围压对土动剪模量和阻尼比影响的简化计算方法

围压对土的初始动剪模量、阻尼比、动剪模量比和阻尼比随剪应变的变化关系（G/Gmax-λ 和h-λ等性质产生明显的影响.通过总结分析已有研究和经验模型,确认围压对初始动剪模量和阻尼比的影响.以M.B.Darendeli的研究为基础,得出了由常规围压状态下土动剪模量和阻尼比与剪应变的关系曲线来分析任意围压状态下其关系曲线的简化计算方法,在不考虑围压引起G/Gmax-λ 和h-λ 曲线形状变化的条件下,直接由剪应变值计算动剪模量和阻尼比.通过试验结果的统计回归分析,提出了砂土和黏性土的主要参数建议值.与已有经验模型和试验结果的分析对比表明,所获得的围压状态下的土性质曲线具有足够的工程精度.将该方法与等效线性化方法结合可方便地进行考虑围压影响的一维场地地震反应分析.上海地区典型深厚场地的地震反应分析表明,由基岩反应谱计算地表反应谱时,在大震作用下宜考虑围压对土性质的影响.

A SIMPLE METHOD CONSIDERING EFFECTS OF CONFINING PRESSURE ON DYNAMIC SHEAR MODULI AND DAMPING RATIO

Confining pressure is one of the key factors that affect dynamic soil properties such as low-amplitude shear modulus and damping ratio,normalized modulus reduction and material damping curves,etc..By analyzing the classical model of dynamic soil properties,predictive equations for estimating low-amplitude shear modulus and damping ratio are presented.Based on the study of M.B.Darendeli,a simple method for calculating normalized shear modulus reduction and damping ratio curves is proposed under certain effective confining pressure from those under referenced confining pressure.According to this method,assuming that the confining pressure has little influence on the shape of the curves;the curves under certain confining pressure can be easily obtained by changing the strain value of the reference curves.The key parameter in this method is deduced with regression analysis of laboratory data collected from various sources.The predicted dynamic shear modulus reduction and damping ratio curves are in excellent agreement with those of the empirical models and test results.Because the strain dependency,the proposed method can be easily combined to the equivalent linear method programs such as DYNEQ for seismic site response analysis.Case study shows that it is reasonable to use effective mean confining pressure dependent soil properties to calculate the surface response spectrum of deep site from rock response spectrum under major earthquake.