Construction method of underground cutoff wall under complex geological conditions

Abstract

The application relates to the technical field of hydraulic construction, in particular to a construction method of an underground anti-seepage wall under complex geological conditions, which comprises geological survey and slot section division, guide wall construction, preparation of high-stability composite mud, slot section construction by a method of sequence division and skip construction, intermittent slot forming operation, mud circulation and mud supplement control, and anti-seepage wall pouring and forming, and has the beneficial effects that the slot section length is optimized from the traditional 6m to 3m, and the sequence division and skip construction method is adopted, so that the disturbance range of single slot forming on deep sand layers is obviously reduced, the slot wall exposure time is shortened, the self-stabilization capacity of the slot wall is effectively improved by the arch effect formed by the short slot section, and the slot wall collapse risk is reduced.

Description

Technical Field

[0001] This invention relates to the field of water conservancy construction technology, specifically a method for constructing underground anti-seepage walls under complex geological conditions. Background Technology

[0002] In infrastructure construction such as water conservancy projects, pumping station projects, and deep foundation pit support, underground anti-seepage wall technology is widely used in foundation reinforcement and retaining works due to its advantages such as high rigidity and good water-stopping properties. Especially in pumping station projects along rivers, geological conditions are often complex, frequently involving soft, loose strata such as silt, fine sand, and medium-coarse sand. These strata typically have characteristics of loose structure, high porosity, high permeability, low bearing capacity, and low cohesion.

[0003] Deep foundation pit construction for pumping stations often involves critical processes such as earthwork excavation and lowering the groundwater level. When constructing underground anti-seepage walls in deep sand layers, the trench walls are highly susceptible to seepage damage under water and soil pressure due to construction disturbance and high-pressure water head. This can lead to trench collapse, grout leakage, and in severe cases, even quicksand, soil erosion, and piping. These problems not only significantly increase the amount of hole cleaning and excessive concrete filling, but may also cause a drop in the surrounding groundwater level, damaging adjacent soil structures and existing building foundations, posing serious safety hazards and quality defects to the project itself and the surrounding environment.

[0004] Underground cutoff walls and their technology are widely used in the retaining and seepage prevention structures of deep foundation pits for pumping stations due to their excellent soil and water retention functions and high rigidity. Existing construction methods for underground cutoff walls typically involve dividing the wall into unit trenches of a certain length along the wall axis, using equipment such as hydraulic grab buckets or impact drilling rigs to perform trenching operations under mud-wall conditions, followed by the installation of a reinforcing cage (or connecting pipe) and underwater pouring of concrete using a tremie pipe method to ultimately form a continuous cutoff wall. Under conventional geological conditions, the length of each unit trench is often designed to be around 6 meters, employing a "two-grab trenching" or "three-grab trenching" process, and using staggered construction to avoid construction interference between adjacent trench sections.

[0005] However, under complex geological conditions such as deep silty sand layers, the above-mentioned conventional construction methods face the following prominent technical challenges: 1. Poor trench wall stability: The 6m long trench section has a large excavation span and long exposure time, which greatly disturbs the loose strata. The negative pressure and disturbance generated when the grab bucket cuts down quickly and grabs the soil can easily destroy the original stress balance of the soil around the trench wall, leading to gradual or sudden collapse of the trench wall. 2. Severe over-pouring of concrete: Over-excavation caused by collapse often results in the concrete pouring volume exceeding the design volume by about 40%, causing serious material waste; 3. A significant conflict exists between trenching efficiency and wall quality: In sandy layers, controlling trench wall stability often requires slowing down construction, but this prolongs the unsupported exposure time of the trench wall, increasing the risk of collapse. Traditional "two-grab-one-trench" or "three-grab-one-trench" processes for 6m trench sections involve redundant steps such as repeated positioning and trench section overlap adjustments, lengthening the single-trench operation cycle and making it difficult to achieve rapid, high-quality wall construction. Furthermore, frequent hole collapses not only increase the workload of trench repair and leak sealing, severely restricting construction progress, but also easily cause quality problems such as mud inclusions and wall breaks, leading to project cost overruns and schedule delays.

