高、低压下砂土剪切带及砂土—结构界面层力学行为演化研究

发布时间：2018-08-20 11:35

【摘要】：论文以岩土材料内部剪切带及岩土体-结构体接触界面层问题为背景，针对应变局部化现象尤为突出的砂土材料，通过室内物理试验、模型理论分析构建了能够将砂土常压下剪胀软化、高压下剪缩硬化特性进行统一的砂土材料模型，并结合无网格方法实现了砂土剪切带、砂土-结构接触界面层产生、发展过程的模拟计算，使得对岩土工程真实的破坏过程进行描述成为可能。通过砂土室内试验，对砂土常压至高压范围内的强度、变形特性进行了系统研究，获得结论有：①因高压条件下砂土出现一定量的颗粒破碎，破碎颗粒充填砂样原有结构，使得砂样孔隙比迅速减小。砂土在0至8MPa范围内等向压缩曲线呈指数衰减型，孔隙比-围压拟合关系可表示为e a b exp c ln p。砂土的压缩特性具有明显的粒径效应，大尺寸砂土颗粒在高压力下较小尺寸颗粒更易破碎，会产生较大的孔隙比变化。②砂土峰值应力比受砂土粒径、围压共同影响，峰值强度公式不再符合经典的M-C强度准则，而残余应力比对应的强度公式则基本不受粒径、围压的影响，符合典型的无粘性摩擦型岩土材料特性。③砂土材料在剪切过程中存在明显的临界状态，不同围压条件下剪切稳定时的孔隙比e与平均应力p间的拟合曲线关系表达式为e l m exp n ln p，这一指数衰减曲线可作为砂土在剪切过程中最终趋于稳定的一个状态线。基于对砂土强度、变形特性的分析，构建了能够将砂土剪胀软化及剪缩硬化特性进行统一的砂土模型。模型采用应力路径相关因子对等向压缩路径获得的屈服面硬化参数进行修正得到与应力路径无关的砂土当前屈服面硬化参数，同时由砂土临界状态参数推导得出能够体现砂土临界特性的潜在屈服面硬化参数。根据当前、潜在屈服面间的关系定义了能够反映砂土当前状态的动态密实参数及潜在强度。模型共有涉及弹性、临界破坏强度、当前屈服面及潜在屈服面的参数共计10个，各参数物理意义明确，均可通过室内三轴试验获得。通过试验得知砂土-结构面接触剪切力学特性受砂土颗粒与结构形貌相对尺度的影响呈三段式，存在着两个明显的特征点，分别定义为“极限相对尺度”与“临界相对尺度”。在“极限相对尺度”Rmax前，砂土-结构面接触力学特性受形貌尺度的影响较为明显，接触剪切作用机制为接触剪切力用于克服砂土颗粒跨越结构表面的粗糙形貌的阻力，论文将这一相对尺度范围内的砂土-结构接触剪切模式定义为“粗糙摩擦接触”机制，相应的结构面类型为“粗糙频”结构面。在“极限相对尺度”Rmax之后，砂土-结构面接触力学特性受形貌尺度的影响基本消失，此时的接触剪切作用机制为砂土颗粒群内部发生翻滚跨越耗能，论文将“极限相对尺度”Rmax之后的砂土-结构面接触剪切模式定义为“形貌约束接触”机制，相应的结构面类型为“形貌频”结构面。论文针对不同结构面类型提出了不同的接触力学特性参数获取方法。针对“粗糙频”结构面，采用砂土-结构面接触剪切试验获得峰值接触剪应力与残余接触剪应力，，得到的峰值接触摩擦角与残余接触摩擦角即对应于计算中的接触静止摩擦角与滑动摩擦角；“形貌频”结构面在接触剪切中对砂土颗粒提供形貌约束边界，其结构表面形貌特征需在计算建模中进行真实描述。论文分析明确砂土剪切带及砂土-结构接触界面层演化模拟计算中的应变软化计算不收敛以及大变形网格畸变两个关键难点。通过分析相关计算理论并比较现有数值计算方法，选用无网格SPH方法作为论文砂土模型的二次开发平台，将模型子程序与之进行对接编译生成新的SPH求解器，克服了计算难点使得对砂土等应变软化材料的大应变问题模拟计算成为可能。利用新生成的求解器开展砂土自身剪切及砂土-结构接触界面剪切试验模拟，分析了不同边界条件下的力学行为以及应力、应变场的发生、发展过程，再现了试验中的应变局部化剪切带及界面层现象与特征，获得结论：①砾砂在常压下内部产生应变局部化剪切带与其应变软化、剪切体胀特性密切相关；高压下试样呈现应变硬化、剪切体缩特性，应变局部化剪切带不再产生。②常压下砂土双轴力学特性受端部边界影响，端部约束则会增大峰值应力，试样内部应变局部化区域也较为集中，产生明显的单对“共轭”对称应变局部化剪切带；端部光滑边界条件下的应变局部化发展相对分散而呈现多对“共轭”对称应变局部化带状区域。③砂土与结构面界面剪切过程中存在着显著的应变局部化区域，剪切力学特性随结构面相对形貌尺度的增大而逐渐趋近于砂土自身剪切力学特性，且随结构面的下移，剪切特性受结构面影响逐渐消失，最终与砂土自身剪切特性相近。
[Abstract]:Based on the problems of internal shear band of geotechnical materials and interface layer between geotechnical body and structure, aiming at the sandy soil material with prominent strain localization phenomenon, a unified sandy soil material model is constructed through laboratory physical experiments, which can soften the sandy soil under normal pressure and perform the shear-dilatancy-hardening characteristics under high pressure. With the meshless method, the sandy soil shear band, the sandy soil-structure interface layer and the development process are simulated, which makes it possible to describe the real failure process of geotechnical engineering.
The strength and deformation characteristics of sand from normal pressure to high pressure are systematically studied by laboratory tests. The conclusions are as follows: (1) Under high pressure, a certain amount of particle breakage occurs in sand and the original structure of sand sample is filled with broken particles, which makes the porosity ratio of sand sample decrease rapidly. The isotropic compression curve of sand in the range of 0 to 8 MPa is exponential. The relationship between porosity ratio and confining pressure can b e expressed as e a B exp C ln P. The compression characteristics of sand have obvious particle size effect. The large size sand particles are easier to break up under high pressure, and the smaller size sand particles will produce larger pore ratio changes. 2 The peak stress ratio of sand is affected by the particle size of sand, confining pressure and peak strength formula. It is no longer in accordance with the classical M-C strength criterion, but the strength formula of the residual stress ratio is not affected by the particle size and confining pressure, which conforms to the characteristics of the typical non-viscous friction geotechnical materials. The relation expression of fitting curve is e l m exp n n n n n n n p, which can be used as a state line of sand which tends to be stable in the process of shearing.
