Mechanical analysis and field measurement of large-section quasi-rectangular pipe jacking buried deeply in soft soils

Deeply buried large-section rectangular and quasi-rectangular pipe jackings are increasingly used in the construction of large-scale urban underground structures such as subway stations in soft soils. The water and earth pressures and internal force responses of the structures can play an important role in ensuring the long-term safety of urban underground space. However, the relevant researches are still limited. Based on the field measurements of Jing'an Temple Station Project of Shanghai Metro Line 14, the spatial distribution of water and earth pressure and structural responses of large-section rectangular pipe-jacking structure deeply buried in soft soils are investigated. Then, the influences of earth pressure theories and soil-structure interaction modes on the internal force responses of pipe-jacking structures are analyzed. The applicability of different design methods are evaluated. The main conclusions include: (1) The measured vertical earth pressures at the top of the structures are close to the theoretical values of soil column weight, and the bending moments of the structures calculated by using the above theoretical earth pressure are consistent with the field measurements, and thus suggesting that the soil arching effects at the top of the pipe jacking in this project may not be significant. (2) The bending moment of the quasi-rectangular pipe jacking shows a "butterfly-shaped" distribution, indicating that the deformation mode of the structures is vertically concave and transversely convex, while the transverse convexity of the waist can further result in horizontal ground reactions. (3) The horizontal soil reactions can have significant impacts on the internal force of the large-section rectangular pipe jacking structures. The existing design specifications for pipe-jacking structures mostly ignore this factor, which can lead to a remarkable overestimation of the structural bending moment (e.g., for the structures in the case study, and the computation error for the bending moment at the waist areas can be a factor of two).