Study Of Site Seismic Response Under The Effects Of Additional Attenuation Of Small Strain Shear Modulus

Gmax is an important parameter in site seismic response analysis, and it was often obtained by Hardin Formula in previous effective stress analysis method. However, Hardin formula is based on static tests, considering only the influence of effective stress and void ratio, but not cyclic stress history. Soil’s Gmaxunder cyclic stress was assumed to be equivalent to the same effective stress static state Gmax. However, many studies have proven the attenuation of Gmax under large dynamic strain, namely the value of Gmax under earthquake is lower than the corresponding one calculated by Hardin Formula with the same effective stress. The attenuation caused by earthquake was named additional attenuation. Currently, few studies had been done on additional attenuation’s influence on site response of earthquake.Previous studies on Gmax under large amplitude cyclic load are reviewed, and two modes of additional attenuation of Gmax are summarized corresponding to strong and weak structural soils. It’s found that the relation between modulus ratio Gmax11/Gmax1(Gmax1refers small strain modulus under static stress state, Gmax11refers additional attenuation modulus under cyclic dynamic loading) and effective stress ratio σm/σm0can be represented by two curves or three curves. Different decay modes are presented in this paper to facilitate for numerical calculations.A refined one-dimensional equivalent linearization method has been composed on the basis of effective stress principle, to account for the influence of additional attenuation of Gmax.It can be observed that the additional attenuation of Gmax has influence both on the response of accelerations and shear stresses. Besides, the additional attenuation of Gmax can speed up the liquefaction of site and expand the final liquefied range. The influences of different site conditions and input predominant period on seismic site response are also investigated. The results indicate that the influence of the additional attenuation of Gmax on site increases as the underground water level decline, but slight changes occur as the thickness of site increases. Moreover, the influence of the additional attenuation of Gmax on strong structural soil site is greater than weak structural soil site. Faster liquefaction due to additional attenuation of Gmax happens in different input predominant periods.To further investigate the influence of Gmax additional attenuation on the seismic response of sites, the program mentioned above was used in the simulation and analysis of dynamic centrifuge model test of saturated sands. Strain control shear test and compression and resilience test were carried out to obtain the pore pressure generation model’s parameters of Fujian sand used in dynamic centrifuge model test, and parameters useful to calculate the body strain increment△εvd and soil resilient modulus Er are obtained. Numerical calculations were carried out to simulate the responses of site model under different seismic acceleration amplitudes, comparison between numerical calculations and tests showed that the responses of both methods agreed with each other perfectly. The growth rate of pore pressure of numerical calculations were slower than that of dynamic tests, which may be due to the initial condition differences between dynamic centrifuge model tests and hollow cylinder tests which determine the pore pressure parameters. No obvious differences of depth of liquefied soil and beginning time of each layer were found in both methods. Conclusions can be drawn as follows, additional attenuation of Gmax makes obvious influence on the seismic response of site when subjected to slight earthquake, but it makes little influence or even no impact on the seismic response of site when subjected to severe earthquake because of soil’s rapid liquefaction.