Treatment method of deep soft soil in earth-rock dam foundation and earth-rock dam

Abstract

The present invention relates to a method for treating deep soft soil in an earth-rock dam foundation and an earth-rock dam. The method for treating deep soft soil in an earth-rock dam foundation comprises determining a dam foundation excavation surface, wherein the dam foundation excavation surface is generally trough-shaped, the trough bottom surface corresponds to the dam anti-seepage body and extends to below the soft soil layer, and the trough walls on the upper and downstream sides of the trough bottom surface are slopes inclined downward toward the trough bottom; before excavating the soft soil in the dam foundation, vibratory crushed stone piles are first constructed on the upper foundation upstream and downstream of the dam anti-seepage body, the pile holes pass through the dam foundation excavation surface, and only holes are drilled above the dam foundation excavation surface without backfilling with crushed stone, and only crushed stone is backfilled below the dam foundation excavation surface, forming crushed stone piles below the dam foundation excavation surface; if the soft soil below the excavation surface is not treated, the soft soil slope cannot be stabilized and construction is also very difficult; and generally, excavating deep soft soil is very difficult, so only after treatment with crushed stone piles can the stability of the excavated slope be guaranteed, and the core anti-seepage body of the dam can be placed on a non-soft soil foundation after excavation.

Description

Technical Field

[0001] The present invention relates to a method for treating thick soft soil in an earth-rock dam foundation and an earth-rock dam, and is suitable for treating thick soft soil in an earth-rock dam foundation in a water conservancy project. Background Art

[0002] Earth-rock dams are the oldest type of dam construction and are the most widely used due to their outstanding advantages, such as strong foundation adaptability and the economical use of locally sourced dam materials. Earth-rock dams can utilize the overburden as their foundation, provided that (after treatment) they meet requirements for deformation, shear strength, seepage stability, and leakage. For example, the core rockfill dam of the Pubugou Hydropower Station, with a maximum height of 186 meters, is built on a gravel overburden layer over 70 meters thick. However, for some special soil foundations, such as soft soil, due to their low permeability, high compressibility, low shear strength, and high sensitivity, low engineering properties are usually the only options, and low dams can usually only be constructed. According to data from completed domestic projects, their heights generally rarely exceed 25 meters. With the development of foundation treatment methods and improvements in technology, the height of dams built on soft soil foundations is also increasing.

[0003] The primary purpose of soft soil foundation treatment is to increase foundation strength, reduce compressibility, minimize settlement and uneven settlement, and prevent sliding failure and large cracks in the dam. Common treatment methods include soil replacement, pressure plate installation, drainage wells, and vibro-stone piles. Soil replacement is generally used when the soft soil layer is not thick. For deep soft soil foundations, due to its extremely low strength, ensuring the stability of the deep foundation pit slopes is extremely difficult if excavation and replacement are performed. Excavation and construction in soft soil are also extremely difficult and costly, making it neither economical nor rational. The pressure plate treatment method is widely used both domestically and internationally, but is generally applicable to dam heights not exceeding 10-15 meters. The drainage well method has poor adaptability to complex foundations and cannot penetrate or bypass obstacles such as boulders in the soft foundation. Furthermore, the sand well treatment depth is limited, generally within 30 meters. Vibroflotation was first used to reinforce and treat sandy foundations. In recent years, researchers at home and abroad have applied vibroflotation crushed stone pile composite foundations to soft soil treatment, achieving promising results. Vibroflotation crushed stone pile composite foundations have been widely used in hydropower project foundation treatment both domestically and internationally. However, to date, earth-rock dams built or under construction on soft soil foundations in China and abroad have not exceeded 55.0 meters in height, and the thickness of the soft soil at the dam foundation is generally within 20 to 25 meters. The construction of high earth-rock dams on deep soft soil foundations (35 to 40 meters) remains a significant challenge. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: in view of the above-mentioned problems, a method for treating deep soft soil in an earth-rock dam foundation and an earth-rock dam are provided to ensure that the dam foundation settlement deformation and shear strength meet the requirements of dam safety.

[0005] The technical solution adopted by the present invention is: a method for treating deep soft soil in earth-rock dam foundation, characterized by:

[0006] Determine the excavation surface of the dam foundation. The excavation surface of the dam foundation is generally trough-shaped. The bottom of the trough corresponds to the part of the dam's anti-seepage body and extends below the soft soil layer. The trough walls on the upper and lower reaches of the trough bottom are all slopes sloping downward toward the trough bottom.

