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Adobe is a term applied to certain California soils, which, when wet are very plastic and when dry are excessively cracked, the individual clods being hard and tough. Figure 1 shows a characteristic soil of this type in its dry state.


Figure 1. Looking down on an enamel-ware pan of dry adobe soil.

Adobe soils, because of their large volume changes under various moisture conditions, form poor subgrades for permanent highways. The present study was undertaken to secure data on these volume changes or amounts of expansion and contraction—as they are frequently called. The following table gives the county from which the various soils were received.

Table I



No information on adobe soils could be found in engineering publications available in the university library. It was necessary therefore, to proceed slowly and try out ideas and schemes before preparing a comprehensive program of investigation.

Since data on volume changes was the principal information desired, that work was given most attention. It was begun with actual volumetric determinations which were soon abandoned for linear shrinkage tests from which volumes could be computed. The linear shrinkage experiments were very satisfactory because it was possible to show moisture-volume relations for a range of values and also check determinations made by actual volume measurements. These tests were therefore made on all soils sent to the laboratory. The Butte County soil was then used to determine the effects of lime and sand on volume changes. When this work was well under way the moisture content, specific weights and sieve analyses were made for each soil. A few specimens were then prepared so that an idea of the tensile and compressive strengths of adobe could be obtained. The last experiments made were probably the most interesting because one served to substantiate, by actual expansion tests, the expansions computed from linear shrinkage tests, while the other showed that confined adobe soil exerted a very high pressure when it absorbed water.

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