A method for improving the weight design and construction of earth-rock dams on deep overburden foundations to reduce liquefaction-induced deformation

Abstract

The application belongs to the technical field of earth-rock dam construction, and provides a weight pressing design and construction method for improving the liquefaction deformation of the earth-rock dam foundation on deep overburden, which fills and compacts the dam weight pressing, and fills the overall section from bottom to top into a ladder shape, each layer of the ladder cross section is trapezoidal, the weight pressing design is n levels, wherein n is greater than or equal to 2, the overall section is filled from bottom to top into a multi-level ladder shape with gradually decreasing length and gradually slowing down slope, that is, only the free surface slope is ensured to be stepped. In the case of the same total weight pressing material, compared with the traditional one-level weight pressing design and construction scheme, the multi-level ladder weight pressing can increase the confining pressure of the overburden under the dam slope and reduce the initial shear stress before the earthquake, can reduce the pore pressure ratio of the overburden during the earthquake, reduce the liquefaction deformation, and improve the seismic safety of the earth-rock dam on deep overburden. At the same time, the increase of the multi-level ladder weight pressing height can increase the protection effect on the downstream dam slope and improve the anti-sliding stability of the downstream dam slope.

Description

Technical Field

[0001] This invention belongs to the field of earth-rock dam construction technology, and relates to a counterweight design and construction method for improving the liquefaction deformation of earth-rock dam foundations on thick overburden layers. Background Technology

[0002] my country's plans prioritize hydropower base construction and development as key tasks. With the continuous advancement of hydropower engineering construction in my country, the geological conditions at dam sites are becoming increasingly complex. The dual challenges of high seismic intensity and overburden foundations have become unavoidable issues in dam construction. Earth-rock dams, with their advantages of adapting to complex geological conditions and strong seismic performance, are the preferred dam type for deep overburden strata. For example, earth-rock dam projects such as the Yele and Altash dams, built on deep overburden strata, have a seismic intensity of no less than 8 degrees. The seismic safety of earth-rock dams on overburden strata is a key concern for the dam engineering community.

[0003] Seismic liquefaction deformation of the overburden foundation is a critical issue jeopardizing the seismic safety of earth-rock dams. Applying weights at the upstream and downstream toes is a relatively effective and economical method, and a commonly used engineering reinforcement measure by designers. However, when overburden liquefaction is a concern, determining the appropriate weight size to ensure both project safety and cost savings is a pressing issue in earth-rock dam design. Insufficient weights will not generate sufficient anti-sliding force and compressive stress on the sand layer, while excessive weights will increase project investment, construction time, and construction difficulty. Therefore, optimizing the weight design is crucial for the seismic safety of earth-rock dams with deep overburden. Summary of the Invention

[0004] To address the issue of ballast pressure required for traditional earth-rock dams with deep overburden layers due to the presence of liquefiable sand layers, this invention provides a ballast pressure design and construction method to mitigate liquefaction deformation in earth-rock dams with deep overburden layers. It alters the cross-sectional shape of traditional ballast pressure by constructing a multi-stage stepped ballast with gradually decreasing length and gentler slope from bottom to top. Compared to the traditional single-stage ballast design, this invention, while maintaining the same amount of soil, increases the confining pressure of the overburden layer at the lower part of the dam slope and reduces the initial pre-earthquake shear stress. It also lowers the pore pressure ratio of the overburden layer during earthquakes, reducing liquefaction deformation. In other words, this invention not only improves the liquefaction deformation problem but also enhances the anti-sliding stability of the downstream dam slope, thereby improving the seismic safety of earth-rock dams with deep overburden layers.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for designing and constructing a counterweight system to improve liquefaction deformation of earth-rock dam foundations on thick overburden layers. The method involves compacting the dam with counterweights, with the overall cross-section constructed in a stepped shape from bottom to top. Each step has a trapezoidal cross-section, and the counterweight is designed in n levels, where n≥2. The overall cross-section is constructed in a multi-level stepped shape with gradually decreasing length and gentler slope from bottom to top. In other words, it is only necessary to ensure that the free surface (one side of the counterweight is connected to the dam body, and the other side is a free surface) is sloped in a stepped shape.

