考虑微观结构的海相软黏土三维弹塑性模型

Soil structure is the main characteristics to distinguuish the soft clays from the other geotechnical materials, and it has a significant effect on the mechanical behavior of clay. However, there is some divergence on the deformation and strength characteristics and their generation mechanism about the soft clay recently because of the distribution of regional and complexity of soil structure. Correspondingly, only a few existing constitutive models can be used to the engineering practice for defects of irrational assumption or hard to detemine the parameter values of constitutive models. Based on the above study situation, a series of laborlatory tests and related theoretical study on the undisturbed and reconstituted samples of marine soft clay, taken from Hangzhou, Ningbo of Zhejiang province and Shanghai, will be carried out in this project. Firstly, a lot of laboratory tests including one-dimension compression tests, triaxial consolidation-drained shear, triaxial consolidation-undrained tests, creep tests, permeability tests and microstructure tests, will be carried on the marine soft clay to verified the destructural mode of soil structure, suggested by the proposer, is rational. Then, according to interaction between the microstructure-macroscopic mechanical of clay, the method to determine the fabric parameter-reference void ratio will be studied and the reference void ratio determined from the above method are verified that it can be used to normolize the test results obtained from the laboratory tests. Lastly, an elasto-plastic model for marine soft clay, based on the above test and theoretical study results, and the fabric effect of structure will be also taken into consideration via the parameters of constitive model, will be set up and its three-dimensonal constitive model, extended to three-dimension stress state based the spatial mobilized plane(SMP)strength criterion by the transformed stress method, will be applied to the finite element software to calculate the stability about the projects of foundation settlement embankment preloading and deep pit excavation built in the marine soft clay environment.

结构性是天然沉积软黏土与其他岩土材料区别的主要特性之一，对土体的力学特性具有重要影响。然而，由于软黏土的区域性及结构性的复杂性，使人们对结构性土的变形、强度特性及其机理的认识存在一些分歧；相应地，构建的本构模型由于假定的局限性或参数的不确定性等弱点，只有少量的模型能运用到工程实践中。基于此，本项目拟以杭州、上海等海相沉积软黏土的原状样和重塑样为研究对象，开展大量的压缩、剪切、蠕变、渗透和微观试验，获取更多的试验数据以佐证申请者提出的结构屈服破坏模式的合理性；根据微观结构-宏观力学的相互联系，研究简单方便确定组构参数-参考孔隙比的方法，且对三种软黏土的试验数据进行归一化。根据试验和理论分析，建立一个考虑微观结构效应的软黏土弹塑性本构模型。用变换应力方法实现模型基于SMP破坏准则的三维化后，将模型嵌入有限元程序中，用于海相软黏土环境下的地基沉降、路堤堆载、基坑开挖等实际工程的分析。

为明确结构性对土体力学特性的影响机理，项目用固结仪、三轴仪、压汞仪等设备对多种海相软黏土的原状样和重塑样进行大量的压缩、剪切和压汞试验研究，并得到如下结论：结构屈服应力之前，原状样中各部分孔径体积的比例变化不大，结构屈服应力后的孔径变化主要为大孔径和中孔径逐渐转化为小孔径，但对于微孔径和超微孔径的影响并不明显；不同制样方式的重塑样在相近孔隙比时的孔径分布曲线存在明显差异，且压实样孔径最大，泥浆样孔径次之，重塑样孔径最小，且重塑过程对土体孔径大小及分布产生的影响并不会因固结压力大小而完全消除；相近孔隙比时，土中孔径大于0.2 μm的孔隙体积量和孔径分布均匀性可合理地解释软黏土不同土样的强度关系为原状样的强度最高，重塑样的强度最低；原状样和重塑样的大孔隙体积含量可合理解释同一孔隙比下原状样的渗透系数将大于相应重塑样的渗透系数的试验结果; 用参考孔隙比e* 10归一化土体的变形和强度特性后发现，不同软黏土原状样和重塑样的压缩指数Cc与参考孔隙比e* 10具有基本相同关系；临界状态时的ef/e* 10-log pf 、ef/e* 10-log qf曲线均可基本归一化为一条曲线，认为原状样受到大于结构屈服应力的荷载后，土体结构中的胶结已基本破坏，原状样和相应重塑样的力学特性差异主要是由于组构差异引起的。最后，在深入理解软黏土力学特性的基础上，对超固结重塑土本构模型基础上考虑原状土的胶结强度及Hvorslev线修正后，建立了结构性土的弹塑性模型，弹塑性模拟的结果表明：模型能很好的反映原状土的力学特性。该研究成果已发表10篇论文，授权1项发明专利，3项实用新型专利。同时，利用该成果可较好的预测软黏土的材料参数，减少勘察设计中土工实验的工作量，提高工作效率，建立的有限元模型嵌入ABAQUS有限元软件进行基坑开挖有限元计算，用于指导工程实践。