[0006] Therefore, there is an urgent need for a construction method that can effectively solve the problem of trench wall stability under complex geological conditions, improve trenching efficiency, and ensure project quality. Summary of the Invention

[0007] The purpose of this invention is to provide a construction method for underground anti-seepage walls under complex geological conditions, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for constructing an underground seepage barrier under complex geological conditions, the method comprising the following steps: S1. Geological Survey and Trench Segment Division: Based on the geological survey report and design drawings, determine the axis position of the anti-seepage wall and the distribution range of the sand layer, and divide the anti-seepage wall in the excavated soil into multiple unit short trench segments with a length of 3m along the axis. S2. Guide wall construction: Excavate guide wall trenches along the axis of the seepage prevention wall. After leveling the bottom of the trench, tie steel bars and set up formwork to pour concrete to form a “┓┏” shaped guide wall structure. After demolding, set up transverse supports between the guide walls. S3. Preparation of high-stability composite mud: The wall protection mud is prepared by using a composite system of sodium-based bentonite, soda ash and CMC thickener; S4. "Sequential skip-section" method for trench section construction: The construction sequence of Phase I and Phase II is adopted, with Phase I trench section constructed first, followed by the adjacent Phase II trench section. S5. Intermittent trenching operation: Move the hydraulic grab bucket along the guide wall track to the trench section to be constructed, adjust the center of the grab bucket to align with the center line of the trench section, and adopt an intermittent trenching process during the trenching process; S6. Mud circulation and replenishment control: During trenching, maintain the mud level at least 0.5m above the groundwater level and no less than 50cm below the top of the guide wall. Replenish mud after each grab bucket operation. If mud leakage occurs in the sand layer, immediately activate the emergency replenishment system and simultaneously add sticky mud balls or sawdust and other sealing materials into the trench. After the recovered mud is separated and desanded by a hydrocyclone, it enters the sedimentation tank and is left to stand for no less than 4 hours. It can be reused only after the test indicators are qualified. Mud with a sand content of more than 8% is directly discharged into the waste mud tank for treatment. S7. Construction and Forming of the Cutoff Wall: After the trench in S5 is dug to the design depth, the hole cleaning and slurry replacement method should be used by pump suction reverse circulation. The amount of slurry replaced should not be less than 1 / 2 of the total amount of mud in the trench. Before the completion of hole cleaning and slurry replacement in Phase II trench, the mud skin at the end of the Phase I trench should be washed in sections with a wire drill bit. 2-4 hours after the concrete is poured, the joint pipe should be pulled out for trial. During the trial pull, the bottom joint pipe should always be buried in the poured concrete. After the concrete strength reaches a level that can withstand the friction of the joint pipe being pulled out, the joint pipe should be pulled out gradually and at a uniform speed. The empty hole section formed by the pipe pulling should be filled with mud in time.

[0009] For the high pressure head conditions that may exist in deep sand layers, appropriately increasing the mud density can further increase the pressure of the slurry column, effectively offset the water pressure in the formation, and prevent the trench wall from being damaged by seepage or collapsing due to excessive head difference.

[0010] Preferably, during trenching of the sand layer section in step S5, the mud density is increased to 1.35 g / cm³. 3 This is to enhance the ability to balance the liquid column pressure of sand layers with high pressure head.

[0011] By dynamically monitoring and controlling the performance indicators of the mud in stages, we can ensure that the mud maintains the best wall protection performance in different construction stages such as trenching and hole cleaning, and avoid trench wall instability or incomplete hole cleaning caused by fluctuations in mud indicators.

[0012] Preferably, the density of the freshly prepared mud in step S3 is 1.3 g / cm³. 3 The density of this mud is increased by 0.1–0.15 g / cm³ compared to conventional mud. 3 The density, viscosity, water loss, and pH value of the mud were tested every 2 hours during construction. The control standards for different stages were as follows: mud density during trenching process: 1.25–1.35 g / cm³ 3 Viscosity 35-65s; mud density after hole cleaning not greater than 1.30 g / cm³ 3 Viscosity 32-50s.

[0013] To ensure that the length of the trench section matches the width of the grab bucket, each trench section only needs to be grabbed once to complete the trenching process, eliminating the need for repeated positioning and overlapping trimming within the trench section, the trenching time for a single trench is reduced by more than 60%. The sequential skip-compartment construction effectively staggers the construction disturbance time of adjacent trench sections, avoiding stress concentration and chain instability of the trench wall caused by simultaneous construction of adjacent trench sections.

[0014] Preferably, the unit short trough section in step S4 has a length of 3m, which is consistent with the width of the hydraulic grab bucket, so as to achieve troughing in one go with a single grab bucket.

[0015] To insert a brief static pause during the dynamic process of grabbing and cutting, the mud is allowed to penetrate into the sand layer of the trench wall under relatively stable hydraulic conditions, forming a mud cementing layer of a certain depth. This pre-reinforces the newly exposed trench wall during subsequent soil grabbing, reducing the risk of trench wall collapse at its source. Preferably, the specific operation sequence of the intermittent trenching process in step S4 is as follows: grab bucket cuts down at low speed → pauses briefly for 3 to 5 seconds → grabs soil at low speed → pauses briefly for 3 to 5 seconds → grab bucket lifts; the pause time is adjusted on-site according to the density and permeability of the sand layer, and the mud circulation is kept uninterrupted during the pause.

