Construction method of plastic concrete deep anti-seepage wall across strong leakage and collapsible backfill stratum

Abstract

The application relates to the technical field of hydraulic engineering, and discloses a construction method of a plastic concrete deep anti-seepage wall for a stratum with strong leakage and easy collapse after backfilling, groove section division; wall stabilizing mud preparation; measurement, lofting and guide wall construction; pre-concentration slurry treatment of a backfilling layer; hole forming and groove forming; hole cleaning and slurry replacement; embedding of a grouting pipe and pouring of a guide pipe; and plastic concrete pouring. The backfilling layer is pretreated, i.e. pre-concentration slurry treatment, so that the anti-seepage wall process can be more efficiently and stably implemented in a complex condition hydropower project, the groove forming guarantee rate is greatly improved, and the additional labor, machinery and time costs caused by frequent hole collapse during groove forming, leakage of the wall stabilizing mud of the backfilling layer with strong leakage and other unfavorable factors are avoided. The bentonite mud slurry of the anti-seepage wall groove hole uses positive electric glue as a tackifier to improve the wall stabilizing effect.

Description

Technical Field

[0001] This invention relates to the field of water conservancy engineering technology, specifically to a construction method for a deep anti-seepage wall made of plastic concrete spanning a strong, easily collapsing, backfilled stratum. Background Technology

[0002] Plastic concrete cutoff walls are formed by continuously drilling holes and creating trenches in loose, permeable foundations or soil-rock backfill layers, then pouring plastic concrete to solidify the walls. Plastic concrete uses far less cement than ordinary concrete and incorporates more bentonite, clay, and other cementing materials, resulting in a fluid concrete with low strength, low elastic modulus, and large ultimate strain. The compressive strength of plastic concrete wall materials is generally 1.0–5.0 MPa, and the elastic modulus is generally 300–2000 MPa. Cutoff walls with a depth greater than 70m and less than 100m are defined as deep cutoff walls, while those with a wall thickness of no more than 400mm are defined as thin cutoff walls.

[0003] In hydropower projects where concrete cut-off walls are widely used, there are problems such as difficulty in drilling and trenching in some areas with complex geological conditions, high content of boulders, boulders, and gravel, thick burial depths, strong leakage backfill layers, or loose and easily collapsible strata. During trenching, the backfill layer frequently collapses, and the wall-stabilizing mud material is severely lost. In landslide dammed lake sedimentary strata and strong mud-forming strata (clay, silty clay), there is also the problem of not being able to effectively control stratum leakage. This application is proposed to address the above problems. Summary of the Invention

[0004] The purpose of this invention is to provide a construction method for a deep, plastic concrete anti-seepage wall spanning easily collapsible backfill strata with high leakage. By pre-grouting the backfill layer below the guide wall, the problems of frequent backfill layer collapse and severe leakage of wall-stabilizing mud material during trenching are solved to a certain extent, ensuring the safety of borehole construction in non-cohesive soil strata. The use of bentonite mud slurry with MMH positively charged dry powder as a thickener effectively controls stratum leakage and improves the wall-stabilizing effect.

[0005] The present invention is achieved through the following technical solution.

[0006] The present invention provides a construction method for a deep anti-seepage wall using plastic concrete in a backfill stratum prone to leakage and collapse, comprising the following steps: trench segment division: dividing the trench into Phase I and Phase II sections and determining the trench length;

[0007] Wall-stabilizing mud preparation: The wall-stabilizing mud composition includes bentonite and positively charged gel dry powder;

[0008] Surveying and setting out and guide wall construction: After the site is leveled, the axis of the anti-seepage wall and the structural outline of the guide wall are set out to make the axis of the guide wall parallel to the axis of the anti-seepage wall;

[0009] Pre-grouting treatment of backfill layer: Pre-grouting holes are evenly arranged on both the upstream and downstream sides of the anti-seepage wall. Pre-grouting is carried out through the pre-grouting holes to inject grout into the backfill layer. Grouting is carried out using grouting pipes. Grout nozzles are evenly arranged radially on the grouting pipes. Conventional continuous grouting is carried out first. Conventional continuous grouting uses cement bentonite grout. After the grouting of the surrounding soil layer is completed, a reinforcement step is carried out. Concrete is used for grouting to form a concrete support in the pre-grouting holes. When the support is formed, high-pressure grouting is carried out to completely squeeze out the cement bentonite grout in the pre-grouting holes.

[0010] Drilling and trenching: The trenching process is selected based on the geological conditions during drilling operations;

[0011] Hole cleaning and slurry replacement: After the second-stage trench section is completed, the joint end holes are first brushed and cleaned, and all trench holes are cleaned and slurry replaced after completion.

[0012] Installation of grouting pipes and pouring guide pipes: Pre-embed the curtain grouting pipes under the seepage barrier wall and fix them, and install the guide pipes in the trench;

[0013] Plastic concrete pouring: Concrete is transported to a trench using a duct for pouring.

[0014] Furthermore, in the solidification mud preparation step, the solidification mud preparation process is as follows: Step 1: Water + bentonite + additive 1 + positive charge dry powder + additive 2, the five components are stirred together for at least 5 minutes; Step 2: Water + bentonite + additive 1, the three components are stirred for at least 5 minutes, then additive 2 and positive charge are added and stirred for at least 5 minutes.

[0015] Furthermore, pre-grouting holes are arranged in one row on each of the upstream and downstream sides of the anti-seepage wall, with the axis of the pre-grouting holes parallel to the axis of the anti-seepage wall.

[0016] Furthermore, in the conventional continuous grouting process of the pre-grouting treatment step of the backfill layer, the pre-grouting is designed to be carried out according to the principle of sequential densification, with two sequences within the row. First, sequence I holes are grouted, followed by sequence II holes. The design is for pressureless or low-pressure grouting, with low pressure referring to a pressure ≤0.2MPa. The pre-grouting is carried out by drilling with the casing to the design elevation, pulling out the drill bit, installing a sealing device at the top of the casing, and performing pure pressure grouting. During grouting, the casing is lifted and grouting is carried out in sections from bottom to top.

