Comprehensive treatment of rock and soil: dewatering construction practice of deep foundation pit
Keywords: sandy soil measures when the water level in deep well can't fall down
1. Project overview
Hangzhou Phoenix City Project is located between Tao Qiu Road, Feiyunjiang Road and China Knitting City, with a total land area of 15923m2, a total construction area of 129978m2, an underground construction area of 263 14 m2 and an overground construction area of 103664 m2. In the form of bored pile foundation, bearing platform, ground beam and raft, the underground building is a two-story basement, the elevation of the first floor of the basement is -4.95m, the elevation of the basement floor is -8.45m, and the height of the ground beam of the basement is 1300 (the thickness of the basement floor is 650, and the thickness of the cushion is 100). The foundation pit area of this project is
2. Engineering geological and hydrological conditions
According to the geotechnical investigation report of Fenghuang provided by Zhejiang Institute of Engineering Geophysical Exploration, the landform of Fenghuang project is: old house foundation in the east and abandoned farmland in the west, with flat terrain, belonging to alluvial plain at the mouth of Qiantang River and turbulent sedimentary environment.
The soil layers from top to bottom are as follows:
1- 1 plain fill with top elevation of 7.39~8.5m and thickness of 0 ~ 2.6m
1-2 miscellaneous fill with top elevation of 5.89 ~ 9.58m and thickness of 0 ~ 4. 1 m;
2- 1 clayey silt, with the top elevation of 2.39 ~ 7.88m and the thickness of 0-11.8m;
2-2 Sandy silt, with the top elevation of-2.52 ~-4.27m and the thickness of 0 ~11.6m;
2- Gravelly sandy silt with top elevation of-8.95 ~-7.69m and thickness of 0 ~ 4.2m;
2-3 blocks of stones, with the top elevation of-11.6 ~-7.09m and the thickness of 0 ~12m;
4- 1 sandy silt, the top elevation is-11.95 ~-3.75m, and the thickness is 0 ~12.1m;
4-2 Silty sand, top elevation-12.9 ~-2.82m, thickness19 ~1.5m;
4-3 Silty sand mixed with sandy silt, with top elevation of-16.6 ~-12.12m and thickness of 0 ~ 7.3m
5 Gravel, top elevation-19.68 ~-17.5m, thickness 0 ~ 2m;
6- 1 fully weathered tuff, with top elevation of-18.96~- 14.92m and thickness of 0 ~ 9.2m
6-2 strongly weathered tuff, with top elevation of-24.16 ~-17.1m and thickness of14 ~ 8.6m;
6-3 Moderately weathered tuff, with top elevation of-28.02 ~-19.4m and thickness of 0 ~ 10. 1 m.
Groundwater in the site is mainly composed of pore phreatic water of loose lithology and fissure water of bedrock. During the survey, the depth of water level measured in the borehole is 0.5~4.7m, and the groundwater is not corrosive to concrete. The dynamic change of water level is controlled by atmospheric precipitation, and the annual average change in this area is 1 ~ 2m. The anti-floating design water level of this site adopts the national elevation of 70m. 2-2 Sandy silt is slightly liquefied and partially severely liquefied within 20m depth of the site, so it is judged as a moderately liquefied site. Deep well dewatering is used to increase the compactness of sand and improve its seismic performance.
3. Introduction to Enclosure Law
This project adopts the combined supporting structure of bored pile and reinforced concrete internal support. There is no waterproof curtain in the foundation pit, gravity deep well dewatering is used in the pit, vacuum deep well dewatering is used outside the pit, and conversion well point dewatering is used in some deep foundation pits.
4. Construction sequence and environment
The foundation pit excavation of this project is divided into Area A, Area B, Area C and Area D. According to the design requirements, the water level in the foundation pit must be reduced to below-1 1.5m before excavation, and all deep wells in Area A will be started one week before excavation in Area A. ..
There are 72 deep wells and 4 water level observation wells/kloc-0 in this project.
5. Deep well dewatering construction technology
5. 1 Deep well construction technology
Construction preparation → well point measurement and positioning → digging wellhead → running casing → drilling rig in place → drilling empty → backfilling bottom sand cushion → hoisting well pipe → backfilling gravel filter layer → washing well → running water pump → installing pumping control loop → trial pumping → normal pumping → sealing the well after dewatering.
5.2 Deep well drilling
In this project, mud is used to protect the borehole. In order to prevent the borehole from collapsing, the borehole is equipped with a casing by digging and burying, and a mud ditch and mud pool are set on one side. Before sinking the pipe, wash the well with compressed air to remove mud and silt in the hole to facilitate sedimentation.
5.3 Deep well sinking construction
PVC pipes are used in deep wells and should be vertical and located in the center of the borehole. The top of the pipeline is about 500mm higher than the natural ground. In order to ensure the alignment of well joints, the outer wall is bound and guided with long bamboo pieces. The filtration part of well pipe should be placed in the appropriate range of aquifer. After running the well pipe, fill the gravel filter material in time. The particles are 3~`8㎜ fine gravel, which must meet the grading requirements. The qualified rate is over 90%, and the impurity content is not more than 3%. It is not allowed to use mechanical direct blanking, and the shovel is used for manual blanking to prevent uneven layering and impact on the shaft wall. The filter material must be filled continuously at one time, about 1m from the bottom to the wellhead, and the upper part is sealed with clay, without sand and gravel. After filling the sand filter material, wash the well before installing the water pump. The method of well washing is to wash the pipe wall with a high-pressure water gun, and at the same time drain the water with a sewage pump to wash out the sediment. Well washing should be carried out within 8 hours before running the completion pipe and filling the filter material in one step, so as to avoid the gradual aging of the borehole wall mud during the hole-forming process and affect the infiltration effect of soil into the deep well.