[0016] In order to ensure that the lateral support can effectively resist the lateral squeezing force on the guide wall during the trenching process, prevent the guide wall from deforming or shifting, and ensure the positioning accuracy and trench stability of the subsequent trenching.

[0017] Preferably, in step S2, transverse supports are promptly installed between the guide walls after demolding, with a spacing of no more than 1.5m between the transverse supports; the guide wall concrete strength can be removed only when it reaches more than 75% of the design strength.

[0018] Preferably, before pouring the anti-seepage wall in step S7, the borehole needs to be cleaned and inspected. One hour after cleaning and replacing the slurry, the thickness of the silt at the bottom of the borehole should not exceed 10cm, and the slurry density should not exceed 1.30g / cm³. 3 The viscosity is 32-50s, and the sand content is no more than 4%. After passing inspection, the joint pipe should be lowered within 4 hours. The distance between the bottom of the joint pipe and the bottom of the trench should be controlled at 100-250mm, with a deviation rate of no more than 0.4%. Plastic concrete should be continuously poured using the tremie method, with one joint pipe configured for every 3m short trench section. The 260mm guide pipe is used, and the distance between the bottom of the guide pipe and the bottom of the trench is controlled between 100 and 250mm. During the pouring process, the rising speed of the concrete surface is controlled to be no less than 2m / h, the depth of the guide pipe embedded in the concrete is maintained between 2 and 6m, and the concrete is poured to 50cm above the design elevation.

[0019] Preferably, the mud preparation ratio per cubic meter in step S3 is: 80 kg of high-quality sodium bentonite, 3.2 kg of soda ash, 0.8 kg of CMC thickener, and 1000 kg of water. The soda ash is pre-prepared into a 20% aqueous solution, and the CMC is pre-prepared into a 1.5% aqueous solution before being added. The mixture is stirred for 3-5 minutes using a high-speed rotary mixer. After thorough mixing, it is transferred to an expansion tank and allowed to stand for at least 24 hours to fully hydrate and swell before use. The density of the freshly prepared mud is controlled at 1.3 g / cm³. 3 The funnel viscosity is 32-55s, the water loss is less than 18mL / 30min, and the mud cake thickness is less than 2.5mm.

[0020] Preferably, in step S5, after the grab bucket cuts down at low speed in the intermittent trenching process, it pauses briefly for 3-5 seconds to allow the mud to seep into the pores of the sand layer and form cement; after the grab bucket grabs the soil at low speed, it pauses again for 3-5 seconds to allow the mud to seep into the newly exposed trench wall surface; the lifting speed of the grab bucket is not greater than 0.3m / s to avoid negative pressure that could cause the trench wall to collapse; the verticality of the wire rope is checked every 2m of excavation to ensure that the hole inclination is not greater than 0.4%.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. By optimizing the trench section length from the traditional 6m to 3m and adopting a phased skip-section construction method, the disturbance range of the deep sand layer caused by a single trenching is significantly reduced, the exposure time of the trench wall is shortened, and the "arch effect" formed by the short trench section effectively improves the self-stabilizing ability of the trench wall and reduces the risk of trench wall collapse. 2. The intermittent trenching process is adopted, with short pauses during the grab bucket cutting and soil grabbing process. Combined with the optimized ratio of high-stability composite mud, the mud has sufficient time to penetrate into the sand layer pores to form cement and quickly form a thin and dense mud cake on the trench wall surface. This dual action achieves temporary reinforcement of the trench wall during construction, fundamentally solving the problems of easy grout leakage and hole collapse in deep sand layers. 3. The 3m short trough section is matched with the width of the hydraulic grab bucket to achieve trough formation with one grab, eliminating the need for repeated positioning and trough section overlap and trimming processes that require two grabs to form a trough in the traditional 6m trough section. The trough formation time of a single trough is reduced by more than 60%. Combined with sequential skip-compartment construction, multiple trough sections can be operated simultaneously in a continuous flow, which greatly improves construction efficiency. 4. The customized high-stability composite mud system, based on sodium-based bentonite, combined with CMC thickener and soda ash, undergoes full hydration and swelling over 24 hours, increasing the density of the freshly prepared mud to 1.3 g / cm³. 3 With low water loss and thin but tough mud cake, it can accurately maintain the dynamic balance between the grout column pressure and the soil and water pressure of the stratum, significantly reducing trench wall collapse and excessive concrete volume, and saving more than 20% in overall construction costs compared with traditional methods. Attached Figure Description

[0022] Figure 1 This is a schematic flowchart of the underground anti-seepage wall construction method in an embodiment of the present invention; Figure 2 This is a schematic diagram of the guide wall in an embodiment of the present invention; Figure 3 This is a flowchart of the mud circulation process in an embodiment of the present invention; Figure 4 This is a construction layout diagram of the anti-seepage wall in an embodiment of the present invention; Figure 5 This is a plan view of the trench layout of the anti-seepage wall in an embodiment of the present invention.

