A construction method for the advanced support of a silt and pebble soil layer foundation pit excavation

Abstract

This invention discloses a method for advanced support construction in the excavation of foundation pits in silt and gravel soil layers, belonging to the field of foundation pit engineering technology. Addressing the problems of soil erosion leading to voids and instability at steep slopes in silt and gravel soil foundation pits, this method first constructs an external retaining wall and connects reinforcing steel bars to the internal retaining wall wire mesh. Corresponding trenches are excavated and the support structure is constructed simultaneously. Different materials are used for trench backfilling (original soil / cement-soil, and shotcrete of the same grade), and foam boards / inflatable airbags are placed inside the retaining wall wire mesh. After the backfill material reaches 70% of its design strength, the foundation pit is excavated in layers. After exposing the retaining wall wire mesh, it is connected with the central wire mesh and fixed with anchor bolts before shotcrete is applied. This invention can improve the foundation pit's resistance to collapse and the strength of the shotcrete structure, effectively enhancing the safety of the support.

Description

Technical Field

[0001] This invention relates to the field of foundation pit engineering technology, specifically a method for advanced support construction in foundation pit excavation of silt and gravel soil layers. Background Technology

[0002] In riverside and lakeside areas, the soil layers are mostly silt or gravel. When using shotcrete with wire mesh for foundation pit support construction, the shotcrete at the bottom of the excavation face can easily wash away the silt or gravel at the bottom of the reinforcing mesh, causing it to fall and clump together at the bottom of the foundation pit. This can easily create voids at the junction of the reinforcing mesh and the foundation pit, making shotcrete with wire mesh ineffective in ensuring the quality of the foundation pit support. When the silty soil layer is limited by the construction boundary, has a short slope distance, and a large slope, the bearing capacity of the silty soil is insufficient, easily leading to foundation pit instability. To ensure the safety of foundation pit excavation and support in silty and gravel soil layers, a pre-excavation support construction method for silty and gravel soil layers has been invented. By implementing pre-excavation protection for the silty and gravel soil layers, the safety of foundation pit support is improved. Summary of the Invention

[0003] The purpose of this invention is to provide a method for advanced support construction in the excavation of foundation pits in silt and gravel soil layers, so as to solve the problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a method for advanced support construction in the excavation of foundation pits in silt and gravel soil layers, comprising the following steps:

[0005] S1. Surveying and Setting Out and Trench Excavation: Using surveying equipment, set out the excavation boundary line of the foundation pit. Excavate the retaining wall trench and connecting reinforcement trench 5-10 meters outward from the excavation edge, at a location that will not affect construction. Excavate the retaining wall wire mesh trench along the excavation boundary line. The depth of the retaining wall trench is consistent with the designed excavation depth of the foundation pit, and the spacing between adjacent retaining wall trenches is controlled at 5-10 meters. The connecting reinforcement trench is perpendicular to the long side of the retaining wall trench. The spacing between adjacent connecting steel bar trenches is 2±0.3 meters. The horizontal distance between the centerline of the outermost connecting steel bar trench in each group and the short side of the nearest back retaining wall trench is no more than 1 meter. The excavation depth of the connecting steel bar trench is 0.5 meters above the design bottom elevation of the foundation pit, and the width is 0.5 meters. The length of the retaining wall wire mesh trench is the same as that of the back retaining wall trench in the same group, or the length is 1 meter longer than each end of the back retaining wall trench and maintains symmetry. The depth of the retaining wall wire mesh trench is the same as that of the back retaining wall trench.

[0006] S2. Fabrication of Reinforcing Steel and Wire Mesh Structure: During trench excavation, the rear retaining wall reinforcing cage, connecting reinforcing steel, and retaining wall wire mesh are fabricated simultaneously. The rear retaining wall reinforcing cage is rectangular. The retaining wall wire mesh is a double-layered wire mesh with a spacing of 3-5 cm between the two layers. The top 5 cm of the retaining wall wire mesh is bent inward at a 90-degree angle to form the retaining wall wire mesh receiving surface, and the bottom 5 cm is bent inward at a 135-degree angle to form the retaining wall wire mesh end. The connecting reinforcing steel is also double-layered.

