Long Term Cumulative Deformation Of Cohesionless Material Induced By Repeated Loading

With the rapid development of traffic engineering, the long term settlement of subsoil induced by repeated loading has received much attention recently. Controlling and reducing settlement have been a common interest to the pavement engineers and the geotechnical engineers. Pavement soil mechanics and traditional soil mechanics have developed as two separate disciplines. As a result, current pavement design methods are mainly based on empirical equations with rough results. Although there are some constitutive models based on fundamental elasto-plasticity theory, most of them can not be used in a large number of repeated loading, e.g. 10~6, which is because that the numerical implementation of such a model through a classical step-by-step procedure proves unrealistic beyond a few load cycles. Therefore, a constitutive model based on shakedown theory that only accounts for the maximum plastic deformation for each load cycle is needed. To settle the puzzle problem, the author firstly summarizes deformation mechanisms in this dissertation, and then formulates an elasto-plastic model within the framework of shakedown theory. Computation results show that the model can consider both computational efficiency and complex deformation features of cohesionless soil. In brief, the research production is of importance both for theoretical analysis and practical engineering.The main achievements of this thesis are as follows:1. In order to investigate the deformation characteristics of sand under static loading, a series of traditional triaxial compression tests of Shanghai silt sand are performed in the laboratory on GDS triaxial apparatus. Test results show that subjected to shear, a sand will exhibit either dilative or contractive behavior under both drained condition and undrained condition. Whether a sand dilates or contracts depends on the initial state of the sand, that characterized by both the density of the sand and the effective mean normal stress applied. The larger the density, the lower the effective mean normal stress, the more dilative response is. And the strain hardening or softening laws has the similar features. What's more, the initial stress anisotropy has significant influence on the strength and dilatancy of sand.2. Secondly, to obtain the cumulative deformation pattern, a series of cyclic triaxial compression tests are performed on the same apparatus. The test results show that the cumulative plastic strain will be larger with the smaller density, the lower effective mean normal stress and the higher dynamic stress level. At the same time, it should be noted that most of cumulative deformation happens mainly at initial stage and approaches to stable deformation gradually. What's more, the mechanical process can be characterized as cyclic densification with volumetric compaction.