Special Issue on Construction on Expansive Soils

Abstract

Where it prevails, expansive clay soil is a monster that can silently wreak havoc on constructed facilities. This dilemma is the catalyst for this special issue on expansive soils. A few years ago, after I brought up the topic of expansive soils at a meeting of the Editorial Board of the Journal of Performance of Constructed Facilities, Professor Kenneth L. Carper, the editor in chief, suggested that I be guest editor for a special issue on the subject. The cumulative deleterious effects that expansive clays have on constructed facilities in the United States annually generally exceed those of all hurricanes, tornados, floods, and earthquakes combined. A recent study found that the annual cost of damage to constructed facilities owing to expansive clays in the United States was approximately $13 billion Puppala and Cerato 2009 . One may wonder how much of that $13 billion is related to residential construction. Expansive clays consist of colloid-sized particles primarily of the montmorillonite mineral variety that have a very large specific surface area per unit volume. For example, the total surface area of 10.0 g 0.35 oz of montmorillonite, about a tablespoon, is on the order of 8,400 m2 90,000 sq ft . The surface and edges of a particle of the clay have a net negative charge. These particular properties and mineralogy enable the individual particles to attract and absorb very large amounts of water onto their surfaces. Depending on the initial moisture content and average colloid size, as the expansive soil absorbs water, it swells, inordinately exerting pressures varying from about 100 kPa 2,000 psf to 1,400 kPa 30,000 psf Chen 2000 . Differential swelling or shrinkage of the clay occurring under a constructed facility that is not properly designed to counteract those deformations often results in excessive foundation movement and cracking of slabs and walls. Depending on geographic location, expansive clays are known by various descriptors and names: e.g., “fat clay,” “clay of high plasticity,” “clay of high shrink-swell potential,” “CH,” and, in Mississippi in particular, “Yazoo clay,” “gumbo,” and “buckshot.” The main problem facing constructed facilities that are found on or above expansive soils, therefore, is moisture content change. During my 34 years of experience inspecting damaged foundations on expansive clay deposits in Mississippi, the cause of damage was often a leak of a buried potable water and/or a sewer line. If the moisture content of the expansive soil can be controlled and kept constant, no problems are likely to develop. Building foundations are designed to support live and dead loads that are generally typical for constructed facilities. When the subgrade on which the foundation bears is a soil of high shrink-swell potential, however, additional loads owing to potential deleterious soil volume change must be included in the structural analysis or handled in some way as to lessen their effects on the structure. Alternatives include drilled pier-grade-beam con-