[0007] S3. Reinforcement treatment of the corners of the foundation pit: The retaining wall wire mesh is continuously arranged in an "L" shape at the inside and outside corners of the foundation pit, and the retaining wall wire mesh on both sides of the "L" shape is connected to the corresponding rear retaining wall steel cage at the corner.

[0008] S4. Lifting and lowering the reinforcing steel structure: Lift the completed rear retaining wall reinforcing steel cage and connecting reinforcing steel as a whole into the corresponding rear retaining wall trench and connecting reinforcing steel trench;

[0009] S5. Trench backfilling: First, backfill the trench connecting the reinforcing bars. If the original soil at the location of the trench connecting the reinforcing bars is silt, use cement-soil mixture for backfilling. If the original soil is not silt, use the original soil for backfilling. Then, use cement-soil mixture to backfill the retaining wall trench. Finally, backfill the retaining wall wire mesh trench. Place foam boards or inflatable airbags on the inside of the retaining wall wire mesh near the foundation pit to make the retaining wall wire mesh fit tightly against the soil behind it. Then, use concrete of the same grade as the foundation pit slope spray anchor to backfill the retaining wall wire mesh trench.

[0010] S6. Layered excavation of the foundation pit: After the backfill material strength of the retaining wall trench, connecting steel bar trench, and retaining wall wire mesh trench all reach 70% of the design strength, the foundation pit is excavated in layers along the edge of the foundation pit. When the excavation reaches a position close to 0.3 meters from the edge of the foundation pit, mechanical operation is stopped and manual excavation is adopted. The blank area without retaining wall wire mesh can be mechanically excavated to the edge of the foundation pit, but the range within 0.5 meters from the edge of the retaining wall wire mesh must be excavated manually. During the excavation process, the foam board or inflatable airbag on the inside of the retaining wall wire mesh should be removed in time.

[0011] S7. Wire Mesh Connection and Shotcrete: After the adjacent retaining wall wire meshes on the same layer are exposed during excavation, the middle wire mesh is used to connect the adjacent retaining wall wire meshes. The overlap length between the middle wire mesh and the retaining wall wire mesh should not be less than 1 meter. The connection method is welding or binding. Fixed anchor rods are driven into the bottom of the middle wire mesh to ensure that the middle wire mesh is in close contact with the retaining wall wire mesh and the soil behind it. Shotcrete is applied to the outer surface of the middle wire mesh and the retaining wall wire mesh until the concrete thickness reaches the design thickness.

[0012] S8. Cyclic construction: Repeat steps S6 and S7. After completing the construction of other parts of the same layer of the foundation pit, continue to excavate downwards in layers and repeat the above construction steps until the foundation pit is excavated to the design depth.

[0013] Preferably, the length of the rear retaining wall trench is 7 meters and the width is 1 meter.

[0014] Preferably, the retaining wall steel cage and retaining wall wire mesh are manufactured in layers, with each layer of retaining wall steel cage and retaining wall wire mesh having a height of 2 meters. The layers are stacked according to the depth of the foundation pit up to the top of the foundation pit slope. The upper and lower adjacent retaining wall steel cages are connected as one unit by binding or welding, and the end of the retaining wall wire mesh of the upper layer rests vertically on the retaining wall wire mesh receiving surface of the lower layer.

[0015] Preferably, the connecting steel bars are fixedly connected to the rear retaining wall steel cage and the connecting steel bars are fixedly connected to the retaining wall wire mesh by welding.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] The pre-support construction method for excavation of foundation pits in silt and gravel soil layers in this invention pre-treats the soil behind the foundation pit and the slope surface of the foundation pit, which improves the foundation pit's resistance to collapse and enhances the structural strength of the shotcrete on the surface of the silt and gravel soil layer, effectively improving the safety of excavation and support of foundation pits in silt and gravel soil layers. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structural plan of each part of the foundation pit;

[0019] Figure 2 Plan layout of each trench and wire mesh;

[0020] Figure 3 A diagram showing the longitudinal layout of each wire mesh;

[0021] Figure 4 This is a schematic diagram showing the connection between the retaining wall wire mesh and the central wire mesh.

