A dam foundation covering layer excavation construction method containing a deep deposition dammed lake facies

Abstract

This invention provides a method for excavating the overburden layer of a dam foundation containing deep sedimentary dammed lacustrine facies, comprising the following steps: step S1 geological survey and identification, step S2 construction road layout, step S3 dewatering system setup, step S4 horizontal step-type vertical excavation, step S5 slope protection, and step S6 step-type waste disposal. This invention provides a method for excavating the overburden layer of a dam foundation containing deep sedimentary dammed lacustrine facies, achieving a dual improvement in excavation efficiency and construction safety.

Description

Technical Field

[0001] This invention relates to the field of construction technology, and in particular to a method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies. Background Technology

[0002] Western my country's river basins, particularly the Jinsha River, Dadu River, and Yalong River regions, are rich in hydropower resources and are key markets for national hydropower development. However, these areas are generally influenced by the Himalayan orogeny, resulting in typical high-mountain canyon landforms. Under these conditions, mountain stability is poor, making them highly susceptible to large-scale landslides. When landslides collapse into rivers, they often block the river channel, forming barrier lakes. After a barrier lake breaks, the floodwaters carrying massive amounts of sediment rapidly accumulate in areas with gentle riverbed gradients, forming a deep and complex lacustrine facies overburden.

[0003] These types of landslide-dammed lacustrine overburden layers, especially the silty sediments within them, are characterized by high natural water content, large porosity, low permeability, weak bearing capacity, and slow consolidation rate. Under natural conditions, their stable slope ratios are extremely gentle (typically around 1:8), and their self-stabilizing capacity is extremely poor. These characteristics present severe safety and schedule challenges to dam foundation excavation and construction under these geological conditions.

[0004] In existing hydropower projects, the treatment of deep overburden layers is mostly concentrated on sand and gravel overburden layers, and the dam body is often constructed directly on top of them, rarely involving large-scale excavation of the full depth of the overburden layer. Even when excavation is necessary, traditional construction methods mainly employ large-area horizontal mining (large-scale horizontal mining). However, applying this traditional method to the excavation of dam foundations containing deep lacustrine facies sediments presents a series of insurmountable technical problems: Limited excavation methods: Due to the extremely gentle self-stabilizing slope of the soft soil layer, in the narrow river valley terrain, the traditional large-scale mining method is difficult to develop enough working face, resulting in low excavation efficiency and a high risk of slope instability accidents.

[0005] Low dewatering efficiency: Silt-like soil has a low permeability coefficient and consolidates slowly. Traditional dewatering systems are often poorly designed, and their drainage efficiency cannot meet the requirement of rapidly reducing the soil moisture content to a safe excavation level, severely restricting construction progress.

[0006] The layout of construction roads is difficult: the construction site is narrow, making the layout of roads into the foundation pit extremely difficult. If the roads are laid directly on the weak soil layer of the landslide dammed lake, they are very likely to sink severely due to insufficient bearing capacity, leading to transportation disruptions and even safety accidents.

[0007] Poor equipment compatibility with soil: Conventional earthwork excavation equipment is not well-suited to silty soil layers with high water content. The unreasonable combination of equipment leads to low construction efficiency and makes it difficult to meet the requirements of high-intensity and tight schedule construction.

[0008] The disposal of waste materials presents a significant challenge: the large amount of waste generated during excavation needs to be disposed of quickly, but the silty waste material itself solidifies very slowly. Traditional one-time, large-scale waste disposal methods are highly likely to cause overall instability of the waste disposal site, posing a huge safety hazard.

[0009] In summary, existing technologies lack a comprehensive construction method to systematically address the aforementioned problems in the excavation of dam foundation overburden containing deep sedimentary lacustrine facies. Therefore, there is an urgent need to develop a novel, integrated construction method to safely, efficiently, and economically complete dam foundation excavation projects under such complex geological conditions. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies, achieving a dual improvement in excavation efficiency and construction safety.

