A new elasto-viscoplastic model for rate-dependent behavior of clays

[摘要] Viscous properties of clay pertain to numerous geotechnical problems, such as excessive settlements associated with constructed fills on marine clays, or rate-dependent cyclic response of offshore foundations under wave loading. Although there are many observations on rate-dependent characteristics of clays in laboratory tests, none of the existing soil models has attained consistent descriptions of the observed strain-rate effects in shearing, or creep and relaxation in compression. This thesis develops a novel elasto-viscoplastic soil model, MIT-SR, to address these limitations. The proposed I-D formulation introduces a physically-based evolution law that attributes the macroscopic viscoplastic strain rate to an internal strain rate associated with the prior strain rate history. By varying the rate-sensitivity parameter the proposed model provides a unified framework that can describe a wide range of observed time-effects in I -D compression behavior, and resolves a long-standing dilemma regarding the coupling of creep and long-term consolidation (;;Hypothesis A vs B;; controversy). The framework has been further enhanced to allow the predictions of volumetric hysteretic behavior as well as the reduction of creep properties after I -D swelling. The MIT-SR soil model features a 3-D surface system, generalized hysteretic formulation and physical-based viscoplastic flow rules, which are grafted together with the framework of the prior MIT-SI elastoplastic soil model. The MIT-SR model adds 5 parameters to the prior formulation in order to represent rate-dependent soil properties. These parameters can be evaluated from a set of defined test procedures in standard laboratory devices. The proposed model has been calibrated for Resedimented Boston Blue Clay. Comparisons of predictions with experimental data have shown that the proposed model can accurately predict anisotropic stress-strain-strength properties and rate-dependent characteristics for a range of undrained shear tests. MIT-SR is the fourth generation of effective-stress soil models developed at MIT. It greatly expands the predictive capabilities of its predecessors and overcomes the limitations of existing rate-dependent soil models in the literature. The proposed model has many potential applications in geotechnical analyses, particularly in problems that involve considerable strain rate effects (e.g., pile penetrations in clays) or long-term ground movements (e.g., embankments founded on soft clays).

[发布机构] Massachusetts Institute of Technology