Keywords: foundation pit; tube well; dewatering 
　　1 Project Overview and Geological Conditions 
　　The excavated soil of a certain project foundation pit is mainly cohesive soil, with an excavation slope of 1:3. Based on the actual site conditions, the excavated slope will not be supported. Dewatering and drainage of the foundation pit are critical to excavation. Before excavation, the groundwater level should be lowered to 0.5m below the excavation surface, and disturbance of the natural foundation is strictly prohibited. A 30cm thick protective layer should be left at the bottom of the foundation pit. Foundation pit dewatering mainly adopts open ditch drainage and tube well dewatering. 
　　The engineering geology is divided into three layers: upper, middle, and lower. The upper part is mainly artificial fill, the middle part is brick-red reticulated clay, and the lower bedrock is mainly sandy conglomerate. 
　　2 Materials and Equipment Investment Plan 
　　Well pipe: Concrete well pipes are used, with an inner diameter of φ300mm and a wall thickness of 50mm. The lower part is a 1.0m settling pipe, and the upper part is a sand-free concrete filter pipe. 
　　Water pump: Two 22kW mud pumps are used for open ditch drainage. Each deep well is equipped with one 150QJ20-26/4 deep well pump, and each pump is equipped with a control switch. 
　　3 Foundation Pit Dewatering Construction Concept 
　　3.1 Overall Construction Method 
　　The excavation slope is 1:3. At this stage, the soil moisture content is moderate, and the foundation pit is excavated directly. In the second stage, when excavating below the 52.0m elevation, open ditch drainage and tube well drainage are used. Before the second stage excavation below 52.0m, deep wells are drilled starting at the 52.0m elevation. Tube well dewatering begins seven days after the completion of the cutoff wall construction to lower the soil phreatic water and improve the efficiency of earthwork excavation below the groundwater level. To enhance the dewatering effect, drainage ditches are used as an auxiliary measure. The third stage is after the foundation pit excavation is completed, using the tube well points as sumps, and adopting both open ditches and tube wells to lower the groundwater, ensuring that structure construction is carried out in dry conditions. 
　　3.2 Open Ditch Drainage Layout 
　　The foundation pit is rectangular, 50.6m long and 21m wide. As the excavation proceeds, when the pit depth approaches 52.0m, drainage ditches are set around the perimeter of the pit (outside the foundation outline, within 0.3m of the pit edge slope toe). At the four corners of the pit or every 30-40m, a sump with a diameter of 0.8m is set. The ditch bottom width is 0.3m, and the ditch bottom is 0.4m lower than the foundation pit. The sump bottom is 0.8m lower than the drainage ditch. As the excavation progresses, the drainage ditches and sumps are set and deepened in stages until the pit bottom reaches the design elevation. After the foundation pit is excavated to the predetermined depth, the drainage ditches and sumps are trimmed and improved. When the ditch walls are unstable, they must be supported with dry-laid brick and stone or permeable sandbags. 
　　3.3 Tube Well Dewatering Layout 
　　The pumping tube wells are arranged along the perimeter of the foundation pit, 1.5m from the outer edge of the pit. One row of tube wells is arranged on each of the left and right sides of the pit, with 3 wells on each side. The number of tube wells can be increased or decreased according to the dewatering effect. The distance from the well center to the building edge is 1.5m, the well spacing is 7m, and the wellhead diameter is 600mm. The well pipes are installed in sections and dismantled section by section as the excavation proceeds, down to 30cm above the excavation surface. During structure construction, the normal water level in the wells is controlled below 48.0m to ensure that the groundwater level in the soil is 0.5m below the pit bottom. 
　　3.4 Deep Well Construction Method 
　　3.4.1 Construction Procedure 
　　Well location setting-out → wellhead preparation and casing installation → drilling rig positioning and drilling → backfilling sand cushion at well bottom → lifting and placing well pipe → backfilling filter layer between pipe wall and hole wall → installing pumping control circuit → trial pumping → normal operation of dewatering well 
　　3.4.2 Hole Formation 
　　XY-600 drilling equipment is used for drilling. According to design requirements, a 600mm diameter three-blade or four-cone drill bit is selected for one-time hole formation. The supporting pump is a BWT450/12 mud pump, with a maximum working pressure of not less than 1.2MPa and a mud delivery rate of not less than 5L/S. The drillability is controlled at 2.5-4m/h. In clay layers, mud can be self-generated; in sandy soil layers, qualified mud from the mud pit is used. The mud specific gravity is strictly controlled at 1.1 to 1.2, mud viscosity at 18-20 seconds, and sand content less than 5.0%. Generally, the specific gravity of mud flowing out of the borehole is 1.2-1.3. After sedimentation and filtration in the high-level pit, it enters the low-level pit and can be pumped to the borehole for circulation only after passing inspection. 
　　3.4.3 Deep Well System Equipment 
　　Well pipe: Concrete well pipes are used, with an inner diameter of φ300mm and a wall thickness of 50mm. The lower part is a 1.0m settling pipe, and the upper part is a sand-free concrete filter pipe. 
　　Water pump: Each deep well is equipped with one 150QJ20-26/4 deep well pump, and each pump is equipped with a control switch. 
　　3.4.4 Lifting and Placing Well Pipe, Filter Material Backfilling, and Clay Water Stop 
　　To keep the borehole and well pipe concentric, guide wooden blocks are tied to the outer wall of the well. The drilling frame is not moved, and the original drilling frame is used to lift the concrete pipe. After re-measuring the bottom elevation of the hole and confirming it is correct, fill in the record. A section of concrete blind pipe is first installed at the bottom end, lifted with a hard wooden pallet and wire rope, and slowly lowered into the hole to the predetermined depth. The blind pipe is connected to the filter pipe, the tongue and groove are aligned, and the outer wall is wrapped with a layer of 80-mesh nylon filter cloth. The joint between the two pipes is 200mm, wrapped with non-woven fabric, and then fixed vertically with 3-4 bamboo strips and tightened with 10# galvanized iron wire. Medium-coarse sand is backfilled outside the pipe. Reposted from 233 Network School Papers

