Keywords: foundation pit; Tube well; deposit
1 general situation of the project and geological conditions
The excavation soil of a foundation pit is mainly cohesive soil, and the excavation slope is 1: 3. According to the actual situation above the construction site, excavation of slope is not supported. Drainage is the key to foundation pit excavation. Before excavation, the groundwater level should be lowered to 0.5m below the excavation surface. It is forbidden to disturb the natural foundation, and a 30cm thick protective layer should be reserved at the bottom of the foundation pit. Dewatering of foundation pit mainly adopts open ditch drainage and tube well dewatering.
Engineering geology is divided into upper, middle and lower layers: the upper part is mainly artificial accumulated fill, the middle part is brick red reticulated clay, and the lower bedrock is mainly gravel.
2 Material and equipment input plan
Well tube: concrete well tube with an inner diameter of φ300mm and a wall thickness of 50mm is used. The lower part of the well tube is a sedimentation tube of 1.0m, and the upper part is a sand-free concrete filter tube.
Water pumps: two 22kW mud pumps are used for open ditch drainage, and each deep well is equipped with 1 150QJ20-26/4 deep well pumps, and each pump is equipped with a control switch.
3 Foundation pit dewatering construction ideas
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 directly excavated. In the second stage, open trench drainage and tube well drainage are used for earthwork excavation below the elevation of 52.0m; In the second stage, before the earthwork excavation below the elevation of 52.0 meters, a deep well was drilled at the elevation of 52.0 meters, and tube well dewatering began 7 days after the cutoff wall construction, so as to reduce soil diving and improve the earthwork excavation efficiency below the groundwater level. At the same time, in order to improve the dewatering effect, drainage ditch is used to assist dewatering. The third stage is to use the tube well point as the water collecting well after the foundation pit excavation, and at the same time use the open ditch and tube well to reduce the groundwater and ensure the construction of dry land structure.
3.2 Open trench drainage arrangement
The foundation pit is rectangular, 50.6m long and 2 1 m wide. With the excavation of the foundation pit, when the depth of the foundation pit is close to 52.0m, drainage ditches are set around the foundation pit (outside the foundation contour line and within 0.3m of the toe of the foundation pit edge), and collecting wells with a diameter of 0.8m are set at the four corners of the foundation pit or every 30~40m, and the bottom of the ditch is 0.3m wide, which is 0.4m lower than that of the foundation pit for water collection. With the excavation of the foundation pit, drainage ditches and collecting wells are set up in stages and deepened until the bottom of the pit reaches the design elevation. After the foundation pit is excavated to a predetermined depth, the drainage ditch and collecting well should be trimmed and improved. If the trench wall is unstable, it should be supported by dry masonry or permeable sandbags.
3.3 Dewatering Arrangement of Tube Well
The pumping tube well is arranged along the periphery of the foundation pit, with a distance of 1.5m from the outer edge of the foundation pit. A row of tube wells are arranged on the left and right sides of the foundation pit, with 3 wells on each side. The number of tube wells increases or decreases according to the drainage effect. The well center is 1.5m away from the building sideline, the well spacing is 7m, and the wellhead diameter is 600mm. The well pipe shall be installed in sections, and removed to 30cm above the excavation surface with the excavation of foundation pit. During structural construction, the elevation of normal water level in the well shall be controlled below 48.0m to ensure that the underground water level of the soil is 0.5m lower than the bottom of the foundation pit.
3.4 Deep well construction method
3.4. 1 construction procedure
Well layout → making wellhead and installing casing → drilling rig in place and drilling → backfilling bottom sand cushion → hanging well pipe → backfilling filter layer between pipe wall and hole wall → installing pumping control circuit → trial pumping → normal operation of dewatering well.
Hole formation
Drill holes with XY-600 drilling equipment. According to the design requirements, select a three-blade or four-cone drill with a diameter of 600mm to form a hole at one time. The matching water pump is BWT450/ 12 mud pump, the maximum working pressure is not less than 1.2MPa, the mud conveying capacity is not less than 5L/S, the drillability is controlled at 2.5 ~ 4 m/h, the clay layer can be self-prepared, and the qualified mud in the mud pit enters the sandy soil layer. Strictly control the mud specific gravity from 1. 1 to 1.2, the mud viscosity 18~20 seconds, and the sand content is less than 5.0%. Generally, the specific gravity flowing out of the borehole is 1.2~ 1.3, which will be precipitated and filtered in the high-level tank, and pumped into the drilling circulation after passing the test.
