A compressible diatomaceous clay, Sacramento Valley, California

Tests on a core sample of a silty diatomaceous clay indicate that it is highly susceptible to compaction, and thus could contribute or perhaps has contributed to land subsidence in the area of a test hole in the Sacramento Valley near Zamora, California. A large compression index is the most distinctive physical property of this diatomaceous clay. In fact, the index is larger than that of any other clay analyzed from the test hole and is comparable to the compression index of a sample from another diatomaceous clay, the Corcoran Clay Member of the Tulare Formation that underlies a large part of the San Joaquin Valley to the south. A high liquid limit, high plasticity index, and rather low specific gravity are also distinctive properties of this clay. The diatoms, which comprise in large part the 60-70 percent of the sample that is amorphous, probably contribute to these properties.