A jet anchor double-pipeline post-grouting construction method suitable for super-thick water-rich silt stratum

Abstract

The present application belongs to the technical field of geotechnical engineering deep foundation pit support and slope prestressed anchoring construction, in particular to a jet grouting anchor cable double-pipeline post-grouting construction method suitable for ultra-thick water-rich silt stratum, which is applicable to loose sandy stratum first-level deep foundation pit support and slope prestressed anchoring engineering with stable confined water and silt layer thickness greater than or equal to 4m, including existing conventional steps: site investigation and process parameter design, anchor cable hole position measurement and placement, jet grouting drill wall hole forming, anchor cable lowering, one-time high-pressure jet grouting, grout maintenance, anchor cable tensioning and locking, quality detection and archiving; also supporting differential double-pipeline post-grouting anchor cable prefabrication process and segmented high-pressure post-grouting process; simultaneously solving technical problems such as silt stratum hole collapse and sand gushing, grout loss, insufficient anchoring force, tensioning hole mouth water seepage, prestress attenuation and the like, and taking into account construction efficiency, engineering cost and long-term foundation pit support safety.

Description

Technical Field

[0001] This invention belongs to the field of deep foundation pit support and slope prestressed anchoring construction technology in geotechnical engineering, and in particular, it is a method for post-grouting construction of jet grouting anchor cable with dual pipelines adapted to ultra-thick water-rich silty sand strata. Background Technology

[0002] Urban new area foundation pit projects often encounter thick saturated silty sand strata. These strata are characterized by loose soil, well-developed pores, high permeability, and stable confined water.

[0003] Currently, during conventional jet grouting anchor construction, due to the lack of self-stabilizing ability of silty sand, bare holes are prone to collapse, sand inrush, and water inrush. Relying solely on jet grout for temporary wall protection is insufficient to maintain the integrity of the hole wall. Before the cement-soil grout formed by a single jet grouting has solidified, pressurized water will continuously scour and carry away the cement grout, creating voids and cracks inside the anchor body. The bond strength between the anchor cable and the soil is highly variable, and the pull-out bearing capacity is insufficient to meet the design requirements of a first-level foundation pit. During the staged tensioning of the anchor cable, tiny separation cracks occur between the anchor body and the surrounding soil. Pressurized water seeps out from the hole opening along these cracks, and long-term scouring of the grout causes continuous attenuation of prestress.

[0004] The repeated jet grouting process involves drilling a conventional single jet grouting drill rod to the designed depth of the anchor cable without removing the drill rod. High-pressure cement grout is continuously sprayed, and the drill rod is repeatedly raised and lowered to cut, mix, and grout the soil in the anchoring section multiple times. The repeated jet grouting expands the diameter of the anchor body and increases the density of the cement-soil mixture. There are no pre-embedded grouting pipelines. All grouting operations are completed by multiple grouting operations during the drilling stage. The grouting process is completed in one go as the drill rod moves forward and backward. However, relying solely on repeated spraying and mixing of the soil with the drill rod cannot seal the deep seepage channels in the strata, and the problem of grout loss cannot be fundamentally solved. The anchoring force is only increased by 10% to 15%. Without sealing the grouting channel at the borehole opening, cracks will form between the free section of the anchor cable and the soil after tensioning, and pressurized water will continue to seep from the borehole opening, resulting in significant prestress loss. The quality is greatly affected by the operator's skill level, and the number of repeated sprayings and the lifting and lowering speed are all based on experience, resulting in high dispersion in the density of the anchor body. In thick, loose, pressurized silty sand strata, even with long periods of bare hole exposure, frequent borehole collapses and sand inrushes still occur. This method is only suitable for thin sandy and cohesive soil strata and cannot be used for water-rich silty sand conditions ≥4m deep.

[0005] The casing-following jet grouting process uses a special casing drilling rig with a protective casing. During the drilling process, the drill rod and steel casing advance synchronously. The casing supports the borehole wall throughout the process, isolating the loose sand and groundwater from the outside, and preventing borehole collapse and sand inrush during the drilling stage. After the borehole is completed, anchor cables are placed inside the casing and jet grouting is carried out. After the grouting is completed, the casing is pulled out in sections, and the borehole shape is maintained by the temporary protective casing. However, this process requires a supporting drilling rig, thick-walled steel casing, and pipe-pulling jacks, increasing costs for casing rental, machinery shifts, and labor. The pipe-pulling process is time-consuming, reducing the daily output of a single drilling rig compared to conventional jet grouting. The casing is easily jammed by sand, leading to difficulties in pipe extraction and situations where the casing becomes stuck in the borehole and is rendered unusable. The casing only serves as temporary wall protection; groundwater will still wash away the cement grout during the grouting stage, and problems such as voids in the deep anchor body and insufficient bond strength persist, limiting the improvement in anchoring force. Furthermore, the lack of an independent borehole seepage prevention structure means that seepage at the borehole opening after tensioning and prestress attenuation defects cannot be eliminated. It is suitable for extremely thick quicksand, gravel layers, areas near important buildings, and special sections where strict control of borehole collapse risk is necessary; it is uneconomical for large-area standard foundation pits.

