Foundation pit dewatering can lower the original groundwater level to below the pit bottom through dewatering construction, keeping the foundation pit dry, thereby improving construction conditions and ensuring the stability and safety of the foundation pit.

　　Compared with the traditional wellpoint dewatering method, the manual punching light wellpoint dewatering construction method has the characteristics of simpler construction technology, easier operation, and better dewatering effect.

　　The Taicang City Planning Exhibition Hall project is located at the intersection of Jiangshen Avenue and Pingzhi Road in the Science and Education New Town in the southern suburbs of Taicang City, on the southwest side of Tianjing Lake, with a total construction area of 18,545 square meters. The main hall of the planning exhibition hall has three floors above ground and one floor underground; the underground floor (garage, equipment room), and three floors above ground (with a three-story-high atrium in the middle, the first floor is the central model exhibition area; the second floor is the special achievement exhibition area; the third floor is the civil air defense exhibition area and the office area within the exhibition hall).

　　The main hall of the project is designed with one basement, and the actual excavation depth of the foundation pit is 2.9 m. Although it does not reach the deep foundation pit standard stipulated in the document "Notice on Issuing the Measures for the Safety Management of Sub-divisional Works with Greater Risks" (Jianzhi [2009] No. 87) of the Ministry of Housing and Urban-Rural Development, because the project is built on the shore of Tianjing Lake and 60% of the construction area is backfilled, in order to prevent lake water from seeping in after foundation pit excavation, the general contractor considered the construction technology of dual-axis mixing pile water-stop curtain combined with manual punching light wellpoint dewatering and slope surface hanging net shotcrete for the foundation pit construction of this project.

　　According to the analysis of the soil layer conditions and excavation depth of this project, the basement foundation of this project is in the lower part of the third soil layer marked in the geological report. The soil of this layer is mostly gray silty soil, fluid-plastic, with high water content (W=42%), large void ratio (e=about 1.2), low soil strength, high sensitivity, and poor permeability. In addition, the fourth layer below the foundation pit bottom is exactly the layer with the most abundant groundwater, which brings certain difficulties to dewatering. In order to ensure the dryness of the foundation pit and the stability of the slope, according to theoretical calculations and design requirements, combined with the characteristics of the dewatering requirements of this project, manual punching wellpoint dewatering was determined to lower the groundwater level of this project, thereby keeping the pit bottom dry, facilitating construction, and meeting the requirements of foundation engineering construction.

　　I. Process Principle of Manual Punching Light Wellpoint

　　1. In foundation pit excavation construction, in order to avoid quicksand and piping, prevent the collapse of the soil mass of the foundation pit wall, and ensure construction safety and engineering quality, underwater operations should generally be avoided as much as possible. When the groundwater level is higher than the foundation pit surface, foundation pit dewatering should be carried out.

　　2. The principle of manual punching light wellpoint dewatering for the foundation pit of the Taicang City Planning Exhibition Hall project is to bury wellpoint pipes outside the foundation pit and on the inner side of the dual-axis mixing pile water-stop curtain deep into the aquifer. The upper ends of every two wellpoint pipes are connected to a centrifugal self-priming pump through connecting bent pipes, and the centrifugal self-priming pump is then connected to the water collection main pipe. After starting the water-pumping centrifugal self-priming pump, groundwater enters the wellpoint pipes through the filter layer on the outer side of the wellpoint pipes under the suction of the centrifugal self-priming pump, and is pumped by the centrifugal self-priming pump into the water collection main pipe and discharged outside the pit, thereby lowering the groundwater level to below the foundation pit bottom.

　　3. According to the application of manual punching light wellpoints in the foundation pit dewatering of the Taicang City Planning Exhibition Hall project, this construction technology has the characteristics of simple machinery and equipment, convenient operation, and good dewatering effect, and can effectively improve the stability of the foundation pit slope and prevent the occurrence of quicksand at the pit bottom. It is especially suitable for soil layers with a permeability coefficient of 0.1–5.0 m/d and soil layers containing large amounts of fine sand and silty sand, and is also suitable for foundation pit dewatering construction where open ditch drainage easily causes quicksand and collapse.

