Ballast design and construction method for improving liquefaction deformation of earth and rockfill dam foundation on deep and thick covering layer

Abstract

The invention belongs to the technical field of earth and rockfill dam construction, and provides a ballasting design and construction method for improving liquefaction deformation of an earth and rockfill dam foundation on a deep and thick covering layer, the method comprises the steps that dam ballasting is rolled and filled, the overall section is filled into a step shape from bottom to top, the cross section of each step is trapezoidal, the ballasting is designed into n stages, n is larger than or equal to 2, and n is larger than or equal to 2; the whole section is filled into a multi-step shape with the length gradually reduced and the gradient gradually reduced from bottom to top, namely, it is only needed to ensure that the free face is sloped into a step shape. Under the condition that the total material of the ballasting is not changed, compared with a traditional one-stage ballasting design construction scheme, the multi-stage stepped ballasting can increase the confining pressure of the covering layer on the lower portion of the dam slope and reduce the initial shear stress before an earthquake, the pore pressure ratio of the covering layer in the earthquake process can be reduced, liquefaction deformation is reduced, and the anti-seismic safety of the earth and rockfill dam on the deep covering layer is improved; and meanwhile, the protection effect on the downstream dam slope can be improved by increasing the height of the multi-stage stepped ballasting, and the anti-sliding stability of the downstream dam slope of the dam is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of earth-rock dam construction, and relates to a weight pressing design and construction method for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer. Background Art

[0002] China plans to list the construction of hydropower bases and hydropower development as key tasks for tackling key problems. With the continuous advancement of China's hydropower project construction, the geological conditions of dam sites faced by high dam construction are becoming increasingly complex. The dual challenges of high seismic intensity and overburden foundation have become unavoidable problems in dam construction. Earth-rock dams have the advantages of adapting to complex geological conditions and strong seismic resistance, and are the preferred dam type on deep overburden layers. For example, the fortification intensities of earth-rock dam projects on deep overburden layers such as Yele and Altashi are not less than 8 degrees. The seismic safety of earth-rock dams on overburden layers is a key concern in the dam engineering field.

[0003] The seismic liquefaction deformation of the overburden foundation is a key problem endangering the seismic safety of earth-rock dams. Conducting weight pressing treatment at the upstream and downstream dam toes is a relatively effective and economical method, and is also a commonly used engineering reinforcement measure by designers. However, in the case of overburden liquefaction problems, how to determine the size of the weight pressing, which can not only ensure the safety of the project but also achieve the purpose of saving investment, is an urgent problem to be solved in the design of earth-rock dams. If the size of the weight pressing is too small, it cannot generate sufficient anti-sliding force and compressive stress on the sand layer. If the size of the weight pressing is too large, it will not only increase the project investment but also increase the project duration and construction difficulty. Therefore, optimizing the design of the weight pressing is crucial for the seismic safety of earth-rock dams on deep overburden layers. Summary of the Invention

[0004] Aiming at the problem that traditional earth-rock dams on deep overburden layers need to set weight pressing to improve foundation liquefaction due to the existence of liquefiable sand layers in the overburden layer, the invention provides a weight pressing design and construction method for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer, which changes the cross-sectional shape of the traditional weight pressing, and fills the dam weight from bottom to top into a multi-stage stepped shape with a gradually decreasing length and a gradually gentler slope. Compared with the traditional single-stage weight pressing design scheme, the invention can increase the confining pressure of the overburden layer at the lower part of the dam slope and reduce the initial shear stress before the earthquake while keeping the amount of soil used unchanged, can reduce the pore pressure ratio of the overburden layer during the earthquake, and reduce the liquefaction deformation. That is, the invention can not only improve the problem of foundation liquefaction deformation but also improve the anti-sliding stability of the downstream dam slope of the dam, and improve the seismic safety of earth-rock dams on deep overburden layers.

[0005] In order to achieve the above object, the technical solution adopted by the invention is as follows:

[0006] A method for designing and constructing a surcharge to improve the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer. In this method, the surcharge of the dam is compacted and filled. The overall cross-section is filled in a stepped shape from bottom to top. The cross-section of each stepped layer is trapezoidal. The surcharge is designed into n levels, where n≥2. The overall cross-section is filled in a multi-stepped shape with gradually decreasing length and gradually gentle slope from bottom to top. That is, it is only necessary to ensure that the slope of the free surface (one side of the surcharge is connected to the dam body, and the other side is the free surface) is in a stepped shape.

