A method for anti-seepage design of foundation with deep overburden layer containing relative water-resisting layer

Abstract

The application provides a kind of anti-seepage design method of foundation with deep overburden layer containing relative water-resisting layer, suspended anti-seepage wall and wall bottom overburden layer grouting curtain jointly bear seepage pressure with downstream water-resisting layer, according to three-dimensional seepage calculation results, through the requirement of wall bottom overburden layer grouting curtain by research, the proportion of water head of suspended anti-seepage wall and wall bottom overburden layer grouting curtain and downstream water-resisting layer is reasonably selected, finally, according to the appropriate proportion, and then the technical requirements of deep overburden layer curtain grouting and the technical requirements of overburden layer relative water-resisting layer outlet counter pressure are put forward, so as to form a set of combined anti-seepage system which is mutually protective and coordinated, which reduces the risk of seepage failure and construction difficulty of anti-seepage bottom overburden layer grouting curtain. The method can greatly save the cost and construction period of engineering construction, and can be applied to the anti-seepage treatment of buildings built on the existing deep and complex structure, multi-layer structure overburden layer.

Description

Technical Field

[0001] This invention relates to the field of water conservancy and hydropower engineering technology, specifically to a method for seepage prevention design of foundations with deep overburden layers. Background Technology

[0002] Due to variations in depositional age and causation, deep overburden foundations often contain relatively impermeable layers such as silty sand and sandy-silty soil layers within the gravel and pebble layers. The permeability coefficient of these relatively impermeable layers is 2 to 4 orders of magnitude lower than that of the overly and below highly permeable overburden layers, often reaching <1×10⁻⁶. -4 A permeability coefficient of cm / s or less.

[0003] For foundations with thick overburden containing relatively impermeable layers, the design of seepage prevention schemes for dam foundations often considers using the relatively impermeable layer upstream of the seepage barrier as a seepage barrier cover, working together with the vertical seepage barrier for joint seepage prevention. Following the design principle of "blocking at the top and draining at the bottom," the relatively impermeable layer downstream of the seepage barrier is usually perforated through drainage holes for drainage.

[0004] With the development of deep overburden dam construction technology, the number of cases involving deep overburden dams is increasing. In some projects, the upstream relative impermeable layer of the vertical cutoff layer is discontinuous and cannot be used as a cutoff layer, while the downstream relative impermeable layer is continuous and of stable thickness. Studies of these cases have revealed that the downstream relative impermeable layer can also be used in conjunction with the dam foundation's vertical cutoff layer for seepage prevention. When the overburden is very deep and the cutoff wall construction cannot completely seal it, the vertical cutoff layer within the overburden often employs a "cutoff wall + grouting curtain of the bottom overburden layer." For high-head dams with highly permeable foundations, multiple rows of grouting curtains of the bottom overburden layer are often required to meet seepage prevention needs. However, considering the combined seepage prevention design with the downstream relative impermeable layer can significantly reduce the need for grouting the overburden layer curtain.

[0005] According to the seepage research results of a certain hydropower station, under the action of high water head, if the grouting curtain of the cover layer under the seepage barrier is too strong, its seepage prevention performance is good, and the water head borne by the curtain is very large, exceeding its allowable seepage gradient, resulting in short curtain durability and a high risk of failure. If the curtain is too weak, its seepage prevention performance is poor, the proportion of seepage borne by the relatively impermeable layer in the downstream cover layer is too high, and the outlet seepage gradient is too high, resulting in seepage failure such as piping at the outlet of the relatively impermeable layer of the downstream dam foundation cover layer. Summary of the Invention

[0006] The main objective of this invention is to provide a method for designing seepage prevention for foundations with relatively deep overburden layers containing water-resistant layers, thereby solving the problems mentioned in the background art.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a suspended seepage barrier wall and a grouting curtain at the bottom of the wall, together with a downstream waterproof layer, jointly bear the seepage pressure. The specific steps are as follows:

[0008] S1. Based on three-dimensional seepage calculations, determine the head sharing ratio between the suspended seepage barrier wall and the grouting curtain of the wall bottom cover layer and the downstream impermeable layer.

[0009] S2. By calculating the seepage prevention strength of the suspended seepage barrier and the grouting curtain of the bottom cover layer, the requirements for the suspended seepage barrier and the grouting curtain of the bottom cover layer are reduced, so that the downstream waterproof layer can bear more of the seepage prevention function.

[0010] S3. Install a reverse filter weight within a certain range downstream of the dam to improve the seepage prevention capacity and reliability of the downstream aquitard.

[0011] In the preferred embodiment, in step S2, the requirements for the grouting curtain of the suspended anti-seepage wall and the wall bottom cover layer are reduced by decreasing the number of rows of grouting curtain at the bottom of the anti-seepage wall or reducing its permeability coefficient.

[0012] In the preferred scheme, the analysis steps for the cover curtain under different permeability coefficients are as follows:

[0013] A1. Profile seepage gradient analysis;

[0014] A2. Analysis of the extent and impact of excessive seepage gradient of the overburden soil and curtain grouting;

[0015] A3. Analysis of areas exceeding permeability limits.

