多向地震荷载作用砂土震陷计算方法研究

Seismic compression is a major cause of earthquake disaster. However, there are many limitations in the existing procedures for evaluating the seismic compression. The overall purpose of this research project is to develop a procedure of evaluating seismic compression induced by multi-directional earthquake motions by the means of theoretical derivation, numerical simulation and lab tests. Dynamic simple shear tests on different types of sands under multi-directional earthquake loading will be simulated with finite element method, and the constitutive model used in the numerical analyses will be the reduced order bound surface hypo-plasticity model. The effects of characteristics of loading sequence, asymmetry of loading, earthquake loading direction, earthquake loading type, input loading dimension, earthquake magnitude, site-to-source distance and properties of sand on seismic compression will be also carefully investigated. Then, based on dynamic simple shear tests simulated by using finite element method, a new method of computation of equivalent number of strain cycles for seismic compression of sand induced by multi-directional earthquake loading will be developed. Thereafter, a series of dynamic simple shear tests will be carried out to validate the Richart-Newmark cumulative damage fatigue hypothesis for computing the number of equivalent strain cycles, and a procedure for evaluating seismic compression induced by multi-directional earthquake motions will be proposed which can take the effect of loading sequence into consideration. Then, we shall develop calibration curves of the parameters for Richart-Newmark cumulative damage fatigue hypothesis. Finally, comparison of the volumetric strains obtained by laboratory tests and those computed by the proposed method will be carried out to validate the procedure for evaluating seismic compression proposed in this study. The research results will contribute to predict the seismic compression in earthquake, improve the basic theories in soil dynamics and provide specific guidelines for earthquake resistant of foundation.

砂土震陷是主要的震灾之一，现行砂土震陷计算方法明显不足。本课题拟通过理论分析、数值模拟和室内试验，研究多向地震荷载作用砂土震陷计算方法。建立弹塑性边界面模型的有限元模型，模拟分析多向地震荷载作用下砂土动单剪试验，研究地震加载序列、加载方向、不对称性及振动波型和单双向加载、不同震级和震中距及砂土相对密度等对震陷的影响；基于有限元数值模拟单双向加载的等效循环周数，实现混合非线性回归分析，建立砂土震陷等效循环周数计算模型；进行不同砂土震陷系列动单剪试验，验证基于Richart-Newmark (R-N)累积损伤原理进行等效循环周数计算的有效性，提出多向地震荷载作用和加载次序影响的砂土震陷计算新方法；建立砂土R-N模型参数数据库和回归分析模型，提供R-N累积损伤模型的参数校核曲线，试验验证多向地震作用砂土震陷计算方法的有效性。本项研究将完善土动力学理论等，为抗震设计提供理论依据和技术保障。

砂土震陷是主要的震灾之一，但地震荷载特性对砂土震陷影响规律不明确，现行砂土震陷计算方法存在严重不足。本课题通过理论分析、数值模拟和室内试验，研究了地震荷载特性评价方法，提出了多维地震荷载等效循环周数计算模型；分析了地震加载序列、加载方向、不对称性、振动波型、地震动维度、强度、脉冲特性及砂土特性和场地参数对震陷的影响，提出了考虑地震荷载特性的非饱和与饱和砂土震陷计算方法。研究了近断层速度脉冲地震动作用下边坡变形规律，提出了基于速度脉冲地震动边坡地震位移统一预测模型。结果表明：多维地震荷载等效循环周数计算模型可准确预估地震荷载；同一砂土相对密度和上覆荷载条件下，振幅相同的振动型地震波引起的砂土震陷比冲击型地震波引起的震陷大；地震动维度和方向对震陷有重要影响，采用单向水平地震荷载计算砂土震陷不准确，水平双向荷载引起的震陷值约为单向荷载震陷的1.5-2.3倍；震陷量与地震动速度谱强度相关性较好，随地震强度增加，震陷量增大；近断层脉冲地震动引起的砂土变形和孔压积累远大于非脉冲地震动。砂土相对密度对震陷有较大影响，增大砂土相对密度可有效减小震陷和地表侧向位移；计算饱和砂土震陷应考虑渗透系数的影响，变渗透模型可提高数值方法模拟饱和砂土地震响应的准确性；干砂阻尼比和频率比的取值对计算场地非线性响应影响显著；相同地震荷载条件下，砂土场地地表侧移量残余值与地表倾角呈线性增长关系；提出的地震荷载作用下砂土震陷计算方法可准确计算砂土震陷值。速度脉冲地震动引发的边坡位移远大于非脉冲地震动引起的位移值，对长周期边坡影响尤为明显；基于边坡参数和脉冲地震动参数建立的预测模型可较为准确的预估边坡地震位移。本项研究可完善土动力学理论等，为抗震设计提供理论依据和技术保障，具有重要的理论价值和工程意义。