Method for treating weak interlayer under dam foundation of gravity dam

Abstract

The present disclosure provides a processing method for soft interlayer under gravity dam foundation, belonging to the field of hydraulic technology in water conservancy engineering. The method comprises: building a check dam downstream of the overflow dam; the foundation of the check dam is buried below the soft interlayer; a first prestressed anchor cable is constructed in the downward stable layer below the lower part of the foundation of the check dam to anchor the first prestressed anchor cable to the downward stable layer of the check dam; a second prestressed anchor cable is constructed in the area between the overflow dam and the check dam to make the second prestressed anchor cable pass through the soft interlayer and be anchored to the downward stable layer of the area between the overflow dam and the check dam. The present disclosure can effectively maintain the deep anti-sliding stability of the dam body by building a check dam and laying two groups of prestressed anchor cables in the case of cutting off the soft interlayer by the downstream scour pit of the built gravity dam.

Description

Technical Field

[0001] This disclosure relates to the field of hydraulic engineering in water conservancy projects, and in particular to a method for treating weak interlayers under the foundation of a gravity dam. Background Technology

[0002] Gravity dams are one of the most widely used dam types in hydraulic engineering, relying on their own weight for stability. Overflow dams, as the main spillway structures of gravity dams, often employ a jet flow energy dissipation method to project the downstream flow to the downstream riverbed. During the design and construction phases of gravity dams, if weak interlayers exist within the dam foundation, they typically need to be excavated or treated with engineering measures. However, in some projects, the weak interlayers beneath the dam foundation were not identified during construction. After the project is operational, the downstream jet flow crater deepens, potentially cutting off the weak interlayer and exposing it. This results in the dam body and the superstructure losing the resistance support from the downstream rock mass, significantly increasing the risk of deep-seated sliding along the weak interlayer. At this point, because the dam is already built, conventional methods for removing the weak interlayer cannot be implemented. Therefore, how to effectively maintain the deep-seated anti-sliding stability of an existing gravity dam under the condition that the downstream jet flow crater has cut off the weak interlayer is a pressing technical problem that needs to be solved in this field.

[0003] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0004] This disclosure provides a method for treating weak interlayers under the foundation of a gravity dam, thereby overcoming, to some extent, the shortcomings of existing technologies that cannot address the problem of exposed weak interlayers after the dam body is built.

[0005] Other features and advantages of this disclosure will become apparent from the following detailed description, or may be learned in part from practice of this disclosure.

[0006] According to one aspect of this disclosure, a method for treating a weak interlayer beneath the foundation of a gravity dam is provided. The gravity dam includes an overflow dam, and the foundation of the gravity dam includes a weak interlayer. The method includes: constructing a backwater dam downstream of the overflow dam; embedding the foundation of the backwater dam below the weak interlayer; constructing a first prestressed anchor cable in a lower stabilizing layer below the foundation of the backwater dam to anchor the first prestressed anchor cable to the lower stabilizing layer of the backwater dam; and constructing a second prestressed anchor cable in the region between the overflow dam and the backwater dam, such that the second prestressed anchor cable passes through the weak interlayer and is anchored to the lower stabilizing layer of the region between the overflow dam and the backwater dam.

[0007] In one exemplary embodiment of this disclosure, the dam includes a dam body and a cushion pool, the cushion pool being used to form a cushion to dissipate energy from the water flow out of the overflow dam.

[0008] In one exemplary embodiment of this disclosure, the crest elevation of the backwater dam is determined according to the required water cushion thickness, and the distance from the crest of the backwater dam to the bottom of the water cushion pool is not less than the water cushion thickness; wherein, the water cushion thickness refers to the vertical depth of the water cushion formed in the water cushion pool.

[0009] In one exemplary embodiment of this disclosure, the slope ratio of the upstream slope of the impounding dam ranges from 1:0.5 to 1:1, the slope ratio of the downstream slope of the impounding dam ranges from 1:2 to 1:4, and the slope ratio of the upstream slope of the water cushion pool ranges from 1:2 to 1:3.

[0010] In one exemplary embodiment of this disclosure, the foundation of the backwater dam is embedded below the weak interlayer, including: The foundation of the dam is buried at least 2 meters below the weak interlayer.

[0011] In one exemplary embodiment of this disclosure, the method further includes: A steel pipe is installed in the anchor hole of the first prestressed anchor cable and the second prestressed anchor cable; The bottom of the steel pipe extends at least 0.5 meters below the stable layer at the bottom of the weak interlayer, and the wall thickness of the steel pipe ranges from 0.8 to 1.2 centimeters, while the inner diameter ranges from 10 to 15 centimeters.

[0012] In one exemplary embodiment of this disclosure, the method further includes: During the construction of the first prestressed anchor cable and the second prestressed anchor cable, corresponding anchor cable holes are formed; Both the first prestressed anchor cable and the second prestressed anchor cable include an inner anchoring section, a free section, and an outer anchoring section; the inner anchoring section is located at the bottom of the anchor cable hole, and the steel strand is bonded to the underlying stabilizing layer by grouting; an isolation sleeve is provided on the outside of the steel strand in the free section; the outer anchoring section is set at the opening of the anchor cable hole and is used to lock the prestress.

[0013] In one exemplary embodiment of this disclosure, the outer anchoring section of the first prestressed anchor cable is embedded inside the foundation slab of the backwater dam; the outer anchoring section of the second prestressed anchor cable is embedded in a trench in the area between the overflow dam and the backwater dam, and the trench is covered with concrete.

