High-pressure jet grouting construction method for concrete cutoff wall for dam

Abstract

A high-pressure jet grouting construction method for a concrete cutoff wall for a dam, comprising the following steps: S1, determining a cutoff wall method; S2, determining a cutoff wall location; S3, optimizing a cutoff wall construction procedure; and S4, performing cutoff wall joint treatment. The construction method employs a series of operations to determine whether a cutoff wall method is feasible; extending a seepage path to reduce a hydraulic gradient when the hydraulic gradient does not meet requirements; optimizing a cutoff wall location by moving the cutoff wall to an upstream side of a dam body; constructing the dam body and the cutoff wall at the same time without mutual interference to accelerate construction progress; and performing cutoff wall cap casting before constructing the high-pressure jet grouting cutoff wall, thereby saving time spent on working platform excavation and secondary backfill, saving construction costs, and reducing the duration of construction.

Description

A method for high-pressure jet grouting construction of anti-seepage walls for concrete dams Technical Field

[0001] This invention relates to the field of concrete dam technology, and in particular to a high-pressure jet grouting construction method for a concrete dam anti-seepage wall. Background Technology

[0002] Given the complex geological conditions, the dam foundation riverbed has a cover layer structure, mainly composed of alluvial deposits, consisting of a layer of boulders, sand, gravel, and pebbles. Within 5 meters of the cover layer, there are boulders and boulders scattered throughout the surface, accounting for over 25% of the content. The remainder consists of pebbles, gravel, and sand. This layer has poor gradation, loose soil, localized voids, and high permeability. Below 5 meters of the riverbed, the soil is mainly composed of pebbles, gravel, and sand, with some boulders. Under these conditions of relatively strong to moderate permeability, the dam foundation anti-seepage wall construction requires the use of concrete diaphragm wall technology. However, the construction time for concrete diaphragm walls is relatively long.

[0003] In some dam cutoff wall structures, high-pressure jet grouting and curtain grouting methods are used for seepage prevention, but these are only employed when the construction period and geological conditions permit. In the aforementioned geological environments, there are currently no known construction cases using high-pressure jet grouting, nor is there any publicly available technology. Summary of the Invention

[0004] The purpose of this invention is to propose a high-pressure jet grouting construction method for concrete dam anti-seepage walls, applicable to construction environments with complex dam geological conditions, deep overburden layers containing boulders, pebbles, sand, gravel, and sand, and dam foundation strata with relatively strong to moderate permeability; improving construction efficiency and ensuring construction timeliness while ensuring construction quality.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A high-pressure jet grouting construction method for a concrete dam cutoff wall includes the following steps:

[0007] S1. Determine the method for constructing the anti-seepage wall;

[0008] S2. Determine the location of the anti-seepage wall;

[0009] S3. Optimize the anti-seepage wall process;

[0010] S4. Treatment of the connection between the seepage prevention wall and the wall.

[0011] In some embodiments, it is applied to construction environments where the dam body has complex geological conditions, the basic geological conditions are deep overburden layers containing boulders, boulders, sand pebbles, and gravel, and the dam base strata are highly to moderately permeable.

[0012] In some embodiments, in step S1:

[0013] By conducting high-pressure jet grouting experiments and quality inspections under similar geological conditions within the engineering area, the feasibility of using high-pressure jet grouting walls as anti-seepage walls for dam foundations was determined.

[0014] In some embodiments, step S1 further includes: determining the parameters of the anti-seepage wall;

[0015] Specifically, this includes: leakage calculation, seepage gradient calculation, and comprehensive analysis to determine the result;

[0016] If the seepage gradient is greater than the specification requirement, and it is still greater than the specification requirement after increasing the depth of the anti-seepage wall, then the seepage gradient can be reduced by extending the seepage path.

[0017] In some embodiments, in step S2:

[0018] The dam body and the cutoff wall are designed to be constructed simultaneously without interference; the axis of the cutoff wall is moved to the upstream side of the dam body.

[0019] The anti-seepage wall is connected to the dam body with a cap concrete, and a PVC waterstop is installed at the connection between the cap and the dam body.

[0020] In some embodiments, in step S3:

[0021] First, the cap of the anti-seepage wall is poured, and then the high-pressure jet grouting anti-seepage wall is constructed.

[0022] In some embodiments, in step S3:

[0023] When performing jet grouting drilling, the casing method is used; the bottom elevation of the hole is more than 30cm lower than the design elevation.

[0024] After the hole depth meets the design drawing requirements, a brittle PVC pipe wall is inserted, which can be crushed by the pressure of the rotary jet, and then the root pipe is pulled out.

[0025] In some embodiments, step S3 further includes: special case control;

[0026] (1) Water inrush treatment;

[0027] To prevent water inrush from washing away fine particles in the borehole and forming cavities, pebbles and coarse sand were backfilled on the outside of the PVC protective pipe while the root pipe was being pulled out, and the borehole opening was sealed with mortar.

[0028] (2) Permeable layer treatment;

[0029] When spraying to a highly permeable layer, reduce the lifting speed, increase the density and feed rate of the slurry, and add 1% to 3% water glass to the cement slurry;

[0030] (3) Treatment of grout return and leakage;

[0031] When large voids in the formation cause no grout return or severe grout leakage, the main measures are: stop raising the jet grouting pipe, reduce the rotation speed, reduce the water pressure, and increase the grout density or grout feed rate; or, add water glass to the grout and jet grout in situ.

