Deep foundation pit support construction method under complex environment

Abstract

The application provides a deep foundation pit support construction method in a complex environment, and belongs to the technical field of building construction. The deep foundation pit support construction method in the complex environment comprises the following steps: surveying a construction site, determining the height difference between the riverbed height of a river and the support elevation, and backfilling the river; surrounding the construction site and performing high-pressure rotary jet pile hole column construction; excavating earthwork on a support installation operation surface; welding and fixing a bracket to the column; installing and fixing a surrounding purlin, a bracket, a non-standard part, and a connecting piece; installing a support closed fish-bellied beam, and arranging monitoring points on the fish-bellied beam; applying tension to the pre-stressed steel strand on the fish-bellied beam, and performing stress detection; excavating the next layer of earthwork; and the construction method can solve the problems of low overall steel degree and stability of a support system, difficulty in accurately controlling the displacement of a foundation pit, and easy deformation of the foundation pit in traditional deep foundation pit support through concrete inner support and steel support.

Description

Technical Field

[0001] This invention belongs to the field of building construction technology, and more specifically, relates to a method for supporting deep foundation pits in complex environments. Background Technology

[0002] With the continuous advancement of urbanization in my country and the expansion of urban construction scale, deep foundation pit engineering, as an important form of urban construction, is increasingly widely used. Deep foundation pit engineering refers to the general term for engineering projects involving the excavation, excavation, support, and drainage of deep underground soil in urban construction. Deep foundation pit engineering involves deep underground soil and is affected by various factors such as geological conditions, groundwater level, soil properties, and underground pipelines during construction. Therefore, deep foundation pit engineering is characterized by high construction difficulty, high safety risks, and long construction periods. Deep foundation pit support construction is one of the key links in deep foundation pit engineering. Deep foundation pit support construction refers to the series of support measures taken during the excavation process of deep foundation pit engineering to ensure the stability of the surrounding soil and prevent foundation pit collapse, forming a support structure to ensure the safe and smooth progress of the deep foundation pit project. The quality of deep foundation pit support construction directly affects the safety and construction efficiency of the deep foundation pit project; therefore, quality control of deep foundation pit support construction is a crucial aspect of deep foundation pit engineering. Deep foundation pit support construction in complex environments refers to the construction of deep foundation pits under complex geological conditions, such as soft soil, quicksand, and rock strata. This construction faces numerous challenges, including poor soil stability, high groundwater levels, complex underground pipelines, and challenging geological conditions. Therefore, more scientific, rational, and safe construction methods and measures are needed for deep foundation pit support construction in complex environments. Deep foundation pit engineering is a high-risk project, with many uncertainties and risks during its construction. Researching the key technologies and management methods for deep foundation pit support construction in complex environments can improve the safety of deep foundation pit projects and reduce the occurrence of accidents. The construction process of deep foundation pit projects is complex and requires significant manpower, material resources, and financial resources. Researching the key technologies and management methods for deep foundation pit support construction in complex environments can improve the construction efficiency and reduce construction costs.

[0003] Traditional deep foundation pit support systems, which use internal concrete supports and steel supports, have low overall rigidity and stability, making it difficult to accurately control foundation pit displacement and prone to deformation. Summary of the Invention

[0004] In view of this, the present invention provides a construction method for deep foundation pit support in complex environments, which can solve the problems of low overall rigidity and stability of traditional deep foundation pit support systems that rely on internal concrete supports and steel supports, making it difficult to accurately control foundation pit displacement and prone to foundation pit deformation.

[0005] This invention is implemented as follows:

[0006] This invention provides a method for constructing deep foundation pit supports in complex environments, comprising the following steps:

[0007] S10: Conduct a site survey to determine the difference between the riverbed height and the support elevation, and carry out river backfilling;

[0008] S20: Enclose the construction site and construct the high-pressure jet grouting pile pilot hole erection column;

[0009] S30: Excavate the earthwork for the support installation work surface;

[0010] S40: Weld the bracket to the column to fix it in place;

[0011] S50: Install and fix walers, brackets, non-standard parts and connectors;

[0012] S60: Support the installation of the closed fish belly beam, and arrange monitoring points on the fish belly beam;

[0013] S70: Apply prestressed steel strand tension to the fish-belly beam and perform stress testing;

[0014] S80: Proceed with the excavation of the next layer of earthwork.

[0015] The technical advantages of the deep foundation pit support construction method provided by this invention in complex environments are as follows: By surveying the construction site, determining the difference between the riverbed height and the support elevation, and backfilling the riverbed; enclosing the construction site and constructing the high-pressure jet grouting pile pilot columns; excavating the earthwork for the support installation work surface; welding and fixing the corbels to the columns; installing and fixing the walers, supports, non-standard parts, and connectors; installing the closed fish-belly beam support and arranging monitoring points on the fish-belly beam; applying prestressed steel strand tension to the fish-belly beam and conducting stress testing; and excavating the next layer of earthwork. This method solves the problems of low overall rigidity and stability of traditional deep foundation pit supports using concrete internal supports and steel supports, making it difficult to accurately control foundation pit displacement and prone to deformation.

[0016] Based on the above technical solution, the deep foundation pit support construction method of the present invention under complex environment can be further improved as follows:

[0017] The specific steps for surveying the construction site, determining the difference between the riverbed height and the support elevation, and carrying out river backfilling include:

[0018] The first step is for the construction workers to survey the surrounding waterways and pipelines at the location of the foundation pit to determine the elevation difference between the riverbed and the support.

[0019] The second step is to determine the relationship between the river channel and the foundation pit.