[0016] In the preferred scheme, the seepage gradient of the cover layer soil and the seepage gradient of the downstream impermeable layer soil are compared under different working conditions to select the cover layer curtain permeability coefficient that meets the requirements.

[0017] In the preferred scheme, step S3 specifically involves implementing reverse filtration and weight-bearing protection and interlayer protection in the downstream impermeable layer area where the seepage gradient exceeds the limit, and calculating the allowable slope i of the soil. 联合抗渗 Compare the slope i calculated by soil mass 实际 with i 联合抗渗 The relative size of the soil surface is used to determine whether the foundation meets the seepage safety requirements.

[0018] In the preferred scheme, the allowable slope i of the soil 联合抗渗 The calculation formula is:

[0019] i 联合抗渗 =i 常规 +Δi 压重及埋深 +Δi 地层保护 1

[0020] Δi 压重及埋深 =α·i 常规2

[0021] α=(i P -i0) / (F s ·i0) 3

[0022] Δi 地层保护 =β·i 常规 4

[0023] β=(i 地层保护 -i 无保护 ) / (F s ·i 无保护 5

[0024] The specific implementation steps are as follows:

[0025] B1. Allowable slope i of soil determined by conventional methods 常规 ;

[0026] B2. Quantitative measures for reverse filter compaction and soil depth on the allowable slope of the soil. 联合抗渗 Contribution Δi 压重及埋深 ;

[0027] B3. Quantifying the effect of combined effects of multi-layered foundations on allowable slope of soil. 联合抗渗 Contribution Δi 压重及埋深 ;

[0028] B4. Based on the calculation results of the above formula, the allowable slope i of the soil is obtained by comprehensively considering multiple factors such as the reverse filter weighting measures, the soil depth, and interlayer protection. 联合抗渗 ;

[0029] B5, according to i 联合抗渗 Based on the three-dimensional seepage calculation results, the safety of downstream outlet seepage is evaluated.

[0030] This invention provides a method for seepage prevention in deep overburden foundations. A suspended cutoff wall and a grouting curtain at the base of the wall, along with the downstream impermeable layer, share the seepage pressure. Based on three-dimensional seepage calculations, the requirements for the grouting curtain at the base of the cutoff wall are modified, and an optimal head-sharing ratio between the suspended cutoff wall, the grouting curtain, and the downstream impermeable layer is selected. Furthermore, technical requirements for the grouting of the deep overburden curtain and the back pressure at the outlet of the overburden relative to the impermeable layer are proposed, thus forming a mutually protective and coordinated joint seepage prevention system. This reduces the risk of seepage failure of the grouting curtain at the base of the cutoff wall and the construction difficulty. This method can significantly save on engineering construction costs and time, and can be widely applied to seepage prevention treatment of buildings constructed on deep, complex, and multi-layered overburden structures. Attached Figure Description

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0032] Figure 1 This is a cross-sectional view of the downstream side of the concrete anti-seepage wall along the axis of working condition 1 of the present invention;

[0033] Figure 2 This is the cross-section along the axis of the concrete anti-seepage wall in working condition 1 of the present invention;

[0034] Figure 3 This describes the excessive seepage slope of the overburden soil in the cross-section at station 0+156.5m of this invention.

[0035] Figure 4 This is the case of excessive seepage slope of the overburden soil at station 0+192.6m in the working condition of this invention;

[0036] Figure 5 This is the case of excessive seepage slope of the overburden soil at station 0+249.9m in the working condition of this invention;

[0037] Figure 6 This is the downstream side section along the axis of the concrete anti-seepage wall in working condition 2 of the present invention;

[0038] Figure 7 This is the cross-section along the axis of the concrete anti-seepage wall in working condition 2 of the present invention;

[0039] Figure 8 This refers to the case of excessive seepage slope of the overburden soil in the cross section at station 0+156.5m of the present invention.

[0040] Figure 9 This is the case of excessive seepage slope of the overburden soil at station 0+192.6m in the second working condition of this invention;

[0041] Figure 10 This refers to the case of excessive seepage slope of the overburden soil at station 0+249.9m in the second working condition of this invention. Detailed Implementation

[0042] Example 1

[0043] like Figures 1-10 As shown in Table 1, seepage prevention methods for deep overburden foundations are employed. Deep overburden often has a multi-layered structure with varying mechanical properties, material composition, density, and cementation conditions in each layer, resulting in significant differences in the permeability coefficient of each layer. The physical and mechanical parameters of the overburden in the dam site area of ​​a certain project are shown in Table 1.

[0044] Table 1: Recommended Geological Values ​​of Physical and Mechanical Parameters of Overburden Layer in Dam Site Area of ​​a Certain Project

[0045]

[0046] Note: A horse trail should be provided for slopes with an elevation greater than 20m.

