Dilatancy relation for overconsolidated clay

A distinct feature of overconsolidated (OC) clays is that their dilatancy behavior is dependent on the degree of overconsolidation. Typically, a heavily OC clay shows volume expansion, whereas a lightly OC clay exhibits volume contraction when subjected to shear. Proper characterization of the stress-dilatancy behavior proves to be important for constitutive modeling of OC clays. This paper presents a dilatancy relation in conjunction with a bounding surface or subloading surface model to simulate the behavior of OC clays. At the same stress ratio, the proposed relation can reasonably capture the relatively more dilative response for clay with a higher overconsolidation ratio (OCR). It may recover to the dilatancy relation of a modified Cam-clay (MCC) model when the soil becomes normally consolidated (NC). A demonstrative example is shown by integrating the dilatancy relation into a bounding surface model. With only three extra parameters in addition to those in the MCC model, the new model and the proposed dilatancy relation provide good predictions on the behavior of OC clay compared with experimental data

Simulation of wellbore construction in offshore unconsolidated methane hydrate-bearing formation

The unconsolidated nature of offshore methane hydrate-bearing formation poses challenges to sustainable methane gas production as the weak formation is susceptible to disturbance during wellbore construction. This could contribute to loss of well integrity which could manifest as sand production and error in the interpretation of downhole tests such as mini-frac tests. In this study, a simulation methodology of wellbore construction process is proposed. A finite element model adopting this methodology is developed in order to assess the effect of wellbore construction process on the integrity of the unconsolidated methane hydrate-bearing formation in the Nankai Trough, Japan. The main objectives are (i) to develop a modelling methodology of well construction process for numerical simulations, (ii) to assess the zone and magnitude of well construction-induced stress/strain disturbance in the formation and (iii) to evaluate relative impact of each well construction stage on the integrity of the formation. The results from this study show that the zone of horizontal stress disturbance from the geostatic state due to wellbore construction could extend to more than three times the radius of the wellbore. Following the wellbore construction, the deviator stress is concentrated in the hydrate reservoir sublayers with high hydrate saturation while plastic deviatoric strain has accumulated in the sublayers with low hydrate saturation. The results also show that modelling of cement shrinkage process is crucial in predicting the concentration of deviator stress in the high hydrate saturation layers

A viscoplasticity model with an enhanced control of the yield surface distortion

A new model of metal viscoplasticity, which takes combined isotropic, kinematic, and distortional hardening into account, is presented. The basic modeling assumptions are illustrated using a new two-dimensional rheological analogy. This demonstrative rheological model is used as a guideline for the construction of constitutive relations. The nonlinear kinematic hardening is captured using the well-known Armstrong-Frederick approach. The distortion of the yield surface is described with the help of a so-called distortional backstress. A distinctive feature of the model is that any smooth convex saturated form of the yield surface which is symmetric with respect to the loading direction can be captured. In particular, an arbitrary sharpening of the saturated yield locus in the loading direction combined with a flattening on the opposite side can be covered. Moreover, the yield locus evolves smoothly and its convexity is guaranteed at each hardening stage. A strict proof of the thermodynamic consistency is provided. Finally, the predictive capabilities of the material model are verified using the experimental data for a very high work hardening annealed aluminum alloy 1100 Al.Comment: 32 pages, 9 figure

Seismic response analysis by subloading surface model

A lot of disaster by liquefaction have been reported in area along the shore of Japan. In particular, liquefaction has occurred in the wide area in the Great East Japan Earthquake of 2011. Various approaches for the liquefaction analysis have been proposed up to present. Among these approaches, the subloading surface model is formulated in the framework of the plasticity model and thus it is expected to provide a highly pertinent simulation of cyclic loading behaviour of materials. Further, the explicit constitutive equation of soils has been formulated to describe the cyclic loading behaviour with the cyclic mobility [1]. In this study, the validity of the liquefaction analysis by the subloading surface model is examined by comparing the simulation by the subloading surface model with the actual record for the acceleration wave in the ground surface to the input of the actual data of the acceleration wave in the soil ground base. The actual data used in the simulation was recorded in the Kushiro earthquake in 1993

Complete formulation of the subloading surface model

The subloading surface model is endowed the noticeable ability to describe the wide classes of irreversible mechanical behavior [1]. However, the past formulations of the subloading surface model have contained several inexact equations, which have been modified repeatedly after the concept of the subloading surface was proposed in 1977 [2]. The exact formulation is presented first in this article for the hypoelastic-based plasticity, which enjoys the distinguished superiority in the both aspects of the description of material behavior in high accuracy and of the numerical calculation in high efficiency

Complete formulation of the subloading surface model

The subloading surface model is endowed the noticeable ability to describe the wide classes of irreversible mechanical behavior [1]. However, the past formulations of the subloading surface model have contained several inexact equations, which have been modified repeatedly after the concept of the subloading surface was proposed in 1977 [2]. The exact formulation is presented first in this article for the hypoelastic-based plasticity, which enjoys the distinguished superiority in the both aspects of the description of material behavior in high accuracy and of the numerical calculation in high efficiency

EXTENDED TANGENTIAL-SUBLOADING SURFACE MODEL FOR GENERAL LOADING BEHAVIOR OF SOILS : APPLICATION TO NONPROPORTIONAL LOADINGS

The conventional elastoplastic model premising that the interior of yield surface is a purely elastic domain is incapable of describing the plastic deformation by the rate of stress inside the yield surface. Thus, it is inapplicable to the description of cyclic loading behavior. Besides, the traditional elastoplastic model is independent of the stress rate component tangential to the yield surface. Therefore, it predicts an unrealistically high stiffness modulus for nonproportionalloading process deviating significantly from proportional one. The extended tangential-subloading surface model proposed by Hashiguchi and Tsutsumi (2001) would be capable of describing the cyclic loading behavior and the inelastic strain rate induced by the stress rate component tangential to the subloading surface. In this article, the extended tangential-subloading surface model is applied to the prediction of deformation behavior of sands subjected to various loading ranging from proportional to cyclic nonproportionalloading. The validity is verified by comparing with the various test data. Then, it is revealed that the incorporation of the strain rate due to the stress rate component tangential to the subloading surface is of importance for the description of nonproportional loading behavior.論文http://purl.org/coar/resource_type/c_650

Subloading surface plasticity model algorithm for 3D subsidence analyses above gas reservoirs

The coupled hydro-mechanical state in soils coming from consolidation/subsidence processes and undergoing plasticity phenomena is here evaluated by means of the subloading surface model. The most important feature of this theory is the abolition of the distinction between the elastic and plastic domain, as it happens in conventional elastoplastic models. This means that plastic deformations are generated whenever there is a change in stress and a smoother elasto-plastic transition is produced.The plasticity algorithm has been implemented in the PLASCON3D FE code (on the basis of a previous 2D version), coupling hydro-(thermo)-mechanical fields within a saturated porous medium (locally partially saturated at reservoir level due to the possible presence of a gas phase) subjected to external loads and water/gas withdrawals from deep layers (aquifers/reservoirs). The 3D model has been first calibrated and validated against examples taken from literature, and then subsidence analyses at regional scales due to gas extractions have been developed to predict the evolution of settlements and pore pressure in soils for long-term scenarios