[0023] In the diagram, 1. Excavated soil; 2. Trench; 3. Anti-seepage wall. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Please see Figures 1 to 5 The present invention provides a technical solution: A construction method for underground seepage barrier walls under complex geological conditions, comprising the following steps: S1. Geological survey and trench segment division: Based on the geological survey report and design drawings, determine the axial position of the anti-seepage wall 3 and the distribution range of the sand layer, and divide the anti-seepage wall 3 in the excavated soil 1 into multiple unit short trench segments with a length of 3m along the axial line; S2. Guide wall construction: Excavate guide wall trenches along axis 3 of the anti-seepage wall. The trench depth shall not be less than 1.5m. After the bottom of the trench is leveled, steel bars shall be tied and formwork shall be erected and concrete shall be poured to form a “┓┏” shaped guide wall structure. The top surface of the guide wall shall be not less than 10cm above the ground. After the formwork is removed, transverse supports shall be set between the guide walls. The curing time shall not be less than 7 days. S3. Preparation of High-Stability Composite Slurry: A composite system of sodium-based bentonite, soda ash, and CMC thickener is used to prepare the wall-protecting slurry. The proportions per cubic meter of slurry are: 80 kg of high-quality sodium-based bentonite, 3.2 kg of soda ash, 0.8 kg of CMC thickener, and 1000 kg of water. The soda ash is pre-prepared into a 20% aqueous solution, and the CMC is pre-prepared into a 1.5% aqueous solution before being added. The mixture is stirred for 3-5 minutes using a high-speed rotary mixer. After thorough mixing, the mixture is transferred to an expansion tank and allowed to stand for at least 24 hours to fully hydrate and swell before use. The density of the freshly prepared slurry is controlled at 1.3 g / cm³. 3 The funnel viscosity is 32-55s, the water loss is less than 18mL / 30min, and the mud cake thickness is less than 2.5mm. S4. "Sequential skip-section" method for trench section construction: The construction sequence of Phase I and Phase II is adopted, with Phase I trench section constructed first, followed by the adjacent Phase II trench section. S5. Intermittent Trenching Operation: Move the hydraulic grab bucket along the guide wall track to the trench section to be constructed, and adjust the grab bucket center to align with the trench section centerline, with a deviation of no more than 3cm; adopt an intermittent trenching process during trenching, that is, after the grab bucket cuts down at low speed, pause briefly for 3-5 seconds to allow the mud to seep into the sand layer pores and form cement; after the grab bucket grabs the soil at low speed, pause briefly again for 3-5 seconds to allow the mud to seep into the newly exposed trench wall surface; the grab bucket lifting speed should not exceed 0.3m / s to avoid negative pressure that could cause the trench wall to collapse; check the verticality of the wire rope every 2m of excavation to ensure that the hole inclination is no more than 0.4%; S6. Mud circulation and replenishment control: During trenching, maintain the mud level at least 0.5m above the groundwater level and no less than 50cm below the top of the guide wall. Replenish mud after each grab bucket operation. If mud leakage occurs in the sand layer, immediately activate the emergency replenishment system and simultaneously add sticky mud balls or sawdust and other sealing materials into the trench. After the recovered mud is separated and desanded by a hydrocyclone, it enters the sedimentation tank and is left to stand for no less than 4 hours. It can be reused only after the test indicators are qualified. Mud with a sand content of more than 8% is directly discharged into the waste mud tank for treatment. The mud recycling process includes: first, mud is prepared; then, the prepared mud is allowed to settle; after settling, the mud is stored for use in subsequent trench construction, including underwater concrete pouring; excess mud is recycled; waste residue is separated by a hydrocyclone; the separated mud is then used to prepare new mud; and the separated sediment is transported out.