[0006] The working principle of single-strand post-grouting pipe anchor cable is that only one independent PVC grouting pipe is pre-embedded in the anchor cable steel strand. The bottom end of the grouting pipe reaches directly to the bottom of the anchoring section, and there is no grouting channel in the free section. First, jet grouting is performed to form the hole, followed by one-time foundation grouting. After the cement and soil have initially set, high-pressure fracturing grouting is performed through the single pre-embedded pipe to fill the voids in the anchoring section, improving the overall density of the anchoring section. There is no seepage prevention grouting structure for the free section at the hole opening. Compared with repeated jet grouting and ordinary jet grouting, the grouting fullness and pull-out bearing capacity of the anchoring section are effectively improved. No casing is required, the incremental equipment investment is small, and the construction efficiency is better than the casing process. However, with only single-pipe grouting in the anchoring section, after the anchor cable is tensioned and loaded, the soil in the free section separates from the anchor body, creating through-seepage fissures. Pressurized groundwater continuously gushes out from the borehole, eroding the grout over a long period. The prestress loss rate is generally high, which cannot meet the long-term stability requirements of the first-level foundation pit. It can only strengthen the deep anchoring force and cannot simultaneously seal the borehole leakage. Only one of the two problems can be solved. The repair of borehole leakage in water-rich thick silty sand strata can only be remedied by additional drilling and grouting in the later stage, which adds extra procedures, extends the construction period, and increases labor costs.

[0007] In summary, none of the existing mainstream anchor cable technologies can simultaneously solve the technical problems of "grout loss in the anchoring section and water seepage at the tensioning orifice" in thick pressure silty sand formations, and there are at least one significant shortcoming in terms of quality, cost, and efficiency. Summary of the Invention

[0008] To address the shortcomings of the existing technologies, a dual-pipeline post-grouting construction method for jet grouting anchor cables, adapted to ultra-thick water-rich silty sand strata, is provided. This method simultaneously solves technical problems such as hole collapse and sand inrush, grout loss, insufficient anchoring force, water seepage at tensioning orifices, and prestress attenuation in silty sand strata, while taking into account construction efficiency, project cost, and long-term support safety of the foundation pit.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by this invention is a post-grouting construction method for jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata. It is applicable to first-level deep foundation pit support and slope prestressed anchoring projects in loose sandy strata with stable confined water and silty sand layer thickness ≥4m. It includes existing conventional steps: site investigation and process parameter design, anchor cable hole location measurement and layout, jet grouting drilling for wall protection and hole formation, anchor cable lowering, one-time high-pressure jet grouting, grout curing, anchor cable tensioning and locking, and quality inspection and archiving; it also includes a differentiated dual-pipeline post-grouting anchor cable prefabrication process and a segmented high-pressure post-grouting process. The on-site operation process of the differentiated dual-pipeline post-grouting anchor cable prefabrication process is as follows: pre-cut steel strands and two high-pressure grouting pipes, and use ring-shaped isolation clamps to bind and fix them in layers. First fix the steel strands, then fix the two high-pressure grouting pipes. After the overall binding is completed, transport it to the hole position for lowering. The differentiated dual-pipeline post-grouting anchor cable prefabrication process is as follows: the anchor cable body adopts Φ15.2 standard prestressed multi-strand steel strands, including a first post-grouting pipe and a second post-grouting pipe that are independent of each other; the first and second post-grouting pipes are high-pressure PVC grouting pipes with an outer diameter of 32mm, a wall thickness of 2.5mm, and a rated pressure resistance of ≥3MPa. The first post-grouting pipe is laid along the entire length of the steel strand, and its bottom end is completely sealed with a rubber plug. Every 50cm along the anchor section, there are small grout outlet holes with a diameter of 8mm. The second post-grouting pipe is fixed to the outside of the steel strand, and its grout outlet is located 2m away from the anchor cable hole. The outside of the grout outlet is wrapped with a 60-mesh metal sand-proof filter to prevent sand from flowing back and blocking the pipe. The two grouting pipes are fixed to the steel strand through isolation brackets and are not connected to each other. The segmented high-pressure post-grouting process is set after the completion of the first high-pressure jet grouting and 4-6 hours after the initial setting of the grout. Two independent horizontal high-pressure grouting pumps are connected to two grouting pipes respectively. The rated working pressure of the grouting pumps is ≥3MPa, and the two-stage grouting process is executed sequentially. S1. Anchorage section reinforcement grouting: Inject 2.0-2.5MPa cement grout into the first rear grouting pipe. After the grouting volume reaches the design value, stabilize the pressure for 3-5 minutes and then stop the pump. If the pressure drop is ≤0.2MPa within 3 minutes during the stabilization stage, the grouting can be considered full. S2. Grouting for seepage prevention and sealing at the borehole: After an interval of 10 minutes, 1.0-1.5 MPa cement grout is introduced into the second grouting pipe to fill the seepage cracks in the soil 2m below the borehole. The pressure is maintained for 3 minutes until there is no backflow of grout or seepage at the borehole, at which point the grouting is completed. The entire grouting process adopts a dual-control acceptance standard of grouting volume and final pressure.