[0007] The specific implementation process includes the following steps:

[0008] 1) Clean the foundation and construct a concrete anti-seepage wall.

[0009] 2) Dam body filling: Lay the dam rockfill material 4, transition material and cushion material layer by layer, and carry out vibratory compaction construction according to the compaction degree or porosity required by the design until the dam crest elevation is reached.

[0010] 3) Implement counterweight filling. Specifically: lay the counterweight material layer by layer, and compact it using vibratory rollers according to the design requirements for compaction degree or porosity. It is recommended that the counterweight filling standard be lower than the dam body filling standard; that is, the counterweight filling requirements can be lower than the dam body requirements, for example, the compaction degree can be lower or the porosity can be higher, to save investment. Specifically:

[0011] The material is laid layer by layer from bottom to top, with the highest surface not exceeding the dam crest and the lowest surface being the foundation surface or a thick overburden layer 6. The first phase of the ballast 1 is filled, with a height of h1 and a length of l1. One end connected to the dam body 4 is sloped, and the other end is sloped. The slope of the first phase of the ballast is i1, and i1 < i0, where i0 is the slope of the downstream dam slope.

[0012] The second phase of the ballast 2 is constructed using the same construction method as the first phase of the ballast 1. The ballast height is h2, the length is l2, and the slope of the second phase of the ballast is i2, where l2 < l1 and i2 ≤ i1 < i0.

[0013] The construction of the three-stage ballast system continues with ballast of 3 or higher until the entire stepped ballast system is completed.

[0014] Furthermore, there are no requirements regarding the order of construction of the dam body and the counterweight. Depending on the cut-fill balance requirements of the project, the dam body and the counterweight can be constructed simultaneously, the dam body can be constructed first and then the counterweight, or the counterweight can be constructed first and then the dam body.

[0015] Furthermore, for core-wall rockfill dams, both upstream and downstream counterweights are applicable; for face-panel rockfill dams, downstream counterweights are applicable.

[0016] The innovativeness of this invention is as follows: The main design feature of this invention is the stepped cross-section of the ballast. This invention uses stepped ballast to improve the liquefaction deformation problem of the earth-rock dam foundation on the thick overburden layer 6. Under the condition that the total ballast material remains unchanged, the shape of the ballast cross-section is changed to improve the degree of improvement on the liquefaction deformation problem of the foundation, so that the ballast can further play its reinforcing role.

[0017] The beneficial effects of this invention are as follows:

[0018] Compared with existing technologies, this invention eliminates the traditional monolithic structure of the counterweight and proposes a stepped counterweight cross-section. Compared with the traditional single-stage counterweight design, this invention mainly increases the confining pressure of the lower overburden layer of the dam slope and reduces the initial pre-earthquake shear stress while maintaining the same amount of soil. It also reduces the pore pressure ratio of the overburden layer during earthquakes, reduces liquefaction deformation, and improves the seismic safety of earth-rock dams with deep overburden layers. At the same time, the increased height of the multi-stage stepped counterweight increases the protection of the downstream dam slope and improves the anti-sliding stability of the downstream dam slope. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a stepped counterweight design scheme for improving the liquefaction deformation of earth-rock dam foundations on thick overburden layers according to the present invention.

[0020] The diagram labels are as follows: 1. Phase I counterweight; 2. Phase II counterweight; 3. Phase III counterweight; 4. Dam body rockfill; 5. Concrete cutoff wall; 6. Thick overburden layer; l1. Phase I counterweight length; l2. Phase II counterweight length; l3. Phase III counterweight length; h1. Phase I counterweight height; h2. Phase II counterweight height; h3. Phase III counterweight height; i1. Phase I counterweight slope; i2. Phase II counterweight slope; i3. Phase III counterweight slope; i0. Downstream slope of the dam.

