基于交变移动本构模型的粗粒土动力特性数值解析

Numerical analysis of dynamic characteristics of coarse grained soils based on Cyclic-mobility constitutive model

粗粒土由于抗剪强度高、易压实、变形较小等优点被广泛应用于各类工程建设中。目前对于粗粒土动力特性的研究较少,理论滞后于工程应用。在工程设计中,通常将粗粒土划分为非液化土类,但在国内外多次强震调查中发现了严重液化的粗粒土,在一定的震动作用下,松散至中密的粗粒土也有可能发生液化。根据国内外地震中发生严重液化现象的粗粒土选取了研究土料的级配,在模型参数标定基础上,使用可描述土体交变移动和液化现象的交变移动(Cyclic-mobility)土体本构模型,探讨了试样端部与试验仪器之间的摩擦、重力场、动剪应力比、围压、动荷载频率等不同因素对饱和粗粒土动力特性的影响。研究表明:Cyclic-mobility土体本构模型能较好模拟粗粒土的应力、应变特性;端部约束条件和重力场将对粗粒土室内单元试验结果造成一定的影响;较低的围压或较高的动剪应力比作用下,试样的动应变幅值越大、内部应变不均匀现象越明显;频率较低时对应的试样内部体应变分布较为相似,高频率和高动剪应力比同时作用时会对试样造成较大的破坏。通过三轴试验研究土体不同位置单元的动力响应时,应注意应变传感器的布置方法,并对结果进行综合考虑和适当修正。

Coarse grained soils have been widely used in engineering construction because of the advantages of high shear strength, easy compactability, small deformation and so on. There is still few research on dynamic properties of coarse grained soils, and theoretical research lagged behind the practices. In engineering designs, coarse grained soils were usually regarded as nonliquefiable soils. However, reports of after-earthquake surveys both in China and abroad show that coarse grained soils were serious liquefied. Therefore, under a given earthquake shaking, coarse grained soils with loose to moderate density have much potential to be liquefied. In this study, a Cyclic-mobility constitutive model, which can describe the cyclic-mobility and liquefaction characteristics of soils, was employed to discover the dynamic characteristics of coarse grained soils. On the basis of model parameters calibration, the different influencing factors which affecting dynamic properties of coarse grained soils were further studied, such as the friction between loading plates and specimen, gravitational force of specimen, cyclic shear ratio, confining pressure and loading frequency.The results show that Cyclic-mobility constitutive model can well reproduce the stress-strain characteristics of coarse grained soils under undrained triaxial test, gravitational field and the friction between loading plates and specimen have some effects on the laboratory element test results of coarse grained soils. Under lower confining pressure or higher cyclic shear stress ratio, the phenomenon of larger dynamic strain and the uneven distribution of dynamic strain inside samples will be more obvious. The distributions of volumetric strain inside samples are similar under lower loading frequencies, however, the combined action of high loading frequency and cyclic shear stress ratio will cause large damage on the sample. In the triaxial test, when studying the dynamic response of soil elements at different locations by triaxial test, attention should be paid to the arrangement of strain sensors, and the results should be considered comprehensively and corrected appropriatel.