[0010] The above-mentioned method for grouting construction using a dual-pipeline jet grouting anchor cable for ultra-thick water-rich silty sand formations involves using a single-pipe high-pressure jet grouting drill with a rated jet grouting pressure ≥30MPa for wall protection and hole formation. The drill rotates at low speed and simultaneously injects cement slurry to form a temporary wall protection, with the drilling speed controlled at 0.6~1.0m / min. Drilling speeds below 0.6 m / min result in low construction efficiency, while speeds above 1.0 m / min prevent the jet grout from forming a complete protective film, making localized hole collapse more likely. After drilling to the designed hole depth, continuous jet grouting for 30 seconds is used to reinforce the bottom sand layer.

[0011] The above-mentioned method for grouting after jet grouting with anchor cables in ultra-thick, water-rich silty sand formations includes the following high-pressure jet grouting process parameters: jet grouting pressure 20-25 MPa; jet grouting pressure below 20 MPa cannot adequately cut and mix the silty sand, while pressure above 25 MPa can easily excessively disturb the formation and expand the seepage channels; the cement slurry uses P.O42.5 ordinary Portland cement, with 3% expansion agent added by the cement mass, a water-cement ratio of 0.8-1.0, and a mixing time of not less than 3 minutes; the drill rod lifting speed is 0.4-0.8 m / min.

[0012] The above-mentioned jet grouting anchor cable dual-pipeline post-grouting construction method adapted to ultra-thick water-rich silty sand strata includes the following anchor cable tensioning and locking procedures: After post-grouting, the anchor body is naturally cured for 7 days until its strength reaches more than 80% of the design strength before graded tensioning is carried out; tensioning is performed using a through-hole jack equipped with a 0.4-grade precision hydraulic gauge, with four levels of loading, the four levels of load being 25%, 50%, 75%, and 100% of the design tension respectively, and the holding time of each level being no less than 5 minutes. After loading to 1.1 times the design tension, compensation tensioning is implemented, and after locking, the orifice is sealed with cement mortar to complete the anchor sealing.

[0013] The above-mentioned method for grouting construction using jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata uses a ring-shaped plastic clamp support, with one set installed every 1.5m. Each set of supports simultaneously clamps the steel strand, the first grouting pipe, and the second grouting pipe, achieving a central separation between the three and preventing the pipeline from being squeezed, bent, or blocked from the grout outlet.

[0014] The above-mentioned jet grouting anchor cable dual-pipeline post-grouting construction method adapted to ultra-thick water-rich silty sand strata involves embedding stress monitoring elements after the anchor cables are tensioned. The stress monitoring elements are CTV-202 type stress monitoring elements with an instrument stress acquisition accuracy ≤0.5kN. Anchor cable stress data are collected continuously for 7 days, and a stress attenuation of less than 8% is considered qualified. Anchor cables are randomly selected for pull-out bearing capacity testing, and an independent construction ledger for each anchor cable is established.

[0015] The above-mentioned jet grouting and anchor cable dual-pipeline post-grouting construction method adapted to ultra-thick water-rich silty sand strata also includes handling of abnormal construction conditions, including: If a large amount of sand gushes from the orifice and grout continues to return during the grouting process: reduce the grouting pressure by 0.2~0.5MPa and extend the pressure stabilization time by 2min; if a large amount of grout still returns, suspend grouting and let it stand for 20min until the surface grout has initially set before re-grouting. Grouting pipe blockage: Remove the filter screen at the end of the grouting pipe and flush with clean water under high pressure. If the blockage is severe, replace it with a spare grouting pipe of the same specification. For areas with silty sand thickness exceeding 6m and high confined water level: increase the upper limit pressure of grouting in the anchoring section to 2.5MPa, extend the pressure stabilization time to 5min, and simultaneously increase the design grouting volume by 15%; After tensioning, a small amount of water seepage occurred at the orifice: low-pressure cement grout was injected through the second grouting pipe to seal the leak.

[0016] The beneficial effects of this invention, a dual-pipeline post-grouting construction method for jet grouting anchor cables adapted to ultra-thick water-rich silty sand strata, are that by using differentiated segmented grouting with two independent grouting pipelines, graded pressure coordinated control, jet grouting hole formation, primary grouting, and secondary reinforcement grouting after initial setting, it simultaneously solves technical problems such as hole collapse and sand inrush, grout loss, insufficient anchoring force, water seepage at tensioning orifices, and prestress attenuation in silty sand strata. It also takes into account construction efficiency, project cost, and long-term support safety of the foundation pit, forming a standardized and replicable complete anchoring construction method.

[0017] This invention employs two completely independent grouting pipelines. The first pipeline extends directly to the bottom of the anchorage section, where high-pressure fracturing grouting fills the grout voids created by pressurized water erosion, enhancing the bond strength between the steel strands and the soil. The second pipeline is used specifically for grouting within a 2m radius below the borehole opening, effectively sealing seepage fissures generated after tensioning, eliminating continuous water leakage at the borehole opening after tensioning, and significantly reducing the prestress attenuation rate of the anchor cable. The two pipelines grout independently without interference, eliminating the risk of grout cross-contamination or pipe blockage.