[0011] The method for automatically generating reports provided by this invention includes the following steps: A method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies includes the following steps: Step S1, Geological Investigation and Identification: Conduct a geological investigation of the dam foundation overburden, identify the excavation layers, and conduct geotechnical tests on the soil at each layer to comprehensively determine the physical properties of the soil. Step S2, Construction Road Layout: The main road to the foundation pit is laid on a hard foundation outside the excavation area. In the inverted sections where the valley is narrow and the slope is steep, a "semi-tunnel excavation + cantilever support" structure is used to lay the road. For the road located on the silty cover layer of the landslide dammed lake, a layered compaction structure of "geogrid + stone chips" is used to improve the bearing capacity. Step S3, Dewatering System Setup: Dewatering manholes and drainage ditches are arranged around and inside the excavation area. The pilot trench of the drainage ditch is advanced synchronously with the road to achieve early drainage. Submersible pumps in the dewatering manholes continuously pump water to control the groundwater level. Step S4, Horizontal bench vertical mining: The horizontal vertical mining method perpendicular to the river direction is adopted to replace the traditional large-area horizontal mining method; the bench height and slope ratio are optimized based on field tests. Step S5, Slope Protection: Protect the excavated slope in the order of "drainage pipe + non-woven geotextile + fine sand protective layer + five-strand steel wire mesh gabion". Step S6, stepped flow waste disposal: The waste disposal site is set up as a stepped structure, and the waste disposal operation is carried out in a layered stacking and compaction manner, and drainage facilities are set up at the same time.

[0012] Furthermore, in step S1, the excavation layers include either layer Ql-3 or layer Ql-2 [zz5.1], which are confirmed on-site by the design geological engineer.

[0013] Furthermore, in step S4, the optimized step height is 5-10 meters, the upstream side slope ratio is 1:4, and the downstream side slope ratio is 1:5.

[0014] Furthermore, in step S4, the combination of mechanical equipment used is preferably selected based on the characteristics of the silty cover layer, including but not limited to wetland bulldozers, long-arm excavators, and dump trucks.

[0015] Furthermore, in step S3, the location, number, and depth of the dewatering wells are determined based on the geological survey results and the excavation depth, and the water level in the wells is controlled below the top elevation of the bottom sand and gravel layer.

[0016] Furthermore, in step S2, the "geogrid + stone chips" road structure is formed through layered paving and compaction.

[0017] Furthermore, in step S5, each layer of slope protection material is laid flat and firmly fixed to prevent slope erosion and instability.

[0018] Furthermore, the construction method is applied in the excavation of the overburden layer of dam foundations in deep river valleys with thick barrier lake sediments.

[0019] Compared with related technologies, the excavation and construction method for the overburden layer of a dam foundation containing deep sedimentary lake facies provided by the present invention has the following beneficial effects: This invention abandons the traditional large-area horizontal mining method and innovatively adopts "horizontal bench vertical mining". This method can effectively deploy the working face in narrow river valley terrain, avoiding the problem of limited excavation face caused by the self-stabilizing slope ratio of weak soil layers. At the same time, through field tests, the bench height (5-10m) and slope ratio (1:4 upstream, 1:5 downstream) are optimized, which significantly reduces the instability risk of high and steep slopes while ensuring excavation efficiency, achieving a dual improvement in excavation efficiency and construction safety.

[0020] To address the challenges of road layout in narrow valleys and steep slopes, this invention employs a "semi-tunnel excavation + cantilever support" structure. This structurally and reliably solves the passage problem on steep, overhanging sections of slopes without occupying additional valley space. Simultaneously, addressing the extremely low bearing capacity of the silty cover layer in landslide-dammed lake facies, an innovative "geogrid + gravel" composite road structure is designed. Through layered compaction, this significantly improves the bearing capacity of soft soil sections, effectively preventing subsidence and damage from heavy transport vehicles and ensuring smooth and safe material transportation.