[0007] Before excavating the soft soil at the dam foundation, vibratory crushed stone piles are first constructed on the upper foundation upstream and downstream of the dam's anti-seepage body. The pile holes pass through the dam foundation excavation surface, and only holes are drilled above the dam foundation excavation surface without backfilling with crushed stone. Only crushed stone is backfilled below the dam foundation excavation surface, forming crushed stone piles below the dam foundation excavation surface. If the soft soil below the excavation surface is not treated, the soft soil slope cannot be stabilized and construction is also very difficult. In addition, excavation of deep soft soil is generally very difficult. Therefore, only after treatment with crushed stone piles can the stability of the excavation slope be guaranteed, so that the core anti-seepage body of the dam can be placed on the non-soft soil foundation through excavation.

[0008] Excavation and replacement are carried out according to the determined dam foundation excavation surface.

[0009] The vibro-stone piles are arranged in a plum blossom shape, a diamond shape or a rectangle shape.

[0010] The drilling depth of the vibro-stone piles gradually becomes shallower from the corresponding anti-seepage body of the dam toward the upstream and downstream (whether it needs to be shallower or not depends on the deformation of the dam foundation and the stability of the slope).

[0011] Before vibro-compacted gravel pile construction, the overburden above the soft soil layer is excavated to a certain depth. The excavation area is the area covered by the dam body (depending on the distribution of soil layers, it may not necessarily be the entire dam foundation). Among them: 1. If there is a relatively poor soil layer above the soft soil surface, it can be excavated, and the depth of the gravel pile can also be appropriately reduced; 2. A certain thickness of normal overburden must be retained above the soft soil, otherwise it will be impossible to construct gravel piles directly on the soft soil surface; 3. If the soft soil is directly on the surface, no excavation is required. A layer of gravel must be placed on top of the soft soil before it can be used as the working surface for gravel pile construction.

[0012] A dam is characterized in that the dam base is trough-shaped as a whole, the trough bottom corresponds to the dam's anti-seepage body and extends deep into the soft soil layer, and the trough walls upstream and downstream of the trough bottom are slopes sloping downward toward the trough bottom; and a number of gravel piles are provided on the trough bottom and downstream of the dam base, penetrating into the soft soil layer and formed by vibro-compacted gravel pile construction.

[0013] The vibro-stone piles are arranged in a plum blossom shape, a diamond shape or a rectangle shape.

[0014] The drilling depth of the vibro-stone piles gradually becomes shallower from the corresponding anti-seepage body of the dam toward the upstream and downstream (determined to be shallower or not shallower according to the needs of dam foundation deformation and slope stability).

[0015] When there is a covering layer below the soft soil layer, a concrete anti-seepage wall located in the covering layer is provided below the dam anti-seepage body; in addition, there is still a covering layer below the soft soil layer. If the covering layer meets the basic requirements, the anti-seepage body can be placed on the covering layer and an anti-seepage wall can be provided to prevent seepage.

[0016] When the soft soil layer is beneath bedrock, a grouting gallery is provided beneath the dam's anti-seepage body and a grouting curtain is constructed downward.

[0017] The beneficial effect of the present invention is that the present invention excavates and replaces the soft soil of the foundation of the dam's anti-seepage body, thereby avoiding the adverse effect on the safety of the dam caused by cracking of the anti-seepage body due to excessive settlement deformation.

[0018] The present invention adopts vibro-compaction gravel piles to treat part of the dam foundation, which not only ensures the stability of the soft soil excavation slope during the construction period, but also can serve as a part of the permanent dam foundation to improve the physical and mechanical properties of the dam foundation bearing layer; the dam foundation gravel piles can greatly improve the stability of the soft soil foundation excavation slope, reduce the excavation range of the foundation pit, shorten the construction period, and save engineering work.

[0019] The present invention ensures the deformation and stability of the dam foundation by excavating and replacing soft soil in a certain range within the foundation, in conjunction with a vibro-replacement gravel pile composite foundation. Since soft soil generally has a high water content, the use of vibro-replacement gravel piles for treatment and forming surface drainage conditions can accelerate the drainage and consolidation of the soft soil and improve the physical and mechanical properties of the soft soil itself. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a typical cross-sectional view of the deep soft soil treatment at the foundation of the earthen anti-seepage core wall dam in Example 1.

[0021] Figure 2 This is a typical cross-sectional view of the treatment of deep soft soil in the foundation of the (asphalt) concrete core dam in Example 1.

[0022] Figure 3 This is a typical cross-sectional view of the treatment of deep soft soil at the foundation of a homogeneous earth dam in Example 1.

