Hillside Materials and Slopes of the San Francisco Bay Region, California

Hillsides in the San Francisco Bay region differ in slope of the ground surface and in the kinds of materials beneath the ground surface. These differences affect engineering use of the land in several ways, including the volume and ease of grading needed to provide flat space for roads and foundations, the stability of that grading, the success of water wells and septic-tank systems, and the susceptibility to geologic hazards such as landsliding and earthquake shaking. This report systematically describes the materials and slopes of hillsides in nine counties (Alameda, Contra Costa, Maria, Napa, San Francisco, San Mateo, Santa Clara, Solano, and Sonoma) that constitute the bay region, so that land-use advantages or problems can be anticipated.

Hillside materials in the region range from soft, young stratified rock to older and harder stratified rock, volcanic rock, hard homogeneous rock, and chaotically mixed assemblages of hard and soft rock. The distribution of these materials is shown on 1:125,000-scale maps based on geologic mapping of the region. Approximately 360 units are described by bedrock composition, physical properties of bedrock (rock hardness, bed thickness, and spacing of fractures and partings), texture of surficial mantle (soil cover), expansivity, and permeability. Descriptions are based on thousands of systematic field observations, about one thousand free-swell tests for expansivity, examination of aerial photographs, and published reports. The units are described in detail and also in summary form, and they are organized and colored on the map according to dominant composition and physical properties, so that the general distribution of material properties can be readily discerned.

Slope of hillsides in the region is portrayed on 1:125,000-scale maps by the distribution of three slope intervals that reflect the level of constraint on development. These slope intervals are superimposed on materials units, so that the various combinations of slope and materials are displayed throughout the region.

Because of limitations of map scale and the heterogeneity of earth materials, the information is not sufficiently specific for design of foundations or grading. Its proper use is in more general considerations, such as general planning, preliminary evaluation of land, and review of proposals or reports. Using inference schemes outlined elsewhere, the data can be used for regional estimates of cut-slope stability, excavatabihty, and character of materials as fill. In addition, the data may prove useful for predicting the extent and severity of hazards such as landsliding and earthquake shaking. For such purposes, this report offers a consistent description of the physical character of hillside terrain pertinent to engineering use of land.