[0018] During the drilling stage, cement slurry is sprayed simultaneously to form a temporary protective wall. When drilling to the bottom of the hole, the loose sand is reinforced by in-situ jet grouting. The foundation anchor body is formed by jet grouting in one go. After the slurry has initially set, high-pressure post-grouting is carried out. The dual control standards of grouting volume and final pressure are used to ensure full grouting. Compared with traditional processes, defects such as hole collapse, sand inrush and grout leakage are significantly reduced.

[0019] This invention only adds two pre-embedded grouting pipelines, eliminating the need for supporting casing and pipe pulling equipment, resulting in minimal equipment investment; the process is simplified, increasing the number of anchor cables that a single drilling rig can construct per day, shortening the overall support period, and significantly reducing the overall cost compared to the casing follow-up process.

[0020] This invention quantifies and limits all parameters, including drilling speed, jet grouting pressure, water-cement ratio, post-grouting pressure, pressure stabilization time, and tensioning grading standards. It also includes long-term stress monitoring after tensioning and the establishment of construction ledgers for each anchor cable. The entire construction data is recordable and traceable, meeting the stringent quality acceptance requirements for Class I foundation pits.

[0021] The entire process is designed for loose silty sand strata with a thickness of ≥4m and containing stable pressurized water. It can be directly applied to deep foundation pits and mountain slope anchoring projects. After all procedures and parameters are solidified, standardized operating documents can be compiled and directly replicated for use in similar geological projects. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the present invention; Figure 2 Diagram of the prefabrication process for differentiated dual-pipeline post-grouting anchor cables; Figure 3 This is a diagram of the dual-pipeline segmented high-pressure post-grouting process; Figure 4 Diagram of anchor cable tensioning and locking process; Figure 5 This is a diagram illustrating the hole-forming process for a rotary jet drilling rig. Figure 6 This is a diagram illustrating the process of high-pressure jet grouting foundation injection. Figure 7 This is a cross-sectional schematic diagram of the anchor cable body. Detailed Implementation

[0023] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1-7 As shown, a dual-pipeline post-grouting construction method for jet grouting anchor cables adapted to ultra-thick water-rich silty sand strata is applicable to first-level deep foundation pit support and slope prestressed anchoring projects in loose sandy strata with stable confined water and silty sand layer thickness ≥4m. The method includes existing conventional steps: site investigation and process parameter design, anchor cable hole location measurement and marking, jet grouting drilling for wall protection and hole formation, anchor cable lowering, one-time high-pressure jet grouting, grout curing, anchor cable tensioning and locking, and quality inspection and archiving. It also includes a differentiated dual-pipeline post-grouting anchor cable prefabrication process and a segmented high-pressure post-grouting process. The prefabrication process of the differentiated dual-pipeline post-grouting anchor cable is as follows: The anchor cable body includes a Φ15.2 standard prestressed multi-strand steel strand 1, a first post-grouting pipe 2 and a second post-grouting pipe 3 that are independent of each other; the first and second post-grouting pipes are high-pressure PVC grouting pipes with an outer diameter of 32mm, a wall thickness of 2.5mm, and a rated pressure of ≥3MPa; the first post-grouting pipe is laid along the entire length of the steel strand, and the bottom end is completely sealed with a rubber plug. Every 50cm along the pipe body in the anchoring section, there are small holes with a diameter of 8mm for grouting; the second post-grouting pipe is fixed to the outside of the steel strand, and its grout outlet is located 2m away from the anchor cable hole. The outside of the grout outlet is wrapped with a 60-mesh metal sand-proof filter to prevent sand from backflowing and blocking the pipe; the steel strand is fixed first, and then the two high-pressure grouting pipes are fixed. The two grouting pipes are fixed to the steel strand 1 by a ring-shaped plastic isolation bracket 4, and the pipes are not connected to each other.

[0025] This system achieves integrated prefabrication of pipelines that combine anchor reinforcement and orifice seepage prevention. The two pipelines are independent of each other, eliminating the risk of grout leakage and eliminating the need for additional drilling and plugging later. It solves the problem of traditional single-pipe anchor cables, which can only reinforce the anchor section but cannot seal tension seepage cracks.

[0026] The segmented high-pressure post-grouting process is set after the completion of the first high-pressure jet grouting and 4-6 hours after the initial setting of the grout. Two independent horizontal high-pressure grouting pumps are connected to two grouting pipes respectively. The grouting pumps are equipped with pressure stabilizing and energy storage devices, with a rated working pressure ≥3MPa. The two-stage grouting process is executed sequentially. S1. Anchorage Section Reinforcement Grouting: Inject 2.0-2.5MPa cement grout into the first post-grouting pipe. After the grouting volume reaches the design value, stabilize the pressure for 3-5 minutes and then stop the pump. If the pressure drop is ≤0.2MPa within 3 minutes during the stabilization stage, the grouting can be considered full. The high-pressure cement grout splits and spreads along the fissures of the silty sand stratum, filling the grout voids formed by the scouring of pressurized water, greatly improving the bond strength between the steel strand and the surrounding soil, and making up for the insufficient density of the anchor body caused by the loss of grout during the first jet grouting stage.