[0017] Furthermore, in the conventional continuous grouting process, if the injection rate of a normal borehole section under pressureless conditions is no greater than x L / min and the unit consumption of the borehole section is no greater than y kg / m, then when the injection flow rate of a certain borehole section after grouting is started is greater than 2 to 4 times x L / min and the duration is relatively long, pressureless grouting is adopted and flow restriction measures are taken. When the unit consumption of the borehole section has reached 2 to 4 times y kg / m and the injection rate has not changed or has not changed significantly, grouting can be continued after an intermittent period until the grouting is completed. The grouting completion standard for this borehole section is: under pressureless conditions, the injection rate is no greater than x L / min, and grouting is completed after continuing to grout for a certain period of time.

[0018] If, after grouting a certain section, the injection rate is low under pressureless grouting conditions (less than x L / min), and the injection rate suddenly increases significantly after pressurization (greater than 3 to 4 times x L / min), the injection rate can be controlled at 2 to 4 times x L / min by adjusting the grouting pressure. When the unit consumption of the section reaches 2 to 4 times y kg / m, the grouting pressure should be reduced to zero, the injection rate should not exceed x L / min, and grouting should continue for a certain period of time before grouting is stopped.

[0019] In general borehole sections, when the injection rate is <1.5 to 2 times x L / min during grouting, the pressure is increased in stages at 0.05 MPa increments, sequentially increasing to 0.1 MPa, 0.15 MPa, and 0.2 MPa. The pressure stabilization time for each stage is 3 to 5 minutes. During the pressure increase process, the final grouting pressure is determined based on the changes in injection rate and unit consumption, but the maximum grouting pressure shall not exceed 0.2 MPa.

[0020] Furthermore, in the reinforcement step of the backfill layer pre-grouting treatment step, the concrete support is formed in sections from bottom to top, and the pressure of the grout nozzle is greater when the bottom is formed than the pressure of the grout nozzle when the top is formed.

[0021] Furthermore, in the pre-grouting process of the backfill layer, high-pressure grouting is first performed, with a pressure greater than 0.8 MPa, while simultaneously rotating. The high-pressure grout is used to impact the surrounding backfill layer within the pre-grouting hole, forming a grouting cavity, expanding the contact area between the grout and the backfill layer, and improving the grout diffusion efficiency. The high-pressure grouting lasts for at least 3 minutes.

[0022] Furthermore, in the hole-making and trenching steps, the upper loose backfill layer I and II trenches are constructed using the drilling and splitting method, the lower barrier lake facies sedimentary layer I trenches are constructed using the drilling and grabbing method, and the II trenches are constructed primarily using the grabbing method, supplemented by the drilling and grabbing method.

[0023] Furthermore, in the hole cleaning and slurry replacement step, the joint hole of the Phase II trench is cleaned using a circular brush. By applying pressure to the hole wall with the brush, the drill rig drives the brush to clean from top to bottom in sections, thereby achieving the purpose of cleaning the hole wall. The end standard is that the brush and drill bit are basically free of mud and the siltation at the bottom of the hole no longer increases.

[0024] Furthermore, in the hole cleaning and slurry replacement step, the hole cleaning is performed sequentially at the main and auxiliary holes in the tank section;

[0025] The mud with a high sand content at the bottom of the tank is treated by a mud purifier and then returned to the tank hole until the slag outlet of the purifier no longer screens out sand particles; the cleaning hole is advanced from the high end to the low end of the bottom hole section of the tank.

[0026] Before the hole cleaning is completed, samples are taken at the return slurry pipe inlet to test the full performance of the slurry, which serves as the basis for slurry replacement indicators;

[0027] The performance indicators and replacement volume of the mud to be replaced are determined based on the mud test results. The replacement volume is determined comprehensively based on the volume of the trench, the performance of the mud in the trench, and the performance of the newly prepared mud. Solid wall mud is used to replace the mixed mud in the trench. The replacement volume is generally 1 / 3 to 1 / 2 of the trench volume.

[0028] In the hole cleaning and mud replacement step, mud replacement is carried out sequentially at the main and auxiliary holes in the trench section. The drilling rig moves from the end away from the return mud pipe to the end close to the return mud pipe, and fresh mud is transported to the trench through the mud delivery pipe. The mud extracted from the bottom of the trench enters the return mud pool through the return mud ditch, and is then recycled as wall-stabilizing mud when the trench is formed.

[0029] Furthermore, in the plastic concrete pouring step, the ball pressing method is used for initial pouring. When pouring begins, an isolation ball is placed in each guide tube. Before pouring concrete, an appropriate amount of cement mortar is injected into the guide tube, and enough concrete is injected so that the bottom end of the guide tube can be buried in the concrete after the isolation ball is squeezed out.

[0030] The beneficial effects of this invention are:

[0031] By pre-treating the backfill layer (i.e., pre-grouting), the anti-seepage wall technology can be implemented more efficiently and stably in complex hydropower projects, significantly improving the trenching success rate. This results in faster project progress, fewer interfering factors, better construction practices, and improved utilization of resources, leading to greater economic benefits. Compared to previous anti-seepage wall construction projects, the new method allows for controlled construction periods and avoids the additional labor, machinery, and time costs incurred due to frequent borehole collapses, strong leakage, and the inability to return slag caused by backfill layer wall slurry loss. This promotes the application of anti-seepage wall technology in the hydropower industry. Compared to similar plastic concrete projects, the only difference is the addition of pre-treatment measures (pre-grouting) within the backfill layer depth before trenching, without altering the size, layout, or compaction requirements of the construction site. Bentonite slurry is prepared using positively charged adhesive as a binder in the anti-seepage wall trenches to enhance the wall slurry's consolidation effect. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0034] Figure 1 A flowchart illustrating the construction process of a plastic concrete anti-seepage wall;

[0035] Figure 2 A schematic diagram of the construction of a plastic concrete anti-seepage wall trench section;

[0036] Figure 3 This is a schematic diagram of the cross-section of the guide wall of the plastic concrete seepage barrier;

[0037] Figure 4 This is a schematic diagram of the arrangement of pre-grouting holes for the backfill layer. Detailed Implementation

[0038] The following is combined with Figure 1-4 The present invention will be described in detail below.