[0021] This invention advances the drainage ditch pilot trench simultaneously with the construction road, achieving "drainage before excavation," significantly extending the pre-drainage time of the soil and effectively overcoming the inherent defects of silty soil, such as low permeability and slow consolidation. Combined with precisely positioned dewatering wells, it enables accurate control of the groundwater level, allowing the soil moisture content to quickly drop to a safe excavation value, creating favorable conditions for subsequent efficient excavation and fundamentally alleviating the pressure of a tight construction schedule.

[0022] This invention employs a "stepped-type flowing waste disposal" method, which effectively resolves the contradiction between the slow consolidation speed of silty waste and high-strength waste through layered dumping and compaction, along with simultaneous drainage facilities. This dynamic and orderly waste disposal method avoids the risk of overall instability of the waste disposal site caused by traditional one-time large-scale dumping, significantly improving the storage capacity and safety stability of the waste disposal site.

[0023] This invention is not an improvement on a single technology, but a systematic integration of the entire process, including road layout, dewatering, excavation, protection, and spoil disposal, forming a complete and replicable construction method. This method has not only been successfully applied to extreme conditions such as the Lawa Hydropower Station, where deep valleys are interspersed with thick lacustrine facies sediments, but its parameters can also be adjusted and optimized according to lacustrine facies sediment layers of different thicknesses and properties, demonstrating strong versatility and potential for wider application. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the stepped excavation of the foundation pit cover layer in an embodiment of the present invention; Figure 2 This is a schematic diagram of the road layout in the lower foundation pit according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the road structure design for a steep slope section in an embodiment of the present invention; Figure 4 This is a schematic diagram of the road structure design for vertical or inverted slope sections in an embodiment of the present invention; Figure 5 This is a schematic diagram of the overall layout of the foundation pit drainage in an embodiment of the present invention; Figure 6 This is a schematic diagram of the pioneer slot arrangement in an embodiment of the present invention; Figure 7 This is a schematic diagram of the culvert structure in an embodiment of the present invention; Figure 8 This is a schematic diagram of the weir back slope protection structure in an embodiment of the present invention; Figure 9 This is a schematic diagram of the layout of the spoil disposal site in an embodiment of the present invention; Figure 10 This is a schematic diagram of the construction of the drainage hole in an embodiment of the present invention; Figure 11 This is a schematic diagram of the laying of the reverse filter geotextile in an embodiment of the present invention; Figure 12 This is a schematic diagram of the reverse filter fine sand filling in an embodiment of the present invention; Figure 13 This is a schematic diagram of a five-strand steel wire gabion used for slope stabilization in an embodiment of the present invention. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] In the specific implementation process, such as Figures 1 to 13 As shown in this embodiment, a method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies includes the following steps: Step S1, Geological Investigation and Identification: A geological investigation is conducted on the dam foundation overburden layer to identify the excavation layers. Geotechnical tests are performed on each layer to comprehensively determine the physical properties of the soil. The physical property parameters are systematically measured, focusing primarily on parameters such as permeability coefficient, consolidation coefficient, natural water content, plastic limit, and liquid limit. The accurate acquisition of these parameters provides scientific and quantitative data support for subsequent road bearing capacity design, dewatering well layout, optimization of step height and slope ratio, and equipment selection, avoiding safety risks and efficiency losses associated with empirical construction.

[0027] The permeability coefficient is a quantitative indicator that characterizes the permeability of soil. Related studies have shown that, whether it is sandy soil or cohesive soil, the void ratio is an important indicator affecting the permeability of soil.

[0028] Void ratio refers to the ratio of the volume of pores in soil to the volume of solid particles. The smaller the void ratio, the denser the soil; the larger the void ratio, the looser the soil and the higher its compressibility. High compressibility of soil indicates poor structural strength.

[0029] The consolidation coefficient is a scalar quantity that reflects the rate of consolidation of laterally confined soil under load.

