The collapse of Daikai station in the Kobe subway system during the 1995 Hyogoken-Nanbu earthquake exhibited that underground structures are at high risk of earthquake with shallow overburden. Though researches have been done on this issue, it is still necessary to investigate further the mechanical behavior of underground structure during an earthquake and corresponding efficient seismic enhancement. In this paper, in order to find an optimum ground-improvement pattern for rectangular-shaped box culvert constructed in soft ground that does not meet the present seismic requirement, numerical tests with nonlinear 3D dynamic FEM are conducted. Different patterns of the ground improvement for the rectangular-shaped box culvert constructed with cut-and-cover method are investigated to find out an optimum pattern that can reduce the impact of earthquake in the most effective way. In the numerical tests the structure of Daikai station is considered as box culvert. Additionally, in the 2D/3D dynamic finite element analysis, the ground is Toyoura sand, typical clean sand, and its nonlinear mechanical behavior is described by Cyclic Mobility model. Validity of the proposed numerical method is firstly confirmed with 1g shaking table test and then numerical tests are conducted to find out the optimum pattern for the ground improvement.
Underground structures, such as subway facilities, lifelines, warehouses, and so on, consist of the major parts of the infrastructure of modern society and play an important role in its development. In the design of some of underground structures like Daikai station, it was considered that underground structures are in minimum seismic risk in comparison to the aboveground structures. The collapse of Daikai station in the Kobe subway system during the 1995 Hyogoken-Nanbu earthquake exhibited that underground structures are also at high risk of earthquake especially those are constructed in soft ground with thin overburden. According to Hashash et al., 2001, “The Daikai station design in 1962 did not include specific seismic provisions”. Thereafter, the failure of the Bola tunnel in 1999 Turkey earthquake and failure of gas and water pipelines in 1999 Chi-Chi earthquake in Taiwan clarified that proper consideration of earthquake load in the design of underground structures is also important.