[0039] I. Methodological Characteristics:

[0040] The construction platform for the plastic concrete anti-seepage wall is formed by filling with mixed stone and rubble. Although it has been compacted in layers, the density of the stratum is low and there are a large number of boulders and pebbles. By pre-filling the backfill layer under the guide wall with thick grout, the problems of frequent collapse of the backfill layer and serious loss of wall-stabilizing mud material during the trenching process of impact drilling have been solved to a certain extent, ensuring the safety of borehole construction in non-cohesive soil strata.

[0041] For loose, easily collapsing, and highly leaky backfill strata, pre-grouting treatment is carried out. The trench is formed by drilling and grabbing, and plastic concrete is poured with mud slurry to solidify the wall. The seepage barrier penetrates the thick lacustrine sedimentary layer and is embedded in the bedrock, forming a concrete seepage barrier with characteristics such as low strength, low elastic modulus and large ultimate strain.

[0042] To facilitate borehole formation in the 50m deep dammed lake sedimentary strata, bentonite slurry with MMH positively charged dry powder as a binder was used for wall stabilization. The slurry mainly consisted of positively charged adhesive, bentonite, soda ash, and water.

[0043] The construction equipment is mature, the construction progress is fast, and the seepage prevention effect is good.

[0044] II. Scope of Application

[0045] The upper construction platform is for the construction of a plastic concrete anti-seepage wall project in complex strata such as deep, low-density, and easily collapsible loose soil and rock backfill layers.

[0046] III. Process Principles

[0047] The main purpose of pre-filling the loose backfill layer with thick grout after the construction of the plastic concrete anti-seepage wall guide wall is to solve the problem of hole collapse during the impact drilling trenching process caused by leakage of the backfill layer wall slurry material or collapse of the loose backfill layer. The hole is formed by drilling splitting method, drilling grab method or grab method and penetrates into the bedrock. Then the hole is cleaned and the grout is replaced by air lift method, pump suction method or bucket extraction method. Plastic concrete is poured in stages by using joint pipe method. The closure of the trench is selected according to the geological conditions to finally complete the construction of the entire anti-seepage wall.

[0048] Positively charged adhesive is used as a thickener in the preparation of bentonite slurry for the anti-seepage wall trenches to improve the wall-stabilizing effect. Positively charged adhesive dry powder is a free-flowing white or slightly yellow granules or powder with strong suspension and sand-carrying capacity. It has good rheological properties, is easy to prepare and maintain. When used as a thickener, it mainly increases the structural viscosity of the system, thereby increasing the shear dilution of the slurry and the efficiency of suspending and carrying sand to clean the wellbore.

[0049] Plastic concrete replaces most of the cement in ordinary concrete with bentonite, forming a flexible wall material. It has an extremely low deformation modulus, and its mix proportion can be artificially controlled to vary its deformation modulus within a wide range. It has a stress-strain curve that is very similar to the soil morphology, and the plastic concrete properties (mix proportion) that match the stress-strain curve of the surrounding soil can be artificially selected. Its ultimate strain value is much larger than that of ordinary concrete. The compressive ultimate strain value of ordinary concrete is εmax = 0.08% to 0.3%, while the ultimate strain of plastic concrete under unconfined conditions exceeds 1%, which is several times or even tens of times larger than that of ordinary concrete. Under triaxial stress conditions, the strength of plastic concrete is greatly improved, and the strength-confining pressure relationship curve is positively correlated. That is, as the confining pressure increases, the strength of plastic concrete increases significantly, further improving the safety of the seepage barrier wall.

[0050] IV. Process Flow and Key Operating Points

[0051] 1. Construction process flow

[0052] The construction of the deep anti-seepage wall made of plastic concrete across the backfill strata prone to leakage and collapse is organized in the following sequence: construction preparation → surveying and setting out → guide wall construction → pre-grouting of thick slurry on both sides of the axis → drilling and trenching with impact drilling rig / hydraulic grab bucket → laying of grouting pipes and pouring guide pipes → pouring of plastic concrete.

[0053] The construction process of plastic concrete anti-seepage wall is as follows: Figure 1 .

[0054] 2. Key Operating Points

[0055] 2.1 Construction Technology Preparation

[0056] In addition to the usual arrangements for office and living spaces, construction ventilation, water and electricity, roads, and lighting, preparations before the construction of the anti-seepage wall also include technical preparations, such as the division of the trench into Phase I and II sections and the determination of technical solutions, as well as the preparation of wall-stabilizing mud, the design of plastic concrete mix proportions, and mud purification and recycling.

[0057] 1. Tank segment division

[0058] When determining the trench length according to the design drawings, factors such as engineering geological and hydrogeological conditions, construction location, trench excavation method, equipment performance, trench excavation time, wall material supply strength, location of pre-reserved holes in the wall, spacing of pouring pipes, and wall plan shape should be comprehensively considered. When using the drill-and-split method, the main hole is drilled first, then the secondary holes are split, and finally the smaller wall is split. When using the drill-and-grab method, the main hole is drilled first with an impact drill, then the secondary holes are grabbed with a grab bucket. The length of the secondary holes should not exceed the grab bucket opening minus the wall thickness. One trench section is divided into 3 main holes and 2 secondary holes. When using the grab bucket and milling methods, the length of the main hole is equal to the grab bucket opening or the milling head width, and the length of the secondary holes should be 1 / 2 to 2 / 3 of the main hole length. Short trenches are preferred for closure, and they should be arranged in shallower areas with better geological conditions. (Construction instructions for trenching using the drill-and-grab method and casting plastic concrete anti-seepage walls using the joint pipe method are provided.) Figure 2 .

[0059] 2. Preparation of wall-stabilizing mud

[0060] In deep trench drilling in complex strata, bentonite mud with MMH positively charged adhesive as a binder must be introduced. This mud has a strong suspension and sand-carrying capacity to improve the wall-stabilizing effect. Key points for its preparation are as follows:

[0061] ① The performance of positive charge slurry largely depends on the mixing procedure and mixing time. Strictly follow the procedure. Procedure 1: Mix water + bentonite + additive 1 + positive charge dry powder + additive 2 together for 5 minutes; Procedure 2: Mix water + bentonite + additive 1 for 5 minutes, then add additive 2 and positive charge slurry and mix for another 5 minutes.