[0022] In the diagram: 1. Rear retaining wall; 2. Retaining wall wire mesh; 3. Middle wire mesh; 4. Retaining wall wire mesh trench; 5. Connecting rebar trench; 6. Connecting rebar; 7. Rear retaining wall trench; 8. Rear retaining wall rebar cage; 9. Fixed anchor bolt; 2-1. Retaining wall wire mesh receiving surface; 2-2. Retaining wall wire mesh end. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention discloses a method for advanced support construction in the excavation of foundation pits in silt and gravel soil layers, which consists of two parts: the external structure of the foundation pit and the internal structure of the foundation pit.

[0025] The external structure of the foundation pit includes a retaining wall 1 and connecting reinforcing bars 6. Retaining wall trenches 7 and connecting reinforcing bar trenches 5 are excavated 5-10 meters outward from the edge of the foundation pit excavation, without affecting the construction location. The size of the retaining wall trench 7 is determined by the size of the foundation pit, generally 7 meters long and 1 meter wide, with a depth consistent with the designed excavation depth. The spacing between adjacent retaining wall trenches 7 is controlled between 5-10 meters. The connecting reinforcing bar trenches 5 are perpendicular to the long side of the retaining wall trenches 7. The spacing between adjacent connecting reinforcing bar trenches 5 should be 2 ± 0.3 meters. The horizontal distance between the centerline of the outermost connecting reinforcing bar trench 5 in each group and the short side of the nearest retaining wall trench 7 should not exceed 1 meter. The excavation depth of the connecting reinforcing bar trenches 5 is 0.5 meters above the designed bottom elevation of the foundation pit excavation, and the width is 0.5 meters.

[0026] The internal structure of the foundation pit includes a retaining wall wire mesh 2. A retaining wall wire mesh trench 4 is excavated along the edge of the foundation pit excavation. The length and depth are the same as the rear retaining wall trench 7 in the same group. The length can be slightly longer than the rear retaining wall trench 7 by 1 meter at each end and maintain symmetry.

[0027] During the excavation of the retaining wall trench 7, connecting rebar trench 5, and retaining wall wire mesh trench 4, the retaining wall rebar cage 8, connecting rebar 6, and retaining wall wire mesh 2 are fabricated simultaneously. The retaining wall rebar cage 8 is rectangular. Depending on the soil conditions, additional rebar mesh can be added in the middle to improve the overall stability of the retaining wall rebar cage 8. The retaining wall wire mesh 2 is a double-layered wire mesh with a spacing of 3-5 cm. The top 5 cm section is bent inward at a 90-degree angle to form the retaining wall wire mesh receiving surface 2-1, and the bottom 5 cm section is bent inward at a 135-degree angle to form the retaining wall wire mesh end 2-2. Each layer of retaining wall rebar cage 8 and retaining wall wire mesh 2 is 2 meters high, and they are stacked in layers up to the top of the foundation pit slope according to the depth of the foundation pit. Adjacent retaining wall rebar cages 8 can be connected by binding or welding. The retaining wall wire mesh end 2-2 of the upper layer of retaining wall wire mesh 2 should fall vertically onto the retaining wall wire mesh receiving surface 2-1 of the lower layer of retaining wall wire mesh 2. The retaining wall reinforcement cage 8 and the retaining wall wire mesh 2 are fixed by connecting steel bars 6, which are set in two layers. The retaining wall wire mesh 2 should be continuously arranged in an "L" shape at the inside and outside corners, and the retaining wall wire mesh 2 on both sides should be connected to the corresponding retaining wall reinforcement cage 8 at the corner.

[0028] The retaining wall reinforcement cage 8 is hoisted and placed into the corresponding trench. First, the connecting reinforcement trench 5 is backfilled with the original soil. When the original soil is silt, a cement-soil mixture should be used for backfilling. Then, the retaining wall trench 7 is backfilled with the cement-soil mixture, which is made by mixing cement and original soil in a volume ratio of 1:6. When the pebbles are large, the cement-soil volume ratio can be increased to 1:3. Finally, the retaining wall wire mesh trench 4 is backfilled. Foam boards or inflatable airbags are placed on the inside of the retaining wall wire mesh 2 near the foundation pit to ensure that the retaining wall wire mesh 2 is tightly attached to the soil behind it. The same level of concrete as the foundation pit slope shotcrete is used for backfilling.