[0032] When the amount of grout returning from the orifice is small, reduce the rotation and lifting speed, decrease the water pressure, and increase the grout density; or, add water glass to the grout to shorten the consolidation time, so that the grout solidifies within a certain soil layer range, and start lifting and jet grouting again when the grout return is normal.

[0033] (4) Handling of boulders and rocks;

[0034] When spraying the boulders, boulders, or pebbles, increase the rotation speed by 50cm above and below and slow down the lifting speed to fully coat the boulders, boulders, or pebbles with cement grout, so that the pile body is continuous and intact.

[0035] In some embodiments, in step S4:

[0036] At the junction of the high-pressure jet grouting cutoff wall and the original concrete diaphragm wall cutoff wall, an additional row of piles is added, making it a three-row jet grouting pile system, so that the jet grouting piles are tightly integrated with the original cutoff wall.

[0037] In some embodiments, in step S1:

[0038] The seepage barrier wall consists of two rows of high-pressure jet grouting piles;

[0039] The pile depth is 20-25m, the spacing between piles is 60cm, and the pile diameter is 75cm.

[0040] Compared with the prior art, the present invention provides a high-pressure jet grouting construction method for concrete dam anti-seepage walls, which has the following beneficial effects.

[0041] 1. This invention determines the feasibility of the anti-seepage wall method by comparing and analyzing dam anti-seepage technologies, supplementing geological exploration, and conducting high-pressure jet grouting pile experiments; when the seepage gradient does not meet the requirements, the seepage gradient is reduced by extending the seepage path.

[0042] 2. In this invention, the location of the anti-seepage wall is optimized by moving it to the upstream side of the dam body and connecting it to the dam body with a cap concrete and installing a waterstop. After the dam body is constructed to a certain height, the construction of the anti-seepage wall begins. The dam body and the anti-seepage wall are constructed simultaneously without interfering with each other, which speeds up the construction progress. At the same time, the original dam section is preserved and backfilled with concrete for use as an access road to the dam.

[0043] 3. This invention optimizes the construction process by first pouring the anti-seepage wall cap and then constructing the high-pressure jet anti-seepage wall; it saves time on platform excavation and secondary backfilling, thus saving construction costs and time. At the same time, it optimizes the process of chiseling off pile heads, further saving construction costs and time.

[0044] 4. In this invention, during jet grouting drilling, the thickness of different strata, the depth of boulders or boulders, and their approximate particle size are recorded to provide a reference for adjusting jet grouting parameters during construction; corresponding treatment methods are designed for special situations to ensure construction results; a row of piles is added at the junction of the high-pressure jet grouting anti-seepage wall and the original anti-seepage wall to ensure a tight bond between the jet grouting piles and the original concrete anti-seepage wall.

[0045] Other advantages, objectives and features of the invention will be set forth in part in the description which follows; and in part will be apparent to those skilled in the art upon examination of the following description; or may be learned from practice of the invention. Attached Figure Description

[0046] Figure 1 shows the design layout of the dam's anti-seepage wall.

[0047] Figure 2 shows the design rendering of the dam section restoration.

[0048] Figure 3 shows the design cross-section of the newly built No. 1 dam section.

[0049] Figure 4 is a schematic diagram of the high-pressure rotary jet design layout.

[0050] In the picture:

[0051] 1. The axis of the concrete diaphragm wall anti-seepage wall of the original No. 2 dam section;

[0052] 2. The original No. 1 dam section;

[0053] 3. Construct the new No. 1 dam crest axis;

[0054] 4. Design and pile layout diagram of the high-pressure jet grouting anti-seepage wall for the new No. 1 dam section;

[0055] 5. Public roads on the right bank;

[0056] 6. Construct new dam section #1;

[0057] 8. Section 2 of the dam;

[0058] 9. Downstream concrete backfill section of dam section #1;

[0059] 10. Backfill concrete in the scour area at the bottom of the original No. 1 dam section;

[0060] 11. Construct a new high-pressure jet grouting anti-seepage wall on the crest of Dam No. 1;

[0061] 12. Concrete cap;

[0062] 13. Original dam body diaphragm wall seepage prevention wall;

[0063] 14. Water-stopping structure between the concrete cap at the top of the high-pressure jet grouting pile and the concrete structure of the dam;

[0064] 15. Schematic diagram of dam section. Detailed Implementation

[0065] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0066] A high-pressure jet grouting construction method for anti-seepage walls of concrete dams is applied in construction environments with complex geological conditions, deep overburden layers containing boulders, boulders, gravel, and sand, and where the dam foundation strata are highly to moderately permeable.

[0067] The concrete gravity dam foundation anti-seepage wall of a certain hydropower station was designed and constructed as a concrete diaphragm wall system. Due to earthquakes and floods, the dam abutments, anti-seepage wall, and gravity dam were severely damaged, rendering the project unable to generate electricity. Considering the necessary timeframe for project repair, the left bank gates and sedimentation basin need to be repaired within one dry season, and the right bank dam section and all structures need to be rebuilt within the second dry season. To ensure the timely achievement of the project milestones within a single dry season, restore power generation as soon as possible, and fulfill the contractual obligations, continuing the original design of the dam anti-seepage wall as a concrete diaphragm wall is not feasible.

[0068] The technical solution provided in this application is used to adjust the construction method and location of the dam anti-seepage wall; through rapid repair measures, the repair of the damaged dam is ensured to be completed on schedule; at the same time, the anti-seepage capacity of the dam foundation and the bonding strength between the dam foundation and the dam body are guaranteed.