[0062] ② Prepare the mud slurry according to the specified mixing ratio, and the error in the amount of each material added shall not exceed 2%.

[0063] ③ The concentration of the auxiliary agent aqueous solution is 20%, and the concentration of the CMC aqueous solution is 1.5%.

[0064] 3. Mix design of plastic concrete

[0065] Before construction, indoor and on-site concrete tests were conducted according to design requirements to determine the appropriate mix proportions for plastic concrete.

[0066] 2.2 Surveying and Setting Out and Guide Wall Construction

[0067] After the site is leveled, a professional surveying engineer uses a total station to locate and lay out the axis of the anti-seepage wall and the structural outline of the guide wall.

[0068] Sleepers and light rails are laid 7.5m upstream of the seepage barrier to serve as a drilling platform; grouting pipes, water pipes and other pipelines are buried upstream of the platform; the downstream 17.5m range serves as a construction platform for tracked equipment access roads, slag dumping platforms, grout trenches, pre-embedded pipes for curtain grouting under the wall, concrete pouring, temporary traffic and other construction purposes.

[0069] The guide wall axis is parallel to the anti-seepage wall axis, with an allowable deviation of ±15mm; the inner wall surface of the guide wall is vertical, with an allowable deviation of ±10mm; the allowable deviation of the guide wall top elevation is ±20mm. The top of the guide wall is straight, and the elevation is in accordance with design requirements. Taking the upstream cofferdam anti-seepage wall of the Lawa Hydropower Station as an example, the guide wall is designed with a trapezoidal cross-section of reinforced concrete, constructed by full-section excavation followed by formwork casting. The top width is 0.5m, the bottom width is 1.0m, and the height is 1.5m. The concrete strength grade of the guide wall is C20.

[0070] See the cross-sectional layout of the guide wall and construction platform of the plastic concrete seepage barrier wall. Figure 3 (Unit: m)

[0071] Before construction begins, measuring stakes are set at both ends of the trench section. The centerline of the trench holes is determined based on the stakes, and this centerline is used to check and verify the error of the centerline of the resulting wall. The allowable deviation of the hole position from the designed centerline of the anti-seepage wall in the upstream and downstream directions shall not exceed 3cm, and this requirement must be met in all directions.

[0072] 2.3 Pre-grouting treatment of backfill layer

[0073] The construction platform for the cutoff wall is a loose backfill layer with poor soil density and localized voids. During construction, this layer is highly susceptible to borehole collapse and leakage of the solidification slurry. Therefore, pre-grouting treatment is applied to this backfill layer. The pre-grouting holes are designed to be arranged in one row on each of the upstream and downstream sides of the cutoff wall, with their axes parallel to the cutoff wall axis, spaced 1.4m apart, and a hole spacing of 2.0m. Cutoff wall trenches with a designed depth of less than 30m are not treated.

[0074] Pre-grouting holes are evenly distributed, and thick grout is pre-injected into the backfill layer through these holes. Grouting is carried out using grouting pipes. Conventional continuous grouting is performed first, using cement-bentonite grout. After the surrounding soil layers are grouted, a reinforcement step is taken, using concrete grouting to form a concrete support in the pre-grouting holes. While the support is forming, continuous high-pressure grouting is performed to completely squeeze out the cement-bentonite grout in the pre-grouting holes. The injected concrete forms the support, constituting the skeleton of the backfill layer, effectively reinforcing the backfill layer and avoiding the problem of frequent collapse of the backfill layer during the drilling and trenching process.

[0075] Preferably, in the reinforcement step, the concrete support is formed in sections from bottom to top. When the bottom is formed, the pressure of the grout nozzle is greater than that when the top is formed. This allows the high-pressure concrete to cut the surrounding soil layer and widen the inside of the pre-grouting hole. The formed support can be fully embedded in the backfill layer, and ideally, it can form a stable cone-shaped structure.

[0076] The arrangement of pre-grouting holes for loose and easily collapsible backfill layers is shown in the figure. Figure 4 (Unit: m)

[0077] The key points of conventional continuous grouting construction are as follows (the grouting parameters are taken as an example of the backfill layer of the upstream cofferdam anti-seepage wall construction platform of the Lawa Hydropower Station):

[0078] ① The pre-grouting is designed according to the principle of sequential densification. The process is divided into two sequences: first, the first sequence hole is grouted, and then the second sequence hole is grouted. The design is for pressureless or low-pressure (≤0.2MPa) grouting. The pre-grouting is carried out by drilling with the casing to the design elevation (1m below the loose backfill layer), pulling out the drill bit, installing a sealing device on the top of the casing, and performing pure pressure grouting. During grouting, the casing is lifted in sections (1.5m) from bottom to top and grouting is carried out in sections.

[0079] ② The pre-grouting uses cement-bentonite grout. An automatic recorder is used to measure the grout mass. The cement:bentonite ratio is 4:1, and the initial water-to-solid material ratio is 0.7:1. The grout mix ratio is adjusted according to the on-site grouting conditions. Based on the actual grouting situation, when the grout absorption rate is <10L / min, continue grouting for 5 minutes to end this section of grouting. After completion, raise the casing by 1.5m and continue grouting the next section until the borehole opening, thus ending the grouting operation for this borehole.

[0080] ③ If, after grouting begins in a certain section (excluding the bottom section), the injection flow rate is large (greater than 40 L / min) and the duration is long, then pressureless grouting should be adopted, and flow-limiting measures should be implemented. When the grout consumption per unit area in that section reaches 300 kg / m and the injection rate remains unchanged or changes only slightly, grouting can be resumed after a 5-minute interval. If 500 L of grout is injected and the injection rate still does not change significantly, grouting can be resumed after a 10-minute interval until grouting is completed. The grouting completion standard for this section is: under pressureless conditions, the injection rate is no greater than 10 L / min, and grouting is continued for 5 minutes before grouting is completed.