[0030] Liquid limit refers to the water content that separates cohesive soil from its plastic and fluid states. It is the upper limit of the water content in the plastic state. When the natural water content is greater than the liquid limit, the soil is in a fluid state.

[0031] The plastic limit refers to the boundary water content between the plastic and semi-solid states of cohesive soil. It is the lower limit of the plastic state. When the natural water content is greater than the liquid limit but less than the plastic limit, the soil is in a plastic state. When the natural water content is less than the plastic limit, the soil is in a semi-solid state.

[0032] Plasticity index = plastic limit - liquid limit, characterizing the range of water content in the plastic state. Based on the plasticity index, soil can be divided into three categories: plasticity index > 17 is clay, 10 < plasticity index < 17 is silty clay, and plasticity index < 10 is silt.

[0033] Step S2, Construction Road Layout: The main road to the foundation pit is laid on a hard foundation outside the excavation area. In the inverted sections where the valley is narrow and the slope is steep, the road is laid out using a "semi-tunnel excavation + cantilever support" structure.

[0034] It is mainly designed for deep river valleys with steep, overhanging banks. For vertical or overhanging slopes, a "semi-tunnel excavation + cantilever support structure" is used; for steep slopes, a "reinforced gabion + stone backfill structure" is used. The structure is as follows: Figure 3 , 4 As shown.

[0035] When encountering a vertical, overhanging slope section, the upper interlocking support is first constructed, with scaffolding erected on the overburden layer as a construction platform. After the interlocking support is completed, excavation is carried out according to the semi-tunnel structure, with a bottom slab width of 3m (which can be widened if conditions permit). Drilling and blasting rigs and YT-28 hand-held pneumatic drills are used for drilling, full-section excavation, and smooth blasting excavation around the perimeter. The semi-tunnel support uses a drill and blasting rig as a construction platform. The tunnel wall is first sealed with C20 concrete, and then supported by a system of anchor bolts. The cantilever slab is constructed simultaneously with the semi-tunnel support. The foundation excavation width at the bottom of the cantilever slab is not less than 0.3m. Drilling holes with YT-28 hand-held pneumatic drills and filling them with rock expansion fracturing agent are used, along with hand-held pneumatic picks for drilling and shaping. The angle between the horizontal anchor bolts and the inclined anchor bolts of the supporting structure is 15°, constructed using YQ-100B down-the-hole drills. Then, steel reinforcement is installed, formwork is installed, and C30 concrete is poured. Finally, a C30 reinforced concrete retaining wall is poured on the supporting cantilever slab.

[0036] For roads located on silty cover layers of landslide dammed lakes, a layered compaction structure of "geogrid + stone chips" is adopted to improve bearing capacity.

[0037] The settlement of the Ql-3 soil layer with a natural moisture content of 33.6% and a load of 58t was statistically analyzed using different paving thicknesses and different compaction passes, as detailed in Table 1 below:

[0038] Table 1 Under the conditions of a natural moisture content of 33.6% in the Ql-3 soil layer and a load of 58t, laying a layer of bidirectional geogrid + 80cm thick stone ballast as a temporary excavation road (12m wide) can meet the road load requirements.

[0039] Step S3, Dewatering System Setup: Dewatering manholes and drainage ditches are arranged around and inside the excavation area. The pilot drainage ditch advances synchronously with the road to achieve pre-drainage. Submersible pumps in the dewatering manholes continuously pump water to control the groundwater level. This achieves "drainage before excavation," allowing sufficient pre-drainage time for the silty soil. The location, number, and depth of the dewatering manholes are precisely calculated based on geological survey results and the maximum excavation depth, ensuring that the bottom of the manholes penetrates the silty layer to reach the lower gravel layer, and that the water level inside the manholes is stably controlled below the top elevation of the bottom gravel layer. This three-dimensional dewatering system overcomes the inherent defects of silty soil, such as low permeability and slow consolidation speed, allowing the soil moisture content to drop below a safe level before excavation, creating the prerequisite for efficient excavation.

