Deformation and stability of deep excavations in artificial offshore island

A two-dimensional finite element method (FEM) numerical simulation was carried out to investigate the deformation behavior and the global stability of the large-diameter steel cylinder bulkhead as well as the steel tube pile retaining structure in combination with the practice of a waterfront tunnel pit excavation in an artificial offshore island. Two numerical models were established respectively with Plaxis to compare the performance of the local excavation scheme and the large excavation scheme under complex construction conditions. The numerical results show that the local excavation scheme can reduce the relative lateral deformation of the steel cylinder bulkhead from 1.41 percent to 0.69 percent, raise the safety factor of global stability from 1.51 to 1.68, and transform the collapse mode of the structures from the type of steel cylinder overturning rotation around its bottom to the type of block slide around the steel tube pile top in the island. Nevertheless, the local excavation scheme may prolong the construction period and increase the project investment owing to the additional construction of temporary retaining structure. Both of the excavation schemes can satisfy the safety control requirements of stability; meanwhile, relatively large deformation of the excavation will not bring negative impact on the surroundings. The large excavation scheme is more favorable in construction period and project investment control than the local excavation scheme.