Method for excavating silty clay foundation pit

Abstract

The invention belongs to the technical field of foundation pit excavation, and particularly relates to an excavation method of a silty clay foundation pit. According to the excavation method of the silty clay foundation pit, the intercepting ditch and the dewatering wells are arranged in the non-excavation area, surface water entering the excavation area is intercepted through the intercepting ditch, underground water in the excavation area can be effectively lowered to the position below the excavation area through the dewatering wells, good construction conditions are provided for follow-up excavation operation, and meanwhile the construction efficiency is improved. The first construction area and the second construction area are excavated in a layered and alternate mode, after excavation of a soil layer in the second construction area is completed, the inclined face is supported, so that safety accidents such as landslide in the excavation process are avoided, the conditions of load influence and deformation on the inclined face are reduced, the construction period is shortened, and the construction cost is reduced; simple, safe and efficient excavation of the silty clay foundation pit is achieved.

Description

Technical Field

[0001] This invention belongs to the field of foundation pit excavation technology, and specifically relates to a method for excavating a silty clay foundation pit. Background Technology

[0002] In the construction of hydropower stations, the excavation of silty clay foundation pits is often encountered. Traditional foundation pit excavation methods can no longer meet the requirements of modern engineering for safety, efficiency, and environmental protection. Especially in silty clay foundations, because silty clay is a material between clay and silt, it is characterized by moderate fineness, good viscosity and plasticity, and high stability. When in water or at a water content above the liquid limit (maximum water content), it has strong fluidity. Its unstable foundation and complex environment make it prone to safety accidents such as collapse and landslides during foundation pit excavation.

[0003] Common methods for excavating foundation pits include slope excavation, basin excavation, and reverse construction. However, due to the high plasticity, weak swelling and shrinkage, low permeability, and tendency to soften and deform of silty clay, the following problems may arise when using these methods to excavate foundation pits in silty clay: The slope excavation method is difficult to ensure slope stability in silty clay foundations, which can easily lead to landslides and poses a significant safety hazard. While basin excavation and reverse construction methods can reduce safety hazards during the excavation process, they are complex, inefficient, costly, and require highly skilled construction personnel. Summary of the Invention

[0004] This invention provides a method for excavating silty clay foundation pits, in order to solve the technical problems that common foundation pit excavation methods in the prior art generally have significant safety hazards, high construction complexity, and low construction efficiency when excavating silty clay foundation pits.

[0005] This invention is achieved through the following technical solution: A method for excavating a silty clay foundation pit includes the following steps: The construction area is determined and divided into excavation area and non-excavation area according to the construction drawings. The longitudinal cross-section of the excavation area is a trapezoid with a larger top and a smaller bottom. The slope connecting the excavation area and the non-excavation area forms a slope after the excavation of the excavation area is completed. In the non-excavation area, intercepting ditches and multiple dewatering wells are set up. The intercepting ditches are laid out along the edge of the excavation area, and the multiple dewatering wells are laid out around the circumference of the excavation area. The excavation area is divided into multiple soil layers from top to bottom; The excavation area is divided into construction zone one and construction zone two, which covers the perimeter of construction zone one. Excavation of construction area 1. After excavation of one layer of soil in construction area 1, a water collection well and a drainage ditch are set on the exposed soil layer in construction area 1. The lower end of the drainage ditch is connected to the water collection well. A water pump is installed in the water collection well to discharge water to the outside of the construction area. Excavate the same soil layer that has been excavated in construction area 1 in construction area 2. After the soil layer in construction area 2 is excavated, the slope is supported. The excavation of construction zone one and construction zone two shall be carried out alternately in the above manner until all soil layers have been excavated.

[0006] To better realize the present invention, the above method is further optimized by circumferentially arranging multiple dewatering wells at equal intervals around the excavation area, with the distance between two adjacent dewatering wells being 8-15m.

[0007] To better realize the present invention, the above method is further optimized by arranging a vehicle passage A on the slope, with both ends of the vehicle passage A extending to the exposed soil layer in construction zone 1 and the non-excavation area, respectively.

[0008] To better realize the present invention, the above method is further optimized, and the driving channel A is in the form of a broken line.

[0009] To better realize the present invention, the above method is further optimized, and the slope of the driving lane A is 8%-12%.

[0010] To better realize the present invention, the above method is further optimized by laying a precast concrete pavement on the driving lane A.