[0029] After the backfill material in each trench reaches 70% of its design strength, excavate the foundation pit along its edge. Excavate the pit in layers. Once the adjacent retaining wall wire mesh 2 is exposed, connect the adjacent retaining wall wire mesh 2 with a central wire mesh 3, ensuring an overlap of at least 1 meter. This connection can be achieved through welding or binding. Drive fixed anchor rods 9 into the bottom to ensure a tighter fit between the central wire mesh 3 and the retaining wall wire mesh 2, and between the central wire mesh 3 and the soil behind it.

[0030] Shotcrete is applied to the outer surface of the central wire mesh 3 and the retaining wall wire mesh 2 until the designed thickness is reached. Then, other parts of the same layer are constructed. After the same layer is completed, excavation continues downward.

[0031] The specific implementation steps are as follows:

[0032] Step 1: Surveying and Trench Excavation. Using surveying equipment, mark the excavation boundary of the foundation pit. Excavate the retaining wall trench 7 and connecting reinforcement trench 5 5-10 meters outward from the excavation edge, without affecting the construction position. The size of the retaining wall trench 7 is determined by the size of the foundation pit, generally 7 meters long and 1 meter wide, with a depth consistent with the designed excavation depth. The spacing between adjacent retaining wall trenches 7 should be controlled between 5-10 meters. The connecting reinforcement trench 5 is perpendicular to the long side of the retaining wall trench 7. The spacing between adjacent connecting reinforcement trenches 5 should be 2 ± 0.3 meters. The horizontal distance between the centerline of the outermost connecting reinforcement trench 5 in each group and the short side of the nearest retaining wall trench 7 should not exceed 1 meter. The excavation depth of the connecting reinforcement trench 5 is 0.5 meters above the designed bottom elevation of the foundation pit, and the width is 0.5 meters. Excavate the retaining wall wire mesh trench 4 along the edge of the foundation pit. The length and depth are the same as the rear retaining wall trench 7 in the same group. The length can be slightly longer than the rear retaining wall trench 7 by 1 meter at each end and maintain symmetry.

[0033] Step 2: Construct the rear retaining wall reinforcement cage 8, connecting reinforcing bars 6, and retaining wall wire mesh 2. The rear retaining wall reinforcement cage 8 is rectangular. Depending on the soil conditions, additional reinforcing mesh can be added in the middle to improve the overall stability of the rear retaining wall reinforcement cage 8. The retaining wall wire mesh 2 is a double-layered wire mesh with a spacing of 3-5 cm. The top 5 cm section is bent inward at a 90-degree angle to form the retaining wall wire mesh receiving surface 2-1, and the bottom 5 cm section is bent inward at a 135-degree angle to form the retaining wall wire mesh end 2-2. The connecting reinforcing bars 6 can be in the form of a reinforcement cage or a single reinforcing bar, and are fixed to the rear retaining wall reinforcement cage 8 and retaining wall wire mesh 2 by welding.

[0034] Each layer of retaining wall reinforcement cage 8 and retaining wall wire mesh 2 is 2 meters high, and is stacked in layers according to the depth of the foundation pit, up to the top of the pit slope. Adjacent retaining wall reinforcement cages 8 can be connected by binding or welding. The end 2-2 of the upper layer's retaining wall wire mesh 2 should vertically rest on the receiving surface 2-1 of the lower layer's retaining wall wire mesh 2. The retaining wall reinforcement cage 8 and retaining wall wire mesh 2 are fixed by connecting reinforcing bars 6, which are arranged in two layers.

[0035] Step 3: Reinforcement of the corners of the foundation pit. The retaining wall wire mesh 2 should be continuously arranged in an "L" shape at the inside and outside corners, and the retaining wall wire mesh 2 on both sides should be connected to the corresponding rear retaining wall steel cage 8 at the corner.