[0069] As shown in Figure 1;

[0070] 1 represents the axis of the concrete diaphragm wall anti-seepage wall of the original dam section 2;

[0071] Section 2 is the original No. 1 dam section, with a dam crest elevation of 1430m. After being damaged, it was demolished and rebuilt in situ according to the initial requirements, and was retained after optimization; and it is used as the access road to the dam.

[0072] 3 is the newly built No. 1 dam crest axis, No. 1 dam section intersects No. 2 dam section at 35°; the dam section elevation starts from 1430m and connects to the right bank road 5 in three steps, with the top elevation being 1445.6m;

[0073] 4 shows the design and pile layout of the high-pressure jet grouting anti-seepage wall for the new No. 1 dam section; the pile depth is 20m, the spacing between rows is 60m, and it is designed in two rows; among them, an additional row of piles is added on the upstream side where it intersects with the No. 2 dam section, that is, it is set as a three-row jet grouting pile; the anti-seepage wall is arranged in a stepped manner along the slope, and the pile positions extend 1m into the road.

[0074] 5 is the public road on the right bank.

[0075] As shown in Figure 2;

[0076] 6 represents the newly constructed No. 1 dam section;

[0077] Section 2 is the original No. 1 dam section. The newly built No. 1 dam section and the original No. 1 dam section will be backfilled and compacted to serve as the access road to the dam.

[0078] 8 refers to dam section #2, with the same crest elevation as the original dam section #1;

[0079] 9 is the downstream concrete backfill section of dam section 1; to protect the newly built dam section and the right slope, the top is backfilled with slag material to 30cm below the original dam top, and the concrete pavement is poured to maintain the same elevation as the original dam section 1, and used as a dam top road.

[0080] 10 is the backfill concrete for the scour area at the bottom of the original No. 1 dam section, to ensure the stability of the original No. 1 dam section.

[0081] As shown in Figure 3;

[0082] 11 is the construction of a new high-pressure jet grouting anti-seepage wall on the top of Dam No. 1, with a depth of 20m. The wall will be excavated in the dam foundation section to the bottom elevation of the original dam No. 1 section bottom diaphragm wall.

[0083] 12 is the concrete cap, with a bottom concrete thickness of 2.7m and other areas having a thickness of 2m;

[0084] 13 is the original diaphragm wall seepage prevention wall of the dam body;

[0085] 14 is the water-stopping structure between the concrete cap at the top of the high-pressure jet grouting pile and the concrete of the dam structure, with a total of two layers arranged;

[0086] Figure 15 is a schematic diagram of the dam section, one of the cross sections when the anti-seepage wall is adjusted and optimized to be outside the dam structure.

[0087] The high-pressure jet grouting construction method for concrete dam cutoff walls includes the following steps:

[0088] S1. Determine the method for constructing the anti-seepage wall;

[0089] S2. Determine the location of the anti-seepage wall;

[0090] S3. Optimize the anti-seepage wall process;

[0091] S4. Treatment of the connection between the seepage prevention wall and the wall.

[0092] In step S1, the method for creating the seepage barrier is determined.

[0093] Specifically, by conducting high-pressure jet grouting experiments and quality inspections under similar geological conditions within the engineering area, the feasibility of using high-pressure jet grouting walls as seepage barriers for dam foundations will be determined.

[0094] For example, in the aforementioned project, it was determined that when the seepage prevention depth of the dam foundation is 20m, it is feasible to use a high-pressure jet grouting wall as the seepage prevention wall for the dam foundation; correspondingly, the seepage prevention wall consists of two rows of high-pressure jet grouting piles with a pile depth of 20~25m, a pile spacing of 60cm, and a pile diameter of 75cm; among which, the preferred pile depth is 20m.

[0095] Step S1 also includes: determining the parameters of the anti-seepage wall.

[0096] Specifically, this includes: leakage calculation, seepage gradient calculation, and comprehensive analysis.

[0097] in:

[0098] The leakage calculation was performed at two different depths of the cutoff wall (e.g., 20m and 25m), and the calculation results were compared. The leakage of the dam foundation should be less than the requirements of the design report.

[0099] When calculating the seepage gradient, the depths of the two cutoff walls mentioned above are used to calculate the seepage gradients of the cutoff wall, the stratum below the cutoff wall, and the stratum downstream of the cutoff wall, respectively.

[0100] Comprehensive analysis determined that if the seepage gradient of the strata below and downstream of the cutoff wall is greater than the standard requirements, and it is still greater than the standard requirements after increasing the depth of the cutoff wall, then based on the relationship between seepage gradient and seepage path, the seepage gradient can be reduced by extending the seepage path.

[0101] Preferably, the leakage rate should be less than 0.2m. 3 / s, the permeability gradient should be less than 0.2.

[0102] Step S1 also includes:

[0103] The original dam section is retained, and a new dam section is constructed that intersects with the original dam section; the area between the dam sections is backfilled and compacted to serve as an access road to the dam.

[0104] In step S2, the location of the seepage barrier wall is determined.

[0105] In the aforementioned project, the original dam cutoff wall was located at the lower part of the dam body. The construction of the cutoff wall had to be completed before the construction of the concrete dam. The construction progress of the cutoff wall directly affected the construction of the gravity dam and related projects. Moreover, the cutoff wall is an underground hidden project. The geological conditions of the dam site are relatively complex, and various factors that arise during construction are uncontrollable, making it impossible to guarantee the progress.

