With the rapid development of social economy, there are more and more construction projects. Deep foundation pit supporting structure is widely used in some projects because of its safety, economy and vast construction space. Deep foundation pit support technology is suitable for some buildings that need to be placed in deep underground space, and it is necessary to excavate the foundation pit. However, there is not enough space for sloping outside the foundation pit plane or there are adjacent buildings, underground pipelines and transportation roads in this space, so it can only be excavated vertically under the protection of the supporting structure, such as the deep foundation pit project introduced in this paper. In this regard, the author introduced it in detail.
1 project overview
A project needs to build a new drainage pumping station to solve the problem that the submerged water cannot be discharged by itself through the sluice when the water level of the outer river is high. The design flood control standard of pumping station is 200-year flood level, and the drainage standard is 20-year flood level and 24-hour flood control. The water level in the intake pool is 5.80m, and the designed flow rate is18.7m3/s. Four submersible axial pumps 1400QZB- 125 are designed and installed, with a single motor of 355KW and a total installed capacity 1420kW. This pumping station is medium-sized.
2 Difficulties in deep foundation pit construction of pumping station
Because of its geographical location, construction requirements and time constraints, the deep foundation pit project of this pumping station is difficult to construct. The main difficulties are as follows: (1) The pumping station of this project is arranged next to the existing sluice, which is located in an open space between the sluice and the sluice management office. There is a high-voltage tower at the intake end of the pumping station. The main building of the pumping station is close to the existing sluice and management room. The construction site is narrow, and the foundation pit excavation is deep, with the deepest point reaching10.22 m. The edge (west side) of the foundation pit is close to the side wall of the sluice structure, and the east side of the foundation pit is only 6m away from the management room. The layout of foundation pit should meet the excavation safety requirements and try to avoid affecting the safety of existing buildings. ② Diversion and traffic diversion schemes have great influence on the project layout. (2) The main body construction time is short, so the underwater structure must be completed in the dry season, and the cofferdam must be removed.
3 Comparison and Selection of Foundation Pit Support Schemes
Before the construction of this project, according to the field investigation, the distance between the marginal land of this project and the sluice and buildings is small, and the caisson structure and the sidewalk on the upper part of the outer river bank need to be broken for the culvert of the outer river outlet tank. In order to reduce the damage to the external river embankment, the length of the broken embankment should be shortened as much as possible. Therefore, in order to ensure that the excavation of foundation pit will not affect the safety of existing buildings and minimize the impact on surrounding buildings, vertical support measures are taken to excavate the intake pool, main pump house and outlet box culvert of the pump station, so as to reduce the land use and ensure the excavation safety. The elevation of the foundation surface of the joint section between the water intake pool and the water intake of the deep foundation pit project of the pumping station is 0.90m, and the elevation of the foundation surface of the pond before water intake is 0.90 ~-2.06 m; The elevation of the foundation surface of the pump room section is -2.06 m, and the elevation of the foundation surface of the outlet tank, the culvert junction section of the outlet tank and the tidal sluice section of Waijiang River is 2.3m m. ..
The ground elevation at the top of the foundation pit is 8.40m, and the excavation depth of the foundation pit is about 6.30 ~10.22m. According to the geological data, the exposed strata in the inlet section and culvert section of the outer river outlet pool are: ① artificial fill layer, ②- 1 scouring and silting sand and gravel layer, ②-3 scouring and silting muddy clay layer, argillaceous sandstone and gravel layer; The exposed strata in the pump house section include: ① artificial fill layer, ②- 1 alluvial silty sand, gravel sand layer, ②-3 alluvial silty clay layer, ②-4 alluvial silty fine sand, gravel medium-coarse sand layer, argillaceous sandstone and gravel. Among the above soil layers, ②-3 alluvial silty clay layer is highly compressible and belongs to weak layer, and its exposed thickness is about1.0 ~ 4.6m. Based on the analysis of geological data, the following two design schemes of deep foundation pit supporting structure of pumping station are drawn up for comparison:
(1) scheme 1- bored pile support plus rotary jet grouting pile enclosure scheme. Punched cast-in-place piles are used as foundation pit supporting structure outside the foundation pit, and jet grouting piles are used to stop water to prevent leakage between piles. Starting from the ground elevation of about 8.4m, slope excavation will be carried out at the elevation of 1∶2.5 to 6.0m~6.50, and the bottom of the foundation pit will be supported vertically by cast-in-place piles at the elevation of 6.0m ~-2.06m. Diameter of punched cast-in-place pile 1.0m, pile spacing 1.20m, design elevation of pile top 6.0m~6.50m, single pile length 12m ~ 2 1m, diameter of jet grouting pile 0.6m, pile spacing 1.20m, single pile.
