the negative second floor exterior wall; Step I4: backfilling the sandy soil (34) and constructing the second filling plain concrete belt (38); Step I5: dismantling and recovering the second anchor cable (9); Step I6: backfilling sandy soil to the bottom of the crown beam (6); Step I7: dismantling and recovering the first anchor cable (7).

5. The PRC prestressed pipe pile anchor cable deep foundation pit construction method according to claim 4 is characterized in that: When constructing the PRC prestressed pipe pile (2) in step A, the steps specifically include the following: Step A1: firstly conduct a trial pile construction to obtain construction process parameters, including pile driving speed, pile driving pressure, pile length and quality standard after pile construction, wherein the pile driving pressure includes the static pressure during the pile driving process and the final pressure when the predetermined elevation is reached; Step A2: Measure and lay out the positions of the PRC prestressed pipe piles to be constructed; Step A3: 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 and lower ends should be ≤0.3%; Step A4: Press the pile to the predetermined elevation and control the pressure. During the pressing process, monitor and control the verticality of the pile body to ensure that the verticality deviation between the upper and lower ends of the pile body is ≤0.3%; Step A5: Move the pile to the next pile position for construction.

6. The PRC prestressed pipe pile anchor cable deep foundation pit construction method according to claim 4, characterized in that: In the step B, water-stop mixing piles are constructed according to construction parameters, the construction parameters including: pile diameter D600-D1000, spacing between two adjacent piles 0.4-0.7m, pile length 11-15m, cement consumption per meter 110-150 kg / m, material water-cement ratio 0.5-0.6, sinking speed 0.8m / min≤v1≤1.2m / min, lifting speed 0.6m / min≤v2≤1m / min, shotcrete flow rate 55-75L / min; the specific steps of using the determined construction parameters and constructing water-stop mixing piles with a four-mixing and four-spraying process include the following: Step B1: Survey and lay out the pile positions using a total station; Step B2: Pile foundation alignment, leveling, and mud preparation; Step B3: Shotcrete mixing and sinking to the designed pile bottom elevation, and prepare cement slurry and pour it into the aggregate hopper before shotcrete mixing; Step B4: The shotcrete mixing is lifted to 30-50cm above the designed pile top elevation; Step B5: Repeat the spraying and stirring to sink to the designed pile bottom elevation, and 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 B6: Repeat the spraying and mixing to raise the pile to 30-50cm above the designed pile top elevation; Step B7: Pull out the mixer shaft above the ground; Step B8: Cleaning construction tools, including aggregate hopper, mortar pump, shotcrete pipe, and mixing head; Step B9: The mixer moves to the next pile position for construction.

7. The PRC prestressed pipe pile anchor cable deep foundation pit construction method according to claim 4, characterized in that: In the steps E and G, when constructing the first anchor cable (7) and the second anchor cable (9), a trial anchoring of the anchor cables is firstly carried out. The construction parameters of the trial anchoring of the anchor cables include: a hole diameter of 120 to 180 mm, a length of 18 to 25 m, a 3+1 bundle or 4+1 bundle prestressed anchor cable with a detachable core, a steel strand diameter of φ12 to φ18, a standard value of the axial internal force of the anchor cable of 250 kN, 380 kN, and 450 kN, and a locking value of 180 kN, 200 kN, and 250 kN. Before the trial anchoring construction, the soil layer distribution and the physical and mechanical properties of each soil layer in the construction area are firstly detected and determined, and the surrounding underground pipelines and structures are identified and avoided. When the slurry strength of the anchor cable anchoring section reaches 20 MPa or reaches 80% of the design strength grade, an anchor cable pullout test is carried out.

8. The PRC prestressed pipe pile anchor cable deep foundation pit construction method according to claim 4, characterized in that: During the anchor cable construction in step E and step G, the anchor cable uses a mechanical casing and adopts the following pipe drilling construction. During the drilling, the drill rod and the casing are synchronously rotated and drilled section by section. The anchor cable construction specifically includes the following steps: Step EG1: determine the anchor hole position, locate the anchor hole, and process and manufacture the anchor cable; Step EG2: Positioning the drilling rig, during positioning, determine the anchoring position and anchoring angle; Step EG3: Drilling, after drilling is completed, check the hole depth and install the anchor cable; Step EG4: grouting, including primary grouting and secondary grouting; Step EG5: Curing after grouting, tensioning and locking after curing.

