城市建设引发地基土卓越周期变异分析

城市建设强度加大，地基土结构、性状发生变化，引发城市区域地基土的振动特性及卓越周期发生变异。地基土卓越周期与结构物自振周期接近时即产生共振破坏, 在震害中出现的许多异常现象利用普遍的认识无从解释。结构物的自振周期通常是一个“定值”，但卓越周期可能是“变量”却往往被忽略。而造成卓越周期变化的主因则来自于城市建设引起的地质体或场地土的效应、边界条件的改变。城市建设的诸因素如地下空间的开发、大量桩基甚至超长桩基的设置，深大基坑的围护、深大基础的运用、软弱土地基的加固与处理、地下水的汲取与污染、城市建设荷载施加给地质体的附加应力、城市规划引起地形地貌的变化等等均会改变场地土的结构与地基土性状，从而局部或整体改变场地的类别，影响覆盖层的特性和剪切波速等参数变化，地基土卓越周期也随之发生变化。因此在抗震设防设计中有必要综合考虑建筑前后场地类别随场地边界条件变化对建筑物的抗震影响。

Analysis of foundation soil predominant period variation induced by city construction

City construction is strengthened, foundation soil structure and properties change, and the predominant period and the vibration property of foundation soil alter in city region. If ground soil predominant period and structure natural period are close to each other, they can produce resonance destruction, resulting in many unusual phenomena that can not be explained with universal knowledge in the earthquake. Structure natural period is usually "definite value", but predominant period which may be "variable" is often neglected. The main reason of predominant period change comes from the effect of geological body or the site soil and the changes of border condition. Many aspects of urban construction, such as development of the underground space of large yardstick in the city , establishment of a large number of piles, especially even overlength piles, the enclose and protect of large deep foundation pit, application of deep and big foundation, reinforcement and treatment of the weak soil ground, the drawing and polluting of groundwater, the additional stress of the geological body induced by; topographical change and city unique environmental effect etc.; will change the structure of site soil and the properties of foundation soil; accordingly change the classification of the place partially or wholly, will influence the characteristics of the covering soil layer and the parameters like shear wave velocity; consequently the predominant period of foundation soil will change. Therefore, in aseismic design it is necessary to comprehensively consider the effect of the variation of site category which changes with site boundary conditions before and after construction, on the aseismic features of buildings.