[0106] The dam body and the cutoff wall are designed to be constructed simultaneously without interference, thus accelerating the construction progress.

[0107] Specifically, through analysis of progress, quality, and construction organization, the axis of the anti-seepage wall was moved to the upstream side of the dam body; the anti-seepage wall was connected to the dam body with a cap concrete, and two PVC waterstops were installed at the connection between the cap and the dam body; the anti-seepage wall was constructed after the dam body was built to a certain height.

[0108] The concrete thickness at the bottom of the pier cap is 2.5~3.0m, and the thickness in other areas is 1.8~2.2m; preferably, the concrete thickness at the bottom of the pier cap is 2.7m, and the thickness in other areas is 2.0m.

[0109] In step S3, the anti-seepage wall process is optimized.

[0110] Specifically, the anti-seepage wall cap is poured first, and then the high-pressure jet grouting anti-seepage wall is constructed.

[0111] In the aforementioned project, due to site constraints, the right bank slope anti-seepage wall was constructed in stages according to the elevation of the walkway. The construction sequence was as follows: filling the construction platform of this stage → high-pressure spraying wall construction → quality inspection and approval → excavation of the construction platform → removal of pile heads → pouring of the cap → backfilling of this stage → high-pressure spraying wall construction of the next stage.

[0112] During each stage of the cap concrete construction, the construction platform must be excavated and backfilled. Repeated backfilling and excavation are all within the linear construction period. After optimizing the process design, the cap of the anti-seepage wall is poured first, and then the high-pressure jet anti-seepage wall is constructed. This saves time on the excavation of the construction platform and the secondary backfilling and compaction, saves construction costs, shortens the construction period, and optimizes the pile head removal process.

[0113] In step S3, the jet grouting drilling process employs the pipe-following method;

[0114] The bottom elevation of the hole is more than 30cm lower than the design elevation; after the hole depth meets the requirements of the design drawings, a brittle PVC pipe wall is lowered in, which can be crushed by the pressure of the rotary jet, and the root pipe is pulled out.

[0115] In addition, during the drilling process, the thickness of different strata, the depth of boulders or boulders, and their approximate particle size are recorded in detail to provide a reference for adjusting the jet grouting parameters during the jet grouting operation.

[0116] Step S3 also includes: special case control.

[0117] (1) Water inrush treatment;

[0118] To prevent water inrush from washing away fine particles in the borehole and creating cavities in the formation, gravel and coarse sand were backfilled on the outside of the PVC protective pipe while the root pipe was being pulled out, and the borehole opening was sealed with mortar.

[0119] (2) Permeable layer treatment;

[0120] When spraying to a highly permeable layer, reduce the lifting speed, increase the density and feed rate of the slurry, and add 1% to 3% water glass to the cement slurry;

[0121] Preferably, 2% water glass is added.

[0122] (3) Treatment of grout return and leakage;

[0123] When large voids in the formation cause no grout return or severe grout leakage, the main measures are: stop raising the jet grouting pipe, reduce the rotation speed, reduce the water pressure, and increase the grout density or grout feed rate; or, add water glass to the grout and jet grout in situ.

[0124] When the amount of grout returning from the orifice is small, reduce the rotation and lifting speed, decrease the water pressure, and increase the grout density; or, add water glass to the grout to shorten the consolidation time, allowing the grout to solidify within a certain soil layer range. Once the grout return is normal, resume the jet grouting process.

[0125] (4) Handling of boulders and rocks;

[0126] When spraying the boulders, boulders, or pebbles, increase the rotation speed by 50cm above and below and slow down the lifting speed to fully coat the boulders, boulders, or pebbles with cement grout, so that the pile body is continuous and intact.

[0127] In step S4, the seepage barrier wall is connected.

[0128] At the junction of the high-pressure jet grouting cutoff wall and the original concrete diaphragm wall cutoff wall, an additional row of piles is added, making it a three-row jet grouting pile system, so that the jet grouting piles are tightly integrated with the original cutoff wall.

[0129] Meanwhile, alternatively, during the jet grouting construction process, measures such as increasing water pressure and rotation speed, reducing lifting speed, and increasing grout concentration can be adopted to increase the diameter of the pile body, so as to further ensure that the jet grouting pile is tightly integrated with the original seepage prevention wall.

[0130] It also includes: detection.

[0131] (1) Compressive strength and impermeability;

[0132] Twenty-eight days after the completion of the high-pressure jet grouting anti-seepage wall construction, core sampling and water pressure tests were conducted; samples were taken at different depths for compressive strength testing, and static head water pressure tests were used to check the seepage prevention in the inspection holes. The water pressure tests were conducted in the whole section and in sections.

[0133] (2) Leakage detection;

[0134] Groundwater level observation wells are set up and piezometers are installed, mainly to monitor the rise and fall of groundwater level during the operation of the power station, and also to monitor the leakage of the dam foundation and dam shoulders; continuous observation is carried out in the initial stage, before water impoundment, during the dam impoundment period, and after water impoundment.

[0135] The following section provides an explanation of this application, using specific engineering examples.

[0136] The Upper Bodhisi Hydropower Station is located on the Bodhisi River in the Sindhupalchok district of north-central Nepal. The dam site is situated on the right bank of the China-Nepal Highway, 110 kilometers from Kathmandu and 5 kilometers from the China-Nepal border (Zhangmu Port). The power station has an installed capacity of 2×22.5 kW and began generating electricity in January 2001. The 8.1 magnitude earthquake on April 25, 2015, caused the pressure steel pipe to rupture and the powerhouse to be flooded. On July 5, 2016, a landslide dam flood destroyed the right abutment of the dam, the adjacent China-Nepal Highway, and the access road to the dam. The upstream right guide wall of the spillway and the right wall of the sedimentation basin were destroyed, and the powerhouse was flooded again.

