Abstract: There are more and more deep excavation projects directly above or very close to the existing or planned subway tunnels. The design and construction aspects of deep excavation support have always a challenge in the industry. This paper presents a case history of deep excavation in soft soils directly above the planned subway line in Hangzhou urban area. The engineering characteristics, deep excavation support design selection and monitoring results are introduced in detail in the paper. Technical difficulties in the design of subway tunnel underneath the existing excavated foundation, including deformation control of supporting structures, soil strengthening at the bottom of excavation, reservation of the shielded tunnel crossing pass and reduction of the influence of various construction stages are also summarized. The monitoring results show that the selection of deep excavation support system is reasonable and its impact to the surrounding environment is minimal. After the basement floor was completed, the shielded tunnel crossing is favorable to the control of deep excavation deformation.

Key words: soft soils, deep excavation, shielded excavation, under crossing, strengthening