[0047] It can be seen that the permeability coefficient of the silty sand and silt layers ③-2 and ②-3 is 10. -3 ~10 -4 cm / s, much smaller than the permeability coefficient of the adjacent fourth layer (10). -1 ~10 -2 The magnitude of cm / s and the 10 of layers ① and ② -2 ~10 -3 The permeability coefficients can vary by 10 to 100 times, or even more, depending on the centimeter / s. When the permeability coefficient of a layer in the cover layer is much smaller than that of the two adjacent layers above and below, a relatively impermeable layer (or a relatively waterproof layer) is formed.

[0048] This relatively impermeable layer will withstand significant seepage pressure. When the seepage pressure exceeds the allowable value, seepage failure will occur, requiring close monitoring during engineering construction. Overburden layers with similar geological structures are also quite common, widely distributed in the upper reaches of major rivers in Southwest China.

[0049] A three-dimensional seepage field analysis was used to determine the head sharing ratio between the suspended cutoff wall, the grouting curtain at the base of the wall, and the downstream impermeable layer. By altering the seepage prevention strength of the suspended cutoff wall and the grouting curtain at the base of the wall, typically by reducing the requirements for the grouting curtain at the base of the suspended cutoff wall (e.g., reducing the number of rows, lowering its permeability coefficient requirements), the downstream impermeable layer assumes a greater seepage prevention role, thereby reducing the difficulty and workload of the grouting curtain at the base of the cutoff wall. In this example, due to the high difficulty of grouting, the permeability coefficient requirement for the grouting curtain was reduced.

[0050] This section focuses on analyzing the treatment effects under two working conditions: Condition 1: The permeability coefficient of the covering layer curtain is 3.0 × 10⁻⁶. -4 cm / s, the permeability coefficients of the deep and shallow bedrock curtains are 3.0×10 cm / s, respectively. -5 cm / s and 5.0×10 -5 cm / s; Condition 2: The permeability coefficient of the covering layer curtain increases to 1.0×10 cm / s; -3 cm / s, the permeability coefficients of the deep and shallow bedrock curtains are 3.0×10 cm / s, respectively. -5 cm / s and 5.0×10 -5 cm / s.

[0051] (1) Condition 1: The permeability coefficient of the covering layer curtain is 3.0×10 -4 cm / s

[0052] The seepage calculation conditions for Condition 1 are as follows: the curtain thickness is considered as three rows, two rows inside the dam's concrete cutoff wall and one row on the downstream side, with a thickness of 2.75m. The permeability coefficient of the cover layer curtain grouting in the riverbed area is 3.0×10⁻⁶ before the reinforcement grouting. -2 The reduction of cm / s to the design index for the curtain grouting of the overburden layer is 3.0 × 10 cm / s. -4 cm / s.

[0053] A) Typical profile permeability gradient

[0054] Table 2 presents the statistical results of the permeability gradient of typical materials in the longitudinal section 3.

[0055] Under operating condition 1, in longitudinal section 3, the maximum seepage gradient and average gradient of the overburden curtain grouting reached 16.45 and 11.25 respectively, both exceeding the allowable gradient of 10 determined in the feasibility study stage; the maximum seepage gradient of the bedrock curtain grouting was 11.51, slightly greater than the allowable gradient of 15, and the average gradient was 5.73, less than the allowable gradient. No exceedances were observed in the seepage gradient of the overburden soil on the upstream side of the dam's concrete cutoff wall. The maximum gradients of the ②-2GSC1 and ②-3 sub-layers of soil below the inflow point were 0.6 and 0.46 respectively, still exceeding their respective allowable gradients, while the average gradients were both less than the allowable gradients.

[0056] Table 2: Permeability gradient statistics of typical materials in longitudinal section 3

[0057]

[0058] B) Evaluation of the extent and impact of excessive seepage slope of the overburden layer ① soil and curtain grouting.

[0059] Under operating condition 1, the extent of excessive seepage gradient in the overburden layer ① is approximately as follows: Near station 0+133m to 0+163m, the maximum gradient of layer ① downstream of the cutoff wall end is between 0.21 and 0.23, exceeding the allowable gradient by 0.12 to 0.15, with an average gradient of 0.01, all significantly less than the allowable gradient. Upstream of the cutoff wall end, and closer to the front of ①TD, the maximum seepage gradient of layer ① is generally between 0.04 and 0.05, with an average gradient of only 0.01, both less than the allowable gradient, and no exceedances were observed. Figure 1 The contour lines of the seepage gradient of the cover layer curtain on the downstream side profile of the cutoff wall end are given.

[0060] Under operating condition 1, the maximum seepage gradient of the overburden curtain grouting is generally located near the end of the cutoff wall. The extent of exceeding the limit is roughly as follows: Around chainage 0+118m to 0+163m, the maximum gradient ranges from 16.6 to 18.9, and the average gradient ranges from 9.3 to 10.7. The maximum gradient exceeds the allowable gradient by 10%, and the average gradient is slightly greater than the allowable gradient. Around chainage 0+178m to 0+213m, the local maximum gradient ranges from 16.5 to 23.1, and the average gradient ranges from 11.3 to 12.4. Both the maximum and average gradients exceed the allowable gradient by 10%. Figure 2 The contour lines of the seepage gradient of the cover layer curtain (section along the axis of the concrete cutoff wall) are given under working condition 1.