[0011] To better realize the present invention, the above method is further optimized by arranging a vehicle passage B on the exposed soil layer in the construction zone; the two ends of the vehicle passage B extend spirally from the center to the periphery. There are two driving lanes A, which are arranged on two opposite slopes. The two ends of driving lane B are connected to the two driving lanes A respectively.

[0012] Compared with the prior art, the present invention has the following advantages: The excavation method for silty clay foundation pits provided by this invention involves setting up intercepting ditches and multiple dewatering wells in the non-excavation area. The intercepting ditches intercept surface water entering the excavation area, and the dewatering wells effectively lower the groundwater in the excavation area below the excavation area, providing favorable construction conditions for subsequent excavation operations. At the same time, the excavation of construction zone one and construction zone two is carried out in a layered and alternating manner. After the excavation of the soil layer in construction zone two is completed, the exposed slope is supported to avoid safety accidents such as landslides during the excavation process, reduce the load and deformation of the exposed slope, thereby shortening the construction period, reducing construction costs, and achieving simple, safe and efficient excavation of silty clay foundation pits. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram showing the distribution of the excavated and non-excavated areas in the excavation method for a silty clay foundation pit according to the present invention.

[0015] Figure 2 This is a schematic diagram of the excavation method for a silty clay foundation pit according to the present invention, which involves stratified and alternating excavation of construction zone one and construction zone two.

[0016] Figure 3 This is a top view of the construction area during the excavation of a silty clay foundation pit using the excavation method of the present invention.

[0017] In the picture: 1. Excavation area; 11. Construction Zone 1; 111. Drainage ditch; 112. Sump well; 12. Construction Zone 2; 2. Non-excavation area; 3. Dewatering wells; 4. Interception ditch; 51. Driving lane A; 52. Driving lane B. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0019] In the description of this invention, it should be noted that, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," "tail," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0020] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0021] In the embodiments of this application, the excavation method for the silty clay foundation pit includes the following steps: The construction area is determined and divided into excavation area 1 and trenchless area 2 according to the construction drawings. (See attached diagram) Figure 1 The longitudinal section of excavation area 1 is a trapezoid with a larger top and a smaller bottom. After excavation of excavation area 1 is completed, this space becomes the foundation pit. The slope connecting excavation area 1 and non-excavation area 2 forms a slope after excavation of excavation area 1 is completed. In the non-excavation area 2, intercepting ditches 4 and multiple dewatering wells 3 are installed. (See below) Figure 3 Interception ditch 4 is laid along the edge of excavation area 1 to intercept surface water entering excavation area 1; multiple dewatering wells 3 are laid around the perimeter of excavation area 1 so that the groundwater in excavation area 1 can be effectively reduced below the excavation area 1 through dewatering wells 3, providing good construction conditions for subsequent excavation operations. The excavation area 1 is divided into multiple soil layers from top to bottom; The excavation area 1 is divided into construction zone 11 and construction zone 2 12 which covers the periphery of construction zone 11; in this embodiment, the longitudinal cross-section of construction zone 11 is a trapezoid with a larger top and a smaller bottom. The soil layers in construction zone 2 12 have the same thickness as the soil layers in construction zone 1 11. In this embodiment, the thickness of each soil layer is no more than 3m. Excavation is carried out in construction zone 11. After excavation of one layer of soil in construction zone 11 is completed, a sump 112 and a drainage ditch 111 are installed on the exposed soil layer in construction zone 11. The lower end of the drainage ditch 111 is connected to the sump 112. See [reference needed]. Figure 3 A water pump is installed in the water collection well 112 to discharge water to the outside of the construction area; the drainage ditch 111 is 0.5-1m away from the slope, 0.6-1m deep, and 0.4-0.6m wide. The bottom slope of the drainage ditch 111 is not less than 0.3%. The construction of the drainage ditch 111 is completed by a combination of mechanical excavation and manual trimming, and the ditch wall of the drainage ditch 111 is reinforced by spraying concrete. Multiple water collection wells 112 are set up, with a spacing of 20-30m between two adjacent water collection wells 112. The bottom depth of the water collection well 112 is 1-1.5m lower than the bottom of the drainage ditch 111. The diameter of the water collection well 112 is 1-2m, and a prefabricated well cylinder is embedded in the water collection well 112 to achieve rapid construction of the water collection well 112. Excavation will be carried out on the same soil layer as the completed excavation in construction zone 11 of construction zone 2. After the excavation of the soil layer in construction zone 12 is completed, the slope will be supported. The support will be formed by prestressed anchor cables (single anchor cable tension not less than 1500KN, length not less than 30m, and spacing not more than 4.5m×4.5m) + anchor spray support + random drainage holes + concrete panel (concrete strength not less than C25). Excavation of Construction Zone 11 and Construction Zone 2 is carried out alternately in the manner described above; that is, construction begins from the center of Excavation Zone 1 and proceeds outwards towards its perimeter. After the soil layer excavation of Construction Zone 11 is completed, the corresponding soil layer of Construction Zone 2 is then excavated. See [link to relevant documentation]. Figure 2 The excavation is carried out using a forward excavation and lateral unloading method. That is, the excavating equipment (hydraulic backhoe excavator) is located above the excavation layer and digs in the forward direction, while the loading equipment (dump truck) is parked on the side of the excavating equipment to load soil. The thickness of each layer is controlled at about 0.5-0.8m each time, and it is carried out in multiple times to ensure that the soil stress distribution of silty clay is uniform and reduce the risk of slope deformation. After the slope support is completed, real-time deformation monitoring is carried out on it; the real-time deformation monitoring uses a total station to monitor the horizontal displacement of the slope, and the data is recorded daily to ensure that the displacement rate does not exceed 2mm / day; The internal force changes of the anchor cable are monitored by strain gauges or fiber optic sensors, with the distance between measuring points not exceeding 5m, to ensure that the support structure is subjected to uniform stress. Water level observation wells are installed inside and outside the foundation pit, with a spacing of no more than 50m, to monitor groundwater level changes daily and ensure that the groundwater level drops to 0.5-1.0m below the excavation surface. Pore water pressure gauges are also used to monitor pore water pressure in the soil, with a spacing of no more than 8m between measuring points to ensure soil stability. Construction parameters are adjusted in real time based on monitoring data. If the slope displacement rate exceeds 2 mm / day or the internal force of the support structure is abnormal, construction is immediately suspended and reinforcement measures are taken. After all soil layers in Construction Zone 11 and Construction Zone 22 have been excavated, the foundation pit will be inspected and accepted.

