Application of Shallow Tunneling Method (STM) for Crossover Construction in Weak Soil (Case Study: Shiraz Subway)

Different methods have been developed and used for the excavation of tunnels. These methods generally have common points, but each of them is different in execution phase and this is the reason why they are categorized. Selection of an optimal method for tunnel excavation includes full knowledge of the tunneling methods and the parameters affecting them. Geometric characteristics, geological and geotechnical properties, project conditions and existing infrastructure, project completion time, and environmental issues are some of the important parameters which must be studied and evaluated. Also, the prediction and control of settlement in terms of safety and economic point of view has considerable importance in urban areas. In recent years, the shallow tunneling method (STM) has been used to excavating underground spaces such as metro stations and similar spaces in different soils, which have many interests in urban areas due to their lack of traffic jams. According to plan of line 2 Shiraz subway, a Crossover is constructed between the Rahmat and Dolat station in the middle of two main railways of the route. The length of the Crossover is about 136.5 m and its overburden is about 7.5 m and its diameter is about 22 m. This paper focuses on selecting the appropriate excavation method for crossover using STM method based on numerical modeling with FLAC3D software. The purposes of three-dimensional numerical modeling are evaluation of settlement during excavation, and determination of the best excavation method with the lowest settlement rate. Different methods have been investigated with different drift and sections division. Finally, two applicable methods are presented in this paper and the best method with the least settlement was introduced as the preferred option. Based on the result, maximum settlement at ground surface was 2.58 cm in the preferred option.