Moisture Increase in Expansive Soils at Developed Sites

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Courtesy of ASTM International

The purpose of this paper is to provide information pertinent to post-development depth-of-moisture increase in expansive soils underlying developed sites. In southern California, the developments are generally accompanied by irrigation around buildings and other improvements. Local experience indicates that the major source of water that contributes to moisture variation in expansive soils is heavy surface irrigation to support lush landscaping. Recent findings indicate that irrigation produces an equivalent annual rainfall of about 178 cm. In contrast, for example, the natural annual rainfall in the San Diego area is about 28 cm. Accordingly, at developed sites, the change in moisture content in soils underlying developed sites does not necesssarily follow a typical seasonal or cyclic pattern. Consequently, the main problem in this region is generally soil swell rather than shrinkage. Due to the introduction of moisture into the subsurface soils on a relatively regular and constant basis, the upper soil profile undergoes an increase in moisture content to a certain depth. Ultimately, at certain times after the completion of the construction, the moisture content stabilizes and becomes nearly constant with depth. The data from six developed sites were evaluated. These sites were developed about 10 years ago. The data indicate that the depth of moisture change was approximately 2 to 3.5 m. The climatic rating (Cw) of the southern California region is about 15, and the Thornthwaite Moisture Index (TMI) generally ranges between +20 to -40, which is similar to many other parts of the country and the world.

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