[0061] The above statistics show that, under working condition 1, the extent of exceeding the limits for the overburden layer ① soil is very limited, with only some exceeding the limits in a local area (near chainage 0+133m to 0+163m) downstream of the end of the anti-seepage wall; the overburden curtain grouting exceeds the limits more significantly, with the seepage gradient exceeding the allowable gradient by 10 in the range of chainage 0+113.5m to 0+214m and elevation 1180m to 1200m; the overburden curtain grouting exceeds the limits mainly in the left bank chainage 0+73m to 0+83m and the highly permeable zone on the right bank.

[0062] The permeability gradient of the cover layer curtain grout decreases as its own permeability increases (equivalent to a decrease in the ratio of the permeability coefficient of layer 1 to that of the cover layer curtain grout, assuming the permeability of layer 1 remains constant), and increases as its own permeability decreases (equivalent to an increase in the ratio of the permeability coefficient of layer 1 to that of the cover layer curtain grout, assuming the permeability of layer 1 remains constant).

[0063] C) Condition 1: Permeability Exceeding Limit Area

[0064] Here, three typical profiles (section 0+156.5m, longitudinal section 11 (section 0+192.6m), and longitudinal section 3-1 (section 0+249.9m)) of the highly permeable zone on the riverbed and right bank are selected to show the extent of exceedance of each overburden soil layer. See below for details. Figures 3-5 The "square grid" section is shown in the middle.

[0065] from Figure 3 As can be seen from the data, under operating condition 1, the seepage gradient of the overburden soil on the upstream side of the cutoff wall did not exceed the limit. Only the seepage gradient of the local ① layer soil and the ③-1GSC soil below the downstream water inflow point on the downstream side of the cutoff wall exceeded the limit. Among them, the exceedance range of the ① layer soil was very small, with a seepage gradient of 0.14 to 0.22 within the exceedance range (allowable gradient 0.12 to 0.15); the exceedance range of the ③-1GSC layer soil was larger, with a seepage gradient of 0.23 to 0.56 within the exceedance range (allowable gradient 0.15 to 0.18).

[0066] from Figure 4 As can be seen from the data, under operating condition 1, the seepage gradient of the overburden soil on the upstream side of the cutoff wall did not exceed the limit, except for the ③-1GSC soil below the downstream water inflow point. The ③-1GSC soil layer exceeded the limit by a large margin, reaching the downstream side of the water inflow point, with the seepage gradient within the excess range of 0.18 to 0.44 (allowable gradient 0.15 to 0.18).

[0067] from Figure 5As can be seen from the data, under operating condition 1, the seepage gradient of a very small area of ​​soil ②-1 upstream of the cutoff wall end is close to the allowable gradient of 0.12 to 0.15, while the other overburden soils do not exceed the limit. However, the seepage gradients of soil ③-1GSC and soil ②-2GSC1 below the downstream water inflow point exceed the limit. Specifically, the seepage gradient of soil ②-2GSC1 within the exceedance range is between 0.23 and 0.36 (allowable gradient 0.2 to 0.25); the seepage gradient of soil ③-1GSC within the exceedance range is between 0.2 and 0.38 (allowable gradient 0.15 to 0.18).

[0068] The results above show that, under condition 1, the seepage gradient of the overburden soil on the upstream side of the cutoff wall is less than its respective allowable gradient, and there is no case of exceeding the limit; there is a case of exceeding the limit in a very small area on the downstream side of the cutoff wall; there is a case of exceeding the limit in the soil layer ① below the water inflow point in the soil layers ②-2GSC1 and ③-1GSC.

[0069] (2) Condition 2: The permeability coefficient of the covering layer curtain is 1.0×10-3cm / s

[0070] Condition 2, based on Condition 1, further increases the permeability of the overburden curtain grout to reduce the seepage gradient borne by the overburden curtain grout. The permeability coefficient of the overburden curtain grout is calculated and increased from 3.0×10-4 cm / s to 1.0×10-3 cm / s, while other permeability parameters and boundary conditions remain the same as in Condition 1.

[0071] A) Typical profile permeability gradient

[0072] Table 3 presents the statistical results of the permeability gradient of typical materials in the longitudinal section 3.

[0073] Under working condition 2, in longitudinal section 3, the maximum seepage gradient of the curtain grouting of the overburden layer is 13.7, which exceeds the allowable gradient of 10 determined in the feasibility study stage, and the average gradient is 9.3, which is lower than the allowable gradient of 10. The seepage gradient of the overburden layer ① soil on the upstream side of the concrete anti-seepage wall of the dam did not exceed the limit. The maximum gradients of the sub-layers ②-2GSC1 and ②-3 soil below the inflow point are 0.92 and 0.70, respectively, which still exceed their respective allowable gradients. The average gradients are all lower than the allowable gradients. However, the average gradient of the ②-2GSC1 soil reaches 0.19, which is close to the allowable gradient.