9. The PRC prestressed pipe pile anchor cable deep foundation pit construction method according to claim 4, characterized in that: In the process of constructing the waist beam (8) and the second anchor cable (9), step G adopts waterproof measures to control the water and sand gushing in the process of drilling and grouting, including the following construction contents of preventing water and sand gushing: G1: After the hole is completed, an expansion water-stop sand bag is added at the hole mouth. Since the expansion water-stop yarn has the characteristic of rapid expansion when it comes into contact with water, but it has no strength and can be arbitrarily shaped, it can be made into an expansion water-stop sand bag after being sealed with non-woven fabric; G2: The casing and drill pipe are drilled synchronously and the wall protection and hole forming process is adopted. This construction method can control the slag discharge, and can squeeze and compact the surrounding soil during the drilling process, reduce the disturbance of water and soil outside the casing, and effectively reduce the accumulation of pore water pressure; G3: When water and sand gushing occurs, fix the steel plate with expansion bolts at the hole position and seal it with quick-drying cement. The steel plate should be 3 to 5 meters thick.

10. The PRC prestressed pipe pile anchor cable deep foundation pit construction method according to any one of claims 4 to 9, characterized in that: The PRC prestressed pipe pile anchor cable deep foundation pit construction method simultaneously carries out foundation pit excavation. The foundation pit excavation includes excavating the first layer of earthwork to 0.1 m below the bottom elevation of the crown beam (6) before the crown beam (6) and the first anchor cable (7) in the construction step E. After the slope protection, the crown beam (6) and the first anchor cable (7) are constructed, the overall excavation is continued to 0.5 m below the bottom elevation of the waist beam, and then the waist beam (8) is constructed and the second anchor cable (9) is installed. After the waist beam (8) and the second anchor cable (9) in the construction step G are completed, the foundation pit is excavated in layers to the bottom of the pit in the construction step H. When the foundation pit is excavated, the foundation pit is divided into two parts according to different functional areas of the plot, including a first area (30) and a second area (31). At the same time, a plurality of dividing lines (32) are set along the deep foundation pit in the horizontal and vertical directions. The dividing lines (32) divide the deep foundation pit into a plurality of strip blocks. The construction steps of the foundation pit excavation include: Step I: excavate the first layer of earthwork, from the original ground to 0.1m below the top beam, with the layer height L1 set between 2 and 2.5m. The layer height L1 includes the height L11 of the slope protection and the height L12 of the top beam. During excavation, start excavating at intervals along a corner of the foundation pit and then remove the intervals. After excavation, construct the slope protection, top beam (6) and the first anchor cable (7); Step II: excavating the second and third layers of earthwork, the second and third layers of earthwork being the earthwork between the crown beam (6) and the waist beam (8), the layer height L2 being set between 3 and 3.5 m, the layer bottom being between 5 and 6 m from the original ground, spraying concrete and constructing the excavation opening of the second zone (31), after excavation, constructing the waist beam (8) and the second anchor cable (9), the waist beam and anchor cable at the position of the excavation opening are not constructed for the time being; Step III: excavating the fourth and fifth layers of earthwork in the first zone (30). After the construction of the waist beam (8) and the second anchor cable (9), excavating the earthwork in the first zone (30) to the base, setting the layer height L3 of the earthwork to between 3.2 and 4 m, and after the excavation is completed, carrying out the pile foundation construction of the existing working surface; Step IV: excavate the fourth and fifth layers of earthwork in the second area (31). After the construction of the engineering piles on the existing working surface is completed, excavate the remaining earthwork in the second area (31) and adjust the position of the excavation port for the construction of the waist beam and engineering piles in the unfinished area.

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