Generally, geologic mapping starts from the preliminary studies phase with gathering of existing maps and data to detailed site-specific construction mapping. Types of maps progress from areal mapping to site mapping to construction (foundation specific) mapping.
An areal map should consist of sufficient area to include the project sites as well as the surrounding area that could influence or could be influenced by the project. The area and the degree of detail mapped can vary widely depending on the type and size of project and on the geologic conditions. Geologic features and information of importance that are to be mapped include:
- Faults, joints, shear zones, stratigraphy.
- Ground-water levels, springs, surface water or other evidence of the ground-water regime.
- Potential cavities due to karstic formations, mines, and tunnels.
- Potential problem rocks subject to dissolving, swelling, shrinking, and/or erosion.
- Potential rock slope instability.
- Gas, water, and sewer pipe lines as well as other utilities.
Site maps should be large-scaled with detailed geologic information of specific sites of interest within the project area to include proposed structure areas. Detailed description of the geologic features of existing rock foundation materials and overburden materials is essential in site mapping and subsequent explorations. The determination and description of the subsurface features must involve the coordinated and cooperative efforts of all geotechnical professionals responsible for the project design and construction.
During construction, it is essential to map the “as built” geologic foundation conditions as accurately as possible. The final mapping is usually accomplished after the foundation has been cleaned up and just prior to the placement of concrete or backfill. Accurate location of foundation details is necessary.
Permanent and easily identified planes of reference should be used. The system of measurement should tie to, or incorporate, any new or existing structure resting on the rock foundation. Foundation mapping should also include a comprehensive photographic record. A foundation map and photographic record will be made for the entire rock foundation and will be incorporated into the foundation report. These maps and photographs have proved to be valuable where there were contractor claims, where future modifications to the project became necessary, or where correction of a malfunction or distress of the operational structure requires detailed knowledge of foundation conditions.