[0020] The third step is to set up a cofferdam at the bifurcation of the river before the river is excavated. The cofferdam is made of sandbags, with double rows of steel pipes arranged in the sandbags and the steel pipes fixed by channel steel.

[0021] The fourth step is to fix a waterproof cloth on the side of the steel pipe closest to the river channel, and to fill the space between the two rows of steel pipes with sandbags.

[0022] The fifth step is to use water pumps to pump water after the cofferdam construction is completed, and to clean the silt from the bottom of the river channel.

[0023] The sixth step is to backfill the cleared riverbed;

[0024] The specific steps for backfilling the cleaned riverbed are as follows:

[0025] The first step is to process the clay by cleaning out the silt, humus and organic matter, and controlling the moisture content of the clay.

[0026] The second step involves backfilling the clay into the river channel in layers using a layered filling and compaction method.

[0027] The third step is to mechanically level and then re-compact the river channel after the overall backfilling is completed.

[0028] The fourth step is to ensure that the backfill height is the same as the revetment height.

[0029] Furthermore, the specific steps for enclosing the construction site and constructing the high-pressure jet grouting pile pilot hole erection column include:

[0030] The first step is to enclose the construction site with fencing;

[0031] The second step is to determine the location and depth of the pilot hole at the construction location of the foundation pit, and then to lay out the lines.

[0032] The third step is to install the rotary spraying equipment in the designated location and perform debugging;

[0033] The fourth step is to use a jet grouting machine to create pilot holes, operating according to the designed depth;

[0034] The fifth step is to carry out pile foundation construction based on the pilot hole.

[0035] The sixth step is to drive columns into the foundation of the piles;

[0036] The specific steps for using a rotary jetting device to create a pilot hole, based on the designed depth, include:

[0037] The first step is to add cement slurry with a water-cement ratio of 1:1.5 into the rotary jet grouting equipment;

[0038] The second step is to control the airflow pressure output by the rotary jetting equipment to be no less than 0.7 MPa and the cement slurry flow pressure to be no less than 20.0 MPa.

[0039] The third step is to inject grout when the grouting pipe is inserted into the borehole and the nozzle reaches the design elevation. After the injection grouting parameters reach the specified values, the grouting pipe is raised and grout is injected from bottom to top.

[0040] Fourth, once the high-pressure jet grouting is complete, the grouting pipe should be pulled out quickly;

[0041] The specific steps for driving columns into the pile foundation include:

[0042] The first step is to use a total station to set out the construction layout, and calculate the coordinate data of each pile based on the positioning drawings and pile location map;

[0043] The second step involves using a robotic arm to construct the columns. After the equipment is in place, two theodolites are used to cross each other at 90° to check the verticality of the pile. The verticality of the pile inserted into the soil should be kept within 1 / 200 of the embedment depth. The deviation of the center position of the column should not exceed 3cm, the deviation between the center of the completed pile and the center of the designed pile position should be less than 5cm, and the deviation between the top elevation of the column and the designed top elevation should be less than 3cm.

[0044] Furthermore, before constructing the pilot holes for the high-pressure jet grouting piles, trial insertions of the pilot holes are conducted around the foundation pit construction site based on the thickness of the silty clay layer to determine the location of the pilot holes.

[0045] Furthermore, the specific steps for welding and fixing the bracket to the column include:

[0046] The first step is to create a welding workspace for the corbel in a localized deep excavation area during the earthwork excavation process.

[0047] The second step is to fix the welding fastener of the bracket to the side wall of the column, and then weld the bracket to the welding fastener.

[0048] Third, during the docking process between the bracket and the welding fastener, the verticality, elevation, and horizontality of the bracket are monitored.

[0049] The specific steps for welding the bracket to fix the corbel on the side wall of the column are as follows:

[0050] Before welding the corbel, drilling and rebar installation are carried out on the column, and the main reinforcement is exposed by breaking the surface and welding one of the steel plates to form the welded fastener.

[0051] Furthermore, the specific steps for installing and fixing the walers, brackets, non-standard parts, and connectors include:

[0052] The first step is to pre-embed bolts in the corbel, and then connect the walers to the bolts in order of length from longest to shortest.

[0053] The second step is to place the wrench on the pre-tightened bolt, insert the inner sleeve into the spline head inside the bolt, then slightly rotate the outer sleeve to align it with the nut, push it to the root of the nut, turn on the power switch, and the inner and outer sleeves will rotate in opposite directions to tighten the bolt.

[0054] The third step is to weld the bracket to the column after positioning it on the column, and connect the crossbeam to the bracket by bolts and spot welding.

[0055] Fourth, after the waler is installed, install the triangular keys and connectors at the ends of the corner braces and the opposite braces, and fix them to the columns with bolts; when the size and shape of the foundation pit are irregular, non-standard parts are used for the connection with the standard parts; during the connection of the corner braces and opposite braces, a special jack is used to pressurize the middle of the two loading beams.

[0056] Furthermore, when the size and shape of the foundation pit are irregular, the specific locations where non-standard parts are used for connection with standard parts include:

[0057] When the bracing and waler are not perpendicular, at the connection point between the bracing and waler, when the angle between the corner bracing and waler is not equal to 45°, at the connection point between the corner bracing and waler, when the angle between adjacent foundation pits is not equal to 90°, and at the connection point between adjacent walers.

[0058] Furthermore, the specific steps for installing the supporting closed fish-belly beam and arranging monitoring points on the fish-belly beam include:

[0059] The first step is for the construction workers to construct and optimize the structural model of the fish-belly beam based on the distance between the columns and the shape of the foundation pit.

[0060] The second step is to prepare components based on the optimized three-dimensional model of the fish belly beam, transport the prepared components to the construction site, and then assemble and hoist them.