(2) Scheme 2-Retaining wall supporting scheme of gravity lattice mixing pile. Starting from the ground elevation of about 8.4, the slope is excavated at 1∶2.5 to the elevation of 6.00 m ~ 6.50 m, and the elevation of 6.0 m ~-2.06 m to the bottom of the foundation pit, and the gravity lattice mixing pile retaining wall is used for vertical support. The retaining wall is 4.0m thick and consists of cement mixing piles with a diameter of 0.5m and a center-to-center spacing of 0.35m and soil between piles. The construction technologies of the above two schemes are mature and feasible. See table 1 for detailed quantities and investment comparison. The advantage of scheme 1 is that the supporting scheme is safe and reliable, the construction technology is mature, and it can cope with different geological conditions, but the cost is high. Scheme 2 can meet the requirements of engineering support depth and the engineering cost is low. Its shortcomings are also obvious. The thickness of gravity grid mixing pile retaining wall is 4m, and the land for this project is limited, so it is difficult to arrange. It has no embedding effect with strongly weathered rocks and cannot bear lateral shear force. Mixing pile has huge workload, long age and long construction period. From the perspective of engineering application, both schemes can meet the engineering requirements. Considering the factors such as project cost, safety and reliability, land for construction period, etc., it is recommended to adopt the supporting scheme of punching cast-in-place pile with a diameter of 1.0m and the retaining scheme of rotary jet grouting pile.
4 Foundation pit support design
The supporting range of punching cast-in-place pile in this project is from the connecting section of water inlet and sump of pumping station to the connecting section of water outlet culvert and outer river of pumping station. The longitudinal length of the support is about 90m, the pile spacing is 1.20m, and 72 piles form * * *. See Table 2 for the top elevation and pile length of each part of cast-in-place pile. In order to further ensure the safety of the foundation pit and reduce the horizontal displacement at the top of the cast-in-place pile, this is the crown beam at the top of the cast-in-place pile, with a width × height of 1.2m×0.6m, and steel pipe internal bracing with a thickness of 12mm is set every 4m. In order to protect the excavated slope from rain erosion, C20 shotcrete with a thickness of 0. 1m is used to protect the excavated slope.
5 foundation pit safety monitoring
In order to ensure the safety of foundation pit excavation and surrounding houses, it is necessary to monitor the foundation pit on the south side of the pumping station. Monitoring items include horizontal and vertical displacement of supporting structure, horizontal and vertical displacement of slope top, deformation of surrounding buildings, groundwater level, etc. The specific arrangement is as follows: four horizontal and vertical displacement monitoring points are set at the top of the slope on the east side of the foundation pit, with a spacing of about 20m; five horizontal and vertical displacement monitoring points are set on the coupling beam at the top of the cast-in-place pile, with a spacing of about 20m. The inclinometer tube is buried in the bored pile body, and three inclinometers are buried, and the spacing between each inclinometer is about 30m, so as to monitor the deep horizontal displacement of the pile body; Three piezometers are buried inside and outside the foundation pit of the pumping station to monitor the change of groundwater level during the excavation of the foundation pit. In the foundation pit and the east management room, there are three settlement and displacement observation leveling points, * * *, which are used to observe the deformation of soil in this part. The time interval of the above monitoring is determined according to the construction progress. When the change rate of monitoring results is large, the observation times should be encrypted.
6 conclusion
To sum up, the construction of deep foundation pit supporting structure is a comprehensive problem, because it is complex and changeable and greatly influenced by the environment. Therefore, if the adopted design scheme is unreasonable, it will easily lead to sudden problems and pose a great threat to the safety of property and personnel. Relevant personnel should study and formulate the most practical design scheme from the objective and practical requirements of deep foundation pit support. The design scheme adopted in the engineering example introduced in this paper has achieved the expected construction effect in the construction of deep foundation pit supporting structure of this pumping station, which can be used as a reference for similar projects.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd