结构性软黏土K0固结不排水强度定量评价与屈服破坏机理分析

Soil structure and anisotropy have significant effects on the engineering properties of natural soft clays. Triaxial K0 consolidation undrained shear tests are performed on both undisturbed and reconstituted clay samples so as to present a quantitative evaluation method that distinguishes the effects of soil structure and K0 consolidation stress-induced anisotropy on undrained shear strength behavior of natural soft clays; Through triaxial K0 consolidation stress path tests on both natural undisturbed and reconstituted clay samples, yield surfaces and change rules can be obtained to distinguish the effects of soil structure and K0 consolidation stress-induced anisotropy on the yield behavior of natural soft clays; Through comparison and analysis of differences in pre-yield and post-yield and undrained shear strength behavior between undisturbed and reconstituted clay samples, mechanisms on soil structure destructuration of natural soft clays can be revealed. In combination with the test data and the analysis of elasto-plastic theory and critical state soil mechanics theory, yield surface equation and the undrained strength formula can be deduced by considering both the natural soil structure and the K0 consolidation stress-induced anisotropy. This project is very critical for its academic contribution and can theoretically lay the foundation in establishing constitutive model for natural structured clays. It can provide an effective forecasting on deformation and strength in soft soil foundations in practical engineering applications. It can also improve the reliability of calculation and design in foundation excavations, tunnels and other geotechnical engineering.

结构性和各向异性对天然软黏土的工程性质有着显著影响，通过开展天然土原状样与重塑样的K0固结不排水剪切试验，提出结构性软黏土不排水强度特性的定量评价方法，区分结构性、K0固结应力诱发各向异性对天然土不排水强度的影响；通过开展原状样与重塑样K0固结应力路径试验，获得天然土与重塑土的屈服面及其演化规律，区分结构性、K0固结应力诱发各向异性对天然软黏土屈服性状的影响；通过比较天然土屈服前后阶段与重塑土不排水强度与屈服特性的变化规律，揭示天然软黏土的结构屈服破坏机理；结合试验分析，在弹塑性理论及临界状态土力学理论框架上，建立考虑结构性、K0固结应力诱发各向异性影响的天然土屈服面方程及不排水强度理论公式。研究成果具有重要的学术理论意义，可为构建天然结构性软黏土的本构关系提供理论依据。在实际工程中将有助于提高软土地基变形和强度的有效预测，提高基坑、隧道等岩土工程计算与设计的可靠性，增加安全性和经济性。

结构性和各向异性对天然软黏土的工程性质有着显著影响，通过开展天然土原状样与重塑样的K0固结不排水剪切试验，结果表明：原状土样K0固结不排水强度随着p’增加经历K0固结屈服前、逐渐屈服段和屈服后三个阶段。土结构性主要影响在第一阶段，原状样不排水强度随着p’略有增长且高于相应重塑土；第二阶段土的结构性影响逐渐消失，表现在不排水强度p’增长逐渐趋于重塑土，第三阶段，土结构性影响完全消失，原状样不排水强度与重塑土增长规律一致。初始含水率高的重塑土样压缩曲线位于低的重塑土样的上方，随着固结压力的增大，各压缩曲线有逐渐趋于重合的趋势。采用孔隙指数可以将不同初始含水率的重塑土样压缩曲线很好地归一化至重塑土固有压缩曲线上。初始含水率对不排水强度指标的影响主要反映在总内摩擦角的不同，总内摩擦角随初始含水率的增加线性减少。对总粘聚力，有效粘聚力和有效内摩擦角的影响较小。同一初始含水率的重塑土样，在不同应力比下固结至相同p'时的含水率有差异，应力比越大，土样固结完成后的含水率越低，土样的不排水强度越高。通过开展原状样K0固结应力路径试验，获得天然土的屈服面及其演化规律。结果表明，原状土达到初始结构屈服后，结构性逐渐丧失，反映为屈服面的变化由沿着K0线对称的倾斜椭圆逐渐顺时针旋转的过程，在达到完全屈服时，屈服面与重塑土一致，为关于p'轴对称的椭圆。采用变化的应力比替换Nakano模型的屈服轨迹方程中固定的K0线应力比，可以反映土结构性逐渐丧失的过程。椭圆形屈服面的对称轴斜率为K线应力比，在土体达到初始屈服后到结构性完全丧失的过程中，K线应力比从初始的等于K0线应力比逐渐减少至0。基于此，建立了考虑结构性和各向异性影响的屈服面方程。研究成果具有重要的学术理论意义，可为构建天然结构性软黏土的本构关系提供理论依据。在实际工程中将有助于提高软土地基变形和强度的有效预测，提高基坑、隧道等岩土工程计算与设计的可靠性，增加安全性和经济性。