Application of rate dependent soil model, MIT-SR, for evaluation of long-term ground movements due to consolidation and creep

Abstract

Saturated clays exhibit rate-dependent behavior (i.e. their stress-strain-strength properties are significantly affected by the applied loading rate). This is of great significance for geotechnical projects, particularly when long-term predictions of settlements of compressible soil strata are based on results of laboratory element tests on small test specimens. Ladd et al. (1977) show that scaling of laboratory consolidation tests to field scale is strongly affected by assumed creep properties of the clay, leading to competing hypotheses of field performance (Hypotheses A and B). Most pre-existing soil models are based on isochrone theory which assumes a unique relationship between effective stress-strain and strain rate - such that the scaling of creep and consolidation conforms to Hypothesis B. Recently Yuan (2016) has developed a novel elasto-viscoplastic effective stress model, MIT-SR, that is capable of accurately predicting a wide range of rate-dependent characteristics of clay. The proposed formulation is able to represent both scaling hypotheses of coupled creep and consolidation. This thesis analyzed the predictive capabilities of MIT-SR model by studying two projects: New Hamilton Partnership Levee Project (NHPL) and Marina Bay land reclamation projects, Singapore. MIT-SR materials were calibrated with laboratory and field tests only and numerical simulations of the projects were performed with the Finite Element Method (FEM). The NHPL project consisted on an 11 ft high embankment, constructed in 1996 in California, on +30ft of highly compressive San Francisco Bay Mud (SFBM). Our re-analyses included the MIT-SR calibration for SFBM and 2D FEM simulations that compare MIT-SR, Soft Soil Model and Soft Soil Creep Model predictions. The results show that MIT-SR offers superior predictions of the lateral spreading and settlement during both the construction and post-construction phases. The Marina Bay area in Singapore was reclaimed by surcharged underlying marine clays with more than 13m of granular fill between 1979 and 1985. Construction of the new Thomson Line subway through the area has measured on-going free-field settlements from the reclamation during the period 2015-208. We analyze these long-term settlements by calibrating MIT-SR parameters for Singapore marine clay and performing a series of 1 D consolidation analyses to represent variations in the soil profile across the area. The model predicts settlement rates that vary with the thickness of marine clay, principally ranging from 10 to 20mm/yr at present time. The model tends to underestimate the measured settlement rates. This may reflect underestimation of creep properties of the clays or thickness of the fill. The results from MIT-SR simulations in both projects were very encouraging and confirmed the strong predictive capabilities of the model with calibration procedures based on laboratory and site tests only. MIT-SR was able to accurately predict deformations and pore pressures in undrained, consolidation and creep stages with a unique set of parameters calibrated.