A fully coordinated team of geotechnical and structural engineers and engineering geologists are required to insure that rock foundation conditions and design are properly integrated into the overall design of the structure and that the completed final design of the structure is safe, efficient, and economical. Foundation characterization and design work should be guided by appropriate principles of rock mechanics.
Design Considerations – Rock Foundation
The design of rock foundations includes two usual analyses, bearing capacity and settlement analyses and sliding stability analyses. Bearing capacity and settlement analyses involve the ability of the rock foundation to support the imposed loads without bearing capacity failure and without excessive or intolerable deformations or settlements. Sliding stability analyses involve the ability of the rock foundation or slope to resist the imposed loads without shearing or sliding. Both analyses must be coordinated and satisfied in a complete design. Basic data that should be obtained during the design stage include
- Strike, dip, thickness, continuity, and composition of all faults and shears in the foundation
- Depth of overburden
- Ground water condition
- Depth of weathering throughout the foundation
- Joint orientation and continuity
- Lithology
- Physical and engineering properties of the rock mass
- Loading conditions.
Potential failure modes and mechanisms must be determined. For foundation sliding stability, an adequate assessment of the stress conditions and sliding stability of the rock foundation must account for the basic behavior of the structure, the mechanism of transmitting loads to the foundation, the reaction of the foundation to the imposed loads and the effects of the foundation behavior on the structure. In addition to the above, the analyses of rock foundations must include an evaluation of the effects of seepage and of grouting performed to reduce seepage and the seepage effects. These evaluations are particularly important as related to assessment of hydraulic structures. Because of the difficulty in determining bedrock seepage, seepage paths, and the effectiveness of grouting, conservative assumptions should be used in these evaluations.