[0022] Specifically, after excavating Zone 11 and Zone 22 to the design elevation, a 20-30cm thick soil layer will be reserved for manual cleaning to ensure that the flatness error of the foundation does not exceed ±5cm. Use a level and measuring rod to measure the base (bottom of the pit) to ensure that the base elevation and flatness meet the design requirements, with the error controlled within ±5cm; The base is compacted using a road roller or tamper. The number of compaction passes depends on the soil conditions, but is usually no less than 3-5 passes, and the compaction degree is no less than 90%. A geomembrane or composite geomembrane with a thickness of 0.5-1.0 mm is laid on the base surface and hot-welded together to ensure seepage prevention. Anchor trenches with a depth of 0.3-0.5 m are set at the edge of the base to anchor the geomembrane in the trenches to ensure the stability of the seepage prevention layer and meet the construction requirements of subsequent structures.

[0023] In some embodiments, multiple dewatering wells 3 are arranged circumferentially around the excavation area 1 at equal intervals, with the distance between two adjacent dewatering wells 3 being 8-15m. See [reference needed]. Figure 3 In this embodiment, the excavation area 1 is rectangular. Along the long axis of the excavation area 1, the depth of two adjacent dewatering wells 3 is 10-15m, and along the short axis of the excavation area 1, the depth of two adjacent dewatering wells 3 is 8-10m.

[0024] Preferably, the dewatering well 3 is drilled vertically with a drilling rig, the diameter of the hole is 600-800mm, and the hole depth penetrates the aquifer to 1-2m below the excavation face. At the same time, mud is kept to protect the hole wall during the drilling process to prevent the hole wall from collapsing. Then, the piston well cleaner is lowered into the dewatering well 3. By repeatedly pulling the piston (frequency 20-30 times / minute), negative pressure is generated to suck up the mud and fine particles in the well until the water is clear. Then connect the well pipes section by section, install a 1-2m sedimentation pipe (with a filter screen) at the bottom, and wrap the outer wall of the well pipe with geotextile to prevent fine particles from seeping in; fill the annular space between the well pipe and the wall of the dewatering well 3 with filter material (3-8mm gravel) in layers, and fill the height to 1m above the top surface of the aquifer, and compact it manually or mechanically. After the filter media is filled, the top is sealed with clay balls or cement grout to prevent surface water from seeping in. The deep well pump is lowered to a position 2-3m above the bottom of the well, and the cable and water outlet pipe are led out along the well pipe. All the deep well pumps in the dewatering well 3 are started at the same time to control the groundwater level to drop to 0.5-1.0m below the excavation surface, so as to provide good construction conditions for subsequent excavation operations.