[0023] Figure 4 This is a typical cross-sectional view of different treatment ranges and treatment depths of the thick soft soil at the foundation of the earthen anti-seepage core wall dam in Example 1.

[0024] Figure 5 This is a typical cross-sectional view of Example 1, in which the core wall of the earthen anti-seepage core dam is placed in the covering layer and the foundation is anti-seepage with an anti-seepage wall.

[0025] Figure 6 This is a typical cross-sectional view of the earthen anti-seepage core wall dam in Example 1 using a combination of vibro-stone pile treatment, local excavation and replacement, and the installation of a suppression platform.

[0026] Figure 7 This is a longitudinal cross-sectional view of the dam of Scheme No. 1 of the present invention in Example 1.

[0027] Figure 8 This is a longitudinal cross-sectional view of the dam of Scheme No. 2 of the present invention in Example 1.

[0028] Figure 9 This is the distribution map of the most dangerous arc slip locations of the dam in Scheme No. 1 in Example 1.

[0029] Figure 10 The most dangerous arc sliding position distribution diagram of the foundation pit of solution No. 2 in Example 1

[0030] Figure 11 This is the seepage field distribution diagram of Dam No. 1 in Example 1.

[0031] Figure 12 This is the vertical displacement distribution diagram of the No. 1 dam during the water storage period in Example 1.

[0032] Figure 13 This is the horizontal displacement distribution diagram of the No. 1 dam during the water storage period in Example 1.

[0033] Figure 14 This is the distribution map of the most dangerous arc slip locations of Dam No. 2 in Example 1.

[0034] Figure 15 This is the seepage field distribution diagram of Dam No. 2 in Example 1.

[0035] Figure 16 This is the vertical displacement distribution diagram of the No. 2 dam during the water storage period in Example 1.

[0036] Figure 17 This is the horizontal displacement distribution diagram of the No. 2 dam during the water storage period in Example 1.

[0037] Figure 18 This is a typical cross-sectional view of the excavation treatment of the earthen anti-seepage core dam foundation in Example 2 in two phases.

[0038] 1. Gravel piles; 2. Dam body; 3. Concrete anti-seepage wall; 4. Suppression platform. DETAILED DESCRIPTION

[0039] Example 1: This example is a method for treating deep soft soil in an earth-rock dam foundation, comprising the following steps:

[0040] Before construction, the dam foundation excavation surface is determined based on the site conditions and dam design requirements. The dam foundation excavation surface is generally trough-shaped, with the bottom of the trough corresponding to the center of the dam (the dam anti-seepage body) and extending below the soft soil layer. The upper and lower trough walls of the trough bottom are all sloped downward toward the trough bottom.

[0041] Before the excavation of soft soil at the dam foundation, vibratory stone piles are constructed on the foundation upstream and downstream of the dam's anti-seepage body. The pile holes pass through the dam foundation excavation surface, and only holes are drilled above the dam foundation excavation surface without backfilling with stone. Only stone is backfilled below the dam foundation excavation surface, forming stone piles below the dam foundation excavation surface (stone piles are mainly arranged in soft soil layers. If the soft soil layer is not distributed throughout the dam foundation, the scope of the stone piles will be reduced accordingly);

[0042] Excavation and replacement filling are carried out according to the determined dam foundation excavation surface to form a foundation pit.

[0043] This embodiment also provides an earth-rock dam, the dam foundation of which is a foundation pit, the dam base surface is trough-shaped as a whole, the trough bottom surface corresponds to the dam anti-seepage body and extends deep below the soft soil layer, and the trough walls on the upper and lower reaches of the trough bottom surface are all slopes inclined downward toward the trough bottom; on the trough bottom surface of the dam foundation surface and in the downstream area, a number of gravel piles formed by vibro-compaction gravel pile construction that penetrate into the soft soil layer are evenly arranged.

[0044] In this example, various types of dam bodies can be set up on the dam base, such as earth-based anti-seepage core wall dam (see Figure 1 ), (asphalt) concrete core dam (see Figure 2 ), homogeneous earth dam (see Figure 3 ), earth impermeable core dam and pressure platform method (see Figure 6 ).

[0045] In this embodiment, the vibro-stone piles are arranged in a plum blossom, diamond or rectangular shape. The holes of the vibro-stone piles are formed from the cover layer above the soft soil layer downward through the soft soil layer, or the depth of the holes of the vibro-stone piles gradually becomes shallower from the corresponding dam anti-seepage body position toward the upstream and downstream sides. The holes near the dam anti-seepage body position are formed from the cover layer above the soft soil layer downward through the soft soil layer, and the holes far from the dam anti-seepage body position are formed from the cover layer above the soft soil layer downward into the soft soil layer (see Figure 4 ), the penetration depth is determined according to the requirements of dam foundation deformation and slope stability.