　　II. Construction Method of Manual Punching Light Wellpoint

　　1. Punching pipe
　　The punching pipe is a steel pipe 2 m–2.5 m long with a diameter of 32 mm. The lower end is a conical pipe tip with 5 mm–10 mm holes, and the upper end is connected to a plastic hose (or rubber hose) matching the diameter of the punching pipe. The length of the plastic hose (or rubber hose) is configured according to actual needs;

　　2. Wellpoint pipe
　　The wellpoint pipe is a polyethylene PE pipe 2 m long with a diameter of 25 mm. Within the 1 m length range at the lower end, plum-blossom-shaped holes of Ф5 mm are drilled (spacing 100 mm). The drilled part and the lower end of the pipe are wrapped with two layers of nylon window screen, and the nylon window screen is tightly wrapped with No. 12 iron wire. The upper end is connected to the water pump through a polyethylene PE pipe connecting pipe.

　　3. Connecting pipe
　　The connecting pipe is a polyethylene PE pipe with a diameter of 25 mm, used for connection between the wellpoint pipe and the water pump, and between the water pump and the water collection and drainage pipe.

　　4. Water collection and drainage pipe
　　The water collection and drainage pipe is a polyethylene PE pipe with a diameter of 40 mm–50 mm. The connecting pipe between the water pump and the water collection and drainage pipe is connected to the water collection and drainage pipe with a tee joint.

　　III. Main Machinery

　　1. Centrifugal self-priming pump
The CPM158 centrifugal pump is used for wellpoint pumping, and every two wellpoints share one water pump. The CPM158 centrifugal pump is a high-efficiency household water pump. It consists of a micro motor, a centrifugal pump, and a high-quality special mechanical seal. It has the advantages of light weight, low power consumption, convenient operation and maintenance, and safety and reliability.

　　2. High-pressure water pump
　　100TSW-7 high-pressure centrifugal pump, equipped with a pressure gauge, used for lowering wellpoint pipes;

　　IV. Construction Technology

　　1. Determine the dewatering scheme according to the groundwater level depth, soil permeability coefficient, and soil distribution noted in the geological exploration report;

　　2. The wellpoint layout is determined according to the plane shape and size of the foundation pit, geological and hydrological conditions, engineering nature, dewatering depth, etc. The wellpoints for the main hall foundation pit of this project adopt ring-shaped wellpoints, and the lakeside side of the platform area adopts single-row wellpoints. The distance between the wellpoint pipes and the pit wall is not more than 0.50 m, the spacing of wellpoints is 1.5 m, and the burial depth is 1.5 m. Support structures with water-blocking effects, such as deep mixing piles, steel sheet piles, concrete cast-in-place piles, or underground diaphragm walls, should be preferentially adopted, and the vertical pipes of dewatering wellpoints should be buried on the inner side of the support wall (the foundation pit side) as much as possible, and the depth of the wellpoint pipes should be shallower than the depth of the support. (Note: This project adopts the construction technology of dual-axis mixing pile support and water-stop curtain (pile length 8 m) combined with manual punching light wellpoint dewatering, and slope hanging net shotcrete with a slope ratio of 1:1.25.)

　　3. The construction process sequence of wellpoint pipes is: setting out and positioning → preliminary earthwork excavation → punching → installing wellpoint pipes → installing centrifugal self-priming pumps → connecting wellpoint pipes to water pumps with bent connecting pipes → installing water collection and drainage pipes → starting centrifugal water pumps for pumping → observing changes in groundwater level.

　　4. Wellpoint punching: manually align the punching pipe with the wellpoint position, start the high-pressure water pump, and control the water pressure at 0.4–0.8 MPa. Under the impact of high-pressure water, the punching pipe begins to sink, and the punching pipe is continuously raised and lowered. Under the action of water jetting and punching pipe cutting, the punching pipe can sink about 2 m within about 5–10 min. If thicker pure clay is encountered, the pipe sinking time will be extended; at this time, the pressure of the high-pressure water pump can be increased to accelerate the pipe sinking speed. The hole diameter formed by punching should reach 100–150 mm to ensure a certain gap between the well hole wall and the wellpoint pipe for filling sand and gravel. The punching depth should be more than 500 mm lower than the designed installation depth of the filter pipe. When there are no buildings, pipelines, or roads near the foundation pit, the wellpoint water level in the pit should preferably be lowered to 1 m below the foundation pit bottom.

