Use of Agricultural Soil Survey Maps for Engineering Geologic Mapping

Abstract

Geologic mapping for construction of canals and pipelines as well as for irrigation and drainage developments, etc., is both expensive and time consuming. Published regional geologic maps are usually too general for such purposes. Due to the frequent uncertainty of the final alinements and location of project structures, the areas mapped during reconnaissance and feasibility studies may be disproportionately large. For example, in California the widths of mapped areas for preconstruction purposes for Folsom South and Tehama-Colusa canals are about 60–150 m, while reconnaissance maps of Mid-Valley Canal cover a stretch 5–7 km wide. Experience developed from several projects in the Central Valley, California, indicates that National Cooperative Soil Survey (NCSS) “agricultural” soil maps provide an excellent, readily available base for “office compilation” of engineering geologic maps for the reconnaissance-feasibility stages of an investigation. Such soil maps are made essentially for agricultural use, but reflect the character and composition of underlying formations, topography, and other data valuable to engineering geology. On the west side of California9s San Joaquin Valley, for example, grouping of different soil types was utilized to delineate the following units: (1) river channel and low flood plain alluvium; (2) alluvium of intermediate flood plains; (3) alluvium of upper flood plains; (4) the outer rim of west side piedmont alluvium; (5) young west side piedmont alluvium; and (6) old west side piedmont alluvium, etc. The method was used for preparation of geologic maps of San Luis Canal and the Westlands Service Area, San Luis Drain and the associated Kesterson and South Dos Palos reservoir sites, Mid-Valley Canal, six wildlife conservation ponds, and other projects. This approach is particularly good in alluviated basins but may also provide some important data on depth and character of hard bedrock in mountainous regions.