珊瑚岛礁场地非线性地震反应特征分析

南海珊瑚岛礁的地震安全问题是一个亟需解决的基础性科学问题。以某南海珊瑚岛礁为研究对象,开展了16组饱和珊瑚砂试样的动三轴试验,发现采用修正Matasovic本构模型描述珊瑚砂的动剪切模量折减和阻尼比增长特性是适宜的,并给出了相应的模型参数。详细考虑岛礁场地的工程地质特性和珊瑚砂的动力非线性特性、自由场非均匀网格划分和人工边界条件,尤其强调了土体应力–应变滞回曲线的不规则加卸载准则,建立珊瑚砂岛礁场地的二维非线性地震反应分析模型,分析了珊瑚岛礁场地峰值加速度放大系数的空间变异特征、地表加速度反应谱的谱形与持时特征。结果表明:①峰值加速度随场地高程的增加总体上呈增大的趋势,10 m以浅放大效应显著,灰沙岛和港池区域尤为显著;②基岩输入地震动频谱特性对珊瑚岛礁地表谱加速度的谱形影响显著,地表谱加速度在周期小于0.7 s部分的反应非常显著。③地表地震动持时较之输入地震动持时均有所减小,地形地貌特征对地表地震动持时有一定程度的影响。该研究结果对南海类似珊瑚砂岛礁军事和民事功能设施的抗震分析具有借鉴意义。

Nonlinear seismic response characteristics of a coral island site

The seismic safety evaluation of the coral islands in the South China Sea is an urgent and fundamental scientific issue. For a coral island in the South China Sea, the cyclic triaxial tests on 16 sets of saturated coral sand samples are performed. The test results show that the modified Matasovic constitutive model is suitable to characterize the shear modulus ratio reduction and damping ratio increase of coral sand, and the parameters of the Matasovic model are also proposed. Considering the engineering geology characteristics of the coral island, nonlinear dynamic behaviors of the coral sand, layout of nonuniform mesh of the free-field site and artificial boundary conditions as well as the special emphasis given to the irregular unloading-reloading rules for the stress-strain hysteresis loop, a two-dimensional nonlinear seismic response analysis model for the coral island is established. The spatial variation characteristics of the peak acceleration amplification factors, shapes of ground surface acceleration response spectra and durations of the coral island are analyzed. The results show that: (1) The peak accelerations of the coral sand site tend to increase as the elevation increases, the peak acceleration amplifications are obvious for the shallow depth less than 10 m, and the amplification effects are especially strong in lime-sand island and harbor basin regions. (2) The shapes of ground surface acceleration response spectra are closely related to the characteristics of the input bedrock motions. The ground surface spectral acceleration is very significant in the period less than 0.7 s. (3) The ground motion durations are shortened to different degrees, which are closely related to the characteristics of seismic bedrock motions, and to some extent related to the topographical and geomorphic characteristics of the coral island. The results will be of use in the construction of future coral islands in the South China Sea.