[0081] ④ If, after grouting a certain section, the injection rate is low (less than 10 L / min) under pressureless grouting conditions, and the injection rate suddenly increases significantly (greater than 40 L / min) after pressurization, the injection rate can be controlled between 20 and 40 L / min by adjusting the grouting pressure. When the grout consumption of that section reaches about 200 kg / m, the grouting pressure should be reduced to zero, the injection rate should not exceed 10 L / min, and grouting should continue for 5 minutes before ending the grouting process.

[0082] ⑤ For general borehole sections, when the injection rate is <15L / min during grouting, the pressure is increased in stages at 0.05MPa increments, sequentially increasing to 0.1MPa, 0.15MPa, and 0.2MPa. The pressure stabilization time for each stage is 3-5 minutes. During the pressure increase, the final grouting pressure is determined based on the changes in injection rate and unit consumption, but the maximum grouting pressure shall not exceed 0.2MPa.

[0083] Another optional implementation: In the pre-grouting treatment step of the backfill layer, high-pressure grouting is first performed, with a pressure greater than 0.8 MPa, while simultaneously rotating. Several high-pressure nozzles are circumferentially arranged on the grouting pipe, and these nozzles are connected to a high-pressure mud pump. High-pressure grout is used within the pre-grouting holes to impact the surrounding backfill layer, forming a grouting cavity, increasing the contact area between the grout and the backfill layer, and improving the grout diffusion efficiency. High-pressure grouting continues for at least 3 minutes. This grouting method can also increase the diameter of the concrete support, enhancing the support effect. During the reinforcement step, only the nozzles at the bottom of the grouting pipe can be used.

[0084] High-pressure grouting is distributed at different heights on the grouting pipe, and cavitation and grouting are carried out simultaneously at different heights of the pre-grouting hole to improve grouting efficiency and effect.

[0085] 2.4 Hole Formation and Grooving

[0086] The trenching process for borehole construction is selected based on the geological conditions. There are four main methods: ① Drilling and splitting method, which uses impact drilling rigs (either positive or reverse circulation) and is mostly used in gravel or boulder-containing bedrock formations. Its efficiency is relatively low. ② Drill-grab method, which uses impact drilling rigs, rotary drilling rigs, and slewing rigs in conjunction with hydraulic or mechanical grab trenching machines. Its overall efficiency is higher than the drilling and splitting method and it is suitable for most complex formations. ③ Grab method, which is a pure grab trenching method and is mostly used in fine-grained formations. A mechanical grab combined with heavy chisels can be used for complex foundation treatments. Its efficiency is slightly higher than the drilling and splitting and drill-grab methods. ④ Milling method, which uses a hydraulic milling machine to mill the formation and forms trenches. It is mostly used in fine-grained formations below gravel and in weak rock formations. This method is highly efficient and produces good trenching quality, but its cost is relatively high. In this embodiment, the upper loose backfill layer (Phase I and II) trenches are constructed using the drilling and splitting method, i.e., three main holes and two auxiliary holes per trench section are drilled using an impact drill. The lower barrier lake facies sedimentary layer (Phase I) trenches are constructed using the drill-grab method, i.e., three main holes per trench section are drilled using an impact drill, and two auxiliary holes per trench section are drilled using a hydraulic grab trenching machine. Phase II trenches primarily use the grab method, i.e., three main holes and two auxiliary holes per trench section are drilled using a hydraulic grab trenching machine, with the drill-grab method used as a supplement. After the concrete pipes of the end holes of Phase I trench sections are pulled out, the end holes of the adjacent Phase II trench sections are formed. The drilling of Phase II trench sections is carried out after the concrete pouring of the adjacent Phase I trench sections is completed.

[0087] After drilling is completed, the hole depth is measured using a dedicated hole depth measuring rope, which is checked and calibrated before use. Formation rock samples are properly preserved, numbered, and recorded, and then inspected and verified by the supervisor, owner, designer, and construction team. For concrete cut-off walls designed to be embedded in bedrock, accurate verification of the final hole's entry into the bedrock surface is crucial to ensure the quality of the concrete cut-off wall.

[0088] 2.5 Hole cleaning and slurry replacement

[0089] After the second-phase trench section is completed, the joint end holes are first brushed, and after all trench holes are completed, the holes are cleaned and the slurry is replaced. The main cleaning methods are the bucket extraction method and the air lift reverse circulation method.

[0090] The bucket extraction method involves using a bucket to extract large amounts of drilling mud containing cuttings from the borehole and replace the mud. The air-lift reverse circulation method uses high-pressure air from an air compressor to enter the cuttings discharge pipe, where a mixer mixes the liquid and air. The density difference and air pressure inside and outside the discharge pipe are used to lift and discharge the mud, carrying away sediment from the bottom of the borehole. The main equipment includes an air compressor, cuttings discharge pipe, air ducts, and a mud purifier. The steps are as follows:

[0091] ① During hole cleaning, following the construction steps, the drilling rig lifts the slag discharge pipe and cleans the main and auxiliary holes in the trench section sequentially. If there is excessive sediment at the bottom of the trench, repeated cleaning is necessary. Mud with a high sand content at the bottom of the trench is treated by a mud purifier and returned to the trench until no more sand particles are screened out from the slag outlet of the purifier. Hole cleaning proceeds from the high end of the trench bottom section to the low end.

[0092] ② Before the hole cleaning is completed, take a sample at the return slurry pipe inlet and test the full performance of the slurry as the basis for the slurry replacement index.

[0093] ③ Determine the performance indicators and replacement volume of the mud to be replaced based on the mud test results. The replacement volume is determined comprehensively based on the trench volume, the properties of the mud in the trench, and the properties of the newly prepared mud. When replacing the mixed mud in the trench with bentonite mud, the replacement volume is generally 1 / 3 to 1 / 2 of the trench volume.

[0094] ④ Mud replacement is carried out sequentially at the main and auxiliary boreholes in the trench section. The drilling rig moves from the end furthest from the return pipe to the end closest to the return pipe, and fresh mud is delivered to the trench through the slurry delivery pipe. The mud pumped out from the bottom of the trench enters the return slurry pool through the return slurry ditch, and is then recycled as wall-stabilizing mud during trench formation.

