IMPACT OF MOISTURE CHANGES IN UNSATURATED SOIL ENGINEERING APPLICATIONS

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

Stability and Evolution of Planar and Concave Slopes under Unsaturated and Rainfall Conditions

Natural slopes are often observed to have a concave, convex, or a combination concave/convex profile, yet constructed slopes are traditionally designed with planar cross-sectional geometry. In this paper, the stability of two planar slopes was compared with that of companion concave slopes, designed to have similar factors of safety (FOS) under gravity loading. The stability of these slopes was then investigated in response to a suction event followed by a precipitation event, and it was shown that both the planar and the concave slopes experienced similar changes in stability. Additional analyses were conducted with a simulated erosion mechanism to investigate how the planar and concave shapes would evolve under a sequence of three similar suction/precipitation/erosion cycles. The results suggest that for these slopes, the second and third simulated weather cycles reduced the stability of the slopes, yet had a lesser effect on the concave slopes than the planar slopes. This is in spite of the fact that the planar slopes became more “concave-like” due to the simulated erosion, and suggests slopes designed to be concave may perform better than the planar slopes

A method to predict desiccation crack depth in a compacted clayey soil

Estimation of crack depths due to desiccation of clayey soils is needed to predict changes in mechanical or hydraulic properties in the cracked layer. Desiccation cracks are associated with increasing suction due to moisture loss accompanied by restrained shrinkage, which results in tensile stresses in near surface soil layers. A simple analytical method is presented to predict crack depths in compacted clayey soil due to changes in matric suction with depth. The model equation is based on the Hookean elastic equation relating incremental strain to incremental stress and incorporates two stress state variables including net normal stress and matric suction. Input to the model includes the tensile strength and elastic parameters, and to complete the prediction of crack depth, the suction change profile of interest is needed. The method validity was investigated by comparing predicted crack depths to those observed in soil compacted in a bench scale apparatus for studying desiccation cracking. Tensile strength and elastic properties were determined from tests conducted on soil during desiccation under approximate uniaxial conditions. Predicted crack depths were obtained based on changes in suction interpreted from water content sensors at various depths in the soil bed and compared favorably to observed desiccation crack depths.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

The Effects of Soil Suction on Shallow Slope Stability / Gerald A. Miller, Amy B. Cerato, Arash Hassanikhah, Maryam Varsei, Roy Doumet, Celine Bourasset, Rifat Bulut

Research work described in this report was undertaken to improve understanding of desiccation cracking and shallow slope failures in clayey slopes subjected to seasonal variations in weather. Research involved field and laboratory testing and computer modeling. Two test sites where shallow slope failures had occurred were instrumented with weather monitoring equipment and sensors to measure variations in soil moisture. The purpose was to examine the variations in soil moisture, and hence shear strength, as a function of time and depth. A primary goal was to evaluate two commercially available computer programs with respect to their ability to predict soil moisture changes and suction. Results of the study indicated that reasonable predictions of soil moisture changes due to weather are possible with commercial software but considerable effort is needed for parameter determination, model calibration and validation. Unsaturated seepage analyses provided insight into pore water pressure development in the slopes considering the impact of desiccation cracking. The results suggest that desiccation cracks may increase the mass hydraulic conductivity of the near surface soils by one to two orders of magnitude. Further, results of seepage analyses suggest that upper layers of the slope soil profile may become nearly saturated in some areas with positive pore water pressure developing over a significant portion of the failure surface. Unsaturated slope stability analyses were conducted using the predicted pore pressure distributions from unsaturated seepage models and unsaturated strength parameters determined from suction-controlled direct shear tests on compacted soil. In addition, traditional slope stability analyses were conducted using drained shear strength parameters and assumed positive pore pressures. Both methods provided reasonable predictions of the failure conditions (factor of safety of 1) for the two sites; however, both have advantages and disadvantages relative to one another. A simple method of predicting the depth of desiccation cracks in compacted soil was developed based on linear elastic theory and shows promise relative to observations at one of the test sites. Tensile strengths used in crack depth predictions were based on measurements in a new apparatus developed and manufactured at the University of Oklahoma. Theoretical predictions of tensile strength based on a micro-structural effective stress model compared favorably to measured strengths.Final report, Oct. 2011-Dec. 2015N