Interaction between a large-scale triangular excavation and adjacent structures in Shanghai soft clay

Despite much attention being paid to the performance of rectangular basement excavation, the deformation characteristics of triangular excavations and their effects on nearby structures are not fully studied. A large-scale triangular basement with a final excavation depth of 22.8 m excavated at two sides of a cut-and-cover tunnel was extensively instrumented in Shanghai soft clay. Differing from the wall deflection in rectangular excavations, the lateral movements at the top of retaining walls in a triangular excavation was up to 70% of the maximum wall defection even though the first concrete props were cast before commencement of the main excavation. Upon completion of the base slabs, the maximum lateral wall deflection (δhm) ranged from 0.05% H to 0.35% H (excavation depth). During subsequent construction of remaining concrete slabs, δhm increased by up to 50%. Prior to completion of the base slabs, heaves were induced in interior column and diaphragm wall resulting from excavation-induced stress relief. Measured maximum heaves in the interior column and diaphragm wall were 0.08% H and 0.06% H, respectively. Different deformation mechanisms were observed in the cut-and-cover tunnel and shield tunnel. Heave was induced in the cut-and-cover tunnel located within excavation zone with a maximum value of 7.9 mm. On the contrary, settlement was observed in the shield tunnel located outside of excavation zone with a maximum value of 8.0 mm. Because of corner effects in basement excavation, three-dimensional deformation mechanisms were observed in the existing pipelines running parallel and behind the retaining walls. Due to post-excavation wall deflection induced ground settlement, the incremental maximum pipeline settlement was up to 120% of that upon completion of the base slabs.