[0014] In one exemplary embodiment of this disclosure, the method further includes: When the trench is covered with concrete, steel bars are installed on the sidewalls of the trench to anchor the covering concrete to the existing concrete or rock in the trench.

[0015] In one exemplary embodiment of this disclosure, both the first prestressed anchor cable and the second prestressed anchor cable are arranged in a quincunx pattern with a spacing of 2 to 5 meters.

[0016] In one exemplary embodiment of this disclosure, a stilling basin is provided downstream of the dam to dissipate energy from the water flow discharged from the dam.

[0017] In one exemplary embodiment of this disclosure, the backwater dam, the first prestressed anchor cable, and the second prestressed anchor cable work together to increase the vertical load on the upper rock mass of the weak interlayer, improve the shear strength index of the weak interlayer, and maintain the deep anti-sliding stability of the overflow dam and the upper rock mass.

[0018] The exemplary embodiments disclosed herein have the following beneficial effects: A buffer dam is constructed downstream of the spillway dam; the foundation of the buffer dam is embedded below a weak interlayer; a first prestressed anchor cable is installed in the stable layer below the foundation of the buffer dam to anchor the first prestressed anchor cable to the stable layer below the buffer dam; a second prestressed anchor cable is installed in the area between the spillway dam and the buffer dam to pass through the weak interlayer and anchor to the stable layer below the area between the spillway dam and the buffer dam. On the one hand, by constructing a buffer dam downstream of the spillway dam and embedding its foundation below the weak interlayer, the buffer dam's own anti-sliding stability and the resistance of the underlying rock can be utilized to provide reliable downstream resistance support for the rock mass above the weak interlayer and the spillway dam, thereby maintaining the deep anti-sliding stability of the dam body. On the other hand, by constructing the first prestressed anchor cable at the base of the impoundment dam foundation and anchoring it to the underlying stable layer, and constructing the second prestressed anchor cable in the area between the spillway and the impoundment dam to pass through the weak interlayer and anchor it to the underlying stable layer, the vertical load on the rock mass above the weak interlayer can be actively increased, significantly improving the comprehensive shear strength index of the weak interlayer, thereby further enhancing the anti-sliding force of the spillway dam and the upper rock mass along the weak interlayer. The impoundment dam and the two sets of prestressed anchor cables work together to form a comprehensive reinforcement system combining passive resistance and active anchoring, which can effectively solve the deep anti-sliding stability risk caused by the downstream scour cutting through the weak interlayer of the existing spillway dam, and ensure the long-term operational safety of the dam.

[0019] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure. It is obvious that the drawings described below are merely some embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0021] Figure 1 A flowchart illustrating a method for treating weak interlayers under the foundation of a gravity dam is shown. Figure 2 A schematic diagram of the cross-section of the spillway dam, bedrock, and downstream scour crater in the prior art is shown. Figure 3 This schematic diagram illustrates a cross-sectional view of the spillway and dam area in this exemplary embodiment. Figure 4 The diagram schematically illustrates a structural design of prestressed anchor cables beneath the foundation of a backwater dam. Figure 5 The diagram schematically illustrates a structural design of prestressed anchor cables in the area between an overflow dam and a backwater dam. Figure 6 A schematic diagram of the external anchorage section of a prestressed anchor cable is shown. Among them, 1. Overflow dam body; 1-1. Weir crest; 1-2. Sloping section; 1-3. Reverse arc section; 1-5. Flow-off nose sill; 2. Downstream apron of the overflow dam; 3. Upper rock mass; 4. Lower rock mass; 5. Weak interlayer; 6. Scour pit; 7. Impoundment dam; 7-1. Water cushion basin; 7-2. Water cushion basin bottom slab; 7-3. Impoundment dam crest; 7-4. Upstream slope of the water cushion basin; 7-5. Impoundment dam foundation; 7-6. Impoundment dam Downstream stilling basin; 7-7, Upstream slope of the impounding dam (downstream slope of the stilling basin); 7-8, Downstream slope of the impounding dam; 8, Prestressed anchor cable; 8-1, Inner anchoring section; 8-2, Free section; 8-3, Outer anchoring section; 8-4, Anchor cable body; 8-5, Pre-installed steel pipe in the anchor cable hole; 8-6, Trapezoidal groove backfill concrete; 8-7, Bearing plate; 8-8, Bearing plate anchor bar; 8-9, Working anchor plate; 8-10, Anchor pad plate. Detailed Implementation

[0022] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided so that this disclosure will be more comprehensive and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0023] The exemplary embodiments disclosed herein first provide a method for treating weak interlayers beneath the foundation of a gravity dam. The gravity dam includes an overflow dam, and the foundation of the gravity dam includes a weak interlayer. A gravity dam refers to a concrete or masonry dam that maintains stability by its own weight, such as a solid gravity dam or a wide-joint gravity dam. An overflow dam is the section of a gravity dam used to discharge water flow and can consist of an overflow weir, a sloping section, a reverse arc section, and a spillway nose, such as an open overflow dam or a breast wall overflow dam. A weak interlayer refers to a thin layer or banded geological body located within the foundation of the gravity dam, whose shear strength and elastic modulus are significantly lower than the surrounding rock mass, such as mudstone interlayers, fractured interlayers, or interlayer shear zones.

[0024] The following is in conjunction with the appendix Figure 1 The exemplary embodiments will be further described as follows: Figure 1 As shown, the method for treating weak interlayers under the foundation of a gravity dam may include the following steps S110~S130: Step S110: Construct a backwater dam downstream of the overflow dam; the foundation of the backwater dam is buried below the weak interlayer.