[0026] S7. Construction and Forming of the Cutoff Wall 3: After the trench in S5 reaches the designed depth, the hole cleaning and slurry replacement method must be used, with the replacement volume not less than 1 / 2 of the total mud in the trench 2. Before the completion of the hole cleaning and slurry replacement in the second phase, the mud skin at the end of the first phase trench 2 should be washed in sections with a wire drill bit until the brush and drill bit are basically free of mud debris and the siltation at the bottom of the hole no longer increases. One hour after hole cleaning and slurry replacement, the thickness of the siltation at the bottom of the hole should not exceed 10cm, and the mud density should not exceed 1.30g / cm³. 3 Viscosity 32-50s, sand content not exceeding 4%; Before pouring the S7 anti-seepage wall 3, the borehole must be cleaned and inspected. After passing the inspection, the joint pipe must be lowered within 4 hours. The distance between the bottom of the joint pipe and the bottom of the trench must be controlled between 100 and 250 mm, and the deviation rate must not exceed 0.4%. Plastic concrete is continuously poured using the tremie method, with one tremie pipe configured for every 3 m short trench section. The 260mm guide pipe is used, and the distance between the bottom of the guide pipe and the bottom of the trench is controlled between 100 and 250mm. During the pouring process, the rising speed of the concrete surface is controlled to be no less than 2m / h, the depth of the guide pipe embedded in the concrete is maintained between 2 and 6m, and the concrete is poured to 50cm above the design elevation. Two to four hours after the concrete is poured, the joint pipe is pulled out for trial. During the trial, the bottom joint pipe is always buried in the poured concrete. After the concrete strength reaches a level that can withstand the friction of pulling out the joint pipe, the joint pipe is pulled out gradually and at a uniform speed. The empty section formed by pulling out the pipe is filled with mud in time.

[0027] By adopting the above technical solutions, in the construction of the seepage barrier wall 3 in the deep silty sand layer, the traditional 6m long trench section was optimized into a 3m short trench section, and a phased skip-compartment construction method was adopted. This significantly reduced the disturbance range and exposure time of the loose sand layer in a single trenching operation. The "arch effect" formed by the short trench section is conducive to the stability of the trench wall. The intermittent trenching process sets short pauses during the grab bucket cutting and soil grabbing process, allowing the highly stable composite mud sufficient time to penetrate into the sand layer pores and form cementation, effectively sealing the seepage channels. At the same time, the optimized mud system... With low water loss and a thin yet tough mud cake, it can quickly form a dense protective layer on the trench wall surface, maintaining a dynamic balance between the grout column pressure and the soil and water pressure of the stratum; the 3m trench section can be formed in one grab, eliminating the redundant processes of repeated positioning and trench section overlap and trimming in traditional processes. Combined with refined flow construction organization, it significantly shortens the operation cycle of a single trench; the synergistic effect of the above comprehensive technical means effectively solves the technical bottlenecks of difficult trench formation, difficult wall stabilization, and easy grout leakage in deep sand layers, achieving a balance of safety, efficiency, and economy in the construction of the anti-seepage wall.

[0028] When trenching the sand layer section as described in step S5, the mud density is increased to 1.35 g / cm³. 3 This is to enhance the ability to balance the liquid column pressure of sand layers with high pressure head.

[0029] By adopting the above technical solution, for the high pressure head conditions that may exist in deep sand layers, appropriately increasing the mud density can further increase the pressure of the slurry column, effectively offset the water pressure in the formation, and prevent the trench wall from being damaged by seepage or collapsing due to excessive water head difference.

[0030] The density of the freshly prepared mud in step S3 is 1.3 g / cm³. 3 The density of this mud is increased by 0.1–0.15 g / cm³ compared to conventional mud. 3 The density, viscosity, water loss, and pH value of the mud were tested every 2 hours during construction. The control standards for different stages were as follows: mud density during trenching process: 1.25–1.35 g / cm³ 3 Viscosity 35-65s; mud density after hole cleaning not greater than 1.30 g / cm³ 3 Viscosity 32-50s.

[0031] By adopting the above technical solutions, and through dynamic monitoring and phased control of mud performance indicators, it is ensured that the mud maintains optimal wall protection performance in different construction stages such as trenching and hole cleaning, thus avoiding trench wall instability or incomplete hole cleaning due to fluctuations in mud indicators.

[0032] The unit short trough section mentioned in step S4 has a length of 3m, which is consistent with the width of the hydraulic grab bucket, so as to realize the trough formation of a single grab bucket in one go.

[0033] By adopting the above technical solution, the length of the trench section is matched with the width of the grab bucket, so that each trench section can be completed in just one grab, eliminating the need for repeated positioning and overlapping trimming processes within the trench section, and reducing the trenching time of a single trench by more than 60%. The sequential skip-compartment construction effectively staggers the construction disturbance time of adjacent trench sections, avoiding stress concentration and chain instability of the trench wall caused by simultaneous construction of adjacent trench sections.

