The mechanical and hydraulic properties of soil above the groundwater table change due to unsaturated conditions. While common, considering soil to be saturated as a simplifying assumption in the design of many important geotechnical applications involving unsaturated soils is often not appropriate. Moisture contents in unsaturated soil profiles can vary due to weather changes or groundwater fluctuations. In this dissertation, the application of unsaturated soil mechanics in three important geotechnical problems involving variable moisture conditions is illustrated: Part 1) development and impact of desiccation cracks on unsaturated soils; Part 2) interface behavior between unsaturated soil and geomembranes and Part 3) lateral load behavior of piles in unsaturated soil. In Part 1 an extensive field and laboratory investigation, and mechanical and hydraulic modeling of a slope were conducted to understand the effect of desiccation cracks on the slope stability. Laboratory testing was conducted to determine soil shear strength, soil water characteristic curves, and moisture flow properties. Two newly developed apparatuses: one for measuring the soil tensile strength during desiccation and one to examine formation of crack depths during desiccation were employed. A simple analytical model was developed for predicting desiccation crack depth and compared with the results of a numerical model using a finite element program and experimental observations. The results of slope stability analyses showed that the increase of permeability in a cracked layer and the loss of soil cohesion during wetting were important triggering mechanisms for shallow slope failures. Research in Part 2 was carried out to investigate the shearing behavior and develop a preliminary constitutive model for unsaturated soil-geomembrane interfaces. Interface shear tests were carried out on soil-geomembrane interfaces involving two types of geomembranes, smooth and textured HDPE. A series of suction-controlled direct shear tests and saturated direct shear tests were conducted on the clayey soil to compare with the interface test results. A constitutive model was used to simulate the mechanical behavior observed in the experimental results. The experimental results showed that the unsaturated shear strength of soil-geomembrane interfaces were lower than the soil shear strength and lowest for the smooth geomembrane-soil interface. In Part 3, the impact of variable soil saturation on the lateral load behavior of integral abutment piles was explored. An unsaturated seepage model was employed to predict variations in soil moisture content using climate forecasts through the end of the century. A technique for calibrating the future weather predictions was developed using the historical weather data. Forecasted weather information was used in the unsaturated seepage modeling to predict future moisture content variations and the associated matric suction profiles surrounding abutment piles. Predicted suction profiles were employed in a numerical model to study the lateral load behavior of abutment piles under varying soil moisture contents.