[0061] The third step is to lift and place individual components on the crossbeam and support after the fish belly beam is pre-assembled. After the support is lifted, both ends are manually pulled to ensure the overall stability of the support.

[0062] The fourth step is to place core-through jacks at the end positions of the steel strands of the fish-belly beam to pull the steel strands.

[0063] Furthermore, the specific steps for applying prestressed steel strand tension to the fish-belly beam and conducting stress testing are as follows:

[0064] The first step is to thread the steel strands through the core-through jack;

[0065] The second step is to install the working anchor and its matching limit plate;

[0066] The third step is to symmetrically tension both ends of the steel strand, with the tension force increasing from 30% to 50%, 70%, and 110%, and then hold the load for 2 minutes before anchoring.

[0067] Furthermore, the steel strands are tensioned sequentially, one strand at a time, with each strand tensioned to the design stress in three stages to ensure that the entire fish-belly beam steel bundle meets the design stress value after tensioning.

[0068] Compared with existing technologies, the beneficial effects of the deep foundation pit support construction method provided by this invention in complex environments are as follows: Before applying prestress to the prestressed fish-belly steel support, the river channel is dewatered, dredged, and backfilled to compensate for stress, preventing displacement of the retaining piles towards the river channel after prestressing, thus avoiding deformation and collapse of the riverbank; high-pressure jet grouting piles are used to form cement-soil mixing piles, reducing the difficulty of column insertion and reinforcing the bottom of the steel columns, enhancing the overall stability of the support; the dimensions of the support installation components are refined to reduce connection joints; the pressure application sequence of the support installation is optimized, proceeding from both ends towards the middle to ensure foundation pit safety; the position of the steel joints is optimized, using bolt holes and pitches that match other components, ensuring uniformity in connection methods, simplifying on-site operation, and improving construction efficiency; it solves the problems of low overall rigidity and stability of traditional deep foundation pit supports using concrete internal supports and steel supports, making it difficult to accurately control foundation pit displacement and prone to deformation. Attached Figure Description

[0069] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0070] Figure 1 This is an operation flowchart for a deep foundation pit support construction method under complex conditions. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0072] like Figure 1The diagram shows an operation flowchart of a deep foundation pit support construction method provided by the present invention in a complex environment. The method includes the following steps:

[0073] S10: Conduct a site survey to determine the difference between the riverbed height and the support elevation, and carry out river backfilling;

[0074] S20: Enclose the construction site and construct the high-pressure jet grouting pile pilot hole erection column;

[0075] S30: Excavate the earthwork for the support installation work surface;

[0076] S40: Weld the bracket to the column for fixation;

[0077] S50: Install and fix walers, brackets, non-standard parts and connectors;

[0078] S60: Support the installation of the enclosed fish-belly beam and arrange monitoring points on the fish-belly beam;

[0079] S70: Apply prestressed steel strand tension to the fish-belly beam and perform stress testing;

[0080] S80: Proceed with the excavation of the next layer of earthwork.

[0081] During use, the construction site is surveyed to determine the height difference between the riverbed and the support elevation, and the river is backfilled; the construction site is enclosed, and the high-pressure jet grouting pile pilot hole columns are constructed; earthwork is excavated for the support installation work surface; the corbels are welded and fixed to the columns; the walers, brackets, non-standard parts, and connectors are installed and fixed; the support enclosure fish-belly beam is installed, and monitoring points are set up on the fish-belly beam; prestressed steel strands are tensioned on the fish-belly beam, and stress testing is carried out; the next layer of earthwork is excavated.

[0082] In the aforementioned technical solution, the specific steps for surveying the construction site, determining the elevation difference between the riverbed and the support elevation, and carrying out river backfilling include:

[0083] The first step is for the construction workers to survey the surrounding waterways and pipelines at the location of the foundation pit to determine the elevation difference between the riverbed and the support.

[0084] The second step is to determine the relationship between the river channel and the foundation pit.

[0085] The third step is to set up a cofferdam at the bifurcation of the river before the river is excavated. The cofferdam is made of sandbags, with double rows of steel pipes arranged in the sandbags and fixed with channel steel.

[0086] The fourth step is to fix a waterproof cloth on the side of the steel pipe closest to the river channel and fill the space between the two rows of steel pipes with sandbags.

[0087] The fifth step is to use water pumps to pump water after the cofferdam construction is completed, and to clean the silt from the bottom of the river channel.

[0088] The sixth step is to backfill the cleared riverbed;

[0089] The specific steps for backfilling the cleaned riverbed are as follows:

[0090] The first step is to process the clay by cleaning out the silt, humus, and organic matter, and controlling the moisture content of the clay.

[0091] The second step is to backfill the clay into the river channel in layers using a layered filling and compaction method.

[0092] The third step is to mechanically level and recompact the riverbed after the overall backfilling is completed.

[0093] The fourth step is to ensure that the backfill height is the same as the revetment height.

[0094] Furthermore, in the above technical solution, the specific steps for enclosing the construction site and constructing the high-pressure jet grouting pile pilot hole erection column include:

[0095] The first step is to enclose the construction site with fencing;

[0096] The second step is to determine the location and depth of the pilot hole at the construction site of the foundation pit, and then to lay out the lines.

[0097] The third step is to install the rotary spraying equipment in the designated location and perform debugging;

[0098] The fourth step is to use a jet grouting machine to create pilot holes, operating according to the designed depth;

[0099] The fifth step is to carry out pile foundation construction based on the pilot hole.

