ABSTRACT
Naturally deposited soils, whether clayey or sandy, generally exist in a ‘structured’ and overconsolidated state. To describe deformation behavior of a soil in this state, we have to start from the base of an elasto-plastic model of a de-structured soil in a state of normal consolidation. Given that a soil in this unstructured and normally consolidated state still possesses anisotropy, we take for our ‘base’ in this paper the Modified Cam-clay model (Roscoe & Burland, 1968) with the introduced addition of the rotational hardening concept of Sekiguchi & Ohta (1977), which treats stress parameter η * and its evolution rule as an expression of anisotropy. The degrees of structure and overconsolidation are then introduced and quantified by means of the two concepts of the superloading surface for structure (Asaoka et al., 1998a, 2000, 2002), and the subloading surface for overconsolidation (Hashiguchi, 1978, 1989; Asaoka et al., 1997). That is to say, the degree of structure is expressed by means of a superloading surface situated on the outside of the Cam-clay normal-yield surface and similar to it (the center of similarity being the origin ′ =p q= 0 and the similarity rate being given by R* (0 1* ), while the overconsolidation
1 INTRODUCTION
There are considerable researches on constitutive equation describing mechanical behavior of unsaturated soils beginning with Alonso et al. (1990), Kohgo et al. (1993), Karube et al. (1996), etc. These constitutive equations, in most cases, were developed through observation of behavior under constant air pressure (e.g., constant suction test). However, since air as well as water cannot flow out of soil skeleton during earthquakes and short-time heavy rains, mechanical tests which are not under constant air pressure have recently come to be conducted (e.g. Kodaka et al., 2006; Oka et al., 2010). Here, simulation of the mechanical behavior of unsaturated silt specimens was attempted by referring to the results of triaxial compression tests carried out on unsaturated silt especially under undrained and various air boundary conditions (e.g., unexhausted, constant-volume etc.) by Kodaka et al. (2006) and Oka et al. (2010). For the simulation, a soil-water-air coupled finite deformation analysis code (Noda & Yoshikawa, 2014) was used on which SYS Cam-clay model (Asaoka et al., 2002) was employed as the constitutive equation of the soil skeleton. In the case of partially and/or fully unexhausted conditions, because volumetric constraints are more severe, compressibility of air has a much greater impact on the mechanical behavior. Here, in order to focus on role of coupling air (i.e., air compressibility and air permeability), expansion of the constitutive equation to account for the suction effect, is not carried out. First a very brief summary of the SYS Cam-clay model is given. Next, after describing the calculation conditions such as material
state is expressed by means of a subloading surface situated on the inside of the superloading surface and again similar to it (center of similarity ′ =p q= 0, similarity rate R (0 1); recipro-
cal 1/ R is the overconsolidation ratio). The closer R * is to 0 the higher the degree of structure, but with the loss of structure that accompanies progressive plastic deformation R * will approach 1 (evolution rule for R *). Similarly, the closer R is to 0 the more overconsolidated the state of the soil, but as R increases toward 1 with plastic deformation, the state of the soil will also approach normal consolidation (evolution rule for R). It can thus be assumed that the decay of structure with progressive plastic deformation brings a simultaneous loss from overconsolidation (a transition to the normally consolidated state), resulting finally in conditions that match those in the Cam-clay model. The relative positions of the three loading surfaces, assuming conditions of axial symmetry, are as shown in Figure 1. If we start from the Modified Cam-clay as our base, given that the current effective stress exists on the subloading surface shown below, we need to adapt relations to the subloading surface through the application of various elastoplastic principles such as the associated flow rule and Prager’s consistency condition.