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

[0008] 1) Clean the foundation and construct a concrete cut-off wall.

[0009] 2) Carry out dam filling: Spread the dam rockfill 4, transition material, and cushion material layer by layer, and perform vibrating compaction construction according to the specified compaction degree or porosity until the dam crest elevation is reached.

[0010] 3) Carry out surcharge filling. Specifically: Spread the surcharge layer by layer and perform vibrating compaction construction according to the specified compaction degree or porosity. The filling standard of the surcharge is recommended to be lower than that of the dam filling, that is, the filling requirements of the surcharge can be lower than those of the dam. For example, the compaction degree can be lower than that of the dam or the porosity can be larger to save investment. Specifically:

[0011] Carry out surcharge layer-by-layer spreading from bottom to top. The highest surface does not exceed the dam crest, and the lowest surface is the foundation surface or the deep overburden layer 6. First, carry out the filling of the first-stage surcharge 1, with a height of h1 and a length of l1. The end connected to the dam body 4 is slope-connected, and the other end is sloped. The slope of the first-stage surcharge is i1, and i1<i0, where i0 is the slope of the downstream dam slope of the dam;

[0012] Use the same construction method as the first-stage surcharge 1 to carry out the filling of the second-stage surcharge 2, with a surcharge height of h2 and a length of l2. The slope of the second-stage surcharge is i2, where l2<l1 and i2≤i1<i0;

[0013] For the third-stage surcharge 3 and above surcharges, do the same by analogy until the construction of the entire stepped surcharge is completed.

[0014] Furthermore, there is no requirement for the construction sequence of the dam body and the surcharge. According to the requirements of the excavation and filling balance of the project, the dam body and the surcharge can be constructed simultaneously, or the dam body can be constructed first and then the surcharge, or the surcharge can be constructed first and then the dam body.

[0015] Furthermore, for a core-wall rockfill dam, it is applicable to both upstream and downstream surcharges; for a concrete-faced rockfill dam, it is applicable to the downstream surcharge.

[0016] The innovative analysis of the present invention is as follows: The main design point of the present invention is the stepped cross-section of the surcharge. The present invention uses a stepped surcharge to improve the liquefaction deformation problem of the foundation of an earth-rock dam on a thick overburden layer. Without changing the total amount of surcharge material, the cross-sectional shape of the surcharge is changed to improve the degree of improvement of the foundation liquefaction deformation problem, enabling the surcharge to further exert its reinforcement effect.

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

[0018] Compared with the prior art, the present invention cancels the integral structural shape of the traditional surcharge and proposes a stepped surcharge section. Compared with the design scheme of the traditional single-stage surcharge, it is mainly reflected in that, without changing the amount of soil material used, it can increase the confining pressure of the overburden layer at the lower part of the dam slope and reduce the initial shear stress before the earthquake, can reduce the pore pressure ratio of the overburden layer during the earthquake, reduce the liquefaction deformation, and improve the seismic safety of the earth-rock dam on the thick overburden layer; at the same time, the increase in the height of the multi-stage stepped surcharge can increase the protection effect on the downstream dam slope and improve the anti-sliding stability of the downstream dam slope of the dam. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of a stepped surcharge design scheme for improving the liquefaction deformation of the foundation of an earth-rock dam on a thick overburden layer according to the present invention;

[0020] Reference numerals in the figure: 1 First-stage surcharge; 2 Second-stage surcharge; 3 Third-stage surcharge; 4 Dam body rockfill; 5 Concrete cut-off wall; 6 Thick overburden layer; l1 Length of the first-stage surcharge; l2 Length of the second-stage surcharge; l3 Length of the third-stage surcharge; h1 Height of the first-stage surcharge; h2 Height of the second-stage surcharge; h - Height of the third-stage surcharge; i1 Slope of the first-stage surcharge; i2 Slope of the second-stage surcharge; i3 Slope of the third-stage surcharge; i0 Slope of the downstream dam slope of the dam.

