Exfoliation: not the first thing that would come to mind when planning a summer vacation. Residents and vacation goers in Twain Harte, California became quite familiar with this geologic phenomenon when the dam at Twain Harte Lake was damaged in August of 2014 due to exfoliation of the granite dome that serves as the south abutment. Condor assisted the Twain Harte Lake Association in identifying the cause of the damage, designing repairs to the dam and south abutment, getting approval from the California Division of Safety of Dams (DSOD) for the repair, and managing the repair of the dam and abutment.
The investigation phase of the work included subsurface exploration of fracture geometry, acoustic and optical observations of rock movement, and borehole extensometer monitoring near the south abutment. Drilling and geologic mapping required the use of high angle fall protection due to the steep inclination of the rock surface.
The design phase of the project had several components. The first priority was to prevent further damage to the dam, preventing a catastrophic release of water. DSOD required prompt modifications to the dam to assure that no water would be impounded. Condor designed a temporary discharge structure that prevented impoundment of water during the subsequent wet season. The discharge structure remains as a permanent feature of the dam and will be used as a secondary system for future draining of Twain Harte Lake, if needed. Subsequent design efforts were focused on structural repair of the dam and securing the dam to the south abutment. The existing exfoliation fractures, and potential for more, created unique design challenges. Condor’s design included extending the dam further into the granite dome, below existing exfoliation fractures, adding uplift/sliding resistance, and providing a “zipper” to act as a preferential failure plane if there are future near-surface exfoliation events.
Condor’s design was approved in record time by the DSOD, allowing construction to commence in April 2015. Condor provided construction management and resident engineer services throughout construction. Construction work was completed under budget and ahead of schedule, allowing the Lake Association to begin filling the lake and open to the public in June 2015.
sheet fracturing of bedrock as a result of stress relief. The bedrock, formed millions of years ago under the immense weight of miles of overburden, begins to “unload” as the overburden is carried away by glaciers, flowing water, wind and other natural phenomena. Scientists speculate that it could take tens of thousands to millions of years for bedrock stresses to be released and exfoliation to occur. Many believe that such an event has never before been witnessed by geological and engineering professionals. Not only was this an exceptionally interesting project to work on, Condor’s team witnessed an exfoliation event first hand and captured it on video. This was not your typical project. See the video player below to watch a FOX 40 News production of Weird Science: Rock Exfoliation at Twain Harte Lake.