Extended TTS Model for Thermal and Mechanical Creep of Clay and Sand

Abstract

In geothermal engineering, the long-term thermomechanical responses of sand and clay are difficult to predict due to the lack of an essential understanding of thermally induced volume change. Recent experimental studies have measured thermal creep of dry sand, glass beads, and clays. However, this universal phenomena of thermal creep has not been properly included in constitutive models of soils. In this paper, the Tsinghua ThermoSoil (TTS) model, previously developed for clay, is extended to describe thermal and mechanical creep of sand. The elemental thermal creep and thermal cyclic behavior of Todi clay and Bangkok sand, reported under hydrostatic stress or oedometric conditions, are reproduced by the proposed formulation. These comparisons show the following key features of behavior described by the extended model (e-TTS): (1) thermal creep of clay can produce either compressive or s1welling behavior depending on the consolidation stress history, (2) the heating rate and temperature range have significant influences on thermal creep, but only small effects on swelling behavior, (3) thermal cycling induces compression of sand, which is dependent on effective stress level and relative density, and (4) the e-TTS model predicts stabilization of thermal creep strains for sands over within 50–100 thermal cycles due to equalization of elastic and locked-in hysteretic strains.