[0025] The spillway dam is an important component of the gravity dam, constructed of cast concrete. Its main function is to discharge water flow. It mainly consists of a spillway weir, a ramp section, a reverse arc section, and a jet nose. The spillway weir controls the upstream water level and ensures smooth flow, guaranteeing that the water is discharged at an appropriate flow rate. The ramp section guides the water flow smoothly down the dam face, preventing unfavorable flow patterns or negative pressure. The reverse arc section smoothly redirects the discharged water flow from the ramp direction to the jet direction, preventing the water flow from leaving the dam face. The jet nose jets the high-speed water flow to the downstream riverbed far from the dam toe, where it is diffused in the air and dissipated by the downstream water cushion, thus protecting the dam foundation from erosion.

[0026] Due to the long-term flow diversion and energy dissipation caused by the spillway dam, the downstream riverbed is eroded by high-speed water flow, forming scour pits. When the depth of the scour pit exceeds the burial depth of the weak interlayer, the weak interlayer will be exposed on the sidewall of the scour pit, causing the upstream dam foundation rock mass to lose the resistance support of the downstream rock mass, thus triggering a significant risk of dam sliding along the weak interlayer. To address this, a backwater dam can be constructed downstream of the spillway dam, with its foundation buried below the weak interlayer. In particular, a backwater dam can be constructed at the location of the flow diversion scour pit downstream of the spillway dam.

[0027] A backwater dam is a water-retaining structure built downstream of an overflow dam to raise the water level and form a water cushion, such as a concrete backwater dam or a masonry backwater dam. Downstream of the overflow dam refers to the riverbed area behind the spillway sill. Backwater dams can be constructed using cast-in-place concrete or by stacking precast concrete blocks. The foundation depth of the backwater dam can be determined based on geological conditions. For example, the elevation of the top surface of the weak interlayer can be determined through borehole exploration, and the foundation bottom of the backwater dam can be designed below that elevation. For instance, in a certain water conservancy project, the weak interlayer is located 8 meters below the dam foundation, and the foundation bottom of the backwater dam is buried 2 meters below the weak interlayer, meaning the foundation bottom is located 10 meters below the ground surface. In another embodiment, if the weak interlayer is deeply buried, the foundation of the backwater dam can be placed directly on a slightly weathered rock layer 3 meters below the weak interlayer, taking into account the integrity of the rock mass.

[0028] Figure 2 The diagram schematically illustrates the cross-sectional views of the spillway dam, bedrock, and downstream scour pit in the prior art. Figure 3 The diagram schematically illustrates a cross-sectional view of the spillway and dam area in this exemplary embodiment.

[0029] Step S120: Construct the first prestressed anchor cable in the lower stable layer below the foundation of the impounding dam to anchor the first prestressed anchor cable to the lower stable layer of the impounding dam.

[0030] A stable layer refers to a stratum that can provide sufficient anchoring force, such as hard bedrock, weakly weathered rock, or dense gravel. A prestressed anchor cable is a tension member that actively applies prestress to the stratum or structure through tension locking, such as a pressure-dispersing anchor cable or a tension anchor cable composed of high-strength steel strands. The construction of prestressed anchor cables may include drilling, cable insertion, grouting, and tension locking. Constructing the first prestressed anchor cable in the stable layer below the foundation of a dam can be achieved by laying prestressed anchor cables below the dam foundation and anchoring them to the underlying hard bedrock, thereby increasing the anti-sliding force between the dam and the foundation by applying tension. For example, a down-the-hole drill can be used to drill anchor cable holes with a diameter of 150 mm in the lower part of the dam foundation, penetrating 8 meters into the stable layer. The anchor cable body is placed into the hole, and cement grout is injected at the bottom to form an inner anchoring section. Tension locking is then performed after the grout reaches the design strength. For stable layers with poor geological conditions, hole enlargement or secondary grouting processes can be used to improve anchoring force. Figure 4 The diagram schematically illustrates a structural design of a prestressed anchor cable beneath the foundation of a dam.

[0031] Step S130: Construct a second prestressed anchor cable in the area between the spillway and the backwater dam, so that the second prestressed anchor cable passes through the weak interlayer and is anchored to the stable layer below the area between the spillway and the backwater dam.

[0032] The area between the spillway and the impoundment refers to the riverbed or bedrock area between them that is not covered by any structures. Passing through a weak interlayer means that the anchor cable borehole must penetrate the weak interlayer, ensuring the anchoring section is located within the stable layer below the interlayer. In this exemplary embodiment, prestressed anchor cables, i.e., second prestressed anchor cables, can also be installed in the area between the spillway and the impoundment. This increases the anti-sliding force of the weak interlayer by applying tension between the rocks above and below it. Exemplarily, a geological drilling rig is used to drill holes from the downstream revetment surface of the spillway. The hole depth must penetrate the weak interlayer and reach the underlying stable layer by at least 5 meters. After the anchor cables are installed, the section of the hole penetrating the interlayer is grouted to enhance the shear resistance at the interlayer. In another embodiment, multiple anchor cable holes can be arranged in this area to form an anchor cable group. The drilling direction can be adjusted according to the rock strata attitude to ensure that the anchor cable passes through the weak interlayer and the anchoring section is located within the stable layer.

[0033] By constructing a backwater dam and embedding it below the weak interlayer, combined with the active anchoring of two sets of prestressed anchor cables, the vertical load on the overlying rock mass of the weak interlayer can be effectively increased, and the shear strength index of the weak interlayer can be improved. This solves the deep anti-sliding stability problem caused by the downstream scour cutting off the weak interlayer of the existing spillway dam, and enhances the safety of the dam.