[0040] Step S4, Horizontal Stepped Vertical Excavation: This step employs a horizontal vertical excavation method perpendicular to the river direction, replacing the traditional large-area horizontal mining method. The step height and slope ratio are optimized based on field tests. Compared to traditional large-scale horizontal mining, this method allows for a more comprehensive working face in narrow river valleys, avoiding the problem of limited excavation face due to excessively gentle slopes caused by weak soil layers. Through field compaction tests and slope stability calculations, the optimized step height is determined to be 5-10 meters, with an upstream slope ratio of 1:4 and a downstream slope ratio of 1:5, ensuring both excavation efficiency and slope stability. Furthermore, considering the high water content and high viscosity of the silty overburden, a combination scheme is selected: wetland bulldozers for aggregate collection, long-arm excavators for loading, and dump trucks for transportation. This effectively solves the problems of ordinary equipment getting stuck, slipping, and having low loading efficiency in soft soil.

[0041] Step S5, Slope Protection: The excavated slope is protected in the following order: drainage pipe + non-woven geotextile + fine sand protective layer + five-strand steel wire mesh gabion. Each layer of slope protection material is laid flat and firmly fixed to prevent slope erosion and instability. Specific implementation details are as follows: like Figures 10 to 13 As shown, first, the back slope of the cofferdam is sloped according to the design slope ratio. Then, as... Figure 10 As shown, a crawler drill was used to drill drainage holes with a diameter of φ110, a depth of >10m, an elevation angle of >5°, and a spacing of 5m. Then, as... Figure 11 As shown, a 100mm blind pipe is wrapped with geotextile and buried in the hole. Geotextile and an 8cm thick layer of fine sand are laid on the slope as a reverse filter for protection. Then... Figure 12 As shown, place 50cm thick five-strand steel wire gabions in a staggered pattern, then... Figure 13 The road is constructed by using a tracked backhoe excavator to fill the slope with stones with a particle size of not less than 30cm, and finally filling it with a 1m thick layer of stone chips.

[0042] Step S6, Stepped Flow Waste Disposal: The waste disposal site is configured as a stepped structure, with waste disposal operations carried out in a layered, compacted manner, and drainage facilities are installed simultaneously. The waste disposal site is configured as a stepped structure, and the operation is carried out in a flow-through manner from bottom to top, layered, and compacted. After each layer of waste is laid and leveled, it is immediately compacted, and intercepting ditches and drainage blind ditches are simultaneously installed on the inner side of each step. This dynamic and orderly waste disposal method avoids the risk of uneven settlement and overall instability of the waste disposal site caused by traditional one-time large-area dumping, significantly improving the storage capacity and safety stability of the waste disposal site.

[0043] Preferably, in step S1, the excavation layers include either layer Ql-3 or layer Ql-2. According to soil texture classification standards, both Ql-3 and Ql-2 sediments belong to loam (between sandy soil and clay). Specifically, layer Ql-3 sediments are predominantly sandy (sand content > 60%), belonging to silty sand, while layer Ql-2 sediments are predominantly silty (silt content > 80%), belonging to sandy clay. Layer Ql-3 has good permeability and high bearing capacity, and can be directly excavated using conventional machinery; layer Ql-2 has high water content, low bearing capacity, and poor self-stability, and must be constructed strictly according to the complete set of special processes of this invention. On-site verification by a design geological engineer is required. Accurate on-site identification of the layer boundaries ensures differentiated construction strategies for different layers, avoiding waste or safety risks caused by a one-size-fits-all approach.

[0044] Preferably, the combination of mechanical equipment used in step S4 is selected based on the characteristics of the silty cover layer, including but not limited to wetland bulldozers, long-arm excavators, and dump trucks.