[0095] One hour after the mud replacement is completed, a mud sample is taken from the borehole using a mud sampler for a mud test. The cleaning and mud replacement is considered complete once the end criteria are met. The end criteria are in accordance with DL / T5199 "Code for Construction of Concrete Cutoff Walls for Water Conservancy and Hydropower Projects". Taking the Lawa Hydropower Station as an example, the end criteria are: the siltation thickness at the bottom of the borehole is less than 100mm, and the bentonite mud at a depth of 0.5m to 1.5m above the bottom of the borehole meets the following requirements: mud specific gravity ≤ 1.15g / cm³. 3 The viscosity of the Marsh funnel is 32s to 50s, and the sand content is ≤4%.

[0096] The cleaning of the joint holes in Phase II involves using a heavy, round steel wire brush. The position of the wire rope is adjusted to apply pressure to the hole wall. A drilling rig drives the brush to clean the hole in sections from top to bottom, thus achieving the goal of cleaning the hole wall. The completion criterion is that the brush and drill bit are essentially free of mud and debris, and the accumulation at the bottom of the hole no longer increases.

[0097] 2.6 Installation of grouting pipes and pouring guide pipes

[0098] 1. Pre-embedded grouting pipe

[0099] According to the design drawings, the curtain grouting pipes are pre-embedded in the anti-seepage wall and fixed with steel reinforcement positioning frames. Generally, the pre-embedded pipe sleeve connection method is used to connect the grouting pipes. During the construction process, the siltation at the bottom of the hole may exceed the standard. If the test results show that it exceeds the design standard, a second cleaning and grouting replacement must be carried out.

[0100] 2. Pre-embedded casting guide pipe

[0101] The duct for pouring plastic concrete uses quick-connect threaded steel pipes, and the duct joints are equipped with suspension devices. Before use, the duct undergoes straightening checks, water pressure tests, roundness inspections, wear tests, and welding inspections. Qualified ducts are marked with prominent markings. Steel sections are used to support the duct at the orifice opening.

[0102] Piping must be done before the conduits are laid. The conduits are laid out in sequence according to the piping diagram, with 2 to 3 conduits laid in each trench section. The conduit installation meets the following requirements: the center distance of the conduit is 1.0m to 1.5m from the end of the trench hole or the wall of the joint pipe, the center distance of the conduits is not greater than 4.0m, and when the height difference between the bottom of the hole is greater than 250mm, the conduit is placed at the lowest point within its control range.

[0103] 2.7 Plastic Concrete Pouring

[0104] 1. Concrete mixer trucks transport concrete to the work site, where it is then distributed to various chutes and enters the duct.

[0105] 2. When pouring begins, insert an isolation plug into each guide pipe. Before pouring concrete, inject an appropriate amount of cement mortar into the guide pipe, then inject enough concrete to squeeze out the plug and bury the bottom of the guide pipe.

[0106] 3. Plastic concrete is continuously poured, with the concrete surface rising at a speed of not less than 2m / h, continuously rising to 0.5m above the designed top elevation of the wall.

[0107] 4. During normal pouring, the depth of the tremie pipe embedded in the concrete is 2m to 6m; when the concrete surface rises rapidly, the depth can be increased appropriately, but not exceeding 10m; when the top surface of the concrete is close to the opening or the design wall top elevation, the depth of the tremie pipe can be reduced appropriately to facilitate concrete flow, but not less than 1m.

[0108] 5. The concrete surface in the slot rises uniformly, with the height difference controlled within 0.5m. The depth of the concrete surface in the slot is measured every 30 minutes, and the depth of the concrete surface in the guide pipe is measured every 2 hours. The measurement frequency is appropriately increased at the beginning and near the end of the pouring process.

[0109] 6. When pouring concrete, cover the opening with a plate to prevent concrete from spilling into the trench. If the bottom of the trench is uneven, start pouring from the lowest point.

[0110] 7. During concrete pouring, arrange for a testing technician to take samples at the prescribed frequency to test the physical and mechanical properties of the concrete.

[0111] V. Construction Status Handling

[0112] 1. Handling of mud loss during construction

[0113] (1) When the plastic concrete anti-seepage wall construction platform is filled, a certain proportion of clay is filled within a 2m width on both sides of its axis, and the filling is carried out in layers and compacted.

[0114] (2) If mud leakage occurs during the construction of the trench section, an appropriate amount of clay or mud viscosity can be added to the affected area to treat it.

[0115] (3) Replenish qualified mud into the slot in a timely manner to ensure that the mud level is not lower than 0.3m below the top elevation of the wall.

[0116] 2. Treatment of hole collapse

[0117] (1) When signs of hole collapse are found, first lift the construction equipment and then take measures to backfill with clay.

[0118] (2) If the collapsed area is large, or if it affects the stability of the entire construction platform, low-strength concrete of no more than C15 can be poured and the hole can be re-drilled. After the concrete has fully set and has a certain strength, the hole can be opened and drilled again.

[0119] (3) If necessary, measures should be taken to shorten the length of the slot.

[0120] 3. Correction of hole deviation

[0121] When the slot inclination is large, the effective thickness of the wall is reduced, which in turn affects the continuity of the wall.

[0122] (1) In-process control: A hydraulic grab bucket trenching machine with an automatic correction device is used. The operator can observe the hole depth, hole inclination, etc. on the display screen in real time. If the hole inclination exceeds the standard, the automatic correction device will be used to deal with it in time.

[0123] (2) Post-construction control: Backfill the top of the skewed section with mixed stone chips and re-drill the hole in that section, increase the frequency of inclination measurement during the drilling process, and strictly control the skew rate.

[0124] 4. Pipe blockage treatment

[0125] When a blockage occurs, a crane is used to repeatedly lift and shake the conduit to clear the blockage. If this is ineffective, the conduit is lifted within the allowable depth range of the conduit burial depth. The pressure difference of the concrete is used to reduce the flow resistance of the concrete, thereby clearing the blockage.

[0126] When the lifting duct method is ineffective, consider re-burying another set of ducts. The bottom of the newly installed duct should be completely inserted below the concrete surface. Then, use a small suction pump to remove the mud from the duct and continue pouring the concrete.