[0036] Step 4: Lowering the reinforcing steel structure. The entire rear retaining wall reinforcing steel cage 8 is lowered into the corresponding trench.

[0037] Step 5: Trench backfilling. First, backfill the connecting steel bar trench 5 with the original soil. When the original soil is silty, a cement-soil mixture should be used for backfilling. Then, backfill the retaining wall trench 7 with the cement-soil mixture. The cement-soil mixture is made by mixing cement and original soil in a volume ratio of 1:6. When the pebbles are large, the volume ratio of cement to original soil can be increased to 1:3. Finally, backfill the retaining wall wire mesh trench 4. Place foam boards or inflatable airbags on the inside of the retaining wall wire mesh 2 near the foundation pit to make the retaining wall wire mesh 2 fit tightly against the soil behind it. Use the same level of shotcrete as the foundation pit slope for backfilling.

[0038] Step 6: Excavation of the foundation pit. After the backfill material in each trench reaches 70% of its design strength, excavate the foundation pit along its edge. Excavate the pit in layers, stopping mechanical work 0.3 meters from the edge of the pit and using manual excavation to prevent disturbance to the retaining wall wire mesh 2 area. Remove any previously placed foam boards or inflatable airbags during the excavation process. Blank areas without retaining wall wire mesh 2 can be mechanically excavated to the edge of the pit, but the area within 0.5 meters of the edge of the retaining wall wire mesh 2 must be excavated manually.

[0039] Step 7: Connecting the wire mesh and applying shotcrete. After the adjacent retaining wall wire mesh 2 is exposed, connect the adjacent retaining wall wire mesh 2 with the middle wire mesh 3, with an overlap length of no less than 1 meter. This connection can be achieved by welding or binding. Drive in fixing anchor rods 9 at the bottom to ensure a tighter fit between the middle wire mesh 3 and the retaining wall wire mesh 2, and between the middle wire mesh 3 and the soil behind. Apply shotcrete to the outer surface of the middle wire mesh 3 and the retaining wall wire mesh 2 until the designed thickness is achieved.

[0040] Step 8: Repeat steps 6 and 7 to construct other parts of the same layer. After completing the same layer, continue excavation downwards.

