直下型地震下水泥土桩复合地基震陷机理试验研究

A series of bidirectional cyclic load tests were performed to simulate the coupling effect of cyclic spherical stress and cyclic shear stress under seismic load, using GDS dynamic triaxial device, GDS resonant column and GDS bender element tests. Dynamic properties of cement soil and cement-soil compound were investigated to obtain the failure criterion and dynamic strength curve. Residual strain and stiffness softening model were established under bidirectional cyclic load. By introducing cyclic spherical stress to modify the Hardin-Drnevich model, dynamic constitutive model under bidirectional cyclic load was established. Cement-soil compound foundation was regarded as macroscopic composite material and the interaction between soil and cement-soil pile was such a kind of internal forces that no specific internal force was needed to analyze. Combined with dynamic tests, residual strain and stiffness softening model of cement-soil composite were gained and sedimentation behavior of cemented soil pile compound foundation under seismic load was analyzed using Hardin-Drnevich model. Dynamic responses of cement-soil pile compound foundation were obtained. Large-scale compound foundation model tests were conducted using bidirectional vibrating table to verify the results from that of the finite element analysis.

本课题利用GDS双向动三轴仪、GDS共振柱及GDS弯曲元试验系统，通过双向激振循环荷载试验模拟土体在直下型地震下循环球应力和循环剪应力的耦合作用，研究水泥土、水泥土复合试样在具有上述特征的动荷载作用下的动力特性，获得水泥土、水泥土复合试样的破坏标准及动强度曲线；建立双向激振下水泥土、水泥土复合试样的残余应变及刚度软化模型；通过引入循环球应力比等参数实现对Hardin-Drnevich模型的修正，建立双向循环荷载作用下土体的动本构模型。将水泥土桩复合地基土体看成宏观上的复合材料，土体和水泥土桩之间的相互作用作为一种内力而不对其进行具体的内力分析，结合动力试验得到水泥土复合试样的残余应变、软化模型及Hardin-Drnevich模型对直下型地震下水泥土桩复合地基的震陷进行分析，获得水泥土桩复合地基的动力响应，并通过双向振动台进行大比例复合地基模型试验实现对有限元分析结果进行验证。

水泥土桩作为一种常见的软土地基处理方式在工程中被广泛地使用。研究水泥土桩复合地基在地震下的动力特性具有重要意义。本项目通过双向激振循环荷载试验模拟土体在直下型地震下水泥土、水泥土复合试样的动力特性。本文主要内容及结论如下：.（1）通过GDS共振柱试验研究水泥土动力特性时间效应影响。试验结果表明：水泥掺量越大，养护龄期越长，水泥土最大动剪切模量越高。围压越大，水泥土时间效应越明显，高围压下水泥土最大动剪切模量显著增大。.（2）研究了双向循环荷载作用下水泥土的动应变、动强度、软化指数。试验结果表明：相位差决定了双向剪应力的耦合方式。以转折应变作为水泥土破坏标准时，随着水泥掺量和相位差的增加，转折应变逐渐降低。应变拐点出现以前，水泥土软化指数与循环次数近似呈线性关系。随着相位差增加，水泥土软化指数和循环强度降低。.（3）对水泥土复合试样进行了一系列的双向循环单剪试验，得到以下结论：相位差和合应力峰值对双向剪切应变均有明显影响，随着相位差和合应力峰值的增加，双向应变得以加速。提高面积置换率可提高复合土的动强度，随着相位差的增大，水泥土芯的增强作用可被削弱；水泥土芯可以限制双向滞回曲线的倾斜；面积置换率和相位差对复合土的刚度软化有显著影响，刚度随着面积置换率的提高而增大，随着相位差的增大而减小。.（4）对加芯复合土体进行了一系列的应变控制的循环单剪试验，得到以下几个结论：面积置换率和竖向压力是影响复合土刚度的主要因素。随着面积置换和竖向压力的增加，动剪切模量增加；竖向压力一定时，最大剪切模量与置换率呈线性关系；复合土的阻尼比随应变等级的增加而增大。引入面积置换率，对Hardin-Drnevich模型进行修正。