CRITERIA USED IN PREPARATION OF THIS MAP

For purposes of this report, geologic units as shown on the 1:2,500,000-scale geologic map of the United States (King and Beikman, 1974) have been classified according to the amount and swelling potential of the clay that they contain. In some areas of the northern U.S., parts of these units are buried beneath glacial, aeolian, or fluvial surficial deposits that vary in thickness from a few feet to as much as 500 ft. These deposits are not shown on the geologic map of the U.S. and, consequently, are not included on the swelling soils map.

The amount of data available for different areas is extremely variable. In local areas, such as areas of construction in and near metropolitan centers and at damsites, abundant information on the amount of clay may be available. However, for large sectors of the country, little information is reported other than field observations of the physical characteristics of clay of a particular stratigraphic unit. Therefore, no fixed criteria for determining the swelling potential could be devised. Hence, the method adopted is largely subjective and the classification presented here is, of necessity, based on the authors' appraisal of pertinent data that, for a single clay body or geologic unit, may vary considerably in quantity and quality from one area to another. For example, one set of data may indicate that, due to its swelling characteristics, a clay body at a particular construction site poses serious engineering problems. A mineralogic analysis of the same clay body at a point many miles distant may indicate that the clay is dominantly composed of montmorillonite, and at still another far-removed point the clay may be described as highly plastic. This information would thus be interpreted as indicating that the clay along its entire extent between the three points has a high swelling potential.

The swelling-clay classification is based mostly on published descriptions of the physical and mineralogic properties of clays as reported in numerous engineering and geologic reports; in part, it is from the authors' or their colleagues' personal knowledge. Some data were obtained from communication with practicing engineering geologists and geotechnical engineers.

Information from published sources is of three types:

1. Descriptions of swelling characteristics and of properties related to the swelling of clay, such as plasticity index, shrinkage limits, and swell pressures, which have been determined from laboratory and field tests and field observations. Such information is commonly contained in engineering reports that, for the most part, pertain to construction sites of small areal extent.
2. Mineralogic analyses of samples from clay deposits. Thousands of x-ray analyses of clay samples from most parts of the country and from all parts of the geologic column are contained in reports on mineralogy, areal geology, stratigraphy, and economic geology of clays, and in some reports on engineering geology of construction sites. Analyses are commonly reported in terms of percent composition, or order of abundance of clay-mineral constituents.
3. Descriptions of the general physical appearance and weathering characteristics of clay deposits. Such information is commonly contained in reports on areal geology and stratigraphy, many of which are regional in scope. High plasticity, stickiness, tenacity, and high water retention are properties common to swelling clays; hence, descriptive terms referring to such properties have been interpreted as indicative of swelling characteristics.

It should be noted that pedologic soils, developed by near-surface weathering of geologic units, often contain swelling clays in the clay-rich B horizon. The distribution of such soils is not shown on the accompanying map, but is included in the reports of others (Witczak 1972; Krohn and Slosson, 1980).