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for pre-support construction of foundation pits in silt and gravel soil layers, characterized in that: Includes the following steps: S1. Surveying and Trench Excavation: The excavation boundary line of the foundation pit is laid out using surveying equipment. At a location 5-10 meters outward from the edge of the foundation pit excavation, the corresponding back retaining wall trench (7) and connecting steel bar trench (5) are excavated at a position that does not affect construction. The retaining wall wire mesh trench (4) is excavated along the excavation boundary line. The depth of the back retaining wall trench (7) is consistent with the designed depth of the foundation pit excavation, and the spacing between adjacent back retaining wall trenches (7) is controlled at 5-10 meters. The connecting steel bar trench (5) is perpendicular to the long side of the back retaining wall trench (7), and adjacent... The spacing of the connecting steel bar trenches (5) is 2±0.3 meters. The horizontal distance between the center line of the outermost connecting steel bar trench (5) of each group and the short side of the nearest back retaining wall trench (7) is no more than 1 meter. The excavation depth of the connecting steel bar trench (5) is 0.5 meters above the design bottom elevation of the foundation pit excavation, and the width is 0.5 meters. The length of the retaining wall wire mesh trench (4) is the same as that of the back retaining wall trench (7) in the same group, or the length is 1 meter longer than each end of the back retaining wall trench (7) and symmetrical. The depth of the retaining wall wire mesh trench (4) is the same as that of the back retaining wall trench (7). S2. Fabrication of reinforcing bars and wire mesh structure: During the trench excavation process, the rear retaining wall reinforcing cage (8), connecting reinforcing bars (6) and retaining wall wire mesh (2) are fabricated simultaneously; the rear retaining wall reinforcing cage (8) is rectangular; the retaining wall wire mesh (2) is a double-layer wire mesh with a spacing of 3-5 cm between the two layers. The top 5 cm of the retaining wall wire mesh (2) is bent inward at 90 degrees to form the retaining wall wire mesh bearing surface (2-1), and the bottom 5 cm is bent inward at 135 degrees to form the retaining wall wire mesh end (2-2); the connecting reinforcing bars (6) are set in two layers; S3, reinforcement treatment of the corner of the foundation pit: the retaining wall wire mesh (2) is continuously arranged in an "L" shape at the inside and outside corners of the foundation pit, and the retaining wall wire mesh (2) on both sides of the "L" shape is connected to the corresponding back retaining wall steel cage (8) at the corner. S4. Lifting and placing the steel reinforcement structure: Lift the completed rear retaining wall steel reinforcement cage (8) and connecting steel reinforcement (6) as a whole into the corresponding rear retaining wall trench (7) and connecting steel reinforcement trench (5); S5. Trench backfilling: First, backfill the connecting steel bar trench (5). If the original soil at the location of the connecting steel bar trench (5) is silt, use cement-soil mixing material for backfilling. If the original soil is not silt, use the original soil for backfilling. Then, use cement-soil mixing material to backfill the retaining wall trench (7). Finally, backfill the retaining wall wire mesh trench (4). Place foam boards or inflatable airbags on the inside of the foundation pit near the retaining wall wire mesh (2) so that the retaining wall wire mesh (2) is close to the soil behind. Then, use concrete of the same level as the foundation pit slope spray anchor to backfill the retaining wall wire mesh trench (4). S6. Layered excavation of the foundation pit: After the backfill material strength of the retaining wall trench (7), the connecting steel bar trench (5), and the retaining wall wire mesh trench (4) reaches 70% of the design strength, the foundation pit is excavated in layers along the edge of the foundation pit. When the excavation reaches a position close to the edge of the foundation pit, mechanical operation is stopped and manual excavation is adopted. The blank area without retaining wall wire mesh (2) is mechanically excavated to the edge of the foundation pit, but the range 0.5 meters away from the edge of retaining wall wire mesh (2) needs to be excavated manually. During the excavation process, the foam board or inflatable airbag inside the retaining wall wire mesh (2) is removed in time. S7. Wire mesh connection and shotcrete: After the adjacent retaining wall wire mesh (2) on the same layer is exposed by excavation, the adjacent retaining wall wire mesh (2) is connected with the middle wire mesh (3). The overlap length between the middle wire mesh (3) and the retaining wall wire mesh (2) is not less than 1 meter. The connection method is welding or binding. Fixed anchor rods (9) are driven into the bottom of the middle wire mesh (3) so that the middle wire mesh (3) and the retaining wall wire mesh (2) and the middle wire mesh (3) are closely attached to the soil behind. Shotcrete is sprayed onto the outer surface of the middle wire mesh (3) and the retaining wall wire mesh (2) until the concrete thickness reaches the design thickness. S8. Cyclic construction: Repeat steps S6 and S7. After completing the construction of other parts of the same layer of the foundation pit, continue to excavate downwards in layers and repeat the above construction steps until the foundation pit is excavated to the design depth.

2. The method for advance support construction of foundation pit excavation in silt and gravel soil layers according to claim 1, characterized in that: The length of the rear retaining wall trench (7) is 7 meters and the width is 1 meter.

3. The method for advance support construction of foundation pit excavation in silt and gravel soil layers according to claim 1, characterized in that: The rear retaining wall steel cage (8) and retaining wall wire mesh (2) are both made in layers. The height of each layer of rear retaining wall steel cage (8) and retaining wall wire mesh (2) is 2 meters. They are stacked in layers according to the depth of the foundation pit to the height of the top of the foundation pit slope. The upper and lower adjacent rear retaining wall steel cages (8) are connected into one piece by binding or welding. The retaining wall wire mesh end (2-2) of the upper retaining wall wire mesh (2) falls vertically on the retaining wall wire mesh receiving surface (2-1) of the lower retaining wall wire mesh (2).

4. The method for advance support construction of foundation pit excavation in silt and gravel soil layers according to claim 1, characterized in that: The connecting steel bars (6) and the rear retaining wall steel cage (8), and the connecting steel bars (6) and the retaining wall wire mesh (2) are all fixedly connected by welding.

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