　　5. Burial of wellpoint pipes along
　After the hole is formed, insert the wellpoint pipe into the well hole so that the wellpoint pipe is exposed 500 mm above the ground, then pour 5–15 mm melon-seed gravel between the well hole wall and the wellpoint pipe so that the pipe bottom has a height of 500 mm, and then evenly place 2–4 mm coarse sand around the wellpoint pipe. Within the upper 1.0 m depth, fill with clay and compact it to prevent air leakage.

　　6. Installation of centrifugal self-priming pump
　　After the wellpoint pipes are buried, install the centrifugal self-priming pump. Every two wellpoint pipes are connected with a tee to one centrifugal self-priming pump, and the water pump and the water collection and drainage pipe are connected with a connecting pipe.

　　7. Equipment commissioning
　　After the wellpoint pipes, connecting pipes, water pumps, water collection and drainage pipes, and other components are connected, the power supply can be connected for trial pumping to check for air leakage, blockage, and whether the water output is normal. When wellpoints are used, the normal water output pattern is "first large then small, first muddy then clear." If no water is pumped, or the water remains muddy, or it becomes muddy again after being clear, etc., it should be checked and repaired immediately before use.

　　8. Foundation pit dewatering
　　When wellpoints are used, the centrifugal self-priming pump should be kept pumping continuously without interruption. Generally, after 3–5 days of continuous pumping, a groundwater lowering funnel forms in the foundation pit, the groundwater level drops and basically tends to stabilize, and then the construction of the foundation and underground structures can be carried out. During wellpoint dewatering, settlement observation should be carried out on buildings within the area where the water level is lowered. If excessive settlement or horizontal displacement is found, protective technical measures should be taken promptly.

　　9. Removal of wellpoints
　　After the construction of the foundation and underground structures is completed, dewatering can be stopped and the wellpoint system can be removed. The wellpoint pipes can be pulled out with machinery, such as excavators, chain hoists, etc. The holes left by pulling out the wellpoint pipes should be plugged with sand or soil.

　　V. Construction Quality Requirements for Manual Punching Light Wellpoints

　　1. The spacing and burial depth of wellpoint pipes should meet the requirements of the "Special Scheme for Foundation Pit Support and Dewatering Engineering," and the filter pipe part of the wellpoint pipe must be buried in the aquifer.

　　2. After the wellpoint hole is formed, the wellpoint pipe should be lowered immediately and filled with melon-seed gravel and medium-coarse sand filter material to prevent hole collapse. If the wellpoint pipe cannot be lowered in time, the hole opening should be covered with a cover plate to prevent objects from falling into the well hole and blocking it. The burial of wellpoints should be free of serious air leakage, blockage, and poor water output.

　　3. During the dewatering period, specially assigned personnel should be arranged on site for frequent observation. If insufficient vacuum is found during the pumping process, the entire pumping system should be immediately checked for air leakage links and eliminated in time.

　　4. During the pumping process, especially at the beginning of pumping, check whether the wellpoint pipes are blocked. This can be checked by methods such as the sound of water flow in the pipe and whether the pipe surface is moist. If the number of blocked wellpoint pipes exceeds 10% of the total number of wellpoint pipes, the foundation pit dewatering effect will be seriously affected, and measures should be taken promptly, using high-pressure water for repeated flushing treatment.

　　5. After the wellpoints are put into use, the pump must not be stopped midway, to prevent the groundwater level from rising due to stopping pumping and causing accidents of flooding the foundation pit. In addition, after the earthwork excavation of the foundation pit, the groundwater level should be kept below the base by 0.5 m to prevent groundwater from disturbing the foundation soil mass.

　　VI. Concluding Remarks

　　The light wellpoint dewatering method is an effective treatment method for soft soil foundations and has been widely used, and the construction technology is already very mature. Compared with the traditional wellpoint dewatering method, the manual punching light wellpoint dewatering construction method described in this paper has the characteristics of simpler construction technology, easier operation, and better dewatering effect. From the application of this technology in the foundation pit dewatering of the main hall of the Taicang City Planning Exhibition Hall project, this construction method, for foundations with high groundwater level and high soil moisture content, not only increases the stability of the slope but also greatly reduces the amount of earthwork excavation, ensuring safe construction while greatly reducing construction costs, and at the same time shortening the construction period.