[0074] Table 3: Permeability gradient statistics of typical materials in longitudinal section 3

[0075]

[0076] Under operating condition 2, the extent of excessive seepage gradient in the overburden layer ① is approximately as follows: Near station 0+106m to 0+163m, the maximum gradient of layer ① downstream of the cutoff wall end is between 0.33 and 0.56, exceeding the allowable gradient by 0.12 to 0.15. The average gradient is between 0.02 and 0.06, both significantly less than the allowable gradient. Upstream of the cutoff wall end, and closer to the front of ①TD, the maximum seepage gradient of layer ① is generally between 0.07 and 0.1, with an average gradient between 0.02 and 0.03, both less than the allowable gradient, and no exceedances were observed. Figure 6 The contour lines of the seepage gradient of the cover layer curtain on the downstream side profile of the cutoff wall end are given.

[0077] Under operating condition 2, the maximum seepage gradient of the overburden curtain grouting is generally located near the end of the cutoff wall. The extent of exceeding the limit is roughly as follows: Around chainage 0+118m to 0+163m, the maximum gradient is between 13.5 and 14.6, and the average gradient is between 7.1 and 8.4. The maximum gradient exceeds the allowable gradient by 10%, and the average gradient is less than the allowable gradient. Around chainage 0+178m to 0+213m, the maximum gradient is between 13.7 and 18.2, and the average gradient is between 9.3 and 10.1. The maximum gradient exceeds the allowable gradient by 10%, and the average gradient is close to or slightly greater than the allowable gradient. Specifically, at chainage 0+213m, the maximum gradient is 18.2, and the average gradient is 10.1. Figure 7 The contour lines of the seepage gradient of the cover layer curtain (section along the axis of the concrete cutoff wall) are given under working condition 1.

[0078] The above statistical results show that, compared with Condition 1 (permeability coefficient of the overburden curtain grouting is 3.0 × 10⁻⁴ cm / s), under Condition 2 (permeability coefficient of the overburden curtain grouting is 1.0 × 10⁻³ cm / s), the permeability gradient of layer ① is significantly increased, with the local maximum gradient exceeding or approaching the allowable gradient. However, the extent of exceeding the limit in layer ① is relatively limited, occurring only in a limited area downstream of the cutoff wall or at the top of the upstream channel. The maximum gradient of the overburden curtain grouting exceeds the limit in multiple locations, with the average gradient generally less than 10% of the allowable gradient.

[0079] C) Areas with excessive permeability under operating condition 2

[0080] Here, three typical profiles (section 0+156.5m, longitudinal section 11 (section 0+192.6m), and longitudinal section 3-1 (section 0+249.9m)) of the highly permeable zone on the riverbed and right bank are selected to show the extent of exceedance of each overburden soil layer. See below for details. Figures 8-10 The "square grid" section is shown in the middle.

[0081] Compared with working condition 1, under working condition 2, the seepage gradient of the overburden soil on the upstream side of the cutoff wall began to exceed the limit; the ① layer of soil on the downstream side of the cutoff wall end had a very small exceedance range, while the exceedance range of the impermeable layers ②-2GSC1 soil and ③-1GSC soil below the water inflow point was larger.

[0082] from Figure 8 As can be seen from the data, under working condition 2, the seepage gradient of the overburden soil ③-1 and ②-3 on the upstream side of the cutoff wall began to exceed the limit; the seepage gradient of the local ① layer soil on the downstream side of the cutoff wall end, the ②-2GSC1 soil at the downstream end of the cofferdam cutoff wall, and the ③-1GSC soil below the downstream water inflow point also exceeded the limit. Among them, the extent of exceeding the limit in layer ① is very small, with a seepage gradient of 0.15 to 0.54 within the excess range (allowable gradient 0.12 to 0.15); the extent of exceeding the limit in layer ②-2GSC1 is 0.31 (allowable gradient 0.2 to 0.25); the extent of exceeding the limit in layer ②-3 is 0.25 to 0.38 (allowable gradient 0.25 to 0.36); the extent of exceeding the limit in layer ③-1 on the upstream side is 0.15 to 0.20 (allowable gradient 0.15 to 0.18); and the extent of exceeding the limit in layer ③-1GSC is relatively large, with a seepage gradient of 0.32 to 0.86 (allowable gradient 0.15 to 0.18).

[0083] from Figure 9 As can be seen from the data, under working condition 2, the seepage gradient of the overburden soil layers ③-1 and ②-3 on the upstream side of the anti-seepage wall begins to exceed the limit; the seepage gradient of the soil layer ②-2GSC1 at the end of the downstream cofferdam anti-seepage wall, and the soil layers ②-2GSC1 and ③-1GSC below the downstream water inflow point also exceed the limit. Specifically, the seepage gradient of the soil layer ②-2GSC1 within the exceedance range is between 0.21 and 0.28 (allowable gradient 0.2 to 0.25); the seepage gradient of the soil layer ②-3 within the exceedance range is between 0.25 and 0.39 (allowable gradient 0.25 to 0.36); the seepage gradient of the soil layer ③-1 on the upstream side within the exceedance range is between 0.15 and 0.26 (allowable gradient 0.15 to 0.18); the exceedance range of the soil layer ③-1 is relatively large, with the seepage gradient within the exceedance range between 0.22 and 0.66 (allowable gradient 0.15 to 0.18).