[0137] Construction began in 2018 on the No. 1 gravity dam and its cutoff wall, the right side wall of the sedimentation basin, upstream and downstream slope protection, repair or replacement of other damaged structures, metal structures and mechanical and electrical equipment in the headworks area, replacement of some pressure steel pipes, replacement of electrical equipment in the switchyard, repair or replacement of all equipment in the powerhouse except for the main unit, and commissioning and trial operation. The contract required the use of an underground concrete continuous wall consistent with the original design as the cutoff wall for the new No. 1 gravity dam, extending 30m into the right bank slope, with a total length of approximately 100m, of which approximately 70m is located on the slope.

[0138] Based on the construction schedule and diversion construction requirements, and a comprehensive analysis of the construction technology and site conditions of the diaphragm wall, if a concrete diaphragm wall is used as the anti-seepage wall for the new No. 1 gravity dam without the availability of a full-section diversion tunnel or other suitable facilities, construction cannot be completed on schedule, directly affecting the project's schedule and power generation target.

[0139] Using high-pressure jet grouting as the seepage prevention structure for the new No. 1 gravity dam would shorten the construction period, reduce construction costs, and ultimately enable the power generation target to be achieved on schedule or ahead of schedule. However, there are currently no known construction cases of high-pressure jet grouting in this environment, nor is there any publicly available technology. A review of previous geological exploration data for the dam site indicates that the strata contain large-diameter boulders and boulders, making the feasibility and reliability of high-pressure jet grouting construction worthy of further investigation.

[0140] Comparative analysis of dam seepage prevention technologies and comparison of power plants in the region.

[0141] By collecting and comparing design data and geological data of seepage prevention walls for hydropower station structures in Nepal, conducting supplementary geological exploration and high-pressure jet grouting tests, and performing leakage analysis and calculations, the feasibility of using high-pressure jet grouting to construct seepage prevention walls for dams was studied.

[0142] By collecting and comparing geological data and anti-seepage wall data of four power stations in Nepal, it was found that the Upper Tamaksi Power Station and the Nasuwakali Power Station adopted high-pressure jet grouting piles as the foundation anti-seepage structure of the buildings; however, the geological conditions of the Upper Bodhisi Hydropower Station were better than those of the dam foundation.

[0143] Further, supplementary geological exploration.

[0144] To gain a deeper understanding of the geological conditions of the foundation of the new No. 1 gravity dam, two supplementary geological exploration boreholes were selected at both ends of the anti-seepage wall axis of the new No. 1 gravity dam to conduct supplementary geological surveys of the foundation strata; the borehole depths were 55m and 60m, respectively.

[0145] Core analysis from the boreholes revealed that the geological strata of the foundation of the new No. 1 gravity dam mainly consist of: the riverbed cover layer is primarily alluvial deposits, consisting of a layer of boulders, sand, pebbles, and gravel. The riverbed cover layer is divided into two rock groups. The surface layer is 2.5m to 5.0m thick and consists of boulders, sand, pebbles, and gravel. Boulders and boulders are abundant on the surface, accounting for more than 25% of the content. The remainder consists of pebbles and sand. This layer has poor gradation, loose soil, localized voids, and high permeability. Below 5m of the riverbed, the soil is mainly composed of pebbles, gravel, and sand, with some boulders. The pebbles and sand layers are basically interbedded or tightly packed.

[0146] Based on the analysis of permeability tests and dynamic penetration tests conducted by the Kathmandu National Laboratory, the dam foundation strata are classified as having strong to moderate permeability, with a permeability of approximately 10%. -2 cm / s~10 -3 cm / s, with an allowable permeability gradient of i = 0.1 to 0.2.

[0147] High-pressure jet grouting pile experiments were conducted.

[0148] To provide a reference for the design of the anti-seepage wall using the high-pressure jet grouting method and subsequent construction, two sets of parameters were selected on-site for high-pressure jet grouting experiments. The high-pressure jet grouting test piles were carried out on the right bank downstream of the dam to ensure that accurate and effective parameters were obtained under the same geological conditions as the dam's anti-seepage wall.

[0149] The experimental piles were arranged in two rows perpendicular to the water flow, totaling 13 piles, each 9m long, with a wall length of 4.3m. The spacing between rows of piles was 60cm, the pile diameter was 75cm, and the wall width was approximately 1.05m. Twenty-eight days after completion, core samples were taken from the interlocking area of ​​the upstream and downstream piles and from the middle of the pile body for pile compressive strength and borehole permeability pressure tests. A total of 5 core samples were taken. The maximum compressive strength of the core samples was 14.2MPa, and the minimum was 7.3MPa, both exceeding the contractual requirement of 6MPa. The pressure test was conducted in sections, each 3m long, with a maximum permeability of 1.85Lu, less than the contractual requirement of 2Lu.

[0150] The minimum compressive strength and maximum permeability were close to the allowable values ​​required by the contract, and both occurred at the interlocking points of the two piles. Therefore, the high-pressure jet grouting pile test results are valuable and can be used as a basis for the design of high-pressure jet grouting cutoff walls.