[0021] Figure 2 is the contour map of the average principal stress of the overburden layer at the end of the earthquake for the traditional single-layer surcharge ( Figure 2a ) and the contour map of the average principal stress of the overburden layer at the end of the earthquake for the case of the present invention ( Figure 2b ), with the unit of MPa;

[0022] Figure 3 is the contour map of the pre-earthquake shear stress of the overburden layer for the traditional single-layer surcharge ( Figure 3a ) and the contour map of the pre-earthquake shear stress of the overburden layer at the end of the earthquake for the case of the present invention ( Figure 3b ), with the unit of MPa;

[0023] Figure 4 is the contour map of the horizontal displacement of the dam body for the traditional single-layer surcharge ( Figure 4a ) and the contour map of the horizontal displacement of the dam body for the case of the present invention ( Figure 4b ), with the unit of m;

[0024] Figure 5 is the contour map of the vertical displacement of the dam body for the traditional single-layer surcharge ( Figure 5a ) and the contour map of the vertical displacement of the dam body for the case of the present invention (Figure 5b ), unit is m;

[0025] Figure 6 is a cloud diagram of the pore pressure ratio of a traditional heavy overburden layer ( Figure 6a ) and the pore pressure ratio cloud diagram of the overburden in the case of the present invention ( Figure 6b ). DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with specific implementation cases.

[0027] The engineering cases are as follows:

[0028] A concrete panel rockfill dam on a thick overburden is used as an engineering case. The dam is 130m high, the dam crest width is 15m, the upstream dam slope of the dam body is 1:1.6, the downstream dam slope is 1:1.7, the concrete panel thickness is 0.2m, and the deep overburden 6 is 400m deep. The dam foundation anti-seepage adopts a suspended concrete anti-seepage wall 5, which is 130m deep and 1.3m thick. For the traditional one-stage ballast design scheme, the downstream ballast is 600m long, 80m high, and the slope ratio is 1:2.4. The ballast of this embodiment adopts a three-layer stepped shape scheme, then the bottom first-stage ballast 1 is 600m long, 50m high, and the slope ratio is 1:2.4; the second-stage ballast 2 is 300m long, 40m high, and the slope ratio is 1:2.6; the third-stage ballast 3 is 150m long, 40m high, and the slope ratio is 1:2.8. The cross-sectional area of the stepped weight is the same as that of the traditional first-stage weight (i.e., the weight area is equal to ensure that the weight materials used in the two schemes are consistent), both of which are 48000m 2 The specific construction process is as follows:

[0029] 1) Clean the foundation and establish a construction site with the central axis of the concrete anti-seepage wall 5 as the center. Use hydraulic milling to excavate the thick covering layer 6 into a trench along the axis of the concrete anti-seepage wall 5 with a depth of 130m and a thickness of 1.3m. Use mud wall protection, and then pour ordinary concrete through the vertical lifting conduit method to complete the main construction of the concrete anti-seepage wall 5.

[0030] 2) Fill the dam rockfill material 4, transition material and cushion material layer by layer, and perform vibration rolling construction until the dam top.

[0031] 3) The dam body is filled while being compacted and rolled in layers. The stepped compaction is used, and three phases of compaction construction are carried out from bottom to top. The length of the first phase of compaction 1 at the bottom is 600m, the height h1 is 50m, and the slope ratio is 1:2.4; the length of the second phase of compaction 2 is 300m, the height h2 is 40m, and the slope ratio is 1:2.6; the length of the third phase of compaction 3 is l3, 150m, the height h3 is 40m, and the slope ratio is 1:2.8.

[0032] The improved design of weight pressing for this project was carried out by using the method proposed in the present invention, and the numerical simulation analysis of the earthquake encountered during the operation period was carried out. The result analysis is as follows:

