Implementation and evaluation of a recently developed rate-dependent effective stress soil model 'MIT-SR'

Abstract

Reliable modeling of rate-dependency in soil properties remains a major challenge for accurate solutions of many geotechnical problems. Although there are many sources of experimental data concerning rate dependent properties of clays, most pre-existing soil models have been found to have severe predictive limitations. Yuan (2016) developed a new elasto-viscoplastic model, MIT-SR, for rate-dependent behavior of clays. The model unifies the existing elasto-plastic framework from prior MIT soil models (3-D surface system and generalized hysteretic formulation) with a physically-based evolution law that attributes the macroscopic viscoplastic strain to an internal strain rate related to the prior strain rate history. MIT-SR has the capability of describing a wide range of observed time-effects within a unified framework and resolves the long-stranding dilemma regarding creep effects at field scale (i.e., Hypothesis A vs B). This thesis implements MIT-SR model to evaluate strain rate effects on two longstanding geotechnical problems: insitu soil characterization from pressuremeter tests and long-term performance of embankments on soft clays. The first part of this research presents a review of the constitutive model framework and discuss the implementation and validation in non-linear finite element analyses using a User Defined Material (UMAT) in the ABAQUSTM program. Part two consists of the investigation of how disturbances (modeled using Strain Path framework; Baligh, 1985) and strain rate effects (modeled by MIT-SR) affect the results of the Full Displacement or cone-pressuremeter (FDPM) and self-boring pressuremeters (SBPM) for Resedimented Boston Blue Clay (RBBC). The results show that that disturbances of the stress field play a vital role in the interpretation undrained shear strength. For the FDPM case, using the same expansion rate, the disturbed NC RBBC can have as low as 40% of the undrained shear strength of the undisturbed NC RBBC. In contrast, for the SBPM case, the disturbed NC RBBC tend to have a slightly higher undrained shear strength (approximately 10%) than the undisturbed NC RBBC. The third part consists of a re-analysis of a well-instrumented test embankment built on a 40m deep layer of Boston Blue Clay. Finite element analyses of embankment performance are conducted using coupled consolidation with the MIT-SR effective stress model. The results were compared with previous numerical results using MIT-E3 presented by Ladd et al. (1994). Overall, MIT-SR contributes a significant improvement in predictions of settlements, but does not significantly improve predictions of lateral spreading. The overall results suggest that the dependent creep properties play only a secondary role in the performance of this particular test embankment.