Date of Award

August 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Sam Helwany

Committee Members

Adeeb Rahman, David Yu, Roshan Dsouza, Konstantin Sobolev

Keywords

Clay-pile interface, Energy piles, Geothermal energy, Saturated clay, Shaft resistance

Abstract

This research aims to qualitatively/quantitively assess the effect of temperature variation on the energy pile shaft resistance. The important mechanisms by which heating can change the shear strength of clay and clay-concrete interface are evaluated using experimental and numerical methods. As for experimental method, temperature-controlled triaxial tests, constant normal load (CNL) direct shear tests and small-scale pile tests were conducted. As for numerical approach, a fully coupled thermo-hydro-mechanical (THM) analysis was performed to simulate the experimental test results so that the capability of such analysis to predict thermo-mechanical behavior of energy piles is evaluated. Reconstituted (HC-77) kaolin clay, one-dimensionally consolidated from slurry, was used in all the studies. Cyclic and monotonic heat ranging between 24° C and 34°C were applied to the clay specimen and interface. The interface was sheared under two stiffness boundary conditions; Constant Normal Load (CNL) and Constant Normal Stiffness (CNS), where the applied normal stresses varied between 100 kPa and 300 kPa. The results of experimental tests indicate that heating improves the shear strength of normally consolidated (NC) clay and NC clay-concrete interface. However, a decrease in strength of over-consolidated (OC) clays was observed, which is thought to be linked to the heat-induced change in the contact stress between clay particles. It was also found out that the increase in the strength of interface under CNL condition, which was about 10%, is exclusively attributed to heat strengthening of clay at the interface. However, the increase in shaft resistance under CNS condition (96% and 49% due to non-cyclic heating and cyclic heating-cooling, respectively) is primarily attributed to the heat-induced increase of effective lateral stress (81% and 35% due to non-cyclic heating and cyclic heating-cooling, respectively). The heat-induced increase in the shear strength of clay (8-10%) also partially contribute to the overall increase of the shaft resistance under CNS condition. It was also observed that there is very good agreement between the results experimentally measured and those numerically predicted. Therefore, fully coupled THM analysis can effectively be used to predict the thermo-mechanical behavior of real energy piles in clays.

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