    3.4.5 Well Washing and Pumping 
After the well pipe is installed, well washing should be carried out immediately without delay. After well washing, when the water output meets the requirements, formal pumping begins, and the groundwater level is measured and recorded through observation holes.

    3.4.6 Well Sealing 

　　After the foundation structure construction of this project is completed, with the approval of the supervising engineer, pumping is stopped and wells are sealed in an orderly manner to ensure quality and leave no hidden dangers. To ensure sealing safety, before unsealing, sand and gravel are used for backfilling, and the upper 0.5m is filled with clay and compacted. 
4 Operation and Management of Tube Wells 
4.1 Organizational Structure 
Before the deep well construction begins, a dedicated dewatering team is established, consisting of 1 responsible leader, 2 construction workers, 2 full-time electricians, and 2 generator operators. On-site duty is arranged with 6 people on 24-hour shifts. 
4.2 Power Supply System Guarantee 
The system power is supplied from a dedicated line on the downstream side of the anti-scour trench and is connected in parallel with the backup power supply. When the system power fails, the backup power supply is started, ensuring normal operation within 5 minutes, guaranteed by full-time electricians and generator operators. 
4.3 Well Operation Guarantee Measures 
4.3.1 Establish strict management systems, decompose responsibilities to each person, and ensure that each specific task on site is implemented by a specific person. 
4.3.2 Establish an on-site work management system and ensure its normal operation, keeping the site under control at all times. 
4.3.3 Set up a dedicated office on site to promptly check the working status of each well and each pump, and handle problems in a timely manner. 
4.3.4 Full-time electricians check the generator and lines daily for integrity, and test-start them once in the morning and once in the evening to ensure the generator operates normally in case of system power failure. At the same time, spare materials such as water pumps, wires, and fuses are prepared. 
5 Safety Management Measures 
5.1 According to the safety construction electricity regulations, meet the requirements of three-level distribution and two-level protection, and ensure the realization of "one machine, one switch, one leakage protector, one box"; 
5.2 According to the construction needs, assign dedicated personnel for pumping, conduct regular inspections, strengthen operation, maintenance, and repair work, and rectify problems promptly to prevent accidents. 
5.3 Private wiring is strictly prohibited; professional electricians are arranged for wiring. 
5.4 To prevent personnel from stepping into the wells, the well pipes during installation and removal should be 30cm above the ground. 
5.5 During deep well construction and operation, no debris is allowed to fall into the pipes. The sand material for the reverse filter layer should be washed clean and must not contain mud blocks. 
5.6 The well pipes above the excavation surface are installed in sections and dismantled section by section as the excavation proceeds, down to 30cm above the excavation surface, and later retained as sumps for the foundation pit.