[0100] The sixth step is to drive columns into the foundation of the piles;

[0101] The specific steps for using a jet grouting machine to create a pilot hole, based on the designed depth, include:

[0102] The first step is to add cement slurry with a water-cement ratio of 1:1.5 into the rotary jet grouting equipment;

[0103] The second step is to control the airflow pressure output by the jet grouting equipment to be no less than 0.7 MPa and the cement slurry flow pressure to be no less than 20.0 MPa.

[0104] The third step is to inject grout when the grouting pipe is inserted into the borehole and the nozzle reaches the design elevation. After the injection grouting parameters reach the specified values, the grouting pipe is raised and grout is injected from bottom to top.

[0105] Fourth, once the high-pressure jet grouting is complete, the grouting pipe should be pulled out quickly;

[0106] The specific steps for driving columns into the foundation of piles include:

[0107] The first step is to use a total station to set out the construction layout, and calculate the coordinate data of each pile based on the positioning drawings and pile location map;

[0108] The second step involves using a robotic arm to construct the columns. After the equipment is in place, two theodolites are used to cross each other at 90° to check the verticality of the pile. The verticality of the pile inserted into the soil should be kept within 1 / 200 of the embedment depth. The deviation of the center position of the column should not exceed 3cm, the deviation between the center of the completed pile and the center of the designed pile position should be less than 5cm, and the deviation between the top elevation of the column and the designed top elevation should be less than 3cm.

[0109] Furthermore, in the above technical solution, before constructing the pilot hole column for the high-pressure jet grouting pile, trial insertion of the column is carried out around the foundation pit construction site according to the thickness of the silty clay layer to determine the location of the pilot hole for the high-pressure jet grouting pile.

[0110] Furthermore, in the above technical solution, the specific steps for welding and fixing the bracket to the column include:

[0111] The first step is to create a welding workspace for the corbel in a localized deep excavation area during the earthwork excavation process.

[0112] The second step is to fix the bracket welding fastener to the side wall of the column, and then weld the bracket to the welding fastener;

[0113] The third step involves monitoring the verticality, elevation, and horizontality of the bracket during the connection process between the bracket and the welding fastener.

[0114] The specific steps for welding fasteners to fix the corbels to the side wall of the column are as follows:

[0115] Before welding the corbel, drilling and rebar installation are carried out on the column, and the main reinforcement is exposed by breaking the skin and welding one of the steel plates to form a welded fastener.

[0116] Furthermore, in the above technical solution, the specific steps for installing and fixing the walers, brackets, non-standard parts, and connectors include:

[0117] The first step is to pre-embed bolts in the corbels, and then connect the walers to the bolts in order of length from longest to shortest.

[0118] The second step is to place the wrench on the pre-tightened bolt, insert the inner sleeve into the spline head inside the bolt, then slightly rotate the outer sleeve to align it with the nut, and push it to the root of the nut. Turn on the power switch, and the inner and outer sleeves will rotate in opposite directions to tighten the bolt.

[0119] The third step is to weld the bracket to the column after positioning it on the column, and connect the crossbeam to the bracket by bolts and spot welding.

[0120] The fourth step is to install the triangular keys and connectors at the ends of the corner braces and the opposite braces after the walers are installed, and fix them to the columns with bolts. When the size and shape of the foundation pit are irregular, non-standard parts are used for the connection with the standard parts. During the connection of the corner braces and opposite braces, a special jack is used to pressurize the middle of the two loading beams.

[0121] Furthermore, in the above technical solution, when the size and shape of the foundation pit are irregular, the specific locations where non-standard parts are used for connection with standard parts include:

[0122] When the bracing and waler are not perpendicular, at the connection point between the bracing and waler, when the angle between the corner bracing and waler is not equal to 45°, at the connection point between the corner bracing and waler, when the angle between adjacent foundation pits is not equal to 90°, and at the connection point between adjacent walers.

[0123] Furthermore, in the above technical solution, the specific steps for supporting the installation of the enclosed fish-belly beam and arranging monitoring points on the fish-belly beam include:

[0124] The first step is for the construction workers to build and optimize the structural model of the fish-belly beam based on the distance between the columns and the shape of the foundation pit.

[0125] The second step is to prepare components based on the optimized three-dimensional model of the fish belly beam, transport the prepared components to the construction site, and then assemble and hoist them.

[0126] The third step is to lift individual components of the fish-belly beam after pre-assembly and place them on the crossbeam and support for assembly. After the support is lifted, both ends are manually pulled to ensure the overall stability of the support.

[0127] The fourth step is to place core-through jacks at the ends of the steel strands in the fish-belly beam to pull the steel strands.

[0128] Furthermore, in the above technical solution, the specific steps for applying prestressed steel strand tension to the fish-belly beam and conducting stress testing are as follows:

[0129] The first step is to thread the steel strands into the core-through jack;

[0130] The second step is to install the working anchor and its matching limit plate;

[0131] The third step is to symmetrically tension both ends of the steel strand, with the tension force increasing from 30% to 50%, 70%, and 110%, and then hold the load for 2 minutes before anchoring.

[0132] Furthermore, in the above technical solution, the steel strands are tensioned sequentially one by one, with each steel strand being tensioned to the design stress in three stages to ensure that the entire fish-belly beam steel bundle meets the design stress value after tensioning.

[0133] Example:

[0134] I. Preparatory Work

[0135] Surrounding environment survey; trial insertion of steel columns based on the thickness of the silty clay layer to determine the pilot holes for high-pressure jet grouting piles. Develop an earthwork excavation plan that coordinates with the support installation sequence, ensuring support is installed before excavation. Determine the location for steel component stacking and the access routes for construction machinery, and clear construction roads and travel routes. Verify that the axis control points and elevation reference points used for installation positioning, as well as the supporting components and embedded parts, meet the requirements of the drawings.

