Lizhou Chen

Numerical Analysis of Vertical Moisture Barriers in Controlling Expansive Soils in Presence of Soil Cracks

Vertical moisture barriers have the potential to stabilize the moisture variations beneath covered areas (e.g., pavements, slab-on-ground foundations, etc.) in response to surface moisture flux boundary conditions, and can affectively be used in controlling the volume changes of expansive soils with wetting and drying cycles. However, vertical cracks formed outside the boundary of the covered area are pathways for moisture intrusion or escape depending on the weather cycle, and can significantly influence the moisture regime beneath the covered area. The paper analyzes the influences of the depths and locations of outside cracks on the distribution of suction and vertical displacement by finite element method. It is found that the performance of vertical moisture barriers can significantly be diminished with the presence of cracks. An outside crack with enough depth can completely counteract the benefits of moisture barrier. Also, how to determine the dimension of horizontal moisture barrier is discussed.

Development and Application of a New Tensile Stress Model for Expansive Soils

Cracks initiate and propagate when tensile stress, caused by an increase in suction, becomes greater than tensile strength in expansive soils. It is important to understand the variation of tensile stress with suction for properly tackling the problems associated with cracks. In this paper, a new practical model is presented for predicting tensile stress from suction, and is verified using finite element analysis. The parametric study shows that drying time and diffusion coefficient control the distribution of tensile stress with depth and the depth to zero tensile stress, while elastic modulus, the difference between final surface suction and initial suction, and suction compression index control the magnitude of tensile stress. The new model can be used to predict the suction where the crack initiates using the soil water characteristic curve (SWCC), to estimate the depth of crack for a specific region, and to estimate the required drying time for a specific crack depth. The new model widens the general application of the SWCC approach in unsaturated soils and can be a useful tool to analyze the interaction between foundation or pavement subgrade and local climatic conditions.

Climatic Parameter TMI in Subgrade Soils

This paper evaluates historical climatic data acquired from Mesonet weather stations across Oklahoma for pavement applications. Thornthwaite Moisture Index (TMI), a climatic parameter, is widely used in geotechnical engineering as well as other disciplines to evaluate the changes in moisture conditions in near surface soils in the unsaturated zone. It has become an important parameter for predicting the equilibrium soil suction beneath the moisture active zone, as well as the depth to constant suction. This paper employs three different TMI calculation methods (Thornthwaite 1948; Thornthwaite and Mather 1955; and Witczak et al. 2006) and produces TMI-based contour maps for Oklahoma using the climatic data from Mesonet stations. The results are analyzed and compared within the three methods. The evaluation of the analysis indicates that caution should be exercised when adopting a TMI-based approach in predicting climate-related unsaturated soil parameters (e.g., depth to constant suction and equilibrium soil suction).