Design of soil nailing wall and anchor cable combined support for Shenzhen Metro Cultural Center Station?

The following is the related content of soil nailing wall and anchor cable combined support design for Shenzhen Metro Cultural Center Station brought by Zhongda Consulting for your reference.

1. Overview of the project: Metro Cultural Center Station is the terminal of Line 4 of Shenzhen Metro Phase I Project. The station site is located on the south side of Lianhua Mountain, with flat terrain and no buildings on the ground. Cut and cover is adopted for the construction of the station. The foundation pit is 169.9m long and 36.1 ~ 73.6 m wide, with an average excavation depth of 16.8m m. The station area is covered with Pleistocene eluvial layer (Oe12) in the fourth system, and the underlying Yanshanian granite (γ35) with diabase dikes locally. The physical and mechanical parameters of the soil layer are as follows:

(1) Pleistocene eluvial layer in the fourth system. Hard to plastic, ~ 13.4 m thick, fk=23kPa.② Gravel cohesive soil. Hard plastic to soft plastic, ~ 11.8 m thick, fk=2kPa.

(2) Yanshanian granite (γ35)

① completely weathered rock. It is gravel-like soil with a thickness of .4 ~ 1.5 m, fk=3kPa.② Strongly weathered rock. Fragmented, .4 ~ 1.5 m thick, fk=5kPa.③ Moderately weathered rock. The core is a short column with a thickness of ~ 1.m, fk=1kPa.④ Weathered rock. The core is columnar, with developed joints and fissures, and the rock is hard, fk=15kPa.

(3) Diabase ① moderately weathered rock. The core is a short column with a thickness of ~ 1.2m, fk=1kPa.② Weathered rock. The core is columnar, with developed joints and fissures, and the rock is hard, fk=15kPa. The groundwater in the station area is Quaternary pore water and a small amount of bedrock fissure water, and the water level is 2.8～4.3m, with a variation of about 1.m The main recharge source is atmospheric precipitation. The permeability coefficient of gravelly cohesive soil is .8m/d, sandy cohesive soil is .8m/d, fully weathered granite is 1.m/d, and strongly weathered granite is 2.5m/d.

2. Design of foundation pit support scheme The construction site of Cultural Center Station is open, there is no building on the ground, and there is basically no interference between construction and traffic, so cut and cover is adopted for construction, and the average excavation depth of foundation pit is 16.8m；: According to the experience of foundation pit support in Shenzhen and the geological data of this station, the scheme is determined. Prestressed anchor cable by soil nailing wall. The slope ratio is 1∶.3. See Table 1.

Table 1 Comparison of advantages and disadvantages of supporting structure schemes

Tongji University is currently engaged in urban rail transit design.

3. Soil nailing wall supporting design of Cultural Center Station

3.1. The supporting form design is restricted by the red line of land use, and combined with the main structural section, the soil nailing wall supporting design of Cultural Center Station has three forms: (1) single slope with a slope ratio of 1: .3, ***13 rows of soil nails, prestressed anchor cables jumping down from the top of the slope to the third and seventh rows, and continuous reinforced concrete is set under the anchor cable position. The size of the concealed beam is 3mm×4mm.

(2) The supporting design is an upper and lower wall with a platform of .63m in the middle. The average height of the upper wall is 1.33m, and the height of the lower wall is 6.42m, with a slope ratio of 1: .3. ***13 rows of soil nails are used, and prestressed anchor cables are jumped down from the top of the slope in the 3rd and 7th rows, and the anchor cables are set under them. The size of the concealed beam is 3mm×4mm.

(3) The supporting design is two sections of walls, with a platform of 2.62～2.78m in the middle. The average height of the upper section wall is 1.33m, and that of the lower section wall is 6.42m, with a slope ratio of 1: .3. * * 13 rows of soil nails are used, and prestressed anchor cables are jumped from the top of the slope to the third row. The size of concealed beam is 3mm×4mm.

3.2. Soil nailing design Soil nailing is a slender metal rod placed in the soil of foundation pit slope and arranged at close intervals. The soil nailing relies on the friction on its contact surface with the soil to form a self-supporting retaining wall system with the surrounding soil. In the design, the grouting anchor is used as soil nail, considering the good soil conditions and easy hole formation. According to the specifications of Shenzhen area and local experience, the soil nail length L=.8h(h is the height of foundation pit) is preliminarily selected, and then the uplift force of soil nail is checked, so that the soil nail length can meet the overall stability of foundation pit and the internal stability of nail body at the same time. Theoretically speaking, the inclination angle of the soil nail is horizontal, but considering many construction factors and grouting effect, the inclination angle between the soil nail and the horizontal plane is designed as 1. C2 cement mortar is used as grouting material for soil nails, and the upper wall section is grouted by gravity pressure. The soil nails in the lower wall section need to have high uplift resistance, so pressure grouting is adopted, and the grouting pressure is .3 ~ .5 MPa. < P > 3.3. The prestressed anchor cable is similar to the anchor cable, but its mechanism is different. The main function of prestressed anchor cable is to restrain the displacement of soil nail wall, which is divided into anchor section and free section. The tensile force generated by the anchor section is transmitted to the waist beam along the free section and through the anchor seat, and then transmitted to the soil nailing wall by the waist beam. The key factor that determines the safety and stability of the anchor structure is whether the adjacent soil can provide enough resistance to limit the structural displacement. Three 7Φ5 steel strands are used for the prestressed anchor cable of the Cultural Center Station, with a drilling hole diameter of 15mm, the ultimate pull-out force of the anchor cable of 4kN, the designed maximum axial force of 25kN, the tension locking force of 2kN.

3.4, and C2 early strength concrete with a thickness of 1mm； is used for the net shotcrete design. Φ8Ⅰ grade steel is selected for the steel mesh, and the spacing is 2mm×2mm. The lap joint between the steel bars is considered to be convenient for construction, and bending steel is not required, and the length is set at 4mm. The joint of the mesh should be welded by lap joint, and the reinforcement should be welded.

3.5. The dewatering well is designed for dewatering at a deep well point outside the pit, and the depth of the well is determined according to the slope of the dewatering curve, which is 24m；; The number of well points is determined by the calculation of water inflow of foundation pit, with ***46 holes and well point spacing of 12 ~ 15m. Due to the wide ends of foundation pit, there are 8 well points arranged in the foundation pit.

3.6, monitoring design monitoring is the basis for the realization of the information design of soil-nailed wall support design, or the guarantee for the success of soil-nailed wall support. There are 7 monitoring items in the foundation pit of Cultural Center Station. See Table 2.

Table 2 Monitoring items and frequency

4. Design summary Soil nailing wall supporting structure has its specific scope of application, which requires high geological conditions, ground and underground space environment. However, due to its low equipment investment, short construction period, low cost, no noise, and information design and construction, dynamic management is easy to ensure the safety of support, it is gradually being studied and popularized.

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