Parameter study on ground motion design of deep soft site under far-field large earthquake

Based on the typical stratigraphic section in the downtown area of Suzhou City, large-scale refined two-dimensional (2D) finite element (FE) nonlinear analytical models for deep soft site are established. Ground motion design parameters of the deep soft site under far-field large earthquake are investigated. The results indicate that: (1) The peak ground acceleration (PGA) under far-field large earthquake is significantly larger than that under artificial wave. The characteristics of input ground motion as well as inhomogeneity and nonlinearity of ground soils which make PGA at different ground positions have obvious variability. (2) The seismic design parameters, α_max，β_max and T_g, of the deep soft site are far greater than those according to the existing national code. Moreover, the value of β_max increases as the ground motion level grows. (3) The PGA reduction factor with depth can be obtained as follows: to the soils with depth smaller than 5 m, it is 1.0, while to the soils 35 m in depth, it is 0.65, and it can be obtained by means of the linear interpolation when the soils are between 5 m and 35 m in depth. (4) The maximum shear strain appears at a narrow-band area around 10 m in depth. Under large and medium earthquakes, the soils between 10 and 20 m in depth behave from elasticoplastic deformation to large one, which may bring adverse influence on the underground structures within this band, especially on the slim-lined underground structures.