[0151] Determine the parameters of the anti-seepage wall.

[0152] Dam foundation seepage calculation: The original cutoff wall depth was 20m. For the dam foundation seepage calculation, cutoff wall depths of 20m and 25m were used respectively, and the calculation results were compared. When the cutoff wall depth was 20m, the dam foundation seepage was 0.143m³. 3 / s; When the depth of the cutoff wall is 25m, the seepage rate of the dam foundation is 0.128m. 3 / s; Both results are less than the 0.2m required by the original design report. 3 / s.

[0153] Permeability gradient calculation: When calculating the permeability gradient, depths of the cutoff wall were taken as 20m and 25m. The permeability gradients of the cutoff wall, the strata below the cutoff wall, and the strata downstream of the cutoff wall were calculated respectively. The permeability gradients of the strata below the cutoff wall and the surface of the strata downstream of the cutoff wall were both greater than the standard requirements. Only the permeability gradient of the cutoff wall itself was less than the standard requirements. When the depth of the cutoff wall increased to 60m, extending into the underground rock strata, the permeability gradients of the strata below the cutoff wall and the surface of the strata downstream of the cutoff wall were still greater than the standard requirements. Furthermore, cutoff walls with a depth greater than 60m are unreasonable, and the site conditions are not suitable for construction.

[0154] Comprehensive analysis determined that, based on the relationship between seepage gradient and seepage path, the seepage gradient can be reduced by extending the seepage path. According to the downstream abutment and slope protection length of the dam, when the depth of the anti-seepage wall is 20m, the seepage gradients at the downstream abutment and slope end of the dam are 0.149 and 0.116, respectively, both less than the design requirements.

[0155] Through comprehensive comparative analysis and research, combined with the results of jet grouting experiments and geological condition analysis experiments, it was finally determined that it is feasible to use the high-pressure jet grouting method to construct the anti-seepage wall of the dam foundation under the adverse geological conditions of the Shangbo Dikexi Dam. Accordingly, the anti-seepage wall consists of two rows of high-pressure jet grouting piles with a pile depth of 20m, a pile spacing of 60cm, a pile diameter of 75cm, and a maximum wall width of 135cm (see Figure 4).

[0156] The original design placed the cutoff wall at the lower part of the gravity dam. Construction of the cutoff wall had to be completed before the gravity dam could begin. The construction progress of the cutoff wall directly affected the construction of the gravity dam and related projects. Moreover, the cutoff wall was an underground concealed project, the geological conditions of the dam site were complex, and various factors that arose during construction were uncontrollable, making it impossible to guarantee the progress.

[0157] Based on a comparative analysis of construction progress and quality impact, the original No. 1 dam section concrete removal required a relatively long time. After design optimization, the original No. 1 dam section was retained, and a new No. 1 dam section was built, which intersected and connected with the No. 2 dam section at a 35° angle. The scour area of ​​the original No. 1 dam section foundation was backfilled with concrete and used as an access road to the dam. The anti-seepage wall of the new dam section was connected to the original No. 2 dam section (see Figure 1).

[0158] The scour and hollowed-out area at the bottom of the original No. 1 dam section was backfilled and compacted with plain concrete. The newly built No. 1 dam section is 10m higher than the original dam section and is set in a stepped manner. The top of the dam is level with the road on the right bank. The top of the original dam section is used as the road to the top of the dam.

[0159] Meanwhile, the axis of the anti-seepage wall was moved to the upstream side of the dam body (see Figure 2). The anti-seepage wall was connected to the dam body with a cap concrete, and two PVC waterstops were installed at the connection between the cap concrete and the dam body. See Figure 3. The anti-seepage wall was moved to the upstream side of the dam body. After the dam body was constructed to a certain height, the construction of the anti-seepage wall began. The dam body and the anti-seepage wall were constructed simultaneously without interfering with each other, which accelerated the construction progress.

[0160] Due to site constraints, the right bank slope anti-seepage wall was constructed in stages according to different walkway elevations. The construction sequence was as follows: filling the construction platform of this stage → high-pressure spraying wall construction → quality inspection and approval → excavation of the construction platform → removal of pile heads → pouring of platform caps → backfilling of this stage → high-pressure spraying wall construction of the next stage.

[0161] During each stage of the cap concrete construction, the construction platform had to be excavated and backfilled, and the repeated backfilling and excavation took up construction time. After discussions and research with design and consulting engineers, it was decided to pour the cap of the cutoff wall first, and then construct the high-pressure jet grouting cutoff wall. This saved time on the excavation of the construction platform and the secondary backfilling and compaction, thus saving construction costs and time, while also optimizing the process of chiseling off the pile heads.

[0162] Rotary jet drilling:

[0163] Drilling was performed using the casing method, with the bottom elevation of the borehole at least 30cm lower than the design elevation. After the borehole depth met the design requirements, a brittle PVC pipe, capable of being crushed by the jet grouting pressure, was lowered to form the borehole wall, and then the casing was pulled out. During the drilling process, the thickness of different strata, the depth of boulders or boulders, and their approximate particle size were recorded in detail to provide a reference for adjusting the jet grouting parameters during the jet grouting construction.

[0164] Special case control: Design corresponding handling methods for special cases.

[0165] (1) Water inrush treatment;

[0166] At a depth of approximately 15-17m underground in some areas of the EL1413.00 platform, there is a highly permeable layer with a thickness of 1-1.5m. After the completion of drilling in some boreholes, water gushing occurred, and sand return occurred inside the casing. To prevent the water gushing from washing away the fine particles in the borehole and forming cavities in the formation, gravel and coarse sand were backfilled on the outside of the PVC casing while the casing was being pulled out, and the borehole opening was sealed with mortar.

