Review of methods for predicting heave/shrink of expansive soils over time

Hana H. Adem, Sai K. Vanapalli

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Expansive Soils

ABSTRACT: Expansive soils are prone to swelling and shrinkage as they are sensitive to changes in their natural water content conditions. As soil moisture changes due to environmental conditions, heave/shrink related volume change behavior of expansive soils also changes with time. This information is of particular importance for the design of foundations for structures placed in/on expansive soils. Due to this reason, several researchers over the past decade directed their studies to develop methods for the prediction of the vertical soil movements over time. In this paper, a state-of-the-art of the current prediction methods is succinctly summarized. The methods are critically reviewed with respect to their strengths and limitations. This review highlights the need for simple methods for predicting heave/shrink behaviour of expansive soils associated with environmental changes over time to use them in practice. RÉSUMÉ Les sols gonflants sont sujets au gonflement et retrait car ils sont sensibles aux changements naturels de leur teneur en eau. Comme la teneur en eau du sol change en fonction des conditions environnementales, le comportement de changement de volume de ces sols gonflants relié au soulèvement / rétrécissement change aussi avec le temps. Cette information est d'une importance particulière pour la conception des fondations de structures placées sur des sols gonflants. Pour cette raison, au cours de la dernière décennie plusieurs chercheurs ont conduit des études visant à développer des méthodes pour la prédiction des mouvements verticaux du sol au fil du temps. Dans le présent article, les méthodes de prévision existantes sont présentées succinctement. Les méthodes sont examinées de façon critique en termes de leur points forts et points faibles. Cette revue souligne le besoin de méthodes simples permettant de prédire le soulèvement/rétrécissement des sols gonflants associés aux changements environnementaux au fil du temps pour les mettre en pratique. 1 INTRODUCTION The heave and the shrink related volume change behavior of expansive soils over time information is required for the practitioners for reliable design of foundations. This information is also helpful for the assessment of pre-wetting and controlled wetting mitigation alternatives. Significant advances were made during the last decade towards prediction of the heave and the shrink related volume change behavior of expansive soils as a function of time (e.g., Briaud et al. 2003, Vu and Fredlund 2004 and 2006, Zhang 2004, Wray et al. 2005, Overton et al. 2006, Nelson et al. 2007, Adem and Vanapalli 2013). The current prediction methods can be classified into three categories based on the state variables used in their constitutive laws: (i) consolidation theory-based methods that use the matric suction and the mechanical stress as state variables, (ii) water content-based methods that use water content as a state variable, and (iii) suction-based methods that use the matric suction as a state variable. This paper reviews a state-of-the-art of the current methods for the prediction of volume change movements of expansive soils with respect to time discussing their strengths and limitations. 2 CONSOLIDATION THEORY-BASED METHODS The volume change behaviour of an unsaturated soil involves two key processes; namely, the transient water flow and the soil volume change. These two processes are linked to each other by the coupled consolidation theory proposed by Dakshanamurthy et al. (1984). The rigorous formulation of consolidation (i.e., volume change) process for unsaturated soils requires the continuity equation coupled with the equilibrium equations. In a 3-D consolidation problem, there are five unknowns of deformation and volumetric variables to be solved. These unknowns are the soil displacements in the x-, y-, and z- directions, the water volume change, and air volume change. The displacements are used to compute the total volume change. The five unknowns can be obtained from three equilibrium equations for the soil structure and two continuity equations (the water and air phase continuities). These equations require constitutive relations for the volume change of unsaturated soils as well as flow laws for fluid phases (air and water phases). However, the pore air pressure is generally assumed to be atmospheric and remains unchanged during the consolidation process. In this case, only stress equilibrium condition and water flow continuity need to be considered in the analysis. The volume change constitutive relationship of an unsaturated soil links the deformation state variables to the stress state variables. The deformation state variables for an unsaturated soil element are the changes in total volume (i.e., soil structure) and the changes in water volume. The stress state variables are the net normal stress ma(u)s- and the matric suction aw(uu)-. Fredlund and Morgenstern (1976) formulated the constitutive relationships for volume change of soil

RÉSUMÉ: w of methods for predicting heave/shrink

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Hana H. Adem; Sai K. Vanapalli (2014) Review of methods for predicting heave/shrink of expansive soils over time in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper341,author = Hana H. Adem; Sai K. Vanapalli,title = Review of methods for predicting heave/shrink of expansive soils over time,year = 2014}