[0034] The anchor bodies of the first and second prestressed anchor cables can be woven from multiple strands of high-strength, low-relaxation steel strands. It should be noted that the terms "first" and "second" in this disclosure are only used to distinguish anchor cables in different locations and do not indicate any difference in order, priority, or importance, nor do they imply that the two are necessarily different in structure or function. In practice, the first and second prestressed anchor cables can use the same or different materials, specifications, and construction techniques, and the specific parameters can be independently determined according to project needs.

[0035] Figure 5 The diagram schematically illustrates the structural design of a prestressed anchor cable in the area between an overflow dam and a backwater dam.

[0036] Based on the above description, in this exemplary embodiment, a backwater dam is constructed downstream of the spillway dam; the foundation of the backwater dam is embedded below a weak interlayer; a first prestressed anchor cable is constructed in the stable layer below the foundation of the backwater dam to anchor the first prestressed anchor cable to the stable layer below the backwater dam; a second prestressed anchor cable is constructed in the area between the spillway dam and the backwater dam to allow the second prestressed anchor cable to pass through the weak interlayer and be anchored to the stable layer below the area between the spillway dam and the backwater dam. On the one hand, by constructing a backwater dam downstream of the spillway dam and embedding the foundation of the backwater dam below the weak interlayer, the anti-sliding stability of the backwater dam itself and the resistance of the rock below it can be utilized to provide reliable downstream resistance support for the rock mass above the weak interlayer and the spillway dam, thereby maintaining the deep anti-sliding stability of the dam body. On the other hand, by constructing the first prestressed anchor cable at the base of the impoundment dam foundation and anchoring it to the underlying stable layer, and constructing the second prestressed anchor cable in the area between the spillway and the impoundment dam to pass through the weak interlayer and anchor it to the underlying stable layer, the vertical load on the rock mass above the weak interlayer can be actively increased, significantly improving the comprehensive shear strength index of the weak interlayer, thereby further enhancing the anti-sliding force of the spillway dam and the upper rock mass along the weak interlayer. The impoundment dam and the two sets of prestressed anchor cables work together to form a comprehensive reinforcement system combining passive resistance and active anchoring, which can effectively solve the deep anti-sliding stability risk caused by the downstream scour cutting through the weak interlayer of the existing spillway dam, and ensure the long-term operational safety of the dam.

[0037] In an exemplary embodiment, the backwater dam includes a backwater dam body and a cushion basin, the cushion basin being used to form a cushion to dissipate energy from the water flow out of the overflow dam.

[0038] A dam consists of the dam body and a stilling basin. The dam body refers to the main water-retaining structure of the dam, such as a concrete gravity dam or a buttress dam. The stilling basin is a pool-like structure located upstream of the dam, used to store water for energy dissipation, such as a rectangular or trapezoidal stilling basin. The stilling basin forms a water cushion to dissipate the energy of the water flowing out of the overflow dam. The water cushion is a layer of still or slowly flowing water of a certain thickness within the stilling basin, its function being to utilize the viscosity and turbulent diffusion of water to dissipate the kinetic energy of the downstream flow. Energy dissipation can be achieved by the downstream flow directly impacting the water cushion to create a hydraulic jump, or by the flow swirling within the water cushion to dissipate energy. For example, when constructing a backwater dam downstream of an overflow dam, the cushion basin is designed as a trapezoidal cross-section with a bottom width of 20 meters and an upstream slope ratio of 1:2. The water depth within the basin is determined by calculation. When the overflow dam discharges water, the high-speed water flow is ejected into the cushion basin. Under the obstruction and swirling effect of the cushion, the flow velocity is rapidly reduced, preventing scouring and damage to the downstream riverbed. In another embodiment, the cushion basin can be integrally cast with the backwater dam body to form a unified structure, with drainage facilities installed at the bottom of the basin to facilitate maintenance. By setting up a cushion basin, the riverbed and banks downstream of the backwater dam can be effectively protected, extending the service life of the backwater dam.

[0039] The downstream impoundment dam is located at the scour pit downstream of the overflow dam. It consists of the impoundment dam body and the stilling basin. Its main functions are twofold: firstly, to raise the water level and form a water cushion of a certain thickness with the stilling basin to dissipate the energy of the water flow from the overflow dam and prevent the high-energy water flow from scouring downstream structures; secondly, the impoundment dam and the stilling basin are large-volume concrete structures with their foundations buried at least 2 meters below the weak interlayer. By utilizing the structure's own anti-sliding stability and the resistance of the downstream rock, it maintains the deep anti-sliding stability of the rock mass above the weak interlayer and the overflow dam.

[0040] In an exemplary embodiment, a stilling basin is provided downstream of the impounding dam to dissipate energy from the water flow discharged from the impounding dam.

[0041] A stilling basin is installed downstream of a dam to dissipate the energy of the water flowing down from the dam. A stilling basin is a water structure located downstream of the dam, designed to dissipate the remaining energy of the flowing water; examples include tailrace stilling basins and combined stilling basins. The length and depth of the stilling basin are determined by hydraulic calculations, typically ranging from 10 to 20 meters in length, with drainage holes at the bottom. For example, in a dam with a downstream slope ratio of 1:3, a 15-meter-long reinforced concrete stilling basin with a depth of 1.2 meters is installed immediately adjacent to the dam toe, and a tailrace is located at the end of the basin. When water overflows from the dam, it undergoes a hydraulic jump within the stilling basin to dissipate energy before flowing out smoothly. In another embodiment, for high-head projects, stilling piers or toe piers can be installed within the stilling basin to enhance the energy dissipation effect. By installing a stilling basin, the scouring effect of the dam's discharge on the downstream riverbed can be further reduced, protecting the downstream environment.