[0136] II. Stress Compensation for River Backfill

[0137] 1. Determine the relationship between the river channel and the foundation pit.

[0138] The project's foundation pit is located adjacent to a scenic area river on its south and east sides, with the closest distance being only 3 meters. The riverbed elevation is -4.15 meters, while the prestressed fish-belly steel supports are at an elevation of -3.70 meters. After the supports are constructed, each brace and corner brace will require a prestress of 800–1700 kN. Stress compensation measures are needed to prevent the retaining piles from shifting and the riverbank from collapsing. Furthermore, the riverbank has good permeability, and the river is approximately 2 meters deep, making leakage a potential concern during underground construction. During the flood season, there is a risk of the river water level rising too high and flowing back into the foundation pit, necessitating urgent treatment of the river.

[0139] 2. Cofferdam construction

[0140] Before excavation, a cofferdam was set up at the river bifurcation. The cofferdam was made of sandbags and arranged in two rows of φ100×4mm steel pipes, with a length of 5m and a spacing of 300mm. The cofferdam was 1m wide. The top of the cofferdam was fixed by welding the two rows of steel pipes with No. 10 channel steel. The highest water level in the river was 100mm at the top of the cofferdam. Waterproof cloth was attached to the inside of the steel pipes, and the space between the two rows of steel pipes was filled with sandbags up to the top of the cofferdam.

[0141] 3. Pumping water and dredging

[0142] After the cofferdam construction is completed, water pumps are used to pump water out of the river. The river water is used for drilling and grouting pile construction as well as for on-site fire fighting. During the pumping process, the cofferdam must be checked for leaks. After the river water is pumped out, the silt in the river channel is removed.

[0143] 4. Riverbed backfilling

[0144] III. Beef Leg

[0145] 1. Steel column

[0146] The steel column under the steel support adopts high-pressure jet grouting pile pilot hole; the bottom of the pile is the bottom of the column, and the top is the ground.

[0147] 2. Parameters of high-pressure jet grouting piles

[0148] The 600mm double-tube high-pressure jet grouting pile technology is adopted, using P.O42.5 ordinary Portland cement with an admixture content of 20% and a cement grout water-cement ratio of 1:1.5. Before high-pressure jet grouting pile construction, the retaining structure within a 20m radius must be completed and reach 80% of the design strength. The double-tube high-pressure jet grouting pile construction process includes: pressure control, with an airflow pressure of not less than 0.7MPa and a cement grout flow pressure of not less than 20.0MPa; a jet grouting lifting speed of 10–15cm / min; and a cement grout flow rate greater than 60L / min. Grouting can begin when the grouting pipe is inserted into the borehole and the nozzle reaches the design elevation. After the grouting parameters reach the specified values, the grouting pipe is lifted according to the jet grouting pile process requirements, and grout is injected from bottom to top. The borehole lifting speed is 15–20r / min, and the overlap length of the grouting pipe sections should be greater than 100mm. If any abnormal situation occurs during high-pressure jet grouting, such as a sudden drop or rise in pressure or grout leakage, the cause should be identified and timely measures taken. After high-pressure jet grouting is completed, the grouting pipe should be pulled out quickly; the verticality deviation of the high-pressure jet grouting pile should not exceed 1 / 200; during the construction of the high-pressure jet grouting pile, ensure smooth grout return and take measures to reduce the impact on the surrounding environment.

[0149] 3. Column driving

[0150] (1) Calculation and verification of layout data: The layout of construction is carried out using a total station. Based on the positioning drawings and pile location map, the coordinate data of each pile is calculated first, the construction form is filled out, and it is verified and signed by an independent calculator and checker.

[0151] (2) The column erection is carried out using a robotic arm. After the equipment is in place, two theodolites are used to cross each other at 90° to check the verticality of the pile. The verticality of the pile inserted into the soil should be kept within 1 / 200 of the embedment depth. The deviation of the column center position should not exceed 3cm, the deviation of the pile center from the designed pile center position should be less than 5cm, and the deviation of the column top elevation from the designed top elevation should be less than 3cm. An S3 level instrument is configured during pile driving, and marks are pre-marked on the pile driving rod to control the pile top elevation.

[0152] (3) The pile top elevation shall be strictly controlled in accordance with the design drawings and over-pile shall be strictly prohibited; the pile top elevation error shall be strictly controlled and 10% of the total number shall be randomly checked during inspection, and no less than 3 column piles shall be checked.

[0153] IV. Bracket Installation

[0154] 1. The corbels at the bottom of the waler are made of welded structural steel and channel steel. When excavating the foundation pit, the working space for welding the corbels is first excavated in a localized deep excavation at the corbel location.

[0155] 2. Before welding the corbel, drill holes and install rebar in the cast-in-place pile or break the skin of the cast-in-place pile to expose the main reinforcement and then weld the steel plate or weld it directly onto the SMW method pile steel. Weld the corbel onto the welded steel plate. Ensure that the three connection points are firm and reliable with sufficient stability. There should be no twisting or false welding.

[0156] 3. During the connection process between the corbel and the existing steel components of the enclosure structure, the verticality, elevation, and horizontality of the corbel must be monitored throughout the process. The allowable error of the horizontal elevation of the corbel is 5mm, the allowable height difference between the top surfaces of adjacent brackets is L / 1500 (L is the spacing), and it should not be greater than 10mm. The elevation angle of the corbel should be 90 to 95 degrees, and should not be less than 90 degrees.