[0084] from Figure 10As can be seen from the data, under working condition 2, the seepage gradient of the overburden soil layers ①, ②-1, and ③-1 on the upstream side of the anti-seepage wall began to exceed the limit; the seepage gradient of the soil layers ②-2GSC1 and ③-1GSC below the downstream water inflow point also exceeded the limit on a large scale. Among them, the seepage gradient of layer ① is very small, with a gradient of 0.18 within the excess range (allowable gradient 0.12-0.15); the seepage gradient of layer ②-1 within the excess range is between 0.18 and 0.27 (allowable gradient 0.12-0.15); the seepage gradient of layer ②-2GSC1 within the excess range is between 0.23 and 0.54 (allowable gradient 0.2-0.25); the seepage gradient of layer ③-1 on the upstream side within the excess range is between 0.15 and 0.25 (allowable gradient 0.15-0.18); and the seepage gradient of layer ③-1GSC within the excess range is between 0.30 and 0.57 (allowable gradient 0.15-0.18).

[0085] Overall, compared with Condition 1, under Condition 2, the seepage gradient of the overburden soil is increased, especially the seepage gradient of the impermeable layer (②-2GSC1 and ③-1GSC) and the soil in ②-3 below the water inflow point is significantly increased, with a large range of exceeding the limit.

[0086] A comparative analysis of the treatment effects under two working conditions was conducted: Condition 1: The permeability coefficient of the overburden curtain was 3.0 × 10⁻⁴ cm / s, and the permeability coefficients of the deep and shallow bedrock curtains were 3.0 × 10⁻⁵ cm / s and 5.0 × 10⁻⁵ cm / s, respectively; Condition 2: The permeability coefficient of the overburden curtain increased to 1.0 × 10⁻³ cm / s, and the permeability coefficients of the deep and shallow bedrock curtains were 3.0 × 10⁻⁵ cm / s and 5.0 × 10⁻⁵ cm / s, respectively. The basic conclusions are as follows:

[0087] Under operating condition 1, the seepage gradient exceeding the limit for the overburden layer ① soil is located between chainage 0+133m and 0+163m. Near the downstream end of the cutoff wall, the maximum gradient of layer ① soil is between 0.21 and 0.23, exceeding the allowable gradient by 0.12 to 0.15, with an average gradient of 0.01, significantly less than the allowable gradient. No gradient exceeding the limit is observed in layer ① soil upstream of the cutoff wall and near the front end of ①TD. Under operating condition 2, the seepage gradient exceeding the limit for the overburden layer ① soil is located between chainage 0+106m and 0+163m. Near the downstream end of the cutoff wall, the maximum gradient of layer ① soil is between 0.33 and 0.56, exceeding the allowable gradient by 0.12 to 0.15, with an average gradient of 0.02 to 0.06, significantly less than the allowable gradient. No gradient exceeding the limit is observed in layer ① soil upstream of the cutoff wall and near the front end of ①TD.

[0088] Under Condition 1, the exceeding range of the overburden curtain grouting is as follows: From chainage 0+118m to 0+163m, the maximum slope ranges from 16.6 to 18.9, and the average slope ranges from 9.3 to 10.7. The maximum slope exceeds the allowable slope by 10%, and the average slope is slightly greater than the allowable slope. Near chainage 0+178m to 0+213m, the maximum slope ranges from 16.5 to 23.1, and the average slope ranges from 11.3 to 12.4. Both the maximum and average slopes exceed the allowable slope by 10%. Under Condition 2, the exceeding range of the overburden curtain grouting is as follows: From chainage 0+118m to 0+163m, the maximum slope ranges from 13.5 to 14.6, and the average slope ranges from 7.1 to 8.4. The maximum slope exceeds the allowable slope by 10%, and the average slope is less than the allowable slope. Around chainage 0+178m to 0+213m, the maximum gradient ranges from 13.7 to 18.2, and the average gradient ranges from 9.3 to 10.1. The maximum gradient exceeds the allowable gradient by 10, and the average gradient is close to or slightly greater than the allowable gradient.

[0089] Under operating condition 1, the following risks still exist for the dam: (a) The ① layer of soil exceeds the limit only in a limited area downstream of the end of the anti-seepage wall. Since the extent of the exceedance is small, it can be considered that the local small-scale fine particle migration phenomenon will not have a significant impact on the overall dam safety; (b) The extent of the exceedance of the ②-2GSC1 soil and ③-1GSC soil below the water inrush point is large, which requires sufficient attention and must be actively and effectively treated (such as increasing the cover weight, increasing the shallow pressure relief drainage holes, and doing a good job of reverse filter protection, etc.). Otherwise, it may trigger local water and sand inrush downstream of the dam again; (c) The maximum slope and average slope of the cover layer curtain grouting at chainage 0+178m to 0+213m both exceed the allowable slope by 10; at chainage 0+118m to 0+163m, the maximum slope exceeds the allowable value, and the average slope is slightly greater than the allowable slope. This should be given sufficient attention to prevent deep erosion of the dam foundation, which could affect dam safety; (d) The maximum and average gradients of the bedrock curtain grouting at chainages 0+73m to 0+83m and 0+233m to 0+263m both exceed the allowable values ​​by 15%. This should be given sufficient attention to prevent localized damage to the bedrock curtain grouting, which could then affect dam safety.