[0021] Figure 2 shows the average principal stress cloud map of the overburden layer at the end of a traditional single-layered pressure earthquake. Figure 2a ) and the average principal stress cloud map of the overburden layer at the end of the earthquake in the case of this invention ( Figure 2b (), the unit is MPa;

[0022] Figure 3 shows the pre-earthquake shear stress cloud map of a traditional single-layered heavy overburden layer. Figure 3a ) and the pre-earthquake shear stress cloud map of the overburden layer at the end of the earthquake in this invention case ( Figure 3b (), the unit is MPa;

[0023] Figure 4 shows the horizontal displacement cloud map of a traditional single-layer counterweight dam body. Figure 4a ) and the horizontal displacement cloud map of the dam body in the case of this invention ( Figure 4b ), the unit is m;

[0024] Figure 5 shows the vertical displacement cloud map of a traditional single-layer counterweight dam body. Figure 5a ) and the vertical displacement cloud map of the dam body in the case of this invention ( Figure 5b (), the unit is m;

[0025] Figure 6 shows the porosity cloud map of a traditional single-layer ballast capping layer. Figure 6a ) and the porosity cloud map of the capping layer in the present invention ( Figure 6b ). Detailed Implementation

[0026] The present invention will be further described below with reference to specific implementation examples.

[0027] The following are some engineering examples:

[0028] Taking a concrete-faced rockfill dam on a deep overburden layer as an engineering case study, the dam is 130m high, 15m wide at the crest, with an upstream slope of 1:1.6 and a downstream slope of 1:1.7. The concrete face is 0.2m thick, and the deep overburden layer 6 is 400m deep. The dam foundation seepage prevention uses a suspended concrete cutoff wall 5, 130m deep and 1.3m thick. For a traditional single-stage counterweight design, the downstream counterweight is 600m long, 80m high, and has a slope ratio of 1:2.4. In this embodiment, the counterweight uses a three-tiered stepped design. The first-stage counterweight 1 is 600m long, 50m high, and has a slope ratio of 1:2.4; the second-stage counterweight 2 is 300m long, 40m high, and has a slope ratio of 1:2.6; and the third-stage counterweight 3 is 150m long, 40m high, and has a slope ratio of 1:2.8. The stepped counterweight cross-sectional area is the same as that of the traditional single-stage counterweight (i.e., by ensuring equal counterweight areas, the material used for counterweights in both schemes is consistent), both being 48,000 m². 2 The left and right sides ensured the same filling volume. The specific construction process is as follows:

[0029] 1) Conduct foundation cleaning, establish a construction site with the central axis of the concrete cutoff wall 5 as the center, and excavate the deep overburden 6 into a trench with a depth of 130m and a thickness of 1.3m along the axis of the concrete cutoff wall 5 by hydraulic milling, and use mud slurry to protect the wall. Then, pour ordinary concrete by the vertical tremie method to complete the main construction of the concrete cutoff wall 5.

[0030] 2) Fill the dam rockfill, transition material and cushion material layer by layer, and compact them with vibration rollers until the top of the dam is reached.

[0031] 3) While the dam body is being filled, counterweight filling is also being carried out. The filling is carried out in layers and a stepped counterweight is adopted. The counterweight construction is carried out in three stages from bottom to top. The first stage counterweight 1 is 600m long and 50m high with a slope ratio of 1:2.4; the second stage counterweight 2 is 300m long and 40m high with a slope ratio of 1:2.6; the third stage counterweight 3 is 150m long and 40m high with a slope ratio of 1:2.8.

