BUILDING ABOVE SUBSURFACE CAVITIES – A CASE HISTORY FROM BEAVER CREEK, COLORADO

Alpine engineering presents a unique set of problems with highly variable soil and bedrock conditions. Several subsurface cavities were found beneath the site to be developed for 4 three-story condominium buildings at the Beaver Creek Resort. These conditions required a special investigation program and special foundation system.

The site to be developed was underlain by Eagle Valley Evaporite bedrock, which can contain cavities due to the soluble nature of the bedrock. During the geotechnical investigation several subsurface cavities measuring up to 13 feet (4 m) in height were found at or near the surface of the bedrock. The resulting recommendation to the client was to support the proposed structures on a special caisson foundation system. The caissons would be supported by the competent bedrock beneath the cavities and the portion of the caisson through the cavity would be permanently cased.

After the foundation system was designed, an additional geotechnical investigation was conducted by investigating the subsurface conditions at each of the 103 caisson locations. The location and depth of cavities for each of the caissons were then documented. Cavities varying in height from 2 feet (1 m) to 13 feet (4 m) were encountered at depths varying from 25 feet (8 m) to 85 feet (26 m) in 15 of the 103 caisson locations. As a result of the additional investigation the caisson contractor was able to provide a more accurate bid and complete the caisson construction on schedule. The drilling and installation of each caisson was observed and recorded. The 4 buildings have shown no movement since construction in 1987.