In the Wanjin Building project, a large area of the retaining slope on the north side of the foundation pit collapsed. This location is in the farmland and ponds of Toujia Village, with large amounts of silt in the subgrade and complex soil layers. After the collapse outside the foundation pit, inspection found extensive sections of sandy soil and silty clay, as well as some improperly compacted backfill. Groundwater is abundant in this area, and there was moderate rain that day; a large-area collapse occurred after the rain stopped. 

　　After the collapse, the owner convened a meeting with the general contractor, supervision unit, engineering department, foundation pit support team, and earthwork team to immediately formulate a treatment plan. 

 

　　Causes of the collapse: 

　　First: engineering geological causes. The collapse area is located at the boundary zone between the sandy soil layer and the silty clay layer. Sandy soil and silty clay have no tensile strength. After the collapse, the section was found to be smooth, with signs of water accumulation in the lower layer. 

　　Second: the foundation pit support plan did not match the site conditions.

　　Third: excavation was carried out without clearly understanding the geological conditions through exploration.

　　Fourth: continuous rainfall. 

　　Fifth: the foundation pit excavation lasted too long, and the toe of the slope was repeatedly soaked in water. 

 

　　Treatment plan: 

　　Decisions made at the meeting: 

　　First: remove the retaining structure and power supply at the top of the foundation pit. Reinstall the water supply pipe to ensure normal water use at the sales office is not affected. 

　　Then: use 5 excavators to handle the foundation pit collapse. Excavate 2 meters into the completed rebar processing area at the top of the original foundation pit, and change the original 1:1 slope to 1:1.5. 

　　Third: adopt emergency measures for rainy season construction, mobilize 50 emergency construction workers, drive 4-meter steel pipes 3 meters into the base soil at the bottom of the foundation pit with a spacing of 200 mm, then place bamboo plywood against the inside of the steel pipes with 1 meter retained, and use sandbags filled with sand to build a retaining wall 800 mm wide, 1200 mm high, and 80 meters long. After completion, set up observation points at the bottom and top of the foundation pit, and keep records of displacement and settlement. 

　　Fourth: clear buried items and materials, harden the area 3—5 meters at the top of the foundation pit, and build a water-retaining slope.

　　Fifth: carry out support treatment. 

 

　　Treatment schematic diagram 

　　Summary and prevention: 

　　Rainy season construction is a period with frequent collapses. During construction, all units should remain vigilant, prepare emergency measures for rainy season construction in advance, increase patrol personnel, and have safety officers and plumbers/electricians conduct more inspections. Once any issue is found, it must be handled immediately or reported to superiors. During collapse treatment, first, attention must be paid to safety, with proper protection to prevent secondary collapse; second, action must be rapid, racing against time and seizing every minute. After reviewing the collapse details, immediately formulate a treatment plan and solve the problem in the shortest possible time. Also inspect the collapsed section, and once any abnormality is found, report and handle it promptly.