[0034] The specific operation sequence of the intermittent trenching process described in step S4 is as follows: grab bucket cuts down at low speed → pauses briefly for 3 to 5 seconds → grabs soil at low speed → pauses briefly for 3 to 5 seconds → grab bucket lifts up; the pause time is adjusted on-site according to the density and permeability of the sand layer, and the mud circulation is kept uninterrupted during the pause.

[0035] By adopting the above technical solution, a brief static pause is inserted into the dynamic process of grabbing and cutting down the grabbing bucket. During the pause, the mud is allowed to penetrate into the sand layer of the trench wall under relatively stable hydraulic conditions, forming a mud cementing layer of a certain depth. This layer plays a pre-reinforcing role on the newly exposed trench wall during the subsequent grabbing process, thereby reducing the risk of trench wall collapse from the root.

[0036] In step S2, transverse supports shall be promptly installed between the guide walls after demolding, with a spacing of no more than 1.5m between the transverse supports; the guide wall concrete strength can be removed only when it reaches more than 75% of the design strength.

[0037] By adopting the above technical solution, the lateral support can effectively resist the lateral squeezing force of the grab bucket operation on the guide wall during the trenching process, prevent the guide wall from deforming or shifting, and ensure the positioning accuracy and trench stability of the subsequent trenching.

[0038] Example 1: Construction of underground seepage prevention wall for pumping station project like Figure 1 As shown, S1. Geological Survey and Trench Segment Division: The pumping station project is located at the mouth of the Desheng River and the Yangtze River, where the geological conditions are extremely complex. The strata mainly consist of silty clay, silty clay, silt, fine sand, and weathered rock. Among them, the fine sand layer is buried at a depth of 7.80m-26.30m and a thickness of 5.49m-25.90m. This sand layer is loose and has extremely poor stability. Under the influence of groundwater level fluctuations and construction disturbances, it is prone to liquefaction, borehole collapse, and grout leakage. Based on the site geological conditions, the entire underground anti-seepage wall is divided into 78 trench segments, with the length of each trench uniformly optimized to 3m. Figure 5 As shown.

[0039] S2. Guide wall construction: such as Figure 2As described above, a guide wall trench is excavated along the axis of the seepage barrier wall 3. The trench depth is not less than 1.5m. After the bottom of the trench is leveled, steel bars are tied and concrete is poured to form a “┓┏” shaped guide wall structure. The top surface of the guide wall is not less than 10cm above the ground. After the formwork is removed, transverse supports are set between the guide walls. The curing time is not less than 7 days. S3. Preparation of High-Stability Composite Slurry: A slurry mixing station was set up on-site to address the characteristics of the fine sand layer. The following formula was used: 80 kg bentonite, 3.2 kg Na₂CO₃, 0.8 kg CMC, and 1000 kg water. After thorough mixing, the mixture was stored in a slurry storage tank for maturation for at least 24 hours. During trenching, the slurry specific gravity was adjusted to 1.30-1.35 g / cm³ based on measured slurry performance indicators. 3 This ensures that the mud crust is dense and effectively prevents groundwater from seeping in.

[0040] S4. "Sequential skip-section" method for trench section construction: The construction sequence of Phase I and Phase II is adopted, with Phase I trench section constructed first, followed by the adjacent Phase II trench section. S5. Intermittent Trenching Operation: Move the hydraulic grab bucket along the guide wall track to the trench section to be constructed, and adjust the grab bucket center to align with the trench section centerline, with a deviation of no more than 3cm; adopt an intermittent trenching process during trenching, that is, after the grab bucket cuts down at low speed, pause briefly for 3-5 seconds to allow the mud to seep into the sand layer pores and form cement; after the grab bucket grabs the soil at low speed, pause briefly again for 3-5 seconds to allow the mud to seep into the newly exposed trench wall surface; the grab bucket lifting speed should not exceed 0.3m / s to avoid negative pressure that could cause the trench wall to collapse; check the verticality of the wire rope every 2m of excavation to ensure that the hole inclination is no more than 0.4%; After the trench is completed, the "heavy hammer method" is used to check the trench depth and verticality to ensure that the deviation is within the design allowable range. Then, the hole is cleaned and the slurry is replaced until the thickness of the sediment at the bottom of the hole meets the requirements.