[0157] V. Installation of walers and force transmission components

[0158] 1. The steel waler is connected to the bored pile retaining structure by cast-in-place concrete waler. Anchor bolts are pre-embedded in the concrete waler and connected to the steel waler with bolts. The steel waler is used as the side formwork of the concrete waler, and the concrete waler is poured afterward. This improves the installation accuracy of the anchor bolts and the fit between the steel waler and the concrete waler.

[0159] 2. The steel waler and the SMW method pile are connected by welding of force transmission components. After the steel waler is installed, the size of the force transmission components is measured on site and then cut and installed. One end plate is set on one side of the force transmission component and bolted to the waler, and the other end is directly welded to the steel of the method pile.

[0160] 3. Waler installation

[0161] The installation of walers should follow the principle of "longer walers first, shorter walers later, to reduce the number of joints". Longer walers, especially standard sections, should be used first to reduce the number of joints.

[0162] The walers are hoisted section by section in sequence with the support frame erection. Manual labor, in conjunction with hoisting equipment, places the steel walers onto the bracket supports. After the walers are in place, check whether the steel brackets have loosened due to impact; if so, immediately weld them in place for reinforcement. The connection and overlapping parts of the walers must meet strength requirements and be fastened with bolts. Bolts arriving on site must have batch inspection certificates and factory inspection reports; only bolts that pass inspection can be used. When assembling bolted connections, the side of the nut with the frustum should face the side of the washer with the chamfer.

[0163] VI. Installation of brackets and support beams

[0164] The installation of the support components must strictly control their horizontal elevation. The top surface horizontal elevation should be determined by reverse calculation from the positioning elevation of the corner braces and purlins, with an error not exceeding 5mm. The elevation difference between the top surfaces of adjacent supports should not exceed L / 1500 (L is the spacing), and should not exceed 10mm. The upper elevation of the support component = center elevation of the support structure - (1 / 2 of the H-beam specification + support beam specification). Strict verticality control is required during the installation of the support components. Even if the column pile is misaligned, the support component must be made vertical by adding shims. After positioning on the column, weld the support component to the column, with a weld seam of not less than 8mm. Then, connect the beam and support component with four bolts. If a four-bolt connection is not possible, weld angle steel at that point to reinforce the connection between the column and beam. After prestressing is applied, it is essential to check and ensure the effective connection between the support beam and the corner braces and walers; in particular, the corner braces and walers must be tightened with high-strength bolts to improve the overall rigidity of the prestressed support system. Four bolts are required at each fastening point. If four bolts cannot be installed for any reason, at least two connecting bolts must be used, and reinforcement measures must be taken.

[0165] VII. Segmented installation of the bracing angles

[0166] After the steel walers are installed, install the triangular keys and connectors at the ends of the corner braces (connecting the braces, figure-eight braces, and wainscoting). Securely connect the loading beams and retaining boxes with bolts. The H-shaped standard braces are not installed at this stage to allow for the provision of an excavation channel. When the size and shape of the foundation pit are irregular, non-standard components are used for connection with the standard components, primarily in the following locations: when the braces are not perpendicular to the walers, at the connection point between the braces and walers; when the angle between the corner braces and walers is not equal to 45°; at the connection point between the corner braces and walers; when the angle between adjacent foundation pits is not equal to 90°; and at the connection point between adjacent walers. For narrow, deep foundation pits, prestressed braces are installed only at both ends. The middle section is sloped and excavated to create an excavation channel. Bracing closure takes precedence over earthwork excavation by 1-2 layers of support. The pressure application for support closure and earthwork excavation are gradually advanced to ensure coordination between earthwork excavation and support construction. During the connection of some corner braces and cross braces, a special jack is used to apply pressure between the two loading beams for reinforcement. During pressure application, a gap is created between the force-holding box and the loading beam. A steel plate of appropriate thickness must be used to tightly seal the gap, transferring and maintaining the prestress / axial force of the support, and preventing overall eccentricity of the support system under load. A cover plate is installed on top of the steel support to improve stability; the cover plates are typically spaced 4–5 m apart and located at the steel splice points. The steel support is fixed to the bottom bracket using U-shaped clips, and lateral displacement is restrained during the prestressing stage.

[0167] 8. Installation of fish-belly beam

[0168] 1. Fish-belly beams are used in large-span sections without bracing. SS-type fish-belly beams are used, with the angle between the steel strands at the end of the fish-belly beam and the waist beam ranging from 30° to 38°. The fish-belly beam consists of waist beams, web members, cable trays, AS connectors, locks, lock plates, steel strands, etc.

[0169] 2. The fish-belly beam should be partially assembled on the ground according to the design span before being hoisted into place. Afterwards, the bolts must be tightened to meet design and specification requirements. Jacks and oil gauges used for installation must be calibrated periodically, and records must be kept.

[0170] When cutting prestressing tendons, use a grinding wheel to cut them. Do not use oxygen, acetylene, or electric welding to cut them, as this may burn the prestressing tendons.

[0171] 3. Requirements for steel strand cutting: The relative difference in length within the same bundle is as follows: For bundle length > 20m, L / 5000 (L is the bundle length) should not exceed 5mm; for bundle length 6m to 20m, L / 3000 should not exceed 4mm; the tensile stress value should meet the design requirements, and the tensile elongation rate should be controlled at 6%.

[0172] 4. After the fish belly beam is pre-assembled, individual components are lifted and placed on the supporting crossbeams and triangular brackets for assembly. After the support is lifted, both ends are manually pulled to ensure the overall stability of the support.

[0173] 5. When tensioning, the line of action of the tensioning force of the jack should coincide with the axis of the prestressing tendon; the difference between the actual elongation and the theoretical elongation should meet the design requirements and should be controlled within 6%. Otherwise, tensioning should be suspended until the cause is found and measures are taken to adjust it before tensioning can continue.

