ENGINEERING GEOLOGIC MAPPING OF DAM FOUNDATIONS

Construction of dams on rock foundations includes significant involvement by engineering geologists. Responsibilities typically include review of excavated rock surfaces at the foundation grade of the dam, approval of foundation treatment prior to placement of dam material, and documentation of the rock at foundation grade. Documentation typically consists of an engineering geologic map, photographs, and a project report describing the rock conditions and procedures used.

Examples of geologic maps from several dam sites are presented to illustrate the key components of engineering geologic maps and to show various methods of documenting foundation conditions. The maps highlight the combinations of characteristics that can be grouped together to create a mappable unit. On large engineering projects such as dams, the features that are most important to document are those that could affect the engineering behavior of the structure such as strength, degree of fracturing, and location and characteristics of discontinuities. A mappable unit may represent one or more of these characteristics, with the combination of characteristics in a mappable unit being based on the unique geologic characteristics at a site.

Engineering geologic mapping procedures and conditions differ somewhat from traditional geologic mapping, the most important difference being that the engineering geologist benefits from the removal of overburden soils and weathered rock by heavy earth moving equipment and high pressure air/water washing of the rock surface. This construction process exposes a cleaned rock surface over the entire map area, often exposing discontinuities, shear zones, and faults that would not be recognized in a natural setting. The process of mapping in a construction zone presents unique challenges and requires the ability to quickly document key geologic features before the surface is covered in concrete.