[0041] S6. Mud circulation and replenishment control: During trenching, maintain the mud level at least 0.5m above the groundwater level and no less than 50cm below the top of the guide wall. Replenish mud after each grab bucket operation. If mud leakage occurs in the sand layer, immediately activate the emergency replenishment system and simultaneously add sticky mud balls or sawdust and other sealing materials into the trench. After the recovered mud is separated and desanded by a hydrocyclone, it enters the sedimentation tank and is left to stand for no less than 4 hours. It can be reused only after the test indicators are qualified. Mud with a sand content of more than 8% is directly discharged into the waste mud tank for treatment. Concrete pouring: Select concrete with a density ≥ 2100 kg / m³ 3 Plastic concrete with a slump of approximately 22cm. It is poured using a Φ260mm tremie pipe method, with one tremie pipe per trench. During pouring, the joint pipe is moved every 30 minutes to ensure a tight connection and prevent pipe jamming.

[0042] S7. The anti-seepage wall 3 is cast and formed.

[0043] Implementation Results: By applying this construction method, the pump station project successfully overcame the construction challenges posed by thick layers of fine sand. Testing revealed an extremely low collapse rate in the trench walls, with the concrete filling coefficient controlled between 1.05 and 1.10 (compared to over 1.4 with traditional methods). The actual construction period was shortened from the planned 35 days to 26 days, directly saving approximately 500,000 yuan in project costs, achieving significant technical and economic benefits.

[0044] Example 2: Reservoir reinforcement project Project Overview: The reservoir is located in the Yellow River basin, with a total storage capacity of 733 million cubic meters. 3 It is a large reservoir. The dam foundation treatment involves the construction of a plastic concrete anti-seepage wall. The geological conditions include silt and sand layers, which requires extremely high stability of the trench.

[0045] Construction Application: The plastic concrete wall trench construction method described in this invention was adopted in the foundation treatment of the left auxiliary dam and the main dam. During construction, the 3m short trench section skip-pour process was strictly implemented, and special bentonite slurry was used for wall protection. Seamless integration was achieved through meticulous organization of each process, including guide wall construction, trenching, hole cleaning, and concrete pouring.

[0046] Implementation Results: Application results show that this method demonstrates good applicability in foundations with deep overburden. No large-scale hole collapse occurred during trenching, the wall joints were of good quality, and the seepage prevention effect met the design W6 standard. Compared with similar projects, construction efficiency was improved by approximately 30%, and project costs were effectively controlled, providing a successful technical reference for the reinforcement and upgrading of similar water conservancy projects.

[0047] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for constructing underground seepage barriers under complex geological conditions, characterized in that: The construction method includes the following steps: S1. Geological survey and trench segment division: Based on the geological survey report and design drawings, determine the axial position of the anti-seepage wall (3) and the distribution range of the sand layer, and divide the anti-seepage wall (3) in the excavated soil (1) into multiple unit short trench segments with a length of 3m along the axis; S2. Guide wall construction: Excavate guide wall trenches along the axis of the seepage prevention wall (3), tie steel bars after leveling the bottom of the trench, and pour concrete to form a "┓┏" shaped guide wall structure. After demolding, set horizontal supports between the guide walls. S3. Preparation of high-stability composite mud: The wall protection mud is prepared by using a composite system of sodium-based bentonite, soda ash and CMC thickener; S4. "Sequential skip-section" method for trench section construction: The construction sequence of Phase I and Phase II is adopted, with Phase I trench section constructed first, followed by the adjacent Phase II trench section. S5. Intermittent trenching operation: Move the hydraulic grab bucket along the guide wall track to the trench section to be constructed, adjust the center of the grab bucket to align with the center line of the trench section, and adopt an intermittent trenching process during the trenching process; S6. Mud circulation and replenishment control: During trenching, maintain the mud level at least 0.5m above the groundwater level and no less than 50cm below the top of the guide wall. Replenish mud after each grab bucket operation. If mud leakage occurs in the sand layer, immediately activate the emergency replenishment system and simultaneously add sticky mud balls or sawdust and other sealing materials into the trench. After the recovered mud is separated and desanded by a hydrocyclone, it enters the sedimentation tank and is left to stand for no less than 4 hours. It can be reused only after the test indicators are qualified. Mud with a sand content of more than 8% is directly discharged into the waste mud tank for treatment. S7. Pouring and forming of the anti-seepage wall: After the trench in S5 is formed to the design depth, the pump suction reverse circulation method is required to clean the hole and replace the slurry. The amount of slurry replaced is not less than 1 / 2 of the total amount of mud in the trench (2). Before the cleaning and replacement of the slurry in the trench (2) of the second stage is completed, the mud skin at the end of the trench (2) of the first stage is washed in sections with a wire drill bit. The joint pipe is pulled out 2-4 hours after the concrete is poured. During the trial pulling process, the bottom joint pipe is always buried in the poured concrete. After the concrete strength reaches the point where it can withstand the friction of the joint pipe being pulled out, the joint pipe is pulled out gradually and at a uniform speed. The empty hole section formed by the pipe is filled with mud in time.

