Muwafaq Awad

Date of Award

Fall 2019

Document Type

Open Access Dissertation


Civil and Environmental Engineering

First Advisor

Inthuorn Sasanakul


Geo-structures are frequently constructed in or on soils with unsaturated conditions and are subjected to variations of moisture contents due to seasonal changes. Even though unsaturated soil mechanics have been established for several decades, it has not been fully implemented in practice today because of a lack of understanding of the interaction between the soil, water, and air phases. Many experimental research studies have been conducted to improve the understanding of unsaturated behavior but these studies are typically limited to small soil elements tested under fully drained conditions. The goal of this research is to evaluate the effect of hydraulics on the mechanical behavior of unsaturated soils by performing soil element testing and centrifuge modeling with a focus on undrained conditions.

The research begins by evaluating the hysteretic effects on the soil water characteristic curve (SWCC), the relationship between soil moisture content and matric suction, using the automated Tempe cell systems. Results show that hydraulic hysteresis increases as the clay content increases and also increases as the flow rate increases. The second part of the research focuses on the hysteresis effects on the mechanical behavior observed in constant water content (CW) triaxial testing. Results show that matric suction decreases during shearing for all of the drying path tests, while results from the drying- wetting path tests show increases in matric suction. Results of the drying path tests for clayey sand exhibit strain-hardening behavior while those for silty soil exhibit a strain- hardening/softening behavior. The shear strength found in drying-wetting path tests is higher than that for drying tests performed at the same initial matric suction due to the effect of matric suction history. This effect causes the increase and decrease of matric suction during shearing for the drying path and the drying-wetting path, respectively. To account for the hysteretic effect, the shear strength can be estimated based on the matric suction at failure. The third part of the research focuses on the investigation of unsaturated soil behavior using a geotechnical centrifuge model testing. The research involves the development of a miniature sensor to investigate the hydraulic effects on the mechanical behavior of unsaturated soils. A modified heat dissipation sensor (HDS) is tested in a geotechnical centrifuge to evaluate its performance at different centrifuge g- levels. Results show that the HDS can be used to measure in-flight water content, while the SWCC for each soil type can be used to estimate soil matric suction. The fourth part of the research focuses on the characterization of unsaturated soil using the cone penetration test (CPT) and HDS for centrifuge modeling tests to investigate the hydraulic effects on the cone tip resistance, qc. Results show that the qc increases as the degree of saturation decreases for the as-compacted condition or after the evaporation process. The degree of saturation also decreases significantly when the model is subjected to rainfall events. There is no hysteresis effect on qc. Thus, a single relationship between the qc and the degree of saturation can be developed for the clayey sand tested in this study.

Moreover, the centrifuge CPT can be used to evaluate relative change in the shear strength of the soil due to the change in hydraulic conditions. The CPT results from the centrifuge model study are comparable with the predictions based on the cavity expansion theory.