[0090] Under operating condition 2, the following risks still exist for the dam: (a) The ① layer of soil exceeds the limit only in a limited area downstream of the end of the cutoff wall. Since the extent of the exceedance is small, it can be considered that the local small-scale fine particle transport phenomenon will not have a significant impact on the overall dam safety; (b) The extent of exceedance of the ②-2GSC1 soil and ③-1GSC soil below the water inrush point is relatively large, which requires sufficient attention and must be actively and effectively treated (such as increasing the cover weight, increasing shallow pressure relief drainage holes, and doing a good job of reverse filter protection, etc.). Otherwise, it may trigger local water and sand inrush downstream of the dam again; (c) There is a small-scale exceedance of the cover soil on the upstream side of the cutoff wall. It is recommended to add seepage prevention cover and other measures on the upstream side to protect the upstream cover soil; (d) The maximum slope of the cover curtain grouting at chainage 0+178m to 0+213m exceeds the allowable slope by 10, and the average slope is close to or slightly greater than the allowable slope. Sufficient attention should be paid to these areas, and grouting quality should be strictly controlled to prevent deep erosion of the dam foundation, which could affect dam safety; (e) In bedrock curtain grouting at chainages 0+73m to 0+83m and 0+233m to 0+249m, both the maximum and average slopes exceed the allowable value of 15. Sufficient attention should be paid to these areas, and grouting quality should be strictly controlled to prevent localized damage to the bedrock curtain grouting, which could then affect dam safety.

[0091] Overall, Condition 2 showed slightly better treatment results than Condition 1. However, even after treatment under both conditions, the dam still faces the possibility of further malfunctions. Under Condition 1, the seepage gradient of the overburden soil on the upstream side of the cutoff wall did not exceed the limit; the average gradient of the overburden curtain grouting exceeded the limit; the average gradient of the bedrock curtain grouting exceeded the limit; and the impermeable layers ②-2GSC1 and ③-1GSC soil below the seepage point showed significant exceedances. Under Condition 2, the seepage gradient of the overburden soil on the upstream side of the cutoff wall began to exceed the limit in a small range; the seepage gradient of the overburden curtain grouting was significantly lower than that under Condition 1, and the average gradient basically did not exceed the limit, except at chainage 0+213m, where the average gradient was 10.1, slightly exceeding the allowable value; the gradient of the bedrock curtain grouting was significantly lower than that under Condition 1, but the average gradient still exceeded the limit; the downstream side of the cutoff wall end ① soil layer had a very small exceedance range, and the exceedance range and magnitude of the impermeable layers ②-2GSC1 soil and ③-1GSC soil below the water inflow point were larger than those under Condition 1.

[0092] In the initial stage of water inrush, due to the persistent flow rate and the presence of sand in the water, the concrete aggregate in the inrush area experienced multiple collapses during the counter-pressure construction. As the inrush area was continuously expanded and treated, it gradually stabilized. No new collapses or leaks occurred after the second and third phases of counter-pressure treatment. The measures and effects of the third phase of counter-pressure treatment are shown in Table 4.

[0093] Table 4: Summary of Three Phases of Back Pressure Treatment Measures and Their Effectiveness

[0094]

[0095]

[0096] For a micro-element at any location within the overburden layer B, considering the concept of multi-layered foundation combined seepage resistance, its allowable slope i 联合抗渗 The slope of the soil is related not only to its particle size distribution but also to the overlying filter press, its burial depth, and the interlayer protection of the overlying strata. The mechanisms of the filter press and the burial depth are consistent and can be simulated by applying a certain overlying pressure. Adding a filter press at the downstream seepage outlet will generate additional stress on the underlying soil, making it more compact and increasing its allowable slope. Similarly, the greater the burial depth, the denser the soil, and consequently, the greater its allowable slope. The interlayer protection of the overlying strata is similar to the effect of a conventional filter layer on the protected soil. To quantify this effect, a multi-layer foundation combined action permeability test, similar to a filter layer permeability test, needs to be conducted. Based on the above basic ideas, the allowable slope i of the soil considering the combined seepage resistance of the multi-layer foundation is... 联合抗渗 It can be expressed as:

[0097] i 联合抗渗 =i 常规 +Δi 压重及埋深 +Δi 地层保护 (1)

[0098] Δi 压重及埋深 =α·i 常规 (2)

[0099] α=(i P -i0) / (F s ·i0) (3)

[0100] Δi 地层保护 =β·i 常规 (4)

[0101] β=(i 地层保护 -i 无保护 ) / (F s ·i 无保护 (5)

