Evolution of Preconsolidation Pressure of Normally Consolidated Clays Over Full Temperature Range

While it has been established that temperature can change the preconsolidation pressure of clays, the current understanding is limited to specific ranges of temperatures, with temperatures above freezing being studied entirely independently of temperatures below freezing. However, as temperature is a continuous domain and clays may be subjected to both above- and below- freezing temperatures over the course of an engineering application, a unified view is necessary. The first goal of this thesis is to develop a single model which can be used to predict the preconsolidation pressure of a normally consolidated clay at any temperature over a wide range which includes both frozen and elevated temperatures. To do so, consolidation tests were run at various temperatures between -7 °C and 50 °C, and the yield stress at each consolidation temperature was determined. As previous studies have established that the temperature response of clays is dependent upon their mechanical stress history, the specimens were consolidated initially at a reference temperature until they reached the normally consolidated state. Subsequently, the temperature of the specimens was changed and the volume changes during the temperature change stage were recorded. Once the specimens stabilized at the new temperature, they were consolidated once again and the preconsolidation pressure determined at the new consolidation temperature. The volumetric strains and changes in preconsolidation pressure for each temperature used in this study align generally with the previous data published for each temperature domain. Heating led to a decrease in the volume of the specimens, cooling to minimal strain, and freezing to an increase in the specimen volume. Changing the consolidation temperature by either heating, cooling, or freezing the specimen led to various degrees of increase in the preconsolidation pressure. A mathematical model was developed to fit the observed preconsolidation pressures at each consolidation temperature. This model can be used to predict the yield stress of NC kaolinite at any temperature within the tested range, and captures the smaller magnitude increases in yield stress which occur upon heating and cooling as well as the large increases which occur upon freezing the clay. With the effects of unidirectional thermal paths having been treated in the previous portion, a second investigation was also undertaken to assess how much of the temperature history of the soil might influence the behavior at its final consolidation temperature. In particular, the impacts of previous freezing on the preconsolidation pressure at elevated temperatures were investigated. The same clay material was first consolidated to the NC state and then frozen to -15 °C. Subsequently, the material was thawed or heated to various final temperatures and consolidated further to determine the preconsolidation pressure. The results of these tests indicate that the preconsolidation pressure was independent of the consolidation temperature for previously-frozen soil. While increasing contractive axial strains were recorded with increasing temperature, there was no accompanying increase in the preconsolidation pressure. These results indicate the thermal history of the clay can alter its behavior at the current temperature, overriding the effects of the most recent thermal path.