Study On The Dilatancy And Constitutive Model Of Sand

Stress-dilatancy and stress hardening are basic characters of soils, which are determined by the arrangement of grains and the friction among them, a good model should reflect the above two characters. Till now, the stress-hardening has been studied and applied successfully, but as for the stress-dilatancy, due to its complicated process and the many influencing factors, much research needs to be carried out to give a satisfactory description of stress-dilatancy.In this dissertation, the mechanism of dilation in sands is researched, and on the basis of this research, we presented a microscope model which could describe sand's dilatant and compressive characteristics in a simple manner. With the help of some available experimental data, a phase transformation line is defined, and a state parameter together with a dilation index is introduced with the phase transformation line as a reference. Then, the dilation index was introduced into the Cam-Clay model and a stress-dilatancy model of sand was obtained. The thermomechanical framework and bounding surface theory could represent some behavior of some sand quite well, and combining with the proposed state parameter or the dilation index, those models could be more perfect, especially for describing compression and dilatancy. Finally, the proposed model based on the bounding surface theory was validated.It's a distinct characteristic for the dissertation to introduce the state parameter and the dilation index, which took the transformation state as reference, into the models to describe the behavior of dilatant sand. The main conclusions are as follows:â‘ Combined with other mechanism, the presented micro-model could explain many characteristics of sands, such as bigger initial void ratio will result in a smaller peak value of stress ratio, loose sand compressed only while dense sand compressed first and then dilates, volume becomes small when unloading, and the state of transformation, the state of critical, the strength of sand can also take into account. Comparing with the previous state parameter which took the critical state as reference, the proposed state parameter and dilation index in the dissertation could represent the effect of the initial void on the state during deformation process, and is more suitable to describe the behavior of dilative sand.â‘¡By introducing the proposed dilation index into the plastic potential function of the Cam-Clay model, a model for sand especially for dense sand was presented, which could describe the dense sand compressing first and then dilating. And as for loose sand, the presented model evolved into the Cam-Clay model.â‘¢Combined with the thermo-mechanical framework, by introducing the proposed dilation index into the yield function of dissipative stress space, a model was brought forward; and it's the notable point of the model to apply the non-associated flow rule naturally and describe the true deformation of sand.â‘£By introducing the proposed sate parameter and dilation index into the plastic hardening modulus expression and the stress-dilatancy equation, a new dilatant constitutive model for sand was developed within the general framework of bounding surface, which could describe the characteristic of dense sand during cyclic loading. What's more, for loose sand, the model could evolve into the simple and classical bounding surface model naturally.