边界条件对土层粘弹性地震反应的影响

边界条件是土层粘弹性地震反应必须考虑的问题。对于地表面，一般作为自由边界，而对于地下深处基岩面，边界条件有两种处理方法：一是静止边界，二是非静止边界的。首先给出了在时间域内单层地基一维土层粘弹性地震反应封闭解析解，然后从理论上分析边界条件对土层粘弹性地震反应的影响，认为采用静止边界进行土层地震反应计算得到的地震动加速度时程符合天然地震记录形式，而采用非静止边界进行土层地震反应计算得到的地震动加速度时程初始阶段出现较大幅度的振荡现象，随着土层厚度增加，这种振荡现象逐渐减弱直至消失。当土层阻尼较小或剪切波速增长时，振荡现象加剧；当土层阻尼比较大或剪切波速减小时，振荡现象减弱。此外，采用非静止边界进行土层地震反应计算，表现出随着土层厚度的增加，土层对基岩输入地震动的放大作用逐步转化为吸收作用。

Influences of boundary condition on the visco-elastic seismic response of soil layer

Boundary condition is essential to the visco-elastic seismic response analysis of ground.Usually two procedures are used to simulate the boundary condition of the underground rock surface:static boundary and unstatic boundary.At first,the analytical solution of visco-elastic seismic response to one-dimensional ground in time domain was conducted in the paper.And then,the effect of boundary condition on seismic response is studied theoretically.It was considered that the response of acceleration in time domain,which based on the static boundary condition is consistent with natural records,while big fluctuation exists based on the unstatic one.With the increase of the ground thickness the fluctuation is reduced progressively to zero.It is also found that the fluctuation becomes larger with the decrease of damping ratio and increase of dynamic shear wave velocity,and the Huctuation becomes weaker in the converse conditions.In addition,the effect of amplification on input motion based on rock will reduce with the increase of ground thickness by using unstatic boundary condition in calculation.