Deep well system equipment
Well tube: concrete well tube with an inner diameter of φ300mm and a wall thickness of 50mm is used. The lower part of the well tube is a sedimentation tube of 1.0m, and the upper part is a sand-free concrete filter tube.
Water pump: each deep well is equipped with 1 150QJ20-26/4 deep well pumps, and each pump is equipped with a control switch.
3.4.4 Lift the well pipe, backfill the filter material, and stop the water with clay.
In order to keep the borehole concentric with the well pipe, the outer wall of the well is bound with guide wood blocks, and the drill frame is not moved. Hang the concrete pipe with the original drilling rig, and fill in the record after the elevation of the hole bottom is correct. First, a section of concrete blind pipe is arranged at the bottom, which is hoisted by hardwood pallet and steel wire rope, and then slowly falls into the hole until the predetermined depth. The blind pipe is connected with the filter pipe, and the tenon and groove are well aligned. The outer wall is wrapped with a layer of 80 mesh nylon filter cloth, and the two pipe joints are 200mm, which are wrapped with non-woven fabrics. The outside of the blind pipe is vertically fixed with 3~4 bamboo wool, fastened with 10# galvanized iron wire, and backfilled with medium coarse sand.
3.4.5 Well washing and pumping
After the well pipe is installed, the well should be washed immediately without delay. After washing the well, the water output meets the requirements, that is, the formal pumping begins, and the groundwater level is measured through the observation well. Measure, analyze, improve and increase the well at the same time when the well is completed until the water level of the foundation pit drops to the required level. For its structural form, see the structural drawing of tubular well column.
3.4.6 Oil well sealing
After the foundation construction is completed, with the approval of the supervision engineer, the project starts to stop pumping and seal the well in order to ensure the quality and leave no hidden dangers. In order to ensure the safety of plugging, gravel shall be used for backfilling before unpacking, and 0.5m clay shall be filled in the upper part for tamping.
4. Application and management of tube well
4. 1 organization
Before the start of deep well construction, a dewatering team was set up, consisting of 1 in charge of the leadership, 2 builders, 2 full-time electricians and 2 generator workers, and 6 people were on duty 24 hours a day.
4.2 Power supply system guarantee
The power consumption of the system is connected by the special line on the downstream side of the scouring tank and connected to the standby power supply. When the system power supply fails, start the standby power supply to ensure normal operation within 5 minutes, with full-time electricians and generators working as the guarantee.
4.3 Well operation guarantee measures
4.3. 1 Establish a strict management system, and divide the responsibilities to everyone, so that each specific job on site is implemented by a specific person.
4.3.2 Establish a field work management system to ensure the normal operation of the system and keep the field under control.
4.3.3 A special office shall be set up at the site to check the working conditions of wells and pumps in time, and handle any problems found in time.
4.3.4 Full-time electricians check whether the generator and circuit are in good condition every day, and try to start them once in the morning and evening. Once the system is cut off, the normal operation of the generator can be guaranteed. At the same time, spare materials such as water pumps, wires and fuses should be prepared.
5 Safety management measures
5. 1 According to the regulations on safe construction electricity consumption, meet the requirements of three-level distribution and two-level protection, and ensure the realization of "one machine, one brake, one leakage and one box";
5.2 According to the needs of engineering construction, assign special personnel to pump water, conduct regular inspection, strengthen operation, maintenance and overhaul, and make timely rectification when problems are found, so as to nip in the bud.
5.3 It is strictly forbidden to pull the wiring without permission, and arrange the wiring of professional electricians.
5.4 To prevent people from stepping into the well, remove the well pipe 30cm above the ground.
5.5 During the construction and operation of the deep well, it is required that sundries shall not fall into the pipeline, and the sand material of the filter layer shall be washed clean without mud.
5.6 The well pipe above the excavation surface shall be installed in sections. With the excavation of foundation pit, the well pipe shall be moved to 30cm above the excavation surface, and the later foundation pit collecting well shall be reserved.
References:
[1] National Water Conservancy and Hydropower Construction Technology Network Water Conservancy and Hydropower Engineering Construction Manual (Volume 1), (Volume 5). China Electric Power Press, 2004.
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