1, Project Overview Metro Cultural Center Station is the terminal of Line 4 of Shenzhen Metro Phase I Project. The site is located in the south of Lianhua Mountain, with flat terrain and no buildings on the ground. The construction of the station adopts cut and cover. The length of the foundation pit is 169.9m, the width is 36. 1 ~ 73.6 m, and the average excavation depth is16.8m. The station area covers Quaternary Pleistocene eluvial layer (Oe 12), and the underlying Yanshanian granite (γ35) partially covers diabase veins.
(1) Quaternary Pleistocene eluvial layer (Qe 12)
① Gravel cohesive soil. Hard plastic, thickness 0 ~ 13.4 m, fk=230kPa. ② Gravel cohesive soil. Hard plastic to soft plastic, thickness 0 ~ 1 1.8m, fk=200kPa.
(2) Yanshanian granite (γ35)
① Fully weathered rock. It is gravelly soil with a thickness of 0.4 ~10.5m and fk=300kPa. ② Strongly weathered rocks. It is in the form of broken blocks, with a thickness of 0.4 ~10.5m and fk=500kPa. ③ Moderately weathered rocks. The core is a short column with a thickness of 0 ~ 1.0m and fk= 1000kPa. ④ Weathered rocks. The core is columnar, with developed joints and cracks, and the rock is hard, fk= 1500kPa.
(3) Diabase ① Weathered rock. The core is a short column with a thickness of 0 ~ 1.2m and fk= 1000kPa. ② Weathered rocks. The core is columnar, with developed joints and cracks, and the rock is hard, fk= 1500kPa. The groundwater in the station area is Quaternary pore water and a small amount of bedrock fissure water. The water level is buried at a depth of 2.8~4.3m, with a range of about1.0m. The main recharge source is atmospheric precipitation. The permeability coefficient of gravelly cohesive soil is 0.8m/d, sandy cohesive soil is 0.8m/d, fully weathered granite is 1.0m/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 are no buildings on the ground, and there is basically no interference between construction and traffic. Therefore, open-cut cover construction is adopted, and the average excavation depth of foundation pit is16.8m; According to the experience of foundation pit support in Shenzhen and the geological data of the station, the scheme is determined. Prestressed anchor cable for soil nailing wall. The slope ratio is 1∶0.3. See table 1.
Table 1 Comparison Table of Advantages and Disadvantages of Supporting Structure Scheme
Tongji University is currently engaged in the design of urban rail transit.
3. Design of soil nailing wall support for Cultural Center Station
3. 1, the support form design is limited by the red line of the land. Combined with the cross section of the main structure, the soil nailing wall support design of the cultural center station is in three forms: (1) single slope, slope ratio 1: 0.3, *** 13 rows of soil nails, and prestressed anchor cable jumps from the top of the slope to the third.
(2) The support is designed as upper and lower walls with a 0.63m platform in the middle. The average height of the upper wall is 10.33m, the average height of the lower wall is 6.42m, and the slope ratio is 1: 0.3, with *** 13 rows of soil nails. Prestressed anchor cables are tapped from the 3rd and 7th rows at the top of the slope.
(3) The supporting design is upper wall and lower wall, with a platform with a width of 2.62 ~ 2.78 m in the middle. Average height of upper wall 10.33m, average height of lower wall 6.42m, slope ratio 1: 0.3, *** 13 rows of soil nails are adopted, and prestressed anchor cable jumps down from the top of slope to the third row.
3.2, soil nailing design Soil nailing is a slender metal rod, placed in the soil of the foundation pit slope, arranged at close intervals. Soil nails form a self-supporting retaining wall system with the surrounding soil by virtue of the friction force on the contact surface between soil and soil. Considering the good soil conditions and easy hole formation, grouting anchor is used as soil nail in the design. According to the specifications of Shenzhen area and local experience, the length of soil nail L=0.8h(h is the height of foundation pit) is preliminarily selected, and then the uplift force of soil nail is checked, so that the length of soil nail can meet the overall stability of foundation pit and the internal stability of nail body at the same time. Theoretically, the inclination of soil nails is the best in the horizontal plane, but considering many construction factors and grouting effect, the inclination of soil nails and the horizontal plane is designed as 10. C20 cement mortar is used as the grouting material for soil nails, and self-weight pressure is used for infiltration grouting in the upper wall section. The soil nails in the lower wall section need high uplift resistance, so pressure grouting is used, and the grouting pressure is 0.3 ~ 0.5 MPa.
3.3, anchor cable design prestressed anchor cable is similar to anchor cable, but the mechanism of action is different. The main function of prestressed anchor cable is to restrain the displacement of soil nailing wall, which is divided into anchoring section and free section. The tensile force generated by the anchor section is transmitted to the waist beam through the anchor seat along the free section, and then transmitted to the soil nailing wall by the waist beam. The key factor that determines the safety and stability of the anchorage structure is whether the adjacent soil can provide enough resistance to limit the displacement of the structure. Three 7 Φ 5 steel strands are used for the prestressed anchor cable of the Cultural Center Station, with the drilling diameter of 150mm, the ultimate pull-out force of the anchor cable of 400kN, the designed maximum axial force of 250kN and the tension locking force of 200kN.
3.4, net shotcrete design shotcrete adopts C20 early strength concrete, thickness100 mm; ; Φ 8 Ⅰ grade steel is selected for the steel mesh, and the spacing is 200mm×200mm. Considering the convenience of construction, the lap joint between steel bars does not need to bend steel bars, and the length is set at 400 mm. The joints of mesh should be lap welded, and the steel bars should be welded.
3.5. The dewatering well is designed for deep point dewatering outside the pit, and the well depth is determined according to the slope of the dewatering curve, which is 24m；; The number of well points is calculated and determined according to the water inflow of foundation pit, which is ***46 holes, and the well point spacing is 12 ~ 15m. Because the two ends of the foundation pit are wide, eight well points are arranged in the foundation pit.
3.6. Monitoring design monitoring is the support for the realization of information design of soil nailing wall support design, and also the guarantee for the success of soil nailing wall support. There are 7 monitoring items in the foundation pit of Cultural Center Station. See Table 2 for details.
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 and underground space environment. However, due to the advantages of less equipment investment, short construction period, low cost, no noise, information design and construction, and dynamic management, it is gradually being studied and popularized.
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