As a popular tool for characterizing the rheological properties of materials, fractional calculus can be applied to describe soil creeping. This study attempted to analyze the one-dimensional (1D) compression behavior of clay by using fractional calculus. A fractional Merchant model (FMM) was established based on viscoelastic-elastic correspondence principle. This model was successfully applied to fit the oedometer test results of Nansha clay, which is widely spread in the Pearl River Delta, China. It is demonstrated that the fractional derivative-based model was able to characterize the compression behavior of soft clay with fewer parameters than classical rheological models. The relationship between the order of fractional derivative and the compression process of clay was discussed. The order of fractional derivative is observed to decrease as load increases, which implies that the creep behavior of clay became less pronounced with increasing load levels. These findings may open up new avenues for theoretical and empirical modeling of clay compression in the framework of fractional calculus.

抄録全体を表示

Firstly, a new elastic framework, called the elastic hysteretic stress-strain relationship, is established to evaluate the stiffness degradation at small strains and the hysteresis loop generated under cyclic loading condition. Secondly, in order to reasonably describe the phenomenon of isotropic compression curve of OC clay when approaching the normal consolidation line (NCL) under reloading, the unified hardening parameter used in the UH model is revised. Thirdly, a new overconsolidation parameter, which is crucial to make the UH model working with the elastic hysteretic stress-strain relationship effectively, is proposed. Then, a new small strain constitutive model, called the small strain UH (SSUH) model is presented in this paper. The proposed model is found to be able to describe highly nonlinear stress-strain relationship and stiffness degradation at small strains as well as the shear dilatancy and strain hardening/softening behaviors of OC clays at large strains. Besides, it needs only two additional parameter to consider the small strain behaviors compared with the modified Cam-clay (MCC) model.

抄録全体を表示

This paper presents the numerical solutions for consolidation of the under-consolidation dredger fills. By a field test the delivery rules of negative vacuum pressure in the PVDs are studied, the relationship between permeability and effective stress of the high water content soil is also studied by an experiment, then a plain strain FEM program was coded for the dredger fills consolidation analysis in which the nonlinearity constitutive relation Duncan-Chang’s model is employed into the Biot’s consolidation theory, and the nonlinear permeability law is also employed in the Biot’s consolidation theory. As an under-consolidated soil, the self weight that is not consolidate before the enforcement and negative vacuum pressure is included as the consolidation load in this paper . At last, two project cases were calculated using this program, the result shows that the program is suitable to analyzing the under-consolidated soil consolidation under vacuum preloading or surcharge preloading method.

抄録全体を表示

A sophisticated elasto-plastic constitutive model of soil skeleton was newly proposed. The proposed model is built on the basis of the SYS Cam-clay model in which soil skeleton structure concept is included. A new loading surface was introduced into the SYS Cam-clay model based on associated flow rule, and a Drucker-Prager model based on non-associated flow rule was mounted on the model. A new framework of double hardening elasto-plasticity was developed for integration of two different types of elasto-plastic models. Although there are some existing constitutive models called “double hardening model” other than the model proposed in this paper, these are the ones produced by simply connecting yield surfaces or by separately utilizing loading surfaces for isotropic and shear components. The proposed model is clearly distinguished from the existing double hardening models in a sense that two types of elasto-plastic models which should exist independently can work dependently showing their combined loading condition. In this paper, basic formulation and essentials of the proposed model were firstly shown. In addition, basic responses of typical sand and clay described by the proposed model were demonstrated compared with experimental results. Especially, some hollow cylinder torsional tests for sand specimen controlled by switching drained condition during shear were focused on, and it was demonstrated that the soil behaviors seen in those tests can be described not by Cam-clay type models but by the proposed model.