[0046] In this case, when the cover layer below the soft soil layer meets the dam foundation requirements, a concrete anti-seepage wall is provided below the dam anti-seepage body, which is located in the cover layer below the soft soil layer (see Figure 5 When the soft soil layer is under the bedrock, a grouting gallery can be set up under the dam's anti-seepage body and a grouting curtain can be constructed downwards. Alternatively, the bedrock can be directly dug, or the covering layer can be used as the foundation of the anti-seepage body and an anti-seepage wall can be used for anti-seepage.

[0047] The following comparison of a clay core rockfill dam foundation treatment scheme in a certain project verifies the effectiveness of this embodiment:

[0048] The clay core dam has an 8.0m crest width, with an upstream slope ratio of 1:2.0 and a downstream slope ratio of 1:1.8. A 4m-wide bridleway is located upstream, and a zigzag access road is provided downstream. The dam fill material consists of the clay core, transition material, filter material, upstream and downstream shell materials, and upstream and downstream slope protection. The clay core has a crest width of 4.0m, with an upstream and downstream slope ratio of 1:0.25. The horizontal widths of the upstream filter and transition layers are 2.0m and 2.0m, respectively, while those of the downstream filter and transition layers are 2.0m and 3.0m, respectively. The shell material is excavated from the dam yard.

[0049] The upper soil layer of the riverbed is ① silt sand and soft soil, with an average thickness of 7.64m; ② clayey gravel sand, with an average thickness of 3.71m and a top burial depth of 4.00 to 11.80m; the lower soil layer is ③-1 silty soil and ③-2 peat soil, which are mainly soft plastic to soft plastic, partially fluid, weak soil, and low bearing capacity, with an average thickness of 30m and a top burial depth of 9.30 to 12.50m; ④ layer of sand and gravel, with an average thickness of 3.19m and a top burial depth of 21.20 to 40.10m.

[0050] Two schemes were initially proposed. Scheme No. 1: Excavate the surface ① silt sand and soft soil layer as the dam foundation, and use anti-seepage wall to treat the dam foundation cover layer. Figure 7 Solution No. 2: Adopt the dam foundation treatment method proposed in this patent, remove the surface ① silt sand and soft soil layer, and treat the dam foundation with vibratory crushed stone piles. The pile diameter is 1.0m, arranged in a plum blossom pattern, with a spacing of 1.5m, a maximum pile depth of 30m, and a replacement rate of 0.4; in the second phase, excavate and replace the silty soil ③-1 and the peat soil ③-2 of the dam core wall, and place the dam core wall on the bedrock. Figure 8 .

[0051] The dam slope stability, seepage and stress deformation calculations were performed for the two schemes respectively:

[0052] (1) Slope stability

[0053] The dam slope stability calculation was performed using the Slope Analysis module in GeoStudio software developed by Geoslope, a Canadian company. According to EM 1110-2-1902, the Spencer calculation method, which takes into account the forces between bars, was used. The results of the dam slope anti-sliding stability calculation are shown in Table 1.

[0054] Calculation results of clay core wall dam slope stability

[0055] Table 1

[0056]

[0057] Note: The allowable value of safety factor is proposed according to the requirements of US standards.

[0058] The calculation results show that the upstream dam slope of Scheme No. 1 meets the anti-sliding stability requirements under various working conditions, but the safety margin is small. The anti-sliding stability coefficient of the downstream dam slope does not meet the requirements of the specification under stable seepage conditions, earthquake conditions and the highest reservoir water level conditions. Figure 9 The figure shows the distribution of the most dangerous sliding arc positions for Scheme No. 1. The upstream and downstream dam slopes of Scheme No. 2 meet the anti-sliding stability requirements under various working conditions. Compared with Scheme No. 1, the upstream and downstream dam slope stability coefficients of Scheme No. 2 are larger under various working conditions, especially under earthquake conditions, and still have a certain safety margin. Figure 13 This is the distribution map of the most dangerous arc slip locations for Scheme No.2.

[0059] Considering the excavation depth of the foundation pit of Scheme No. 2 is 40.0m, which is a deep and large foundation pit. Therefore, a stability analysis of the temporary slopes formed after the gravel pile construction is completed and the foundation pits upstream and downstream of the dam are excavated is conducted. The analysis results are shown in Table 2. The distribution diagram of the most dangerous sliding arc is shown in Figure 10 shown.