[0025] In some embodiments, a driveway A51 is arranged on the slope, see [reference]. Figure 3 The two ends of the driving passage A51 extend to the exposed soil layer in construction zone 11 and the non-excavation area 2, respectively, to facilitate the passage of vehicles (excavation equipment and loading equipment) and to quickly transport the soil excavated from the soil layer, thereby improving the construction efficiency of the silty clay foundation pit.

[0026] In some embodiments, the driving lane A51 is zigzag-shaped and has a width of 3m, so that vehicles can move smoothly from the slope to the non-excavated area 2 or to the excavated location.

[0027] Preferably, the slope of the driving lane A51 is 8%-12% to ensure the safe driving of vehicles.

[0028] In some embodiments, a precast concrete pavement is laid on the driving lane A51 to prevent the driving lane A51 from sinking and causing vehicles to overturn, thereby improving the safety of the process of the silty clay foundation pit.

[0029] In some embodiments, a driveway B52 is arranged on the exposed soil layer in construction zone 11; the two ends of the driveway B52 extend spirally from the center to the periphery, see [reference]. Figure 3 ; There are two driving lanes A51, which are located on two opposite slopes. Both ends of driving lane B52 are connected to the two driving lanes A51. Vehicles can enter and exit the pit from both directions and run in an orderly manner to improve the excavation efficiency of the silty clay pit.

[0030] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for excavating a silty clay foundation pit, characterized in that: Includes the following steps: The construction area is determined and divided into an excavation area (1) and an excavation-free area (2) according to the construction drawings. The longitudinal section of the excavation area (1) is a trapezoid with a larger upper section and a smaller lower section. The slope connecting the excavation area (1) and the excavation-free area (2) forms a slope after the excavation of the excavation area (1) is completed. In the non-excavation area (2), a water interception ditch (4) and multiple dewatering wells (3) are set up. The water interception ditch (4) is laid along the edge of the excavation area (1), and multiple dewatering wells (3) are laid around the circumference of the excavation area (1). The excavation area (1) is divided into multiple soil layers from top to bottom; The excavation area (1) is divided into construction zone 1 (11) and construction zone 2 (12) which covers the periphery of construction zone 1 (11). Excavation of construction area 1 (11) is carried out. After the excavation of one layer of soil in construction area 1 (11) is completed, a water collection well (112) and a drainage ditch (111) are set on the exposed soil layer in construction area 1 (11). The lower end of the drainage ditch (111) is connected to the water collection well (112). A water pump is installed in the water collection well (112) to discharge water to the outside of the construction area. Excavate the same soil layer that has been excavated in construction area 2 (12) corresponding to construction area 1 (11). After the excavation of the soil layer in construction area 2 (12) is completed, the slope is supported. The excavation of construction zone 1 (11) and construction zone 2 (12) shall be carried out alternately in the manner described above until all soil layers have been excavated.

2. The method for excavating a silty clay foundation pit according to claim 1, characterized in that: Multiple dewatering wells (3) are arranged circumferentially around the excavation area (1) at equal intervals, with the distance between two adjacent dewatering wells (3) being 8-15m.

3. The method for excavating a silty clay foundation pit according to claim 1, characterized in that: A driving lane A (51) is arranged on the slope, with both ends of the driving lane A (51) extending to the exposed soil layer in construction area 1 (11) and the non-excavation area (2), respectively.

4. The method for excavating a silty clay foundation pit according to claim 3, characterized in that: The driving lane A (51) is zigzag-shaped.

5. The method for excavating a silty clay foundation pit according to claim 4, characterized in that: The slope of the driving lane A (51) is 8%-12%.

6. The method for excavating a silty clay foundation pit according to claim 3, characterized in that: A precast concrete pavement is laid on the driving lane A (51).

7. The method for excavating a silty clay foundation pit according to claim 3, characterized in that: A driving lane B (52) is arranged on the exposed soil layer in construction zone 1 (11); the two ends of the driving lane B (52) extend spirally from the center to the periphery; There are two driving lanes A (51), which are arranged on two opposite slopes. The two ends of driving lane B (52) are connected to the two driving lanes A (51).

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