抄録全体を表示

Many studies in unsaturated soil mechanics have focused attention on suction difference. It is also necessary to take into account the flow and high compressibility of the pore air. During rainfall, earthquakes, or other conditions under which air cannot be sufficiently exhausted, changes occur in the pore air pressure, causing changes in the pore air volume. There is an increasing need for methods of testing and analysis capable of dealing with cases where the pore air pressure varies from atmospheric pressure. Based on the background, numerical simulation of unsaturated silty triaxial tests under undrained and unexhausted conditions and under undrained at various controlled air pressure levels was carried out using a soil-water-air coupled analysis code taking the triaxial test as an initial value/boundary value problem. The processes of back pressure increase, suction variation, consolidation, and shear were simulated beginning from a “single initial condition” as in the case of the experiments. As a result, in the simulations of the undrained and unexhausted shear tests, it was shown that since the mechanical behavior is significantly affected by the compressibility of air, it is possible, by simply taking into consideration air coupling, to describe the differences in the mechanical behavior corresponding to the differences in the degree of saturation. Through the simulations of the undrained shear tests with air pressure control, it was found to be possible to reproduce soil mechanical behaviors that change from hardening to softening corresponding to the level of volumetric constraint. Softening behavior can be portrayed on the basis of the loss of structure described by the SYS Cam-clay model. In addition, behaviors that could not be explained without the effects of factors other than air coupling such as suction were also shown.

抄録全体を表示

This paper presents main data from both oedometer tests and triaxial tests on Hong Kong Marine Clay (HKMC) and a mixture of bentonite-silica. The oedometer tests include (i) multi-staged loading tests with unloading/reloading and enough time for creep and swelling in 1D straining and (ii) step-changed constant rate of strain compression tests with unloading/reloading in 1D straining as well. The triaxial tests include K_o-consolidated undrained compression or extension tests with constant (or stepped changed) strain rates. This paper will discuss the time effects and rate effects with a special attention to the non-linear and elastic visco-plastic behavior. Based on the test data presented above, this paper presents a brief review of the works of elastic visco-plastic (EVP) modelling of the time-dependent stress-strain behaviour of soils in one-dimensional straining (1D) and in 3D stress state. A few important concepts and their physical meanings are explained. The 1D EVP model is briefly reviewed with a comparison with the classic Maxwell’s rheological model. It is found that Yin and Graham’s 1D EVP model is an extension of Maxwell’s rheological model for considering the nonlinear behaviour of soils. The recent extension of the EVP modelling framework to consider the swelling of a saturated soil is introduced. A 3D EVP model is also introduced and discussed. A nonlinear creep function proposed by the author is presented. This function has been used in refined 1D and 3D EVP models. It is concluded that the time and rate effects of clayey soils shall be considered in a suitable constitutive model.

抄録全体を表示

Seismic energy is scattered in the form of seismic waves from the underground source to the surficial structures. These waves are known as body waves and will generate volumetric deformations by compressional waves or distortional deformations generated by rotational or shear wave. However, the body waves will generate other deformations at the interface of layers or the ground surface due to specific boundary conditions. Boundary conditions corresponding to the free surface allows additional answers to the equations of motion obtained. Since the earthquake engineering examines effects of seismic waves on the structures which are in or near ground surface and because surface waves are attenuated slower than body waves with distance, the surface waves are more important. Problem of wave propagation in saturated porous media was discussed first by Biot in the mid-twentieth century, after the formulation of the theory of saturated porous medium. Extension of the theory of elastic wave propagation in unsaturated soils which are made of a solid porous skeleton that the empty space is filled with water and air, founded the basis for solving the most complex problems of waves propagation in unsaturated soils. It was found that three compression waves (P1, P2 and P3) and a shear wave propagate in unsaturated porous media. Furthermore, Rayleigh wave are identified as the superposition of P and S-waves at the free surface. This results in propagation of three Rayleigh waves (R1, R2 and R3) at the free surface. The purpose of this research is to obtain the Rayleigh wave characteristics and the governing equations, according to the compression and shear waves. The theoretical results are demonstrated through examples at the end.

抄録全体を表示