论文标题:基坑开挖过程的试验与数值模拟及土的微观结构研究
Experimental and Numerical Simulation of Excavation Process and Microstructure Study
论文作者 刘熙媛
论文导师 闫澍旺,论文学位 博士,论文专业 岩土工程
论文单位 天津大学,点击次数 494,论文页数 121页File Size2108k
2003-12-01论文网 http://www.lw23.com/lunwen_371487/ 基坑开挖;侧向卸荷;三轴试验;微观结构;分形;有限元
excavation;lateral unloading;triaxial test;microstructure;fractal; finite element method
随着高层建筑、地铁、核电站等大型重要工程的建设和发展，深基坑工程已成为岩土工程领域的重要课题。土体在开挖卸荷条件下的微观结构和宏观力学特性是确定深基坑工程模拟分析和设计计算特征参数的依据。长期以来，深基坑工程数值分析与设计计算所采用的土体强度参数均由常规三轴试验方法获得，忽略了深基坑开挖卸荷的特定条件，使得基坑支护结构的土压力及变形的实测值与计算值存在一定的差异。本文结合基坑开挖工程的实际特点，利用光学显微镜和分形理论研究了土体经过卸荷过程后的微观结构。对比常规三轴试验方法，对基坑开挖过程进行了试验和数值模拟。主要研究内容和成果如下：1. 针对深基坑开挖卸荷的实际应力状态，通过对常规应变控制式三轴仪的改造，设计了符合基坑开挖特点的三轴试验方法。选取天津市区同一地点不同深度的饱和土体进行模拟基坑开挖过程的试验研究，并与相同土体的常规三轴试验结果进行了对比分析。研究表明：不同应力路径三轴试验方法得到的抗剪强度指标明显不同，说明不同应力路径对天津市区典型饱和土体抗剪强度指标具有一定影响。2. 土的抗剪强度与土的微观结构密切相关。本文从土的微观结构入手，利用光学显微镜对不同应力路径三轴试验的土样进行孔隙大小及分布的对比研究。对比原状土的微观结构特性，定量地分析了不同应力路径三轴试验土体抗剪强度指标不同的原因。3. 基于分形理论适合于研究复杂不规则图形的特点，本文对分形理论在土体微观结构研究中的作用进行了深入分析。定义了研究土体中孔隙分布状况的平面分形维数。利用MATLAB语言编写程序，对土体微观结构图片进行了平面分维计算，分析了不同应力路径三轴试验后土体微观结构的分形特征。4. 采用本次模拟基坑开挖过程的试验结果，利用Plaxis大型有限元程序进行了典型的深基坑开挖工程实例计算。程序选用莫尔—库仑屈服条件，并模拟基坑的分步开挖过程，得到了采用不同抗剪强度指标的计算结果。通过分析，为今后深基坑支护结构设计中土体强度参数的选择提供了指导性建议。
With construction and development of some large important engineering projects, such as high-rise buildings, subways, nuclear power stations and so on, deep excavation has been a critical question in geotechnical engineering. In unloading condition, the microstructure characteristics and macroscopic mechanical properties for a specified soil are the basis of determining parameters required to simulating analysis and design of deep excavation. For a long time, parameters of shear strength of soil used in design of excavation are obtained by normal triaxial test. Unloading condition in the excavation process is usually ignored. It results in the difference of measured earth pressure and displacement and calculated value on the supporting structure of excavation.Based on the reality characteristic of excavation, microstructure characteristics of soil after lateral unloading are studied by applying the optical microscope and the fractal theory. Compared with normal triaxial test results, triaxial tests and numerical calculation of simulating deep excavation have been studied. The main research content and achievement have been obtained as follows:1. According to the realistic stress path of deep excavation, normal triaxial apparatus of strain control was transformed and triaxial test conforming to deep excavation characteristic was designed. These tests were done with saturated soil that located in the same site and the different depth of Tianjin urban districts. The results of this triaxial tests were compared with that of normal triaxial tests. The study indicated that the parameters of shear strength of soil were different in the two types of triaxial tests, and that the different stress path will affect the parameters of shear strength of saturated soil in Tianjin urban districts.2. The shear strength of soil is closely related with its microstructure. Based on the microstructure of soil, pore size and distribution of soil in the different stress path tests were studied by optical microscope. The reasons of the difference of shear strength parameters were explained quantitatively according to the microstructure of undisturbed soil sample.3. Because the fractal theory is suitable for the study of complex and irregular figure, the functions of studying microstructure of soil were analyzed. The plane fractal dimension used to study the pore distribution was defined. The program was design with MATLAB and the plane fractal dimension of microstructure figures of soil were calculated. The fractal characteristic of soil was analyzed in the different stress path tests.4. According to the triaxial test results, typical practical example of deep excavation was analyzed by finite element program of Plaxis. In the program, Mohr-coulomb yield condition was used and the excavation process was simulated in several separate excavation stages. Finally, the calculation results were obtained with the different parameters of shear strength. Some useful suggestions were provided for selecting the parameters of shear strength of soil in the design of deep excavation.
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