[0060] The calculation results show that the stability coefficients of the upstream and downstream slopes of the foundation pit are greater than the allowable values ​​under general construction conditions and rainstorm conditions. It can be seen that the stability coefficients of the excavated slopes of the composite foundation after treatment with gravel piles meet the requirements under all conditions.

[0061] Stability analysis of temporary slope of foundation pit in scheme No.2

[0062] Table 2

[0063]

[0064] Note: The allowable value of safety factor is proposed according to the requirements of US standards.

[0065] (2) Seepage analysis

[0066] The dam slope stability calculation uses the Seep module in GeoStudio software developed by Geoslope, Canada. The seepage calculation results of each calculation condition are shown in Table 3. Figure 11 and Figure 15 These are the seepage field distribution diagrams of the dam during the stable seepage period for Scheme No. 1 and Scheme No. 2 respectively.

[0067] Seepage calculation results table

[0068] Table 3

[0069]

[0070]

[0071] From the seepage calculation, we can get:

[0072] ① Under both schemes, the seepage field distribution within the dam body and dam foundation is relatively normal, and the equipotential lines are reasonably distributed;

[0073] ② The overall leakage of Plan No. 1 and Plan No. 2 is not large;

[0074] ③ The seepage slope of the clay core wall and the contact clay area of ​​Scheme No. 2 is smaller than that of Scheme No. 1. At the same time, the maximum seepage slope of the dam foundation cover is also smaller than that of Scheme No. 1, and the overall seepage stability is better.

[0075] (3) Two-dimensional finite element analysis

[0076] Two-dimensional finite element analysis was performed on Scheme No. 1 and Scheme No. 2, respectively. The stone pile replacement ratio was set to 0.4, and the basic parameters of the stone pile were considered to be K = 800. The static calculation model was the Duncan model. The calculation results are shown in Table 4. Figure 13 、 Figure 14 They are the distribution diagrams of vertical displacement and horizontal displacement of the dam during the water storage period of Scheme No.1. Figure 16 、 Figure 17 They are the distribution diagrams of the vertical displacement and horizontal displacement of the dam during the water storage period of Scheme No. 2.

[0077] Two-dimensional finite element analysis results table

[0078] Table 4

[0079]

[0080]

[0081] The calculation results show that Scheme No. 1 has large settlements in the dam body and core wall during the completion and water storage periods, and a large horizontal displacement of the dam body downstream during the water storage period, which does not meet safety requirements. Scheme No. 2 has small settlements in the dam body and core wall during the completion and water storage periods, and a small horizontal displacement of the dam body downstream during the water storage period. The core wall is less likely to crack due to excessive deformation, and the dam safety meets the requirements.

[0082] Example 2: This example is basically the same as Example 1, except that the dam foundation excavation can be carried out in two phases. First, the foundation surface is excavated in the first phase, then vibro-stone pile construction is carried out, and then the dam foundation is excavated in the second phase.

[0083] like Figure 18 As shown, in this embodiment, before the vibro-stone pile construction is carried out, the cover layer above the soft soil layer is excavated to a certain depth, and the excavated area is the cover area of ​​the dam body.

Claims

1. A method for treating deep soft soil in an earth-rock dam foundation, characterized by: Determine the excavation surface of the dam foundation. The excavation surface of the dam foundation is generally trough-shaped. The bottom of the trough corresponds to the part of the dam's anti-seepage body and extends below the soft soil layer. The trough walls on the upper and lower reaches of the trough bottom are all slopes sloping downward toward the trough bottom. Before the soft soil of the dam foundation is excavated, vibro-stone piles are constructed on the foundations upstream and downstream of the dam's anti-seepage body. The pile holes pass through the dam foundation excavation surface, and only holes are drilled above the dam foundation excavation surface without backfilling with gravel. Only gravel is backfilled below the dam foundation excavation surface, forming gravel piles below the dam foundation excavation surface. Excavation and replacement are carried out according to the determined dam foundation excavation surface.

2. The method for treating deep soft soil in earth-rock dam foundation according to claim 1, characterized in that: The vibro-stone piles are arranged in a plum blossom shape, a diamond shape or a rectangle shape.

3. The method for treating deep soft soil in earth-rock dam foundation according to claim 1, characterized in that: The drilling depth of the vibro-stone piles gradually becomes shallower from the corresponding anti-seepage body of the dam toward the upstream and downstream.

4. The method for treating deep soft soil in earth-rock dam foundation according to claim 1, characterized in that: Before the construction of vibro-stone piles, the covering layer above the soft soil layer is excavated to a certain depth. The excavated area is the covering area of ​​the dam body.

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