[0042] In this exemplary embodiment, the overflow section of the gravity dam, the stilling basin, the impoundment dam, and the downstream stilling basin can be arranged sequentially along the water flow direction (i.e., from upstream to downstream). The stilling basin is located upstream of the impoundment dam, and the upstream slope of the impoundment dam is also the downstream slope of the stilling basin. The high-speed water jet ejected from the overflow dam's jet sill first falls into the stilling basin, where initial energy dissipation occurs within the water cushion formed by the stilling basin. After energy dissipation in the stilling basin, the water flow is further impeded by the impoundment dam, raising the water level and further dissipating energy. The water then overflows the top of the impoundment dam and flows downstream. A stilling basin is located downstream of the impoundment dam to provide final energy dissipation for the downstream flow, ensuring a smooth flow into the downstream riverbed and preventing scouring damage. The combined arrangement of the above-mentioned "water cushion pool - backwater dam - stilling basin" can effectively eliminate the energy of the high-speed water flow, while the structure of the backwater dam and the anchor cable system can provide deep anti-sliding support for the upstream weak interlayer rock mass.

[0043] In an exemplary embodiment, the crest elevation of the impounding dam is determined according to the required water cushion thickness, and the distance from the crest of the impounding dam to the bottom of the water cushion pool is not less than the water cushion thickness; wherein, the water cushion thickness refers to the vertical depth of the water cushion formed in the water cushion pool.

[0044] The dam crest elevation refers to the elevation of the water-retaining surface at the top of the dam, such as the design dam crest elevation marked on the construction drawings. The required water cushion thickness refers to the minimum water depth determined by hydraulic calculations to meet energy dissipation requirements, such as the value calculated using the formula for the thickness of the water cushion for energy dissipation in the "Code for Design of Concrete Gravity Dams". The distance from the dam crest to the bottom of the water cushion basin should not be less than this water cushion thickness; that is, the effective water-retaining height of the dam must be greater than or equal to the water depth required for energy dissipation. The water cushion thickness refers to the vertical depth of the water cushion formed within the water cushion basin, i.e., the vertical distance from the bottom of the water cushion basin to the upper surface of the water cushion. For example, if a project calculates that the required water cushion thickness for energy dissipation is 5.5 meters, then the dam crest elevation should be at least 5.5 meters higher than the bottom elevation of the water cushion basin. In actual design, the distance from the dam crest to the basin bottom is taken as 6.0 meters to allow for a certain safety margin. In another embodiment, the required water cushion thickness may vary for floods of different frequencies. An overflow outlet can be installed on the dam crest to adjust the backwater height, ensuring the water cushion thickness meets energy dissipation requirements under common flood conditions. By limiting the distance from the dam crest to the pool bottom to be no less than the required water cushion thickness, sufficient energy dissipation depth in the water cushion pool can be guaranteed under any discharge conditions, preventing high-energy water flow from scouring and damaging downstream structures.

[0045] In an exemplary embodiment, the slope ratio of the upstream slope of the impounding dam ranges from 1:0.5 to 1:1, the slope ratio of the downstream slope of the impounding dam ranges from 1:2 to 1:4, and the slope ratio of the upstream slope of the cushion pool ranges from 1:2 to 1:3.

[0046] In this context, the upstream slope of a dam refers to the slope of the dam on the side facing the stilling basin, while the downstream slope refers to the slope of the dam facing away from the stilling basin. The upstream slope of the stilling basin refers to the slope of the stilling basin on the side furthest from the dam. The slope ratio is the ratio of vertical height to horizontal distance; for example, 1:0.5 means that for every 1 meter of vertical elevation, the horizontal slope is reduced by 0.5 meters. The upstream slope ratio of a dam ranges from 1:0.5 to 1:1, meaning the slope can be designed to be a steeper 1:0.5, a gentler 1:1, or any ratio in between. The downstream slope ratio ranges from 1:2 to 1:4, meaning a gentler slope that facilitates a smooth flow of water into the stilling basin. The upstream slope ratio of the stilling basin ranges from 1:2 to 1:3, a slope that promotes a stable hydraulic jump within the basin. For example, the upstream slope of a certain impoundment dam adopts a slope ratio of 1:0.75, the downstream slope adopts a slope ratio of 1:3, and the upstream slope of the stilling basin adopts a slope ratio of 1:2.5. Model tests have verified that the energy dissipation effect is good. In other embodiments, for situations with low foundation bearing capacity or high requirements for anti-sliding stability, gentler slope ratios can be selected to improve the stress conditions of the dam body, such as upstream and downstream slopes of 1:1 and 1:4 respectively. By limiting the range of slope ratios, the impoundment dam structure can possess good stability and hydraulic performance under different geological and discharge conditions.

[0047] In one exemplary embodiment, the foundation of the impoundment dam is buried below a weak interlayer and may include: The foundation of the impounding dam is buried at least 2 meters below the weak interlayer.