[0033] The calculated values of the settlement and horizontal displacement of the dam crest, the pore pressure ratio of the underlying stratum of the dam body, and the horizontal displacement of the top of the cutoff wall at the end of the earthquake are shown in Table 1, and the result distributions are shown in Figures 2 to 6. It can be seen from the calculation results that the average principal stress of the underlying stratum of the traditional first-class weight-pressing dam body before the earthquake is about 2.2 MPa, and the average principal stress of the underlying stratum of the stepped weight-pressing dam body before the earthquake is about 2.6 MPa, increasing by about 18%; at the same time, the shear stress of the underlying stratum at the downstream toe of the traditional first-class weight-pressing dam before the earthquake is 0.37 MPa, and the shear stress of the underlying stratum at the downstream toe of the stepped weight-pressing dam before the earthquake is 0.31 MPa, decreasing by about 19%. After the earthquake, the pore pressure ratio of the underlying stratum under the axis of the traditional first-class weight-pressing dam is about 0.33, and the pore pressure ratio of the underlying stratum under the axis of the stepped weight-pressing dam is about 0.23, decreasing by about 30%; the horizontal and vertical displacements of the dam crest of the traditional first-class weight-pressing dam are 0.272 m and 0.512 m respectively, and the horizontal and vertical displacements of the dam crest of the stepped weight-pressing dam are 0.192 m and 0.435 m respectively, decreasing by about 30% and 15% respectively; the horizontal displacement of the top of the cutoff wall of the traditional first-class weight-pressing dam is 0.081 m, and the horizontal displacement of the top of the cutoff wall of the stepped weight-pressing dam is 0.055 m, decreasing by about 32%.

[0034] To sum up, the design proposed in the present invention effectively reduces the liquefaction degree of the underlying stratum, improves the seismic deformation of the dam, and enhances the seismic safety of the dam.

[0035] Table 1 The influence of the weight-pressing design scheme on the seismic response of the dam

[0036]

[0037] The above-described embodiments only represent the implementation manners of the present invention, but should not be construed as limiting the scope of the patent of the present invention. It should be noted that for those skilled in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A weight pressing design and construction method for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer, characterized in that The designed and constructed weight pressing method presses and compacts the dam by filling, with the overall cross-section filled in a stepped shape from bottom to top, and the cross-section of each stepped layer being trapezoidal; the overall weight pressing is filled in a multi-stepped shape with the length gradually decreasing and the slope gradually becoming gentler from bottom to top.

2. A method for designing and constructing surcharge for improving liquefaction deformation of earth-rock dam foundation on deep overburden layer according to claim 1, characterized in that The designed and constructed weight pressing method includes the following steps: Step 1) Clear the foundation and construct a concrete cut-off wall. Step 2) Conduct dam filling. Step 3) Conduct weight pressing filling. The weight pressing is designed into n levels, and the materials are laid layer by layer for weight pressing from bottom to top. Specifically: First, conduct the first-stage weight pressing filling, with a height of h1 and a length of l1. The end connected to the dam is slope-connected, and the other end is slope-released. The slope of the first-stage weight pressing is i1, and i1 < i0, where i0 is the downstream dam slope of the dam. Use the same construction method as the first-stage weight pressing to conduct the second-stage weight pressing filling, with a weight pressing height of h2 and a length of l2. The slope of the second-stage weight pressing is i2, where l2 < l1 and i2 ≤ i1 < i0. The third-stage weight pressing and the subsequent weight pressings are carried out in the same way until the construction of the entire stepped weight pressing is completed.

3. A weight loading design and construction method for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer according to claim 2, characterized in that, Specifically, Step 2 is as follows: Layer by layer lay the dam rockfill, transition material, and bedding material, and conduct vibration compaction construction according to the designed compaction degree or porosity requirement until the dam crest elevation is filled.

4. A weight pressing design and construction method for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer according to claim 2, characterized in that In Step 3, n ≥ 2.

5. A method for designing and constructing a surcharge for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer according to claim 2, characterized in that, In Step 3, the materials for weight pressing are laid layer by layer, and vibration compaction construction is carried out according to the designed compaction degree or porosity requirement. It is recommended that the filling standard for weight pressing be lower than that for dam filling.

6. A method for designing and constructing a surcharge for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer according to claim 2, characterized in that, The construction sequence of the dam body and the weight pressing can be adjusted. According to the requirements of excavation and filling balance of the project, the dam body and the weight pressing can be constructed simultaneously, or the dam body can be constructed first and then the weight pressing, or the weight pressing can be constructed first and then the dam body.

7. A weight pressing design and construction method for improving the liquefaction deformation of the foundation of an earth-rock dam on a deep overburden layer according to claim 2, characterized in that, For core-wall rockfill dams, it is applicable to both upstream and downstream weight pressings; for concrete-face rockfill dams, it is applicable to downstream weight pressings.

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