[0127] VI. Materials and Equipment

[0128] 1. Materials used in the project

[0129] The main materials used in this project are cement bentonite slurry for pre-grouting, bentonite mud for wall consolidation, and plastic concrete.

[0130] The performance indicators of the cement-bentonite slurry used for pre-grouting shall comply with DL / T5267 "Technical Specification for Grouting of Cover Layer in Hydropower and Water Conservancy Projects", the performance indicators of the bentonite used shall comply with GB / T5005 "Specification for Drilling Fluid Materials", and the performance indicators of the wall-fixing bentonite mud and the material proportioning requirements of the plastic concrete wall shall comply with DL / T5199 "Construction Specification for Concrete Anti-seepage Wall in Hydropower and Water Conservancy Projects".

[0131] The wall-stabilizing mud must have good rheological properties, stability, inhibition, and good suspension and cuttings carrying capacity. The soil material for mixing the mud should be bentonite, clay, or a mixture of both, with bentonite being the preferred main material.

[0132] Taking the mix proportion of the plastic concrete wall material of the upstream cofferdam of the Lawa Hydropower Station as an example, its main design indicators are: 28-day compressive strength of 7.5MPa~10.0MPa, elastic modulus of 1500MPa~2500MPa, permeability coefficient ≤1.0×10-7cm / s, and concrete density of not less than 2100kg / m3. Through experiments, the unit dosage (kg / m3) of the mix proportion design is obtained as follows: water:cement:fly ash:bentonite:sand:small stone (5~20mm) = 290:252:63:105:968:522. With the addition of appropriate amounts of water-reducing agent and air-entraining agent, the bulk density of the plastic concrete is 2200kg / m3.

[0133] 2. Equipment used in the project

[0134] The main equipment in this project includes grouting pumps, impact drilling rigs, hydraulic grab trenching machines, mud purifiers, and hydraulic pipe pulling machines.

[0135] VII. Key Points for Environmental Protection Construction Control

[0136] The construction of anti-seepage walls will generate construction wastewater and waste residue.

[0137] ①Composition of construction wastewater

[0138] As the drilling of the trench section progresses, some of the drilling mud, carrying drilling cuttings, is extracted from the trench and discharged into the slag discharge ditch, forming construction wastewater. After the trench section is completed and concrete is poured, as the concrete surface rises, the solidifying mud, carrying drilling cuttings, is expelled from the trench and flows into the slag discharge ditch, forming more construction wastewater. The construction wastewater mainly consists of drilling mud and drilling waste (cuttings). The drilling mud includes bentonite slurry and clay slurry.

[0139] (2) Treatment of construction wastewater

[0140] To avoid pollution from construction wastewater and to prevent significant waste of slurry raw materials, a slurry return tank is constructed at the construction site. Construction wastewater flows by gravity through a slag discharge ditch to the three-stage sedimentation slurry return tank. The process flow is as follows: the slurry return tank is divided into three slurry pools by an overflow trough. The slurry pool connected to the slag discharge ditch is the inlet slurry pool, and the slurry outlet slurry pool is located on the other side of the overflow trough. The overflow trough is 1m to 1.5m lower than the surrounding walls of the slurry return tank. Its function is to intercept the sand and small stones settled in the inlet slurry pool, allowing the slurry above to overflow the overflow trough and flow by gravity into the outlet slurry pool. One mud purifier is installed on one side of the slurry inlet tank to purify the waste slurry discharged into the slurry inlet tank. After screening the mud and sand, the treated mud is directly discharged into the slurry removal tank. One mud pump is installed in the slurry removal tank, and a slurry distribution valve is installed to connect to the slurry removal pipe of the trench and the slurry return pipe to the pulping station. If the mud in the slurry removal tank meets the standards for reuse after inspection, it is directly discharged into the trench through the slurry removal pipe. If it does not meet the standards, it is returned to the pulping station through the slurry return pipe for appropriate treatment.

[0141] Construction wastewater is discharged into the slurry return tank through the slag discharge ditch. However, as drilling cuttings continuously settle, waste slag is formed at the bottom of the slag discharge ditch. At the same time, waste slag is also formed in the slurry inlet of the slurry return tank due to the sedimentation of drilling cuttings. The waste slag is discharged from the slag discharge ditch and slurry return tank using a backhoe, piled up in a unified manner, and transported to a designated spoil disposal site by dump trucks.