[0167] (2) Permeable layer treatment;

[0168] During the jet grouting process, based on the drilling records, when the jet grouting reaches the highly permeable layer, the lifting speed is reduced, the density of the grout and the amount of grout fed are increased, and 2% water glass is added to the cement grout.

[0169] Adding water glass shortens the setting time of cement grout, allowing it to solidify quickly and preventing it from being washed away by groundwater.

[0170] (3) Treatment of grout return and leakage;

[0171] When large voids in the formation cause no grout return or severe grout leakage, the main measures are: stop raising the jet grouting pipe, reduce the rotation speed, reduce the water pressure, and increase the grout density or grout feed rate; or, add water glass to the grout and jet grout in situ.

[0172] When the amount of grout returning from the orifice is small, reduce the rotation and lifting speed, decrease the water pressure, and increase the grout density; or, add water glass to the grout to shorten the consolidation time, allowing the grout to solidify within a certain soil layer range. Once the grout return is normal, resume the jet grouting process.

[0173] (4) Handling of boulders and rocks;

[0174] According to the drilling records, when the jet grout reaches the layer of boulders, boulders, or pebbles, the rotation speed is increased by 50cm above and below while the lifting speed is slowed down. This ensures that the boulders, boulders, or pebbles are fully covered with cement grout, thus making the pile body continuous and intact.

[0175] (5) Treatment of the connection between the wall and the original anti-seepage wall;

[0176] At the junction of the high-pressure jet grouting anti-seepage wall and the original concrete diaphragm wall anti-seepage wall, an additional row of piles is added, that is, three rows of jet grouting piles are set up to ensure that the jet grouting piles are tightly connected with the original concrete anti-seepage wall.

[0177] Meanwhile, alternatively, during the jet grouting construction process, measures such as increasing water pressure and rotation speed, reducing lifting speed, and increasing grout concentration can be adopted to increase the diameter of the pile body, so as to further ensure that the jet grouting pile is tightly bonded to the original concrete anti-seepage wall.

[0178] Application status.

[0179] The high-pressure jet grouting cutoff wall construction project utilized one set of equipment. Drilling commenced on March 6, 2019, and jet grouting was completed on July 29, 2019. A total of 275 holes were drilled, with a total jet grouting length of 5923.7m and a wall length of 78.08m. Construction was divided into four phases based on platform elevation. The actual construction of the high-pressure jet grouting cutoff wall took 109 days, averaging 3 piles per day, and 2 piles per day in areas with particularly poor geological conditions.

[0180] Twenty-eight days after the completion of the high-pressure jet grouting anti-seepage wall construction, core samples and water pressure tests were conducted on nine piles. The core samples showed continuous pile bodies, dense cement grout, and tight connections between the cement grout and the stones within the sample. Twenty-three core samples taken from different depths of the nine piles were tested for compressive strength; the minimum strength was 7.3 MPa, all exceeding the contractual and design requirement of 6 MPa. Each inspection well underwent a static head water pressure test for seepage prevention. The water pressure test was conducted along the entire section and in sections; the maximum permeability was 1.21 Lu, less than the contractual and design requirement of 2 Lu.

[0181] Four groundwater level observation wells were installed downstream of the new No. 1 gravity dam, with two piezometers installed. These wells are primarily used to monitor groundwater level fluctuations during power station operation and also to monitor leakage in the dam foundation and abutments. Continuous observations were conducted during the initial stage, before impoundment, during dam impoundment, and after impoundment. The water level in the wells tended to stabilize as the water level in front of the dam rose. The maximum water level rise in the wells downstream of the dam was 3.18m. The maximum water level elevation in each well during impoundment was less than the design requirements. Piezometer readings remained unchanged during and after dam impoundment. No water flow was observed from the drainage wells installed on the downstream slope of the right bank and the drainage wells on the abutment downstream of the dam.

[0182] Through core sampling for compressive strength testing and water pressure testing, as well as water level observation holes and seepage detection behind the dam, all indicators were better than the design indicators and met the contract requirements. The high-pressure jet grouting construction method for the dam's anti-seepage wall was successfully applied and is worth learning from.

[0183] Application prospects.

[0184] A comparative analysis of the equipment and construction period used during the initial construction of the underground concrete cutoff wall in this project demonstrates that the high-pressure jet grouting method for cutoff walls offers advantages such as rapid construction progress, lack of site constraints, minimal equipment requirements, and flexible equipment mobility. Furthermore, it is feasible to use high-pressure jet grouting walls as the seepage control structure for dam foundations under complex geological conditions, including large-diameter gravel, pebbles, large boulders, and highly permeable layers. With strengthened construction process control, its seepage control performance can fully meet design specifications. The research and application conducted in this project suggest that the high-pressure jet grouting method has significant potential for wider application in dam cutoff walls under complex geological conditions.

