Research On Elastoplastic Constitutive Model For Saturated Sands Based On State Parameter

Under static or dynamic load, the saturated sand has such properties as pressure sensitivity, dilatancy, strain hardening, and strain softening. Studying the de-formation characteristics of saturated dense to medium dense sand under shear stress to establish an elastoplastic model for saturated sand with the effect of state parameters being taken into consideration has become an important research field and direction in the international geotechnical engineering circle in recent years and has important aca-demic value and practical significance. Based on a large quantity of domestic and over-sea experimental results, the author of this paper studied systematically and deeply the deformation characteristics and dilatancy laws of saturated dense to medium dense sand under static loads in a project for the Beijing Natural Science Funds(Project No.:8112024) and the Technology Development Program of Beijing Municipal Com-mission of Education,"Experimental Study on Elastoplastic Model for Saturated Sand"(Project No.:KM200910009008). The obtained research results are mainly as follows:(1) The dilatancy property of dense to medium dense sand and its influencing fac-tors are described. By analyzing and collating existing experimental results and consi-dering the characteristics of volumetric deformation process of dense to medium dense sand, it was proposed that the critical state point of phase transition, or phase-transition point in short, of such sand should be focused on. It was pointed out that the phase-transition point is a key state of such sand during deformation.Many experiments show that the phase-transition void ratio and the phase-transition confining pressure have a linear relationship on the plane formed by them, that is, the equation of phase transformation line is linear. Therefore, the authors of this paper established a phase-transition-line equation or mathematical expression characterized in a linear change, and set up a new expression of state parameter with the phase-transition line as a reference datum and the phase-transition void ratio as the specific reference variable. The state parameter indicates the distance from the current state point to the phase-transition line with the influence of stress level and material state being compre-hensively considered. The author presented the definitions of and the calculation equa-tions for phase-transition void ratio and phase-transition stress ratio. With the phase-transition void ratio and the critical-state void ratio being considered, the concept of state parameter can be used to accurately judge the dilatancy trend of sand at the ini- tial time point and at any time, so the whole process of volume change of soil body can be reflected more accurately. The derived elastoplastic sand model can truly reflect the change process and deformation characteristic of sand volume, i.e., contraction followed by dilatancy.(2) For sandy soil, the dilatancy ratio cannot be regarded as the unique function of the stress ratio, this is different from cohesive soil, since it is not only related to the three-axis stress variables q and p'but also associated with the internal state of material, i.e., the compactness of soil; The dilatancy model for sandy soil established following the above idea can break through some obstacles encountered in the past before particle breaks so as to reflect the actual status of stress and deformation of soil body. The dila-tancy ratio calculation equation constructed in this paper can describe two kinds of vo-lume change processes, i.e., contraction and dilatancy.(3) The author of this paper established a dilatancy ratio calculation equation for saturated sand with the influence of state parameter being considered so as to realize and achieve the target of considering influence of the compactness on the dilatancy ratio, thereby they further set up a constitutive sand model that can truly reflect the volume change process of saturated dense to medium dense sand.(4) The author of this paper regarded the dilatancy ratio as a state-dependent para-meter in the critical-state soil mechanics framework, and established a macroscopic elastoplastic constitutive model, which shows good simulation capability that has been proven through comparison of experimental data of sand responses under various initial states and loading conditions.