[0027] S2. Grouting for seepage prevention and sealing at the borehole opening: After a 10-minute interval, 1.0-1.5 MPa cement grout is introduced into the second grouting pipe to fill the seepage cracks in the soil 2m below the borehole opening. The pressure is maintained for 3 minutes until there is no grout return or seepage at the borehole opening, at which point the grouting is completed. Low-pressure grouting precisely seals the seepage channels generated in the free section after tensioning, blocking the seepage path of pressurized water and eliminating the long-term prestress attenuation of the anchor cable from the root. This is different from the shortcomings of traditional processes that can only reinforce the anchor section and cannot solve the problem of water leakage at the borehole opening.

[0028] The entire grouting process employs a dual-control acceptance standard of grouting volume and final pressure. Grouting can only be terminated when both conditions are met simultaneously: the total grouting volume reaches the design value and the pressure drop is ≤0.2MPa. A single qualified indicator does not constitute a successful grouting assessment. This avoids judging grouting fullness solely based on a single grouting volume or pressure, prevents insufficient grouting due to differences in formation porosity, and ensures quantifiable and controllable grouting quality for each anchor cable.

[0029] The rotary grouting drilling rig used for hole formation and wall protection employs a single-tube high-pressure rotary grouting rig with a rated rotary grouting pressure ≥30MPa. The rig rotates at low speed while simultaneously injecting cement grout to form a temporary protective wall. The drilling speed is controlled at 0.6–1.0 m / min. Drilling speeds below 0.6 m / min result in low construction efficiency, while speeds above 1.0 m / min prevent the rotary grout from forming a complete protective film, easily leading to localized hole collapse. After drilling to the designed hole depth, in-situ rotary grouting is continuously applied for 30 seconds to reinforce the silty sand layer at the bottom of the hole. The rotary grout forms a protective layer on the hole wall in real time, inhibiting hole collapse and sand inrush in the water-rich silty sand layer. In-situ rotary grouting at the bottom of the hole reinforces loose soil, reduces bottom voids, and minimizes subsequent grout loss, ensuring hole integrity without the need for casing.

[0030] The process parameters of the one-time high-pressure rotary jet grouting are as follows: the rotary jet pressure is 20 to 25MPa. If the rotary jet pressure is lower than 20MPa, the silty sand cannot be fully cut and stirred; if the rotary jet pressure is higher than 25MPa, the formation is likely to be excessively disturbed and the seepage channel is expanded. P.O 42.5 ordinary Portland cement is used for the cement slurry, 3% expansive agent by mass of the cement is mixed, the water-cement ratio is 0.8 to 1.0, and the stirring time is not less than 3min; the lifting speed of the drill pipe is 0.4 to 0.8m / min. A continuous foundation anchorage body is formed by high-pressure cutting and stirring of silty sand and cement slurry. The standardized parameters avoid differences in manual operation, ensure uniform forming of the foundation anchorage body, and provide a complete matrix for subsequent segmented post-grouting.

[0031] The anchor cable tensioning and locking process is as follows: after the post-grouting is completed, natural curing is carried out for 7d, and graded tensioning is carried out after the strength of the anchorage body reaches more than 80% of the design strength. A center-hole jack matched with a 0.4-level precision oil pressure gauge is adopted for tensioning, the tensioning is loaded in four stages, and the four-stage loads are 25%, 50%, 75% and 100% of the design tension in sequence. The load holding time of each stage is not less than 5min. Compensatory tensioning is implemented after loading to 1.1 times the design tension, and after locking, cement mortar is used to seal the orifice to complete anchor sealing. Sufficient curing ensures that the strength of the post-grouted body meets the standard, graded loading slowly releases the soil stress, and compensatory tensioning offsets the initial stress loss, further controlling the long-term prestress attenuation of the anchor cable and meeting the long-term support stability requirements of the first-level foundation pit.

[0032] The isolation support is an annular plastic clamp-type support, one set is arranged every 1.5m, each set of supports simultaneously clamps the steel strand, the first grouting pipe and the second grouting pipe, and is used to separate the steel strand from the two grouting pipes, so as to avoid the pipes from being squeezed, bent and blocking the grout outlet. It ensures that the steel strand and the two grouting pipes are centered in the hole, prevents the pipes from being squeezed and blocks the grout, ensures that the post-grouting slurry can flow out uniformly from each grout outlet, and guarantees stable grouting effects of the two sections.

[0033] After the tensioning of the anchor cable is completed, a CTV-202 type stress monitoring element is embedded, the stress collection accuracy of the instrument is ≤0.5kN, the anchor cable stress data is collected continuously for 7 days, and it is qualified if the stress attenuation is less than 8%. Anchor cables are randomly selected for pull-out bearing capacity testing, and an independent construction account for a single anchor cable is established. The whole-process data is traceable, the long-term stress stable state of the anchor cable is verified in real time, the anchoring bearing capacity is verified through sampling testing, a complete closed loop of quality control is formed, and the acceptance specification requirements for first-level foundation pits are met. Example 1

[0034] Foundation pit anchor cable project of Laigao · Xinyue Times Commercial Complex, Laiwu, Jinan Engineering basic conditions The project is located in Laiwu High-tech Zone, Jinan City. The excavation depth of the foundation pit is 10.0~12.0m, and the safety level of the pit sidewall is Grade I. The site contains a 4.9m thick layer of coarse-grained silty sand rich in water, with stable confined water within the stratum. The soil is loose and highly permeable. During conventional single-jet jet anchor cable construction, a large amount of grout was lost, resulting in inconsistent anchor cable pull-out strength, continuous water seepage at the borehole after tensioning, and a first-time acceptance rate of only 83%. The construction method of this invention is used to construct two layers of anchor cables, with 434 anchor cables in each layer, totaling 868 anchor cables.