[0102] In the formula: i 联合抗渗 Allowable slope gradient for soil layers considering combined seepage resistance of multi-layered foundations; 常规 The allowable gradient is determined using conventional methods. 常规The influence of factors such as reinforcement measures like reverse filter presses, soil depth, and interlayer protection of surrounding strata on the allowable slope of the soil was not considered; Δi 压重及埋深 To account for the increase in the allowable slope of the soil considering the effects of filter weight and burial depth, α is the contribution coefficient of the allowable slope of the soil considering the effects of weight and burial depth, i P Δi and i0 represent the critical slope of the soil under a certain overburden pressure P and zero pressure conditions, respectively; 地层保护 β represents the increase in allowable slope gradient of the soil considering the effect of inter-layer protection, and i represents the contribution coefficient of allowable slope gradient of the soil considering the effect of inter-layer protection. 地层保护 and i 无保护 These represent the critical slope of the soil under conditions with and without soil protection, respectively; Fs is the safety factor, and the allowable slope of a soil is generally obtained by dividing the critical slope of the soil by the safety factor Fs.

[0103] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for designing seepage prevention for foundations with relatively deep overburden layers and impermeable layers, characterized by: The suspended cutoff wall and the grouting curtain at the base of the wall, together with the downstream impermeable layer, share the seepage pressure. The specific steps are as follows: S1. Based on three-dimensional seepage calculations, determine the head sharing ratio between the suspended seepage barrier wall and the grouting curtain of the wall bottom cover layer and the downstream impermeable layer. S2. By calculating the seepage prevention strength of the suspended seepage barrier and the grouting curtain of the bottom cover layer, the requirements for the suspended seepage barrier and the grouting curtain of the bottom cover layer are reduced, so that the downstream waterproof layer can bear more of the seepage prevention function. S3. Install a reverse filter weight within a certain range downstream of the dam to improve the seepage prevention capacity and reliability of the downstream aquitard. In step S3, the specific steps are to implement reverse filter and weight protection and bottom layer interlayer protection in the downstream impermeable layer area where the seepage gradient exceeds the limit, and to calculate the allowable slope of the soil. i 联合抗渗 Calculate slope by comparing soil mass i 实际 and i 联合抗渗 The relative size of the soil surface is used to determine whether the foundation meets the seepage safety requirements. Allowable slope of soil i 联合抗渗 The calculation formula is: i 联合抗渗 = i 常规 +∆ i 压重及埋深 +∆ i 地层保护 ∆ i 压重及埋深 = α·i 常规 α= ( i P - i 0) / ( F s ·i 0) ∆ i 地层保护 = β·i 常规 β= ( i 地层保护 - i 无保护 ) / ( F s ·i 无保护 ) In the formula: i 联合抗渗 Allowable slope gradient for soil layers considering the combined seepage resistance of multi-layered foundations; i 常规 The allowable gradient is determined using conventional methods; ∆ i 压重及埋深 The allowable increase in soil gradient is calculated to account for the effects of filter press weight and burial depth. α The soil allowable slope contribution factor that takes into account the effects of weight and burial depth. i P and i 0 represents a certain overburden pressure P Critical slope of soil under zero pressure conditions; ∆ i 地层保护 The increase in allowable slope of the soil, taking into account the effect of interlayer protection, β The soil allowable slope contribution coefficient, which takes into account the influence of interlayer protection, i 地层保护 and i 无保护 These represent the critical slope of soil under conditions with and without soil protection, respectively. F s For safety factor; The specific implementation steps are as follows: B1. Determine the allowable slope of the soil using conventional methods. i 常规 ; B2. Quantitative measures for reverse filter compaction and soil depth on the allowable slope of the soil. i 联合抗渗 The amount of contribution; B3. Quantifying the effect of combined effects of multi-layered foundations on allowable slope of soil. i 联合抗渗 The amount of contribution; B4. Based on the calculation results of the above formula, the allowable slope of the soil is derived by comprehensively considering the effects of multiple factors such as the reverse filter weighting measures, the soil depth, and the interlayer protection. i 联合抗渗 ; B5. According to i 联合抗渗 Based on the three-dimensional seepage calculation results, the safety of downstream outlet seepage is evaluated.

2. The seepage prevention design method for foundations with relatively thick overburden containing a water-resistant layer according to claim 1, characterized in that: In step S2, the requirements for the grouting curtain of the suspended anti-seepage wall and the wall bottom cover layer are reduced by decreasing the number of rows of grouting curtain or reducing its permeability coefficient.

3. The seepage prevention design method for foundations with relatively thick overburden containing a water-resistant layer according to claim 2, characterized in that: The analysis steps for the overburden curtain under different permeability coefficients are as follows: A1. Profile seepage gradient analysis; A2. Analysis of the extent and impact of excessive seepage gradient of the overburden soil and curtain grouting; A3. Analysis of areas exceeding permeability limits.

4. The seepage prevention design method for foundations with relatively thick overburden containing a water-resistant layer according to claim 3, characterized in that: By comparing the seepage gradient of the overburden soil and the seepage gradient of the downstream impermeable layer soil under different working conditions, a suitable overburden curtain permeability coefficient is selected.

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