Soil and Rock Anchors

Soil and rock anchors are tension elements that derive their resistance to load through cement grouting or bearing in soil or rock, and are commonly used for excavation support or foundation purposes. They can be used in temporary or permanent applications and are often prestressed to control deflection of the structure that they are designed to anchor. A variety of proprietary and commodity systems can be used to construct soil and rock anchors. Consequently, it can be advantageous to specify soil and rock anchors in a manner that contractor-proposed alternative systems can be entertained or to delegate design to the contractor through a performance specification.

Soil and rock anchors are commonly used to provide lateral support for temporary and, sometimes, permanent earth retention systems, as well as underpinning. When site conditions permit their use, the use of soil and rock anchors can reduce or eliminate the obstructions that lateral bracing can present, simplifying construction staging.

The use of soil and rock anchors to resist uplift in foundations is perhaps an underutilized application. Some engineers will insist that piles or drilled shafts are necessary whenever a foundation is subject to uplift, whether from groundwater pressure, wind or earthquake loads. However, if the soil or rock is competent for bearing, incorporating tiedown anchors into footings or a mat foundation is typically much more economical. If the uplift is subject to reversals under service conditions, such as for wind loads, the tiedowns must be prestressed. As permanent elements, foundation tiedowns should usually be corrosion-protected.

The specification and design of soil and rock anchors is an interdisciplinary pursuit. Geotechnical engineering capabilities are required to adequately understand the subsurface conditions and evaluate the resistance provided by soil or rock. However, structural engineering knowledge is needed to properly specify the tiedown type for the demands of the structure and to properly detail the connections. Richard J. Driscoll, Consulting Engineer (RJDCE) specializes in these sorts of problems and has provided both recommendations for soil and rock anchor systems, as well as detailed design for construction. RJDCE is independent of anchor system manufacturers and can provide technology-neutral recommendations, allowing the details of the anchor system to be selected according to the structural demands, subsurface conditions and installer preferences.

See also: Delegated Design, Design Phase and Construction Phase Service, Foundation / Geostructural Engineering, Support of Excavation