[0035] The construction steps are as follows: (1) Site survey and parameter design: The investigation clarified the thickness of the silty sand layer, the groundwater level, and the soil permeability coefficient, determining the total length of the anchor cables and the length of the anchoring section. A complete set of process parameters was established: a single jet grouting pressure of 20-25 MPa and a water-cement ratio of 0.8-1.0; a post-anchoring grouting pressure of 2.0-2.5 MPa; a borehole sealing grouting pressure of 1.0-1.5 MPa; and a post-grouting pressure stabilization period of 3-5 minutes. A complete set of quantitative parameters was customized based on the characteristics of the confined silty sand strata at the site, ensuring strong process adaptability and eliminating the need for repeated on-site trials, thus shortening the initial testing period.

[0036] (2) Differentiated dual-pipeline anchor cable prefabrication: The anchor cable body uses Φ15.2 prestressed steel strand, equipped with two 32mm outer diameter, 2.5mm wall thickness pressure-resistant PVC post-grouting pipes. The first post-grouting pipe is arranged along the entire length of the steel strand, sealed with a rubber plug at the bottom, and has an 8mm grout outlet hole every 50cm in the anchoring section. The second post-grouting pipe is fixed to the outside of the steel strand, with the grout outlet 2m away from the anchor cable hole, and is wrapped with a 60-mesh sand-proof filter screen. A set of annular plastic isolation brackets is installed every 1.5m to simultaneously separate the steel strand and the two grouting pipes, preventing pipe compression and blockage. The system is prefabricated uniformly in the factory / on-site, with clear functional zoning of the two pipes. The brackets ensure the pipes are centered and unobstructed, requiring no secondary adjustments after lowering, making construction convenient and solving the problem of traditional single-pipe anchor cables lacking orifice-type seepage prevention channels.

[0037] (3) Rotary jet grouting drilling rig wall protection and hole formation: A single-tube rotary grouting drill with a rated pressure of 30MPa was selected. Anchor cable hole positions were accurately marked, and the drill was driven at low speed while simultaneously spraying cement grout to form a temporary borehole wall. The drilling speed was controlled at 0.6~1.0 m / min. After reaching the designed hole depth, in-situ rotary grouting was continued for 30 seconds to reinforce the loose silty sand layer at the bottom of the hole and reduce sand inrush. This continuous rotary grouting wall protection significantly reduced the probability of hole collapse and sand inrush in thick, pressure-bearing silty sand layers, eliminating the need for expensive casing equipment and resulting in significantly higher hole-forming efficiency than the casing-following process.

[0038] (4) Anchor cable lowering and primary high-pressure jet grouting: The prefabricated double-pipe anchor cable is lowered into the anchor hole as a whole, ensuring the steel strand and grouting pipe are centered. The drill rod is slowly raised while 22MPa high-pressure jet grouting is carried out simultaneously. The drill rod raising speed is 0.6m / min. The cement grout uses P.O42.5 cement with 3% expansion agent and a water-cement ratio of 0.9. It is used after stirring for 3 minutes. The cement grout cuts and mixes the surrounding silty sand to form the basic cement-soil anchor body. After grouting, the laitance at the hole opening is cleaned, the hole opening is temporarily sealed, and it is left to cure for 5 hours until the grout initially sets. The high-pressure jet grouting forms a complete basic anchor body. During the initial setting and static stage, the cement grout has preliminary strength, which will not destroy the basic anchor body during subsequent grouting, ensuring the effect of secondary reinforcement grouting.

[0039] (5) Segmented high-pressure post-grouting: Equipped with two independent horizontal high-pressure grouting pumps (rated pressure 3MPa), each connected to one of two grouting pipelines: ① Anchorage section reinforcement grouting: Start the grouting pump of the first grouting pipe, control the grouting pressure at 2.3MPa, and continue grouting until the grouting volume reaches the design value. Stabilize the pressure for 4 minutes, monitor the pressure throughout the process, and stop the pump only if the pressure drop does not exceed 0.2MPa within 3 minutes. High-pressure splitting grouting fills the voids in the sand layer, makes up for the loss of grout in the first grouting, and greatly improves the overall pull-out bearing capacity of the anchor cable.

[0040] ② Grouting for seepage prevention and sealing at the borehole opening: After a 10-minute interval, start the second grouting pipe pump at a grouting pressure of 1.2 MPa to fill the seepage fissures in the soil 2 meters below the borehole opening. Stabilize the pressure for 3 minutes. Grouting is complete when there is no backflow of grout or seepage at the borehole opening. Record the grouting pressure, total grout volume, and stabilization time for each anchor cable throughout the process. Perform dual control acceptance of grout volume and final pressure. The two-stage grouting, which is performed in sequence and at different pressures, do not interfere with each other. The low-pressure sealing stage precisely blocks the seepage channels during tensioning. The dual control standard ensures that each anchor cable is fully grouted, significantly reducing the defect rate.

