CONSTITUTIVE MODELS AND HYPERGRAVITY PHYSICAL SIMULATION OF SOILS$^{\bf 1)}$

Numerical simulation and physical simulation are two main methods to analyze the settlement and stability of soil mass. As the mathematical equations of the soil stress-strain relationships, constitutive models are the foundations of numerical simulation. Soil is a type of granular materials, leading to three essential characteristics of it including compressive hardening, shear dilatancy and friction. They are the main characteristics differing soils from metals and should be considered in constitutive model of soils. Traditional soil mechanics, which are widely applied in engineering at present, analyze the deformation and strength of soils separately by elastic theory and limit equilibrium theory based on rigid plasticity, respectively. However, the accuracy of their calculation results is generally difficult to satisfy the requirement of quantitative analysis because the essential characteristics of soils cannot be fully reflected. Cam-clay model is the first elasto-plastic constitutive model that can fully reflect the essential characteristics of soils. It unifies the deformation and strength of soils and can well describe the stress-strain relationships of normal consolidated clays; thus, Cam-clay model is regarded as the beginning of modern soil mechanics. Through introducing a unique unified hardening parameter, unified hardening model further develops the Cam-clay model and enlarges the application scope to over-consolidated clays. The authors believe that the challenge of constitutive model research in the future is how to consider the phase change of soil skeleton and the multi-field coupling in soils, to solve significant geotechnical problems in the field of energy, traffic, environment and hydraulic engineering, which cannot be analyzed quantitatively by current models. Due to the effects of scale compression and time compression, hypergravity physical simulation can overcome the disadvantage that stress level in small-scale model is lower than the prototype level in normal gravity physical simulation. Especially, hypergravity physical simulation is very appropriate to the problems of large scale and long duration. Compared with numerical simulation, hypergravity physical simulation has the advantages of being able to test the rationality of soil constitutive models and reveal the unknown features that cannot be described by current models. Finally, an engineering case of large-diameter steel pipe pile analyzed by combining numerical simulation and hypergravity physical simulation was presented.