基于波速和剩余强度退化的膨胀土疲劳损伤特性研究

发布时间：2018-10-18 06:52

【摘要】：膨胀土疲劳损伤裂隙是土体强度下降的主要原因之一,其疲劳损伤的实质是一个由于干湿循环作用下基质吸力反复加卸载,导致土体内部结构产生大量的微裂隙形核,并且微裂隙随着荷载循环次数增加而逐渐扩展,最终形成宏观裂缝的过程。而宏观裂隙产生是导致土体强度降低的关键因素和膨胀土边坡滑塌的主要肇因。基于此,本文利用非金属超声波仪(TH204型)对等幅度干湿循环条件下的膨胀土进行声波测试,提出了用纵波速度定义膨胀土疲劳损伤的损伤变量,探索了干湿循环作用下膨胀土的损伤演化规律,研究了干湿循环条件下膨胀土的损伤变量随循环次数、循环幅度、控制含水率及低应力条件下的抗剪强度的变化关系,最终基于波速和剩余粘聚力定义损伤变量,提出能反映循环次数和循环幅度变化的非线性疲劳损伤模型。主要研究结论如下:(1)膨胀土试样的损伤变量随着干湿循环次数的增加呈非线性增长,增长幅度趋于平缓。当控制含水率越大,相应的损伤变量要小。循环幅度越大,损伤变量越大。第一次循环后,初始含水率增大,损伤变量整体呈减小趋势,说明初始基质吸力对试件的损伤变化起重要作用,初始含水率越大,损伤度越小。2~6次循环后,其损伤变化值随初始含水率变化规律性不明显。(2)膨胀土粘聚力值和损伤变量值具有良好的对数衰减关系。同一初始含水率条件下,循环幅度越大,粘聚力衰减随损伤变量衰减程度越大。得到的膨胀土的疲劳损伤强度衰减函数,可为膨胀土疲劳损伤模型的建立提供参考依据。(3)利用室内常规试验和非金属声波测试系统量测结果建立干湿循环作用下膨胀的非线性疲劳损伤模型,简单明了,便于应用,但该模型仅适用于等幅度情况,变幅度模型的建立,有待于提出合理的试验方案。本文基于干湿循环作用下膨胀土剩余粘聚力的试验数据,给出了反映剩余粘聚力变化的二次多项式,给出了等幅干湿循环作用下膨胀土疲劳损伤的定义和表达式。由剩余强度定义损伤变量与疲劳损伤的原始定义相符合。
[Abstract]:The fatigue damage crack of expansive soil is one of the main reasons for the decrease of soil strength. The essence of the fatigue damage is the repeated loading and unloading of the matrix suction under the action of dry and wet cycles, which leads to a large number of micro-fissures in the soil structure. And the microcracks expand gradually with the increase of load cycle times, and finally form the process of macroscopic cracks. The occurrence of macroscopic fissures is the key factor leading to the decrease of soil strength and the main cause of landslide of expansive soil slope. Based on this, the sonic test of expansive soil under the condition of non-metallic ultrasonic instrument (TH204) under the condition of dry and wet cycle with equal amplitude is carried out, and the damage variable of fatigue damage of expansive soil is defined by the longitudinal wave velocity. The law of damage evolution of expansive soil under dry-wet cycle is explored. The relationship between damage variables of expansive soil under dry-wet cycle with the number of cycles, cyclic amplitude, controlled moisture content and shear strength under low stress is studied. Finally, based on wave velocity and residual cohesion, a nonlinear fatigue damage model is proposed, which can reflect the variation of cycle number and cyclic amplitude. The main conclusions are as follows: (1) the damage variable of expansive soil increases nonlinear with the increase of dry-wet cycle times, and the increasing range tends to be gentle. The larger the water content is, the smaller the damage variable is. The larger the cycle amplitude, the greater the damage variable. After the first cycle, the initial moisture content increases and the damage variable decreases, which indicates that the initial matrix suction plays an important role in the damage change of the specimen. The greater the initial moisture content, the smaller the damage degree. The change of damage value with the initial moisture content is not obvious. (2) there is a good logarithmic attenuation relationship between the cohesion value of expansive soil and the damage variable value. Under the same initial moisture content, the greater the cyclic amplitude, the greater the attenuation degree of cohesive force with the damage variable. The fatigue damage strength attenuation function of expansive soil is obtained. It can provide reference for the establishment of fatigue damage model of expansive soil. (3) the nonlinear fatigue damage model of expansive soil under dry and wet cycle is established by using the results of indoor routine test and non-metallic acoustic wave testing system, which is simple and clear, and is easy to be applied. However, the model is only suitable for the condition of equal amplitude, and the establishment of variable amplitude model needs to be put forward a reasonable test scheme. Based on the experimental data of residual cohesive force of expansive soil under dry and wet cycles, a quadratic polynomial reflecting the change of residual cohesive force is given, and the definition and expression of fatigue damage of expansive soil under the action of equal amplitude dry and wet cycle are given. The definition of damage variable by residual strength is consistent with the original definition of fatigue damage.
【学位授予单位】：长沙理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TU443

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