[0174] 6. Tensioning of the steel strands should be carried out sequentially, one strand at a time. Considering the influence of frictional resistance between the steel strands, tensioning should be performed in three stages to reach the design stress, ensuring that the entire fish-belly beam meets the design stress value after tensioning. Necessary preventative measures must be in place at the construction site to ensure the safety of all personnel and equipment.

[0175] IX. Application of Prestress

[0176] The principles for applying prestress in this case are: zoning, grading, and cyclic application.

[0177] 1. Apply pressure to the support

[0178] As prestress is applied to newly installed supports, the stress of adjacent, already installed supports may decrease. Therefore, prestress can be added again according to design requirements. Thus, supports must have a prestressing device. When the horizontal displacement rate of the retaining piles exceeds the warning value, prestress can be appropriately increased to control deformation. When applying prestress, the connection status of each joint must be checked promptly, and records of the applied prestress must be kept. Eccentric compression of the support due to uneven contact with embedded parts after prestressing is strictly prohibited. After the support is under load, it is essential to strictly check and prevent gradual changes due to non-perpendicularity between the support and the pressure surface, which could lead to a continuous increase in the horizontal displacement of the retaining wall and even support instability. To control the extension length of the jack cylinder to within 10cm, a steel plate can be placed behind the jack to adjust the cylinder length during pressurization. Pressurization of the support must be strictly performed according to the axial force provided on the design drawings; underloading or overloading is not permitted.

[0179] 2. Applying prestress to the steel strand

[0180] When tensioning steel strands, each strand must be tensioned individually using a phased over-tensioning method to avoid incomplete tensioning in some areas. It is crucial to ensure a unified and evenly distributed tension. Initially, apply 50% and 70% of the tension force specified on the drawings. After stabilization, increase the load to 110% and hold this load for 15 minutes. Once no displacement of the anchor head is observed, lock the strand. Prestressing of the steel strands is achieved using a core jack, over-tensioning to 110% of the prestress, then inserting wedge-shaped clamps. This causes the steel strands to contract and lock, ensuring the prestress meets design requirements.

[0181] 10. Deformation Monitoring

[0182] 1. Deformation monitoring of tool-type combined internal support systems is generally carried out simultaneously with foundation pit monitoring, and the monitoring content includes changes in the axial force of components.

[0183] 2. Component axial force monitoring: String reaction gauges or strain gauges are directly placed at the main stress points of the prefabricated steel support components. The deformation stress is integrated through the transmission cable. The monitoring frequency is 1 to 2 times / day under normal conditions and 3 to 6 times / day under abnormal conditions.

[0184] 3. The monitoring results will be analyzed by professionals and promptly reported to all parties. The process will be continuously monitored until the excavation of the foundation pit is completed and the structural concrete reaches the predetermined strength. The control of alarm values ​​should be strictly in accordance with the design specifications.

[0185] Specifically, the principle of this invention is as follows: A site survey is conducted to determine the difference in elevation between the riverbed and the support, and the riverbed is backfilled; the construction site is enclosed, and the high-pressure jet grouting pile pilot hole columns are constructed; earthwork is excavated on the support installation work surface; the corbels are welded and fixed to the columns; the walers, brackets, non-standard parts, and connectors are installed and fixed; the closed fish-belly beam is installed, and monitoring points are arranged on the fish-belly beam; prestressed steel strands are tensioned on the fish-belly beam, and stress is tested; the next layer of earthwork is excavated.

Claims

1. A method for constructing deep foundation pit supports in complex environments, characterized in that, Includes the following steps: S10: Conduct a site survey to determine the difference between the riverbed height and the support elevation, and carry out river backfilling; S20: Enclose the construction site and construct the high-pressure jet grouting pile pilot hole erection column; S30: Excavate the earthwork for the support installation work surface; S40: Weld the bracket to the column to fix it in place; S50: Install and fix walers, brackets, non-standard parts and connectors; S60: Support the installation of the closed fish belly beam, and arrange monitoring points on the fish belly beam; S70: Apply prestressed steel strand tension to the fish-belly beam and perform stress testing; S80: Proceed with the excavation of the next layer of earthwork; The specific steps for surveying the construction site, determining the difference between the riverbed height and the support elevation, and carrying out river backfilling include: The first step is for the construction workers to survey the surrounding waterways and pipelines at the location of the foundation pit to determine the elevation difference between the riverbed and the support. The second step is to determine the relationship between the river channel and the foundation pit. The third step is to set up a cofferdam at the bifurcation of the river before the river is excavated. The cofferdam is made of sandbags, with double rows of steel pipes arranged in the sandbags and the steel pipes fixed by channel steel. The fourth step is to fix a waterproof cloth on the side of the steel pipe closest to the river channel, and to fill the space between the two rows of steel pipes with sandbags. The fifth step is to use water pumps to pump water after the cofferdam construction is completed, and to clean the silt from the bottom of the river channel. The sixth step is to backfill the cleared riverbed; The specific steps for backfilling the cleaned riverbed are as follows: The first step is to process the clay by cleaning out the silt, humus and organic matter, and controlling the moisture content of the clay. The second step involves backfilling the clay into the river channel in layers using a layered filling and compaction method. The third step is to mechanically level and then re-compact the river channel after the overall backfilling is completed. The fourth step is to ensure that the backfill height is the same as the revetment height.