[0045] Preferably, the construction method is applied to the excavation of the overburden layer of dam foundations in deep river valleys with thick lacustrine sediments. The successful verification of this solution in actual projects such as the Lawa Hydropower Station demonstrates its effectiveness and reliability, providing a replicable and scalable system solution for dam foundation excavation under similar geological conditions. It possesses significant engineering application value and substantial economic and social benefits.

[0046] In summary, this invention systematically solves the long-standing technical bottlenecks in the excavation of dam foundation overburden containing deep sedimentary lake facies, such as high safety risks, low construction efficiency, and uncontrollable construction period, through a series of interrelated innovative technical means. It achieves the construction goals of safety, efficiency, and economy, and has outstanding substantive features and significant progress.

[0047] It should be understood that, unless otherwise expressly stated herein, there is no strict order restriction on the execution of the above steps, and these steps may be executed in other orders. Moreover, at least some steps in the processes involved in the above embodiments may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but may be executed at different times. The execution order of these steps or stages is not necessarily sequential, but may be performed alternately or in turn with other steps or at least some of the steps or stages in other steps.

[0048] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description. Therefore, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0049] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment includes only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies, characterized in that, Includes the following steps: Step S1, Geological Investigation and Identification: Conduct a geological investigation of the dam foundation overburden, identify the excavation layers, and conduct geotechnical tests on the soil at each layer to comprehensively determine the physical properties of the soil. Step S2, Construction Road Layout: The main road to the foundation pit is laid on a hard foundation outside the excavation area. In the inverted sections where the valley is narrow and the slope is steep, the road is laid out using a "semi-tunnel excavation + cantilever support" structure. For the road located on the silty cover layer of the landslide dammed lake, a layered compaction structure of "geogrid + stone chips" is used to improve the bearing capacity. Step S3, Dewatering System Setup: Dewatering manholes and drainage ditches are arranged around and inside the excavation area. The pilot trench of the drainage ditch is advanced synchronously with the road to achieve early drainage. Submersible pumps in the dewatering manholes continuously pump water to control the groundwater level. Step S4, Horizontal bench vertical mining: The horizontal vertical mining method perpendicular to the river direction is adopted to replace the traditional large-area horizontal mining method; the bench height and slope ratio are optimized based on field tests. Step S5, Slope Protection: Protect the excavated slope in the order of "drainage pipe + non-woven geotextile + fine sand protective layer + five-strand steel wire mesh gabion". Step S6, stepped flow waste disposal: The waste disposal site is set up as a stepped structure, and the waste disposal operation is carried out in a layered stacking and compaction manner, and drainage facilities are set up at the same time.

2. The method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies as described in claim 1, characterized in that, In step S1, the excavation layers include either layer Q1-3 or layer Q1-2, which are confirmed on-site by the design geological engineer.

3. The method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies as described in claim 1, characterized in that, In step S4, the optimized step height is 5-10 meters, the upstream side slope ratio is 1:4, and the downstream side slope ratio is 1:

5.

4. The method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies as described in claim 1, characterized in that, In step S4, the combination of mechanical equipment used is optimized for the characteristics of the silty cover layer, including but not limited to wetland bulldozers, long-arm excavators, and dump trucks.

5. The method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies as described in claim 1, characterized in that, In step S3, the location, number, and depth of the dewatering wells are determined based on the geological survey results and the excavation depth, and the water level in the wells is controlled below the top elevation of the bottom sand and gravel layer.

6. The method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies as described in claim 1, characterized in that, In step S2, the "geogrid + stone chips" road structure is formed through layered paving and compaction.

7. The method for excavating the overburden layer of a dam foundation containing deep sedimentary lake facies as described in claim 1, characterized in that, In step S5, each layer of slope protection material is laid flat and firmly fixed to prevent slope erosion and instability.

8. The application of the construction method as described in any one of claims 1 to 7 in the excavation of the overburden layer of a dam foundation in a deep valley with thick barrier lacustrine sediments.

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