5.4 Water pump installation
Submersible pumps are used in deep wells. Before installing the pump, the pump and the control system should be thoroughly inspected, including the rotation direction of the motor, whether the bolts of each part are tightened, whether the seal of the cable joint is loose, and whether the cable is damaged or broken. Then leave it idle on the ground for 3 ~ 5 minutes, and put it into the well for later use when there is no problem. After the installation, try to pump water and put it into normal use after it meets the requirements. The well point power supply system should adopt double lines and set up standby generator sets to prevent sudden power failure or failure from flooding the foundation pit.
6. Monitoring:
In this project, * * * observation well 14 is set, and the professional monitoring unit is responsible for observing and recording the water level change every day, and observing the horizontal displacement and settlement of the observation point, so as to grasp the influence of water level change and precipitation on the surrounding environment in time.
7. There is a problem:
7. After the start of1a area 10, the water level of 24 deep wells (including out-of-pit 10 vacuum deep wells and in-pit 14 artesian wells) began to drift from -7m to-10m, which was-1from the design precipitation requirement. ..
7.2 Cause analysis:
1. This project is divided into four blocks, with Area A as the first block, and the deep wells in the other three blocks have not yet started, so the precipitation pressure in Area A itself is high.
2. Observing the water output of each deep well, it is found that the artesian well in the pit has a large water output, almost all of which are pipes. On the contrary, the water output of the vacuum pump outside the pit is small, and only half of the vacuum wells outside the pit in Area A 10 have water output. Analysis reason: There are 5 ~ 7.5m cohesive silt in the southeast corner of Area A, with a permeability coefficient of 4.7E-05㎝/S and weak water permeability. The distribution of clay silt in other parts is thin, the distribution of sandy silt in the lower soil is thick, and the sandy silt has good water permeability. Due to the negative pressure of vacuum pump, clay particles in clay silt are easy to block the filter screen.
3. During the construction of Area A, the pile foundations of Area C and Area D are still under construction, and there is a stone layer on this site. During construction, the method of flushing and excavation is used to penetrate the stone layer, and sandy silt with good water permeability is under the stone layer, which provides a channel for the construction water to replenish underground.
Eight measures taken:
8. 1 Start other artesian wells near Area A as soon as possible, cut off the water supply in Area B, Area C and Area D, and relieve the precipitation pressure in Area A. ..
8.2 Five vacuum deep wells with small water inflow from the foundation pit in the southeast corner were changed into artesian wells, which were washed again with high-pressure water guns, and the effluent effect was slightly improved. Six artesian wells were added to the foundation pit in Area A, and two artesian wells were added to the foundation pit in Area B ... In view of the unsatisfactory pumping effect of light well point test, the light well point dewatering of elevator shaft deep foundation pit was changed to artesian well dewatering.
By taking the above measures, the monitoring results show that the water level continues to decline and satisfactory results have been achieved.
9 Deep well stop drilling time and plugging method
9. 1 According to the design anti-floating requirements, the deep well in the pit can stop pumping after the basement roof is covered with soil and the three-story podium structure is completed, and the deep well outside the pit can stop pumping after the basement external wall backfill is completed.
9.2 The design of post-cast strip for settlement of this project requires that the main body reach the top before it can be closed. If the anti-floating requirements are met, the precipitation will be stopped immediately, the water level will rise, and the post-pouring belt will flood the basement. During the construction of this project, except for the concrete pouring in the deep well of elevator shaft, the remaining deep wells of foundation pit are reserved to continue pumping water.
9.3 Because the diameter of PVC pipe in deep well is 300, it directly passes through the basement floor. If the joint treatment is not done, it will inevitably bring difficulties to the sealing work in the future. The joints used in this project are shown in the figure. Before binding reinforcement, round steel with a diameter of 80 was used to replace the PVC pipe passing through the bottom plate, and two water-stop steel plates were welded around the round steel, and the submersible pump was replaced by a self-priming pump.
9.4 After the precipitation stops meeting the design requirements, high-pressure grouting is used to fill the gaps between PVC pipes and steel pipes. Welded steel plates are added to the top of the steel pipe for plugging to prevent leakage.
Summary and experience of 10
In the sandy area of 10. 1, it is technically feasible to replace the water-stop curtain in the retaining system of deep foundation pit with deep well dewatering under the condition of good surrounding environment, and certain economic benefits can be achieved.
10.2 The types of deep wells should be reasonably selected according to the soil conditions of different sites, and the types of deep wells should be determined through tests if necessary.
10. The drainage pipeline of this project adopts embedded pipes. In the actual use process, the embedded pipeline is broken many times due to mechanical load, which delays the progress of the project and is labor-consuming and time-consuming. It is suggested to use open ditch drainage pipeline.
10.2 Rain and snow often affect outdoor water pumps and electric boxes, resulting in power failure. Therefore, it is necessary to strengthen the inspection of the circuit by special personnel in rainy and snowy weather.
10.3 Because there are many wells in this project, the wires and outlet pipes are often mixed together. Once there is a circuit fault in a deep well, it is extremely inconvenient to check. It is suggested that the wires and outlet pipes of each deep well be numbered uniformly.