Based on the analysis of the strength and deformation characteristics of sand, a sand model which can unify the dilatancy softening and shear hardening characteristics of sand is constructed. The potential yield surface hardening parameters are derived from the critical state parameters of sand. According to the relationship between the potential yield surfaces, the dynamic compaction parameters and the potential strength are defined to reflect the current state of sand. The total number is 10, and the physical meaning of each parameter is clear, which can be obtained by indoor three axis test.
The results show that the shear behavior of sand-structure interface is affected by the relative scale of sand particles and structure morphology in three-stage form, and there are two distinct characteristic points, which are defined as "limit relative scale" and "critical relative scale", respectively. The mechanism of contact shear is that contact shear force is used to overcome the resistance of sand particles across the roughness of the structure surface. In this paper, the sand-structure contact shear model is defined as "rough friction contact" mechanism, and the corresponding structure surface type is "rough frequency" structure. Surface. After Rmax, the influence of morphology scale on the contact mechanical properties of sandy soil-structural plane disappears, and the mechanism of contact shear action is the energy dissipation of rollover within sandy soil particles. In this paper, the contact shear mode of sandy soil-structural plane after Rmax is defined as morphology constraint. In this paper, different methods for obtaining contact mechanical parameters are proposed for different types of structural planes. For "rough frequency" structural planes, the peak contact shear stress and residual contact shear stress are obtained by sand-structural plane contact shear test, and the peak joint is obtained. The contact friction angle and the residual contact friction angle correspond to the contact static friction angle and the sliding friction angle in calculation, and the "topography frequency" structure plane provides the topography constraint boundary for sand particles in contact shear, and the topographic characteristics of the structure surface need to be described in the calculation model.
In this paper, two key problems in the simulation of sand shear band and the evolution of sand-structure interface layer, i.e. non-convergence of strain softening calculation and large deformation mesh distortion, are analyzed and clarified. A new SPH solver is generated by connecting and compiling the model subroutine with it, which overcomes the computational difficulties and makes it possible to simulate the large strain problem of strain softening materials such as sand. The new solver is used to simulate the self-shearing of sand and the shearing test of sand-structure interface. The forces under different boundary conditions are analyzed. The phenomena and characteristics of strain localization shear band and interfacial layer are reproduced by the occurrence and development of stress and strain field. The results show that: (1) strain localization shear band is closely related to strain softening and shear dilatancy in gravel sand under normal pressure; the specimen shows strain hardening and shear volume shrinkage under high pressure. The strain localization shear bands do not occur at normal pressure. 2. The biaxial mechanical properties of sandy soils are affected by the end boundary, and the peak stress increases with the end constraint. The strain localization region in the specimen is also concentrated, resulting in a single pair of "conjugate" symmetrical strain localization shear bands. (3) There is a significant strain localization region in the interfacial shear process between sandy soil and structural plane, and the shear mechanical properties of sandy soil gradually approach the shear mechanical properties of sandy soil with the increase of the relative morphological scale of structural plane, and shear with the downward movement of structural plane. The influence of the structural plane gradually disappeared, which was similar to the shear property of the sand itself.
【学位授予单位】：中国矿业大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TU43

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