2. The method for constructing an underground seepage barrier under complex geological conditions according to claim 1, characterized in that: The density of the freshly prepared mud in step S3 is 1.3 g / cm³. 3 The density of this mud is increased by 0.1–0.15 g / cm³ compared to conventional mud. 3 The density, viscosity, water loss, and pH value of the mud were tested every 2 hours during construction. The control standards for different stages were as follows: mud density during trenching was 1.25–1.35 g / cm³. 3 Viscosity 35-65s; mud density after hole cleaning not greater than 1.30 g / cm³ 3 Viscosity 32-50s.

3. The method for constructing an underground seepage barrier under complex geological conditions according to claim 1, characterized in that: The unit short trough section mentioned in step S4 has a length of 3m, which is consistent with the width of the hydraulic grab bucket, so as to realize the trough formation of a single grab bucket in one go.

4. The method for constructing an underground seepage barrier under complex geological conditions according to claim 3, characterized in that: The specific operation sequence of the intermittent trenching process described in step S4 is as follows: grab bucket cuts down at low speed → pauses briefly for 3 to 5 seconds → grabs soil at low speed → pauses briefly for 3 to 5 seconds → grab bucket lifts up; the pause time is adjusted on-site according to the density and permeability of the sand layer, and the mud circulation is kept uninterrupted during the pause.

5. The method for constructing an underground seepage barrier under complex geological conditions according to claim 1, characterized in that: In step S2, transverse supports shall be promptly installed between the guide walls after demolding, with a spacing of no more than 1.5m between the transverse supports; the guide wall concrete strength can be removed only when it reaches more than 75% of the design strength.

6. The method for constructing an underground seepage barrier under complex geological conditions according to claim 1, characterized in that: In step S5, when trenching the sand layer section, the mud density is increased to 1.35 g / cm³. 3 This is to enhance the ability to balance the liquid column pressure of sand layers with high pressure head.

7. The method for constructing an underground seepage barrier under complex geological conditions according to claim 1, characterized in that: Before pouring the S7 cutoff wall (3), the boreholes must be cleaned and inspected. One hour after cleaning and replacing the slurry, the thickness of the silt at the bottom of the borehole should not exceed 10cm, and the slurry density should not exceed 1.30g / cm³. 3 The viscosity is 32-50s, and the sand content is no more than 4%. After passing inspection, the joint pipe should be lowered within 4 hours. The distance between the bottom of the joint pipe and the bottom of the trench should be controlled at 100-250mm, with a deviation rate of no more than 0.4%. Plastic concrete should be continuously poured using the tremie method, with one joint pipe configured for every 3m short trench section. The 260mm guide pipe is used, and the distance between the bottom of the guide pipe and the bottom of the trench is controlled between 100 and 250mm. During the pouring process, the rising speed of the concrete surface is controlled to be no less than 2m / h, the depth of the guide pipe embedded in the concrete is maintained between 2 and 6m, and the concrete is poured to 50cm above the design elevation.

8. The method for constructing an underground seepage barrier under complex geological conditions according to claim 2, characterized in that: The S3 slurry mix ratio per cubic meter is: 80 kg of high-quality sodium bentonite, 3.2 kg of soda ash, 0.8 kg of CMC thickener, and 1000 kg of water. The soda ash is pre-prepared into a 20% aqueous solution, and the CMC is pre-prepared into a 1.5% aqueous solution before being added. The mixture is stirred for 3-5 minutes using a high-speed rotary mixer. After thorough mixing, it is transferred to an expansion tank and allowed to stand for at least 24 hours to fully hydrate and swell before use. The density of the freshly prepared slurry is controlled at 1.3 g / cm³. 3 The funnel viscosity is 32-55s, the water loss is less than 18mL / 30min, and the mud cake thickness is less than 2.5mm.

9. A method for constructing an underground seepage barrier under complex geological conditions according to claim 6, characterized in that: In the S5 intermittent trenching process, the grab bucket pauses briefly for 3-5 seconds after cutting down at low speed, allowing the mud time to seep into the sand layer pores and form cement; after grabbing the soil at low speed, the grab bucket pauses again for 3-5 seconds, allowing the mud to seep into the newly exposed trench wall surface; the grab bucket lifting speed is no more than 0.3m / s to avoid negative pressure that could cause the trench wall to collapse; the verticality of the wire rope is checked every 2m of excavation to ensure that the hole inclination is no more than 0.4%.

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