[0032] The method proposed in this invention was used to improve the weight-bearing design of the project, and a numerical model analysis of an earthquake encountered during operation was conducted. The results are analyzed as follows:

[0033] The calculated values ​​of dam crest settlement and horizontal displacement at the end of the earthquake, pore pressure ratio of the overburden layer below the dam body, and horizontal displacement of the cutoff wall are shown in Table 1, and the distribution of the results is shown in Figures 2 to 6. The calculation results show that the average principal stress of the overburden layer below the traditional single-stage ballast dam body before the earthquake was approximately 2.2 MPa, while the average principal stress of the overburden layer below the stepped ballast dam body before the earthquake was approximately 2.6 MPa, an increase of about 18%. Meanwhile, the pre-earthquake shear stress of the overburden layer at the downstream toe of the traditional single-stage ballast dam body was 0.37 MPa, while the pre-earthquake shear stress of the overburden layer at the downstream toe of the stepped ballast dam body was 0.31 MPa, a decrease of about 19%. After the earthquake, the pore pressure ratio of the overburden layer below the axis of a traditional single-stage ballast dam was approximately 0.33, while that of a stepped ballast dam was approximately 0.23, a reduction of about 30%. The horizontal and vertical displacements of the crest of a traditional single-stage ballast dam were 0.272m and 0.512m, respectively, while those of a stepped ballast dam were 0.192m and 0.435m, respectively, a reduction of about 30% and 15%. The horizontal displacement of the top of the traditional single-stage ballast cutoff wall was 0.081m, while that of the stepped ballast cutoff wall was 0.055m, a reduction of about 32%.

[0034] In summary, the design proposed in this invention effectively reduces the liquefaction level of the overburden, improves the seismic deformation of the dam, and enhances the seismic safety of the dam.

[0035] Table 1. Impact of counterweight design schemes on dam seismic response

[0036]

[0037] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for designing and constructing a counterweight to improve liquefaction deformation of earth-rock dam foundations on thick overburden layers, characterized in that, The aforementioned counterweight design and construction method involves compacting and filling the dam with counterweight, forming a stepped cross-section from bottom to top, with each step having a trapezoidal cross-section. The overall counterweight is constructed from bottom to top in a multi-level stepped shape with gradually decreasing length and gentler slope. This includes the following steps: Step 1) Clean the foundation and construct a concrete waterproof wall; Step 2) Construct the dam body; lay the dam rockfill, transition material and cushion material layer by layer, and perform vibratory compaction according to the compaction degree or porosity required by the design until the dam crest elevation is reached. Step 3) Perform compaction filling, with n levels of compaction designed, and the material is laid layer by layer from bottom to top. Specifically: The first phase of compaction filling is carried out, with a height of [missing information]. h 1. Length is l 1. One end connected to the dam body is treated with a slope joint, and the other end is treated with a sloping design. The initial ballast slope is [missing information]. i 1, and i1 < i0, where i0 is the slope of the downstream slope of the dam; The second phase of counterweight filling will be carried out using the same construction method as the first phase, with a counterweight height of [missing information]. h 2. Length is l 2. The slope of the second phase of ballast is... i 2, of which l 2< l 1. i2≤i1<i0; The same principle applies to the third and subsequent stages of ballast, until the entire stepped ballast system is completed.

2. The weighting design and construction method for improving liquefaction deformation of earth-rock dam foundations on thick overburden layers, as described in claim 1, is characterized in that... In step 3, n ≥ 2.

3. A method for designing and constructing a counterweight to improve liquefaction deformation of an earth-rock dam foundation on a thick overburden layer, as described in claim 1, is characterized in that... In step 3, the ballast material is laid layer by layer, and vibratory compaction is carried out according to the compaction degree or porosity required by the design. It is recommended that the filling standard of the ballast material be lower than that of the dam body.

4. A method for designing and constructing a counterweight to improve liquefaction deformation of an earth-rock dam foundation on a thick overburden layer, as described in claim 1, is characterized in that... The construction sequence of the dam body and the counterweight is adjustable. Depending on the cut-fill balance requirements of the project, the dam body and the counterweight can be constructed simultaneously, the dam body can be constructed first and then the counterweight, or the counterweight can be constructed first and then the dam body.

5. A method for designing and constructing a counterweight to improve liquefaction deformation of an earth-rock dam foundation on a thick overburden layer, as described in claim 1, is characterized in that... For core-wall rockfill dams, both upstream and downstream counterweighting are applicable; for face-wall rockfill dams, downstream counterweighting is applicable.

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