[0041] (6) Maintenance, graded tensioning, locking and anchor sealing: After post-grouting, the soil was allowed to cure naturally for 7 days until the anchor body strength reached more than 80% of the design strength before tensioning. Tensioning was performed using a through-hole jack and a 0.4-grade hydraulic gauge, with four stages of loading: 25%, 50%, 75%, and 100% of the design tension, each held for 5 minutes. Compensation tensioning was then implemented after the load reached 1.1 times the design tension. After tensioning, the borehole was checked for water seepage and no significant shrinkage of the steel strands. The borehole was then sealed with cement mortar to complete the anchor sealing. Staged tensioning reduced instantaneous soil deformation, and compensation tensioning offset initial prestress loss. Combined with the initial borehole sealing grouting, the prestress loss was monitored after 7 days and was far below the standard limit.

[0042] (7) Quality monitoring and record keeping: After the anchor cables are tensioned, CTV-202 stress monitoring elements (acquisition accuracy ≤0.5kN) are installed, and anchor cable stress data are collected continuously for 7 days. Randomly selected anchor cables are used for on-site pull-out bearing capacity testing. Hole depth, inclination angle, grouting parameters, and tensioning data are recorded separately for each anchor cable, and an independent construction log is established. This achieves full traceability for each anchor cable throughout the entire process. Long-term stress monitoring verifies the long-term stability of the process, and sampling tests provide direct evidence that the anchorage bearing capacity meets the standards, facilitating data reuse for final acceptance and similar projects.

[0043] Construction Abnormal Condition Handling Plan: 1. During the grouting process, a large amount of sand gushing from the orifice and continuous grout return occur: reduce the grouting pressure by 0.2~0.5MPa and extend the pressure stabilization time by 2min; if a large amount of grout still returns, suspend grouting and let it stand for 20min until the surface grout initially sets before re-grouting; 2. Grouting pipe blockage: Remove the filter screen at the end of the grouting pipe and flush with clean water under high pressure. If the blockage is severe, replace it with a spare grouting pipe of the same specification. 3. Local silt thickness > 6m and confined water level too high: Increase the upper limit pressure of grouting in the anchoring section to 2.5MPa, extend the pressure stabilization time to 5min, and simultaneously increase the design grouting volume by 15%; 4. Slight water seepage at the orifice after tensioning: Low-pressure cement grout is injected through the second grouting pipe to seal the leak.

[0044] The technical advantages of this embodiment are: the anchor cable construction achieves a 100% first-time acceptance rate, significantly reducing defects such as hole collapse, sand inrush, and grout leakage; the anchor cable pull-out bearing capacity is stable with no discrete non-compliance issues, and there is no continuous water seepage at the hole opening after tensioning; the settlement and displacement monitoring data of the foundation pit throughout the entire process are all within the allowable range of the first-level foundation pit specification, and there are no safety hazards such as hole collapse or excessive foundation pit deformation throughout the construction process; the construction procedures are stable and controllable, and the process parameters can be directly converted into standardized operation instructions, which are applicable to the replication and promotion of similar confined silty sand foundation pit projects in the region. Example 2

[0045] A permanent slope reinforcement project was undertaken on a mountain slope with a 6.2m thick silty sand layer, containing continuous pressure-bearing groundwater. The support was classified as Class I safety, and four layers of anchor cables were installed, with 286 cables in each layer. Parameter adjustments were made: the grouting and pressure stabilization time in the anchoring section was extended to 5 minutes, and the design grouting volume was increased by 15%. All other pipe materials, equipment, and tensioning parameters remained consistent with Example 1. Construction results: 100% first-time acceptance rate for anchor cables; average prestress loss over 7 days was 4.9%; slope displacement remained within acceptable limits throughout the year; and there were no issues with hole collapse or tensioning seepage.

[0046] Quantitative Comparison Table of Parallel Experiments of Multiple Processes

