Application of combined steel sheet pile (HSW) in foundation pit in soft soil area
1.0 Application of combined steel sheet pile (HSW) in foundation pit in the soft soil area
The foundation pit support project of Nanjing Chengnan Water Plant is located in the Nanjing Chengnan Water Plant, which is on the southwest side of the intersection of Yangzijiang Avenue and Youyi Road. In this project, there is a six-floor multiple-use building with a frame structure. Also, there is a ground floor garage under the venue. The excavation area of the foundation pit is about 1200 m^2, and the support circumference is about 150 m. Based on the structural foundation plan of the main body, the foundation pit is 5.45 to 6.15 m deep.
2.0 Geological and hydrogeological conditions
2. 1 Geological conditions
Based on the result from a reconnaissance trip, the following soil layers have been affected:
1-1: Miscellaneous fill: variegated, the upper layer is cement pavement, and the lower layer is cobblestone and clay.
2-1: Silty clay: grayish yellow, plastic, soft, low dry strength and toughness, partially mixed with floury soil.
2-2: Silty clay: gray, grayish brown, plastic flow, and low intensity and toughness.
3-1: Silty sand: gray, saturated, medium dense, partial slightly dense, feldspar, quartz particles, partially fine sand.
3-2: Silty sand: gray, saturated, compacted, mixed fine sand, quartz particles and mica fragments.
3-2A: Silty sand with silty clay: gray, clay is soft plastic.
4: Medium sand: gray, saturated and compacted, partial coarse sand, mixed with a small amount of small gravel, feldspar, quartz particles and mica fragments at the bottom.
5: Intense-weathered mudstone: fuchsia. It can be broken by hand, and easy to soften in water.
2.2 Hydrogeological conditions
The shallow groundwater of the site is located at 2-2 layers of soil. There is a layer rich in water, which is found at 0. 90- 1.50 m deep. Deep groundwater is mainly distributed in 3 and four layers of soil, and it is confined water. The impact of confined water on the foundation pit is limited, due to the shallow excavation of the foundation pit. Also, there is a thick layer of silt soil below the pit. It means that there is no need to consider the problem of the uprush of Foundation Pit. Atmospheric precipitation is the main source for groundwater supply, and the water will flow to the adjacent ditch or pond. The groundwater level varies as season changes, and the annual variation of groundwater level is about 1. 0m.
3. 0 The design scheme of foundation pit support.
3. 1 Feature and main problems in the foundation pit
The proposed excavation depth of foundation pit is 5. 45 m and 6. 15 m.
There are some other factors to affect this project:
1) The soil in this area is soft, and it is adverse to the stability of the support structure.
2) There are important pipelines in the west and north sides of the plant, and they are an important protection target in this project.
3) Because of the short construction period, low cost and convenience are factors to affect the designing scheme of support scheme.
3. 2 Support scheme
The original supporting scheme of this project is the combination of drilled pile and two rows of reinforcing concrete support. Under this plan, the construction cost is about 3 million yuan, and the construction period is eight months. However, after considering the need to save time and reduce cost, assembled steel sheet pile (HSW method) is finally adopted. The project cost changes to about 2 million yuan and the construction period reduces to 5 months.
The concrete content of the support scheme includes the combination of a steel sheet pile and H-beam. As a system for soil retaining and water sealing, an H-beam supports the horizontal system, and the pipe pile is used as the column and column pile. In this case, steel sheet pile uses 12m(single section)NSP-Ⅲw steel sheet pile, and structural steel is 12m long 588×300×12×20 H steel. The purlin and support in the pit use 400×400×12×21H section steel.
3. 3. Design and calculation of foundation pit supporting structure
On the premise of ensuring the safety and stability of the foundation pit and surrounding environment, there is a need to reduce the cost of the project. Based on the different geological conditions and the surrounding environment, five sections are selected. Calculate the worst situation of load for each section to avoid the waste of investment caused by calculating the load of the entire project under the same assumption. Generally, the ground overload is considered as 20 kPa.
1) Calculation of the soil pressure:
This design takes the permeability of the rock and soil layer and other geotechnical conditions into full consideration. During the calculation, Rankine’s earth pressure theory is applied. The soil friction between piles is ignored during the calculation, and the active and passive earth pressure is not adjusted.
2) Calculation of supporting structure:
The length of supporting pile is determined by the overall stability requirements, and the reinforcement design of pile is conducted by the maximum bending moment of pile body. The length of steel sheet pile of water sealing is determined by the requirements of impermeability, piping resistance and formation of the watertight waterproof curtain.
3) Support structure design in different working conditions:
Calculate the internal force and deformation of the support structure under each working condition strictly by the actual construction conditions, and design under the worst condition to ensure the safety of the support structure under various working conditions.
4.0 Site monitoring of foundation pit
4. 1 The content of monitoring
Since there are many uncertain factors involved in the design and construction of foundation pit engineering, it is impossible to take all of them into consideration during the design process. Also, only the geological data of the main site is provided, which will continuously change as the construction progresses. Controlling the foundation pit construction during the change requires careful and detailed information on the construction site. It is a deep foundation pit in this case, so the soil displacement caused may have a significant impact on the surroundings. To effectively prevent the adverse effect of foundation pit construction on the surroundings, it is necessary to carry out foundation pit construction tracking and monitoring, ensuring the safety of the surrounding environment and foundation pit during the construction
The field monitoring contents of this construction process includes vertical and horizontal displacement monitoring of retaining pile, the lateral inclination of the pile, horizontal and vertical deformation of the column, the axial force of support, differential settlement of the two ends, slope displacement, and underground water level outside the pit, etc.