[0048] The foundation of the impounding dam must be buried at least 2 meters below the weak interlayer. This means that the vertical distance from the bottom surface (or lower interface) of the weak interlayer to the bottom surface of the impounding dam foundation must be no less than 2 meters. This requirement is to ensure that the foundation of the impounding dam can be embedded in the stable strata below the weak interlayer, allowing the impounding dam to provide sufficient downstream resistance to the upstream area through its own weight and the resistance of the foundation. For example, if the thickness of the weak interlayer is 0.8 meters and its bottom surface is 9 meters below the ground surface, then the bottom surface of the impounding dam foundation should be buried at least 11 meters (i.e., 9 meters + 2 meters) below the ground surface. In another embodiment, when the geological conditions are complex and the weak interlayer has large undulations, the burial depth can be controlled according to the most unfavorable location, that is, the foundation of the entire impounding dam must be placed 2 meters below the lowest point of the weak interlayer. This required burial depth ensures that the backwater dam is in direct contact with the underlying stable layer, preventing the dam itself from sliding along the weak interlayer and thus providing stable resistance to the upstream dam body.

[0049] In one exemplary embodiment, the method may further include: A steel pipe is installed in the anchor hole of the first prestressed anchor cable and the second prestressed anchor cable; The bottom of the steel pipe extends at least 0.5 meters below the stable layer at the bottom of the weak interlayer, and the wall thickness of the steel pipe ranges from 0.8 to 1.2 centimeters, while the inner diameter ranges from 10 to 15 centimeters.

[0050] This exemplary embodiment may include steel pipes installed within the anchor holes of both the first and second prestressed anchor cables. An anchor hole refers to a channel drilled for the construction of the prestressed anchor cable, such as a 130 mm diameter hole drilled using a down-the-hole drill. The steel pipe refers to a pre-processed circular metal pipe, such as a seamless or welded steel pipe. The bottom of the steel pipe extends at least 0.5 meters below the stable layer at the bottom of the weak interlayer; that is, the lower end of the steel pipe must pass through the weak interlayer and enter the lower stable layer by at least 0.5 meters. Specifically, this can mean that the bottom of the steel pipe is higher than the top of the inner anchoring section and extends at least 0.5 meters below the bedrock or concrete-bedrock contact surface at the bottom of the weak interlayer, with the top of the steel pipe flush with the top opening of the anchor hole.

[0051] The wall thickness of the steel pipe refers to the thickness of the pipe wall, such as 0.8 cm, 1.0 cm, or 1.2 cm. The inner diameter refers to the diameter of the internal passage of the steel pipe. This range covers commonly used specifications for steel pipes used in anchor cable hole protection in conventional engineering. For example, a project selects a steel pipe with a wall thickness of 1.0 cm and an inner diameter of 12 cm to match the diameter of the anchor cable body (composed of 7 bundles of 15.2 mm steel strands), ensuring both smooth cable threading and sufficient shear stiffness. In another embodiment, for projects with higher anchoring force requirements, a steel pipe with a wall thickness of 1.2 cm and an inner diameter of 15 cm can be selected, while the number of steel strand bundles is increased to 12. By limiting the range of wall thickness and inner diameter, it can be ensured that the steel pipe has sufficient shear strength and appropriate cable passage space to adapt to the requirements of different anchoring loads.

[0052] The functions of the steel pipes can include guiding the borehole to prevent deviation; protecting the borehole wall to prevent collapse; and enhancing shear resistance by utilizing the high shear strength of steel to improve the shear performance of the anchor cable hole when passing through weak interlayers. For example, when constructing prestressed anchor cables in a dam foundation containing muddy interlayers, steel pipes with a wall thickness of 1 cm and an inner diameter of 12 cm are welded in sections and lowered into the borehole, ensuring the bottom of the pipe penetrates 1 meter into the stable layer. Then, the anchor cable is inserted into the steel pipe and grout is injected. In another embodiment, for anchor cables with larger borehole depths, multiple steel pipes can be welded end-to-end, with guide clamps at the joints to ensure smooth lowering. By using steel pipes, the shear resistance of the anchor cable near weak interlayers can be significantly improved, preventing the anchor cable from being sheared under horizontal thrust and ensuring the anchoring effect.

[0053] In one exemplary embodiment, the method may further include: When constructing the first and second prestressed anchor cables, corresponding anchor cable holes are formed; Both the first and second prestressed anchor cables include an inner anchorage section, a free section, and an outer anchorage section. The inner anchorage section is located at the bottom of the anchor cable hole, and the steel strand is bonded to the underlying stabilizing layer by grouting. An isolation sleeve is fitted on the outside of the steel strand in the free section. The outer anchorage section is located at the opening of the anchor cable hole and is used to lock the prestress.

[0054] Forming corresponding anchor holes refers to drilling ducts at their respective locations during the construction of the first and second prestressed anchor cables. Prestressed anchor cables include an inner anchorage section, a free section, and an outer anchorage section. The inner anchorage section is located at the bottom of the anchor hole. Grouting is used to bond the steel strands to the underlying stable layer. Grouting can use pure cement grout or cement mortar. For example, injecting cement grout bonds the steel strands to the underlying hard bedrock or stable rock mass, forming a reliable anchorage section. The height of the inner anchorage section can be 4 meters to 6.5 meters. An isolation sleeve is fitted around the outside of the steel strands in the free section. The purpose of the isolation sleeve is to prevent adhesion between the steel strands and the hole wall, ensuring that this section can freely and elastically elongate. The isolation sleeve can be a high-density polyethylene pipe or a corrugated pipe. The outer anchorage section is located at the opening of the anchor hole and is used to lock the prestress. It typically includes a bearing plate, anchor plate, working anchor, and anchorage device. For example, during the construction of the first prestressed anchor cable, the drilling depth is 20 meters, with the bottom of the hole penetrating 6 meters into the stable layer. The anchor cable body consists of 12 steel strands. No isolation sleeve is installed within 4 meters of the bottom of the hole, forming the inner anchoring section, which is filled with M30 cement mortar. The section from the inner anchoring section to the hole opening is a free section, with plastic sleeves covering the steel strands. A 2-cm thick steel plate bearing plate and a 6-hole anchor are installed at the hole opening, and the cable is locked after tensioning to the design load. In another embodiment, the second prestressed anchor cable can be designed as a pressure-dispersing type, i.e., multiple bearing plates are installed in the inner anchoring section to disperse and transfer the prestress to the stable layer at different depths, improving anchoring efficiency. By clearly defining the structural composition of the anchor cable, it is possible to ensure that the prestress is effectively transferred to the stable layer, achieving active anchoring of the superstructure. Figure 6 The schematic diagram shows a detailed structural diagram of the external anchorage section of a prestressed anchor cable.

[0055] In an exemplary embodiment, the outer anchoring section of the first prestressed anchor cable is embedded inside the foundation slab of the impounding dam; the outer anchoring section of the second prestressed anchor cable is embedded in a trench in the area between the spillway and the impounding dam, and the trench is covered with concrete.

[0056] The foundation slab of the impounding dam can be a reinforced concrete slab at the bottom of the impounding dam, for example, a foundation slab with a thickness of 1.5 meters to 3 meters. "Embedded within the foundation slab" means that after the outer anchoring section is tensioned and locked, this part is directly poured into the foundation concrete of the impounding dam, without the need for a separate protective structure.

[0057] The outer anchorage section of the second prestressed anchor cable is embedded in a trench in the area between the spillway and the impoundment dam, and the trench is covered with concrete. The trench refers to a manually excavated trench in this area, such as a rectangular or inverted trapezoidal trench with a width of 0.8 meters and a depth of 0.6 meters. Covering the trench with concrete means completely filling it with concrete, thus protecting the outer anchorage section. For example, for a second prestressed anchor cable located on the downstream apron of the spillway, a trapezoidal trench 2 meters long, 1 meter wide, and 0.8 meters deep is excavated first. The outer anchorage section of the anchor cable is placed at the bottom of the trench. After locking the prestress, the trench is backfilled with C30 concrete until it is level with the original ground level. In another embodiment, if the area is a rock foundation, the trench can be directly chiseled into the rock. After the anchor cable is installed, micro-expansion concrete is used for backfilling, making the outer anchorage section an integral part of the rock. By adopting different concealment treatments for the outer anchorage sections of the two sets of anchor cables, it is possible to avoid the outer anchorage sections being exposed and affecting the flow or being damaged by water erosion, while ensuring the long-term effectiveness of the prestress.

[0058] This exemplary embodiment employs differentiated concealed protection treatment for the outer anchorage sections of prestressed anchor cables in different regions. For the first prestressed anchor cable located below the impoundment dam foundation, its outer anchorage section can be directly cast into the bottom slab of the stilling basin, utilizing the concrete of the bottom slab for permanent protection. For prestressed anchor cables located in the area between the spillway and the impoundment dam, a trapezoidal groove can be excavated in the existing concrete bottom slab (rock), and the outer anchorage section can be placed within the groove. After the anchor cable construction is completed, the trapezoidal groove is backfilled and compacted with concrete, while reinforcing bars are inserted into the sidewalls of the trapezoidal groove. This ensures reliable anchorage between the backfill concrete and the existing bottom slab concrete (rock), enhancing the connection between the two. Through these methods, the exposed outer anchorage section can be effectively prevented from affecting flow or being damaged by water erosion.

[0059] In one exemplary embodiment, the above method may further include: When using concrete to cover the trench, steel bars are installed on the sidewalls of the trench to anchor the covering concrete to the existing concrete or rock inside the trench.

[0060] Reinforcing steel bars are installed by drilling holes in the sidewalls of the trench, inserting reinforcing bars, and grouting them for anchoring. A portion of the reinforcing bars protrudes into the trench and is integrated with the overlay concrete. The existing concrete in the trench refers to the concrete structure that existed in the area before trenching, such as the downstream apron concrete of an overflow dam. "Rock" refers to natural bedrock. The reinforcing steel bars prevent cracks from forming between the overlay concrete and the original foundation due to shrinkage or temperature changes, thus avoiding concrete detachment. For example, holes with a diameter of 25 mm and a depth of 0.3 m are drilled every 0.5 m on both sides of the trench. HRB400 reinforcing bars with a length of 0.5 m are inserted, and the protruding ends are welded to the reinforcing mesh in the overlay concrete before concrete is poured. In another embodiment, for rock sidewalls, rebar anchoring adhesive can be used to anchor the reinforcing steel bars, ensuring the anchoring depth meets pull-out resistance requirements. By installing reinforcing steel bars, a reliable anchoring connection is formed between the overlay concrete and the original foundation, capable of withstanding the impact and pulsating pressure of water flow, ensuring the long-term safe operation of the outer anchoring section.

[0061] In one exemplary embodiment, both the first prestressed anchor cable and the second prestressed anchor cable are arranged in a quincunx pattern with a spacing of 2 to 5 meters.

[0062] The quincunx pattern arrangement can be achieved by arranging anchor cables on a plane in the form of vertices of an equilateral triangle or a regular hexagon, with adjacent rows of anchor cables staggered, resembling a quincunx. The spacing refers to the horizontal distance between the center points of two adjacent anchor cables, which can be between 2 and 5 meters, such as 2.5 meters, 3 meters, or 4 meters. For example, below the foundation of a dam, the first group of prestressed anchor cables is arranged with a row spacing of 3.5 meters, a column spacing of 3.5 meters, and adjacent rows staggered by 1.75 meters, forming a quincunx grid. In another embodiment, in the area between the spillway and the dam, due to concentrated stress, a smaller spacing of 2.5 meters can be used for the second group of anchor cables, while a 4-meter spacing is used in the edge areas. This quincunx arrangement allows for more anchor cables to be placed per unit area, while reducing stress concentration, resulting in a more uniform distribution of anchoring force and improved overall anti-sliding stability.

[0063] In an exemplary embodiment, the backwater dam, the first prestressed anchor cable, and the second prestressed anchor cable work together to increase the vertical load on the upper rock mass of the weak interlayer, improve the shear strength index of the weak interlayer, and maintain the deep anti-sliding stability of the spillway dam and the upper rock mass.

[0064] The combined action of the impoundment dam, the first prestressed anchor cable, and the second prestressed anchor cable refers to their coordinated operation. The impoundment dam provides passive resistance through its own weight and foundation embedment. The first prestressed anchor cable increases the anti-sliding force by anchoring the impoundment dam foundation. The second prestressed anchor cable applies prestress by penetrating the weak interlayer, increasing the normal compressive stress of the overlying rock mass. Vertical load refers to the pressure perpendicular to the weak interlayer, such as the pressure transmitted from the anchor cable tension and the gravity of the impoundment dam. Shear strength index refers to the shear resistance parameters of the weak interlayer, including cohesion and internal friction angle. Deep anti-sliding stability refers to the safety of overall sliding along the weak interlayer under the dam foundation. For example, a gravity dam spillway section has a muddy interlayer under its foundation. The calculated anti-sliding stability coefficient along the interlayer is only 1.05, which does not meet the code requirements. After constructing the impoundment dam using this method and installing two sets of prestressed anchor cables, the normal stress of the interlayer increased by approximately 0.5 MPa through anchor cable tensioning, resulting in an overall shear strength increase of about 30% and a sliding stability coefficient improvement to 1.35, meeting safety requirements. In another embodiment, for weak interlayers with different dip angles, the anchor cable angle can be adjusted to make the prestress direction as perpendicular as possible to the interlayer surface to achieve the best stabilization effect. Through the combined effect of these three factors, the deep sliding stability problem caused by the downstream scour of the spillway cutting off the weak interlayer can be effectively solved, ensuring the long-term safe operation of the dam.

[0065] It should be understood that this disclosure is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this disclosure is defined only by the appended claims.

Claims

1. A method for treating weak interlayers under the foundation of a gravity dam, characterized in that, The gravity dam includes an overflow dam, and the dam foundation includes a weak interlayer; the method includes: A backwater dam is constructed downstream of the overflow dam; the foundation of the backwater dam is buried below the weak interlayer. A first prestressed anchor cable is constructed in the lower stabilizing layer below the foundation of the dam to anchor the first prestressed anchor cable to the lower stabilizing layer of the dam. A second prestressed anchor cable is constructed in the area between the spillway and the backwater dam, so that the second prestressed anchor cable passes through the weak interlayer and is anchored to the stabilizing layer below the area between the spillway and the backwater dam.

2. The method according to claim 1, characterized in that, The dam includes a dam body and a cushion basin, the cushion basin being used to form a cushion to dissipate energy from the water flow out of the overflow dam.

3. The method according to claim 2, characterized in that, The elevation of the dam crest is determined based on the required water cushion thickness, and the distance from the dam crest to the bottom of the water cushion pool is not less than the water cushion thickness; wherein, the water cushion thickness refers to the vertical depth of the water cushion formed in the water cushion pool.

4. The method according to claim 2, characterized in that, The slope ratio of the upstream slope of the impounding dam ranges from 1:0.5 to 1:1, the slope ratio of the downstream slope of the impounding dam ranges from 1:2 to 1:4, and the slope ratio of the upstream slope of the water cushion pool ranges from 1:2 to 1:

3.

5. The method according to claim 1, characterized in that, The foundation of the dam is embedded below the weak interlayer, including: The foundation of the dam is buried at least 2 meters below the weak interlayer.

6. The method according to claim 1, characterized in that, The method further includes: A steel pipe is installed in the anchor hole of the first prestressed anchor cable and the second prestressed anchor cable; The bottom of the steel pipe extends at least 0.5 meters below the stable layer at the bottom of the weak interlayer, and the wall thickness of the steel pipe ranges from 0.8 to 1.2 centimeters, while the inner diameter ranges from 10 to 15 centimeters.

7. The method according to claim 1, characterized in that, The method further includes: During the construction of the first prestressed anchor cable and the second prestressed anchor cable, corresponding anchor cable holes are formed; Both the first prestressed anchor cable and the second prestressed anchor cable include an inner anchoring section, a free section, and an outer anchoring section; the inner anchoring section is located at the bottom of the anchor cable hole, and the steel strand is bonded to the underlying stabilizing layer by grouting; an isolation sleeve is provided on the outside of the steel strand in the free section; the outer anchoring section is set at the opening of the anchor cable hole and is used to lock the prestress.

8. The method according to claim 7, characterized in that, The outer anchoring section of the first prestressed anchor cable is embedded inside the foundation slab of the backwater dam; the outer anchoring section of the second prestressed anchor cable is embedded in the trench between the overflow dam and the backwater dam, and the trench is covered with concrete.

9. The method according to claim 8, characterized in that, The method further includes: When the trench is covered with concrete, steel bars are installed on the sidewalls of the trench to anchor the covering concrete to the existing concrete or rock in the trench.

10. The method according to claim 1, characterized in that, Both the first and second prestressed anchor cables are arranged in a quincunx pattern, with a spacing of 2 to 5 meters.

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