[0047] The two embodiments together verify that the present invention is suitable for two types of first-level anchoring projects: 4m and above pressure-bearing silty sand foundation pits and slopes. All quantitative indicators are superior to the three existing mainstream processes, achieving unexpected technical effects.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for post-grouting construction of jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata, applicable to first-level deep foundation pit support and slope prestressed anchoring projects in loose sandy strata with stable pressurized water and silty sand layer thickness ≥4m, including existing conventional steps: site investigation and process parameter design, anchor cable hole location measurement and marking, jet grouting drilling for wall protection and hole formation, anchor cable lowering, one-time high-pressure jet grouting, grout curing, anchor cable tensioning and locking, and quality inspection and archiving; characterized in that: It also includes a differentiated dual-pipeline post-grouting anchor cable prefabrication process and a segmented high-pressure post-grouting process; The on-site operation process of the differentiated dual-pipeline post-grouting anchor cable prefabrication process is as follows: pre-cut steel strands and two high-pressure grouting pipes, and use ring-shaped isolation clamps to bind and fix them in layers. First fix the steel strands, then fix the two high-pressure grouting pipes. After the overall binding is completed, transport it to the hole position for lowering. The differentiated dual-pipeline post-grouting anchor cable prefabrication process is as follows: the anchor cable body adopts multiple bundles of steel strands and includes a first post-grouting pipe and a second post-grouting pipe that are independent of each other. The first post-grouting pipe is laid along the entire length of the steel strand, and the bottom end is completely sealed with a rubber plug. Every 50cm along the anchor section, there are small grout outlet holes with a diameter of 8mm. The second post-grouting pipe is fixed to the outside of the steel strand, and its grout outlet is located 2m away from the anchor cable hole. The outside of the grout outlet is wrapped with a metal sand-proof filter to prevent sand from flowing back and blocking the pipe. The two grouting pipes are fixed to the steel strand through isolation brackets and are not connected to each other. The segmented high-pressure post-grouting process is set after the completion of the first high-pressure jet grouting and 4-6 hours after the initial setting of the grout. Two independent horizontal high-pressure grouting pumps are connected to two grouting pipes respectively. The rated working pressure of the grouting pumps is ≥3MPa, and the two-stage grouting process is executed sequentially. S1. Anchorage section reinforcement grouting: Inject 2.0-2.5MPa cement grout into the first rear grouting pipe. After the grouting volume reaches the design value, stabilize the pressure for 3-5 minutes and then stop the pump. If the pressure drop is ≤0.2MPa within 3 minutes during the stabilization stage, the grouting can be considered full. S2. Grouting for seepage prevention and sealing at the borehole: After an interval of 10 minutes, 1.0-1.5 MPa cement grout is introduced into the second grouting pipe to fill the seepage cracks in the soil 2m below the borehole. The pressure is maintained for 3 minutes until there is no backflow of grout or seepage at the borehole, at which point the grouting is completed. The entire grouting process adopts a dual-control acceptance standard of grouting volume and final pressure.

2. The post-grouting construction method for jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata as described in claim 1, is characterized in that... The rotary grouting drilling rig is a single-tube high-pressure rotary grouting drilling rig with a rated rotary grouting pressure ≥30MPa. The drilling rig rotates at low speed and simultaneously sprays cement slurry to form a temporary protective wall. The drilling speed is controlled at 0.6~1.0m / min. After drilling to the designed hole depth, the grouting is continuously sprayed in situ for 30s to reinforce the silty sand layer at the bottom of the hole.

3. The method for post-grouting construction using jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata as described in claim 2, is characterized in that... The parameters for the single high-pressure jet grouting process are as follows: jet grouting pressure 20-25 MPa, cement grout using P.O42.5 ordinary Portland cement, 3% expansion agent by weight of cement added, water-cement ratio 0.8-1.0, and mixing time not less than 3 minutes. The drill pipe lifting speed is 0.4–0.8 m / min.

4. The jet grouting anchor cable dual-pipeline post-grouting construction method adapted to ultra-thick water-rich silty sand strata as described in claim 3, wherein the anchor cable tensioning and locking process is as follows: after post-grouting, the anchor body is naturally cured for 7 days, and graded tensioning is carried out after the anchor body strength reaches more than 80% of the design strength; the tensioning is divided into four levels of loading, with the four levels of load being 25%, 50%, 75%, and 100% of the design tension in sequence, and the holding time of each level is not less than 5 minutes. After loading to 1.1 times the design tension, compensation tensioning is carried out, and after locking, the orifice is sealed with cement mortar to complete the anchor sealing.

5. The post-grouting construction method for jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata as described in claim 4, is characterized in that... The isolation bracket is a ring-shaped plastic clamp bracket, with one set installed every 1.5m. Each set of brackets simultaneously clamps the steel strand, the first grouting pipe, and the second grouting pipe, achieving a centered separation between the three.

6. The post-grouting construction method for jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata as described in claim 5, is characterized in that... After the anchor cable tensioning is completed, stress monitoring elements are installed, and anchor cable stress data are collected continuously for 7 days. The stress attenuation is less than 8% to be considered qualified. Anchor cables are randomly selected for pull-out bearing capacity testing, and an independent construction ledger for each anchor cable is established.

7. The post-grouting construction method for jet grouting anchor cables with dual pipelines adapted to ultra-thick water-rich silty sand strata as described in claim 6, is characterized in that... It also includes handling abnormal construction conditions, including: If a large amount of sand gushes from the orifice and grout continues to return during the grouting process: reduce the grouting pressure by 0.2~0.5MPa and extend the pressure stabilization time by 2min; if a large amount of grout still returns, suspend grouting and let it stand for 20min until the surface grout has initially set before re-grouting. Grouting pipe blockage: Remove the filter screen at the end of the grouting pipe and flush with clean water under high pressure. If the blockage is severe, replace it with a spare grouting pipe of the same specification. For areas with silty sand thickness exceeding 6m and high confined water level: increase the upper limit pressure of grouting in the anchoring section to 2.5MPa, extend the pressure stabilization time to 5min, and simultaneously increase the design grouting volume by 15%; After tensioning, a small amount of water seepage occurred at the orifice: low-pressure cement grout was injected through the second grouting pipe to seal the leak.

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