Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

John S. McCartney

Second Advisor

Dobroslav Znidarčić

Third Advisor

Ning Lu

Fourth Advisor

Richard Regueiro

Fifth Advisor

Harihar Rajaram

Abstract

An improved understanding of the thermo-hydro-mechanical behavior of unsaturated soils under elevated temperatures and high suction magnitudes is needed to interpret the behavior of thermally-active geotechnical systems where very dry conditions may be encountered. Examples of relevant thermally-active geotechnical systems include ground-source heat exchangers, energy foundation systems, heat dissipation embankments, containment systems for nuclear waste, or backfills for buried electric cables. Important variables governing the performance of these systems include the change in volume and shear strength of a soil element during changes in suction or temperature. Further, the role of effective stress under high suctions and temperatures needs to be established to understand if it is possible to extend thermo-hydro-mechanical constitutive models developed for saturated soils to unsaturated soils.

A new triaxial cell was developed in this study to understand the effects of heating and cooling on the volume change and shear strength of unsaturated silt under high suction magnitudes. The triaxial cell incorporates the vapor flow technique of Likos and Lu (2003) to control high suction magnitudes, and builds upon concepts developed by Uchaipichat and Khalili (2009) for elevated temperature testing including a set of resistance heaters in the cell, a glass cell, cell fluid circulation, and redundant approaches to measure specimen volume change. Three series of triaxial compression tests were performed on specimens of compacted silt under the same high suction magnitude but following different temperature application paths. The testing series involved ambient temperature conditions, application of a change in temperature before application of suction, and application of a change in suction before application of a change in temperature.

The results from isothermal tests on the compacted silt were used to evaluate the role of effective stress state at high suctions. Further they were used to interpret the role of suction-induced hardening in unsaturated soils. A brittle-stress strain curve was observed in all tests performed on unsaturated silt under high suctions. A sharp decrease in shear stress was observed after reaching the peak shear strength and it was not possible to reach critical state conditions at larger displacements. As expected, the tests on unsaturated silt under high suctions had significantly higher shear strength than the soil under saturation conditions for the same consolidation stresses. In this case, application of suction likely led to an increase in the preconsolidation stress of the soil, and the peak shear strength values were used to estimate the change in preconsolidation stress using established constitutive relationships for unsaturated soils. The stress state in the unsaturated specimens was successfully described using the single-value effective stress principle using the suction-stress characteristic curve predicted from the shape of the soil water retention curve (SWRC). The suction stress characteristic curve (SSCC) was observed to increase significantly with increasing suction, and did not tend toward an asymptotic value for the range of suctions investigated in this study. This confirms that the application of high suctions can have a major impact on the effective stress state and the associated shear strength of the soil. The results indicate the importance of carefully understanding the shape of the SWRC before estimating the SSCC.

The results from nonisothermal tests indicate that the path of testing has a major impact on the volume change and shear strength of compacted silt under high suction magnitudes. The change in temperature led to an increase in shear strength when it was applied after the change in suction, which was contradictory to observations for tests on compacted silt under low suctions reported in other studies. The reason for the contradictory behavior is

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