Numerical modeling of the Seismic Response of Shallow Circular Tunnels in Homogenous Layered Ground Using FEM Code ABAQUS and Compare the Results with the Analytical Solutions

Summary
Seismic response of circular tunnels embedded in homogenous ground has been studied using the FEM code abaqus. Results show that the tunnel embedded in one layered ground has a completely different behavior in comparison with the same tunnel embedded in two( or more) layered ground.
In this study, the effect of ground stratification on the seismic response of circular tunnel''s lining is investigated, as most practice-oriented studies consider homogeneous ground. The above mentioned situation has been argued detailed in the coming content.
Methodology and Approaches
A finite element plain-strain model of a circular tunnel''s cross-section embedded in an one and two-layered ground is used to highlight the influence of stratification on the tunnel''s seismic response at the crown, springline and invert points. The finite-element mesh grid simulates a 30-meter soil mass in a plain-strain condition with amount of 31226, 4-node rectangular elements in a rugged limestone rock. Ground and tunnel both are simulated with Single-phase linear elastic materials and Wang’s closed form solution is used to predict the seismic response of tunnel’s cross section. The model only considers the vertical propagation of shear waves in the visco elastic layers on a hard rock bed. The degrees of freedom at the bottom of the floor are completely restrained, because displacements are calculated from the base.
Results and The main parameters govern the calculated loads derived from analytic solutions are the compactness C, the F flexibility ratios, and the maximum rate of free shear strain ɣ. Results prove that the conditions of the ground layers, specially the amount of layers, play an important role in the situation of seismic forces encountering the tunnel lining. The layers interface was placed at the crown, center and invert level. . Models clarify that when the tunnel is fully embedded in one of the two consecutive layers, the seismic forces created in lining may vary significantly in comparison with the case that we have only one layer for tunnel to be embedded in. Furthermore if the tunnel intercepts both layers, maximum lining forces aggravation occurs when the lower layer is very stiff. When the tunnel passes through both layers of soil, the values of R follow the single-layer lining response for , otherwise less R values are obtained. For the lower stiffness of the layers, the R values obtained are almost unaffected by the higher hardness values, EV.