[0142] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand and implement the present invention. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A construction method for a deep anti-seepage wall using plastic concrete across a strong, easily collapsing, backfilled stratum, characterized by: Includes the following steps: Slot division: Divide the slots into Phase I and Phase II, and determine the slot length; Wall-stabilizing mud preparation: The wall-stabilizing mud composition includes bentonite and positively charged gel dry powder; Surveying and setting out and guide wall construction: After the site is leveled, the axis of the anti-seepage wall and the structural outline of the guide wall are set out to make the axis of the guide wall parallel to the axis of the anti-seepage wall; Pre-grouting treatment of backfill layer: Pre-grouting holes are evenly arranged on both the upstream and downstream sides of the anti-seepage wall. Pre-grouting grout is injected into the backfill layer through the pre-grouting holes. Grouting is carried out using grouting pipes. Conventional continuous grouting is carried out first. Conventional continuous grouting uses cement bentonite grout. After the grouting of the surrounding soil layer is completed, a reinforcement step is carried out. Concrete is used for grouting to form a concrete support in the pre-grouting hole. When the support is formed, high-pressure grouting is carried out to completely squeeze out the cement bentonite grout in the pre-grouting hole. Drilling and trenching: The trenching process is selected based on the geological conditions during drilling operations; Hole cleaning and slurry replacement: After the second-stage trench section is completed, the joint end holes are first brushed and cleaned, and all trench holes are cleaned and slurry replaced after completion. Installation of grouting pipes and pouring guide pipes: Pre-embed the curtain grouting pipes under the seepage barrier wall and fix them, and install the guide pipes in the trench; Plastic concrete pouring: Concrete is transported to a trench using a duct for pouring; In the conventional continuous grouting process of the pre-grouting treatment step of the backfill layer, the pre-grouting is designed to be carried out according to the principle of sequential densification. The process is divided into two sequences: first, sequence I holes are grouted, and then sequence II holes are grouted. The design is to use pressureless or low-pressure grouting, where low pressure refers to a pressure ≤0.2MPa. The pre-grouting is carried out by drilling with the casing to the design elevation, pulling out the drill bit, installing a sealing device at the top of the casing, and performing pure pressure grouting. During grouting, the casing is lifted and grouting is carried out in sections from bottom to top. In the conventional continuous grouting procedure, if the injection rate of a normal borehole section under no-pressure conditions is no greater than x L / min and the unit consumption of the borehole section is no greater than y kg / m, then when the injection flow rate of a certain borehole section after grouting is started is greater than 4 times x L / min and the duration is relatively long, no-pressure grouting is adopted and flow restriction measures are taken. When the unit consumption of the borehole section has reached 2 to 4 times y kg / m and the injection rate has not changed or has not changed significantly, grouting can be continued after an intermittent period until the grouting is completed. The grouting completion standard for this borehole section is: under no-pressure conditions, the injection rate is no greater than x L / min, and grouting is completed after continuing to grout for a certain period of time. If, after grouting a certain section, the injection rate is low under pressureless grouting conditions (less than x L / min), and the injection rate suddenly increases significantly after pressurization (greater than 4 times x L / min), the injection rate can be controlled between 2 and 4 times x L / min by adjusting the grouting pressure. When the unit consumption of the grouting section reaches between 2 and 4 times y kg / m, the grouting pressure should be reduced to zero, the injection rate should not exceed x L / min, and grouting should continue for a certain period of time before grouting is terminated. In general borehole sections, when the injection rate is less than 1.5 times x L / min during grouting, the pressure is increased in stages at 0.05 MPa increments, sequentially increasing to 0.1 MPa, 0.15 MPa, and 0.2 MPa. The pressure stabilization time for each stage is 3 to 5 minutes. During the pressure increase process, the final grouting pressure is determined based on the changes in injection rate and unit consumption, but the maximum grouting pressure shall not exceed 0.2 MPa.

2. The construction method of the deep anti-seepage wall using plastic concrete across a strong, easily collapsing backfill stratum as described in claim 1, characterized in that: The solidification mud preparation process is as follows: Procedure 1: Water + bentonite + additive 1 + positive charge dry powder + additive 2, stir the five components together for at least 5 minutes; Procedure 2: Water + bentonite + additive 1, stir the three components for at least 5 minutes, then add additive 2 and positive charge dry powder and stir for at least 5 minutes.

3. The construction method for a deep anti-seepage wall using plastic concrete across a strong, easily collapsing backfill stratum, as described in claim 1 or 2, is characterized in that: The pre-grouting holes are arranged in one row on each of the upstream and downstream sides of the anti-seepage wall, with their axes parallel to the axis of the anti-seepage wall.

4. The construction method of the deep anti-seepage wall made of plastic concrete across a strong, easily collapsing backfill stratum as described in claim 1, characterized in that: In the reinforcement step of the pre-grouting treatment of the backfill layer, the concrete support is formed in sections from bottom to top, and the pressure of the grout nozzle is greater when the bottom is formed than the pressure of the grout nozzle when the top is formed.

5. The construction method of the deep anti-seepage wall made of plastic concrete across a strong, easily collapsing backfill stratum as described in claim 1, characterized in that: In the pre-grouting process of the backfill layer, high-pressure grouting is first performed, with a pressure greater than 0.8 MPa. At the same time, rotation is carried out, and the high-pressure grout is used to impact the surrounding backfill layer in the pre-grouting hole to form a grouting cavity, thereby increasing the contact area between the grout and the backfill layer and improving the grout diffusion efficiency. The high-pressure grouting lasts for at least 3 minutes.

6. The construction method of the deep anti-seepage wall made of plastic concrete across a strong, easily collapsing backfill stratum as described in claim 4, characterized in that: In the process of creating holes and trenches, the upper loose backfill layer I and II trenches are created using the drilling and splitting method, the lower barrier lake facies sedimentary layer I trenches are created using the drilling and grabbing method, and the II trenches are created primarily using the grabbing method, supplemented by the drilling and grabbing method.

7. The construction method for a deep anti-seepage wall using plastic concrete across a strong, easily collapsing backfill stratum, as described in claim 1 or 6, is characterized in that: In the hole cleaning and slurry replacement step, the joint hole of the Phase II trench is cleaned with a circular brush. By applying pressure to the hole wall with the brush, the drill rig drives the brush to clean from top to bottom in sections, thereby achieving the purpose of cleaning the hole wall. The end standard is that the brush and drill bit are basically free of mud and the siltation at the bottom of the hole no longer increases.

8. The construction method of the deep anti-seepage wall made of plastic concrete across a strong, easily collapsing backfill stratum as described in claim 7, characterized in that: In the hole cleaning and slurry replacement step, the hole cleaning is carried out sequentially at the main and auxiliary holes in the tank section. The mud with a high sand content at the bottom of the tank is treated by a mud purifier and then returned to the tank hole until the slag outlet of the purifier no longer screens out sand particles; the cleaning hole is advanced from the high end to the low end of the bottom hole section of the tank. Before the hole cleaning is completed, samples are taken at the return slurry pipe inlet to test the full performance of the slurry, which serves as the basis for slurry replacement indicators; The performance indicators and replacement volume of the mud to be replaced are determined based on the mud test results. The replacement volume is determined comprehensively based on the volume of the trench, the performance of the mud in the trench, and the performance of the newly prepared mud. Solid wall mud is used to replace the mixed mud in the trench. The replacement volume is generally 1 / 3 to 1 / 2 of the trench volume. In the hole cleaning and mud replacement step, mud replacement is carried out sequentially at the main and auxiliary holes in the trench section. The drilling rig moves from the end away from the return mud pipe to the end close to the return mud pipe, and fresh mud is transported to the trench through the mud delivery pipe. The mud extracted from the bottom of the trench enters the return mud pool through the return mud ditch, and is then recycled as wall-stabilizing mud when the trench is formed.

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