[0185] In this invention, the feasibility of the anti-seepage wall method is determined through comparative analysis of dam anti-seepage technologies, supplementary geological exploration, and high-pressure jet grouting pile experiments. When the seepage gradient does not meet the requirements, the seepage path is extended to reduce the seepage gradient. The location of the anti-seepage wall is optimized by moving it to the upstream side of the dam and connecting it to the dam body with a cap concrete and installing a waterstop. Construction of the anti-seepage wall begins after the dam body has reached a certain height. The dam body and anti-seepage wall are constructed simultaneously without interference, accelerating the construction progress. Simultaneously, the original dam section is preserved, backfilled with concrete, and used as an access road to the dam. The construction process involves first pouring the cap of the anti-seepage wall, followed by the construction of the high-pressure jet grouting anti-seepage wall. This saves time on platform excavation and secondary backfilling, reducing construction costs and time. The optimized pile head removal process further reduces costs and time. During jet grouting drilling, the thickness of different strata, the depth of boulders or boulders, and their approximate particle size are recorded to provide a reference for adjusting jet grouting parameters. Corresponding treatment methods are designed for special situations to ensure construction effectiveness. An additional row of piles is added at the junction of the high-pressure jet grouting anti-seepage wall and the original anti-seepage wall to ensure a tight bond between the jet grouting piles and the original concrete anti-seepage wall.

[0186] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0187] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0188] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A high-pressure jet grouting construction method for a concrete dam cutoff wall, characterized in that, Includes the following steps: S1. Determine the method for constructing the anti-seepage wall; S2. Determine the location of the anti-seepage wall; S3. Optimize the anti-seepage wall process; S4. Treatment of the connection between the seepage prevention wall and the wall.

2. The high-pressure jet grouting construction method for the concrete dam anti-seepage wall according to claim 1, characterized in that, It is applicable to construction environments where the dam body has complex geological conditions, the basic geological conditions are deep overburden layers containing boulders, boulders, sand and gravel, and the dam base strata are highly to moderately permeable.

3. The high-pressure jet grouting construction method for the concrete dam anti-seepage wall according to claim 1, characterized in that, In step S1: By conducting high-pressure jet grouting experiments and quality inspections under similar geological conditions within the engineering area, the feasibility of using high-pressure jet grouting walls as anti-seepage walls for dam foundations was determined.

4. The high-pressure jet grouting construction method for the concrete dam anti-seepage wall according to claim 3, characterized in that, Step S1 also includes: determining the parameters of the anti-seepage wall; Specifically, this includes: leakage calculation, seepage gradient calculation, and comprehensive analysis to determine the result; If the seepage gradient is greater than the specification requirement, and it is still greater than the specification requirement after increasing the depth of the anti-seepage wall, then the seepage gradient can be reduced by extending the seepage path.

5. The high-pressure jet grouting construction method for the anti-seepage wall of a concrete dam according to claim 1, characterized in that, In step S2: The dam body and the cutoff wall are designed to be constructed simultaneously without interference; the axis of the cutoff wall is moved to the upstream side of the dam body. The anti-seepage wall is connected to the dam body with a cap concrete, and a PVC waterstop is installed at the connection between the cap and the dam body.

6. The high-pressure jet grouting construction method for the anti-seepage wall of a concrete dam according to claim 5, characterized in that, In step S3: First, the cap of the anti-seepage wall is poured, and then the high-pressure jet grouting anti-seepage wall is constructed.

7. The high-pressure jet grouting construction method for the anti-seepage wall of a concrete dam according to claim 1, characterized in that, In step S3: When performing jet grouting drilling, the casing method is used; the bottom elevation of the hole is more than 30cm lower than the design elevation. After the hole depth meets the design drawing requirements, a brittle PVC pipe wall is inserted, which can be crushed by the pressure of the rotary jet, and then the root pipe is pulled out.

8. The high-pressure jet grouting construction method for the anti-seepage wall of a concrete dam according to claim 1, characterized in that, Step S3 also includes: special case control; (1) Water inrush treatment; To prevent water inrush from washing away fine particles in the borehole and forming cavities, pebbles and coarse sand were backfilled on the outside of the PVC protective pipe while the root pipe was being pulled out, and the borehole opening was sealed with mortar. (2) Permeable layer treatment; When spraying to a highly permeable layer, reduce the lifting speed, increase the density and feed rate of the slurry, and add 1% to 3% water glass to the cement slurry; (3) Treatment of grout return and leakage; When large voids in the formation cause no grout return or severe grout leakage, the main measures are: stop raising the jet grouting pipe, reduce the rotation speed, reduce the water pressure, and increase the grout density or grout feed rate; or, add water glass to the grout and jet grout in situ. When the amount of grout returning from the orifice is small, reduce the rotation and lifting speed, decrease the water pressure, and increase the grout density; or, add water glass to the grout to shorten the consolidation time, so that the grout solidifies within a certain soil layer range, and start lifting and jet grouting again when the grout return is normal. (4) Handling of boulders and rocks; When spraying the boulders, boulders, or pebbles, increase the rotation speed by 50cm above and below and slow down the lifting speed to fully coat the boulders, boulders, or pebbles with cement grout, so that the pile body is continuous and intact.

9. The high-pressure jet grouting construction method for the anti-seepage wall of a concrete dam according to claim 1, characterized in that, In step S4: At the junction of the high-pressure jet grouting cutoff wall and the original concrete diaphragm wall cutoff wall, an additional row of piles is added, making it a three-row jet grouting pile system; this ensures that the jet grouting piles are tightly integrated with the original cutoff wall.

10. The high-pressure jet grouting construction method for the concrete dam anti-seepage wall according to any one of claims 1 to 4, characterized in that, In step S1: The seepage barrier wall consists of two rows of high-pressure jet grouting piles; The pile depth is 20-25m, the spacing between piles is 60cm, and the pile diameter is 75cm.

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