Internal Pore-Water Pressure Measurements and Effective Stress Analysis  in Partially Frozen Soil
Creators: Liang, Yawu
Publisher: name: University of Alberta Library
Year: 2023

<p>The thawing of permafrost is one of the major challenges facing coastal and northern communities in Canada’s North as it is associated with the deterioration of overlying infrastructures. In addition, the industrial development and expansion of Inuit communities demand more climate-resilient infrastructures. However, current engineering design practice utilized in cold permafrost regions is less robust in warm permafrost regions (i.e., &gt; –2 oC). The “near surface” soils in these warm regions, particularly in ice-rich and/or saline areas, are at a partially frozen state with unfrozen water and ice coexisting in the pore space. Further, if there is sufficient unfrozen water to provide a continuous water phase within the pore network, the effective stress that controls the strength and deformation of a soil, should be established via pore water pressure (PWP) measurement.  For the prediction of behavior of existing infrastructures and the design for new soil structures to withstand climate change, the measurement of PWP and conducting an effective stress analysis are desirable to properly understand the mechanical properties of partially frozen soil.  Thus, the main focus of this study is to develop a new method for measuring internal PWP and establishing the effective stress within the partially frozen soil.<br>
Firstly, a new triaxial testing method for internal PWP measurement within soil using the filter-less rigid piezometers (FRPs) is developed. Then, this method is extended to partially frozen soil in order to examine the continuity in the water phase. A series of consolidated undrained and drained triaxial tests were performed on both dense and loose sand with a salinity of 30 ppt, at different temperatures (-3, -5, and -10 oC) and strain rates (1 %/min and 0.1%/min). Continuity in the unfrozen water phase was confirmed by comparing PWP measurements between the FRPs and the base transducer, at -3 and -5 oC. During shear, the decreases in PWP of partially frozen, loose and dense sand are mainly attributed to the shear-induced dilation of the pore ice and the soil skeleton respectively. It was shown that the temperature only affects the effective cohesion, not the effective friction angle of sand. The pore ice stress and its relationship with PWP were also estimated using Ladanyi and Morel’s (1990) postulate on the internal stresses within frozen soil. Based on Ladanyi and Morel’s (1990) concept of internal confinement in frozen sand, a Mohr–Coulomb model that uses effective failure and residual friction angles from unfrozen sand to estimate the strength of partially frozen sand is presented. <br>
Finally, a unique CSL is established in both stress (q-p^&#39;) and void ratio (e-p^&#39;) space for the partially frozen sand at -3 oC. The results also show that the critical state friction angle (φ<em>cs^&#39;) is not affected by temperature while the critical state cohesion (c</em>cs^&#39;) is a function of temperature, strain rate, and failure mode. In e-p^&#39;space, the slope λ_d represents the dilatancy of partially frozen sand, which increases with decreasing temperature and increasing strain rate. </p>