2. The method for constructing deep foundation pit support in complex environments according to claim 1, characterized in that, The specific steps for enclosing the construction site and constructing the high-pressure jet grouting pile pilot hole erection column include: The first step is to enclose the construction site with fencing; The second step is to determine the location and depth of the pilot hole at the construction location of the foundation pit, and then to lay out the lines. The third step is to install the rotary spraying equipment in the designated location and perform debugging; The fourth step is to use a jet grouting machine to create pilot holes, operating according to the designed depth; The fifth step is to carry out pile foundation construction based on the pilot hole. The sixth step is to drive columns into the foundation of the piles; The specific steps for using a rotary jetting device to create a pilot hole, based on the designed depth, include: The first step is to add cement slurry with a water-cement ratio of 1:1.5 into the rotary jet grouting equipment; The second step is to control the airflow pressure output by the rotary jetting equipment to be no less than 0.7 MPa and the cement slurry flow pressure to be no less than 20.0 MPa. The third step is to inject grout when the grouting pipe is inserted into the borehole and the nozzle reaches the design elevation. After the injection grouting parameters reach the specified values, the grouting pipe is raised and grout is injected from bottom to top. Fourth, once the high-pressure jet grouting is complete, the grouting pipe should be pulled out quickly; The specific steps for driving columns into the pile foundation include: The first step is to use a total station to set out the construction layout, and calculate the coordinate data of each pile based on the positioning drawings and pile location map; The second step involves using a robotic arm to construct the columns. After the equipment is in place, two theodolites are used to cross each other at 90° to check the verticality of the pile. The verticality of the pile inserted into the soil should be kept within 1 / 200 of the embedment depth. The deviation of the center position of the column should not exceed 3cm, the deviation between the center of the completed pile and the center of the designed pile position should be less than 5cm, and the deviation between the top elevation of the column and the designed top elevation should be less than 3cm.

3. The method for constructing deep foundation pit support in a complex environment according to claim 2, characterized in that, Before constructing the pilot holes for the high-pressure jet grouting piles, trial insertion of the pilot holes is conducted around the construction site of the foundation pit based on the thickness of the silty clay layer to determine the location of the pilot holes.

4. The method for constructing deep foundation pit support in a complex environment according to claim 3, characterized in that, The specific steps for welding and fixing the bracket to the column include: The first step is to create a welding workspace for the corbel in a localized deep excavation area during the earthwork excavation process. The second step is to fix the welding fastener of the bracket to the side wall of the column, and then weld the bracket to the welding fastener. Third, during the docking process between the bracket and the welding fastener, the verticality, elevation, and horizontality of the bracket are monitored. The specific steps for welding the bracket to fix the corbel on the side wall of the column are as follows: Before welding the corbel, drilling and rebar installation are carried out on the column, and the main reinforcement is exposed by breaking the surface and welding one of the steel plates to form the welded fastener.

5. The method for constructing deep foundation pit support in a complex environment according to claim 4, characterized in that, The specific steps for installing and fixing the walers, brackets, non-standard parts, and connectors include: The first step is to pre-embed bolts in the corbel, and then connect the walers to the bolts in order of length from longest to shortest. The second step is to place the wrench on the pre-tightened bolt, insert the inner sleeve into the spline head inside the bolt, then slightly rotate the outer sleeve to align it with the nut, push it to the root of the nut, turn on the power switch, and the inner and outer sleeves will rotate in opposite directions to tighten the bolt. The third step is to weld the bracket to the column after positioning it on the column, and connect the crossbeam to the bracket by bolts and spot welding. Fourth, after the waler is installed, install the triangular keys and connectors at the ends of the corner braces and the opposite braces, and fix them to the columns with bolts; when the size and shape of the foundation pit are irregular, non-standard parts are used for the connection with the standard parts; during the connection of the corner braces and opposite braces, a special jack is used to pressurize the middle of the two loading beams.

6. The method for constructing deep foundation pit support in a complex environment according to claim 5, characterized in that, When the size and shape of the foundation pit are irregular, the specific locations where non-standard parts are used for connection with standard parts include: When the bracing and waler are not perpendicular, at the connection point between the bracing and waler, when the angle between the corner bracing and waler is not equal to 45°, at the connection point between the corner bracing and waler, when the angle between adjacent foundation pits is not equal to 90°, and at the connection point between adjacent walers.

7. The method for constructing deep foundation pit support in a complex environment according to claim 6, characterized in that, The specific steps for installing the supporting closed fish-belly beam and arranging monitoring points on the fish-belly beam include: The first step is for the construction workers to construct and optimize the structural model of the fish-belly beam based on the distance between the columns and the shape of the foundation pit. The second step is to prepare components based on the optimized three-dimensional model of the fish belly beam, transport the prepared components to the construction site, and then assemble and hoist them. The third step is to lift and place individual components on the crossbeam and support after the fish belly beam is pre-assembled. After the support is lifted, both ends are manually pulled to ensure the overall stability of the support. The fourth step is to place core-through jacks at the end positions of the steel strands of the fish-belly beam to pull the steel strands.

8. The method for constructing deep foundation pit support in a complex environment according to claim 7, characterized in that, The specific steps for applying prestressed steel strand tension to the fish-belly beam and conducting stress testing are as follows: The first step is to thread the steel strands through the core-through jack; The second step is to install the working anchor and its matching limit plate; The third step is to symmetrically tension both ends of the steel strand, with the tension force increasing from 30% to 50%, 70%, and 110%, and then hold the load for 2 minutes before anchoring.

9. The method for constructing deep foundation pit support in a complex environment according to claim 8, characterized in that, The steel strands are tensioned sequentially, one strand at a time, with each strand tensioned to the design stress in three stages to ensure that the entire fish-belly beam steel bundle meets the design stress value after tensioning.

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