How Big is My Rock Toe? A Geophysical Success Story

摘要

The size and configuration of an existing rock toe was a key piece of information required for evaluating possible dam safety modifications for Rampart Dam, Colorado. Geophysical investigations consisting of six Electric Resistivity Imaging (ERI) survey lines were successfully used to define the extent of the rock toe. Seven drill holes were completed to confirm the results of the ERI survey. The geophysical investigations reduced the total cost of the field investigations and limited the extent of intrusive investigations. Rampart Dam, an approximately 250-ft high zoned embankment dam, is classified as a high hazard structure. The facility is owned, operated, and maintained by Colorado Springs Utilities (Utilities). Rampart Dam provides terminal water supply storage for the Utilities' system and is a critical component of the water supply system. The rock toe is located at the downstream toe of the dam. Seepage has exited the downstream face of the embankment above the rock toe since original construction of the dam. Concerns regarding the observed seepage at this location are: 1) unfiltered seepage exiting the downstream slope above the rock toe, and 2) apparent lack of filters between the embankment shell (Zone 2) material and the rock toe. Seepage gradients within the dam appear to be low, seepage conditions have been consistent for a long period of time, and no evidence of internal erosion has been reported. However, seepage is exiting the dam at unfiltered locations, so the possibility of development of internal erosion cannot be ruled out. Utilities desired to evaluate alternatives to address the seepage concerns. The rock toe was built as part of the original construction completed in 1969, and the size and configuration of the rock toe was not clear, based on information from design and construction and previous subsurface investigations. It became clear that the size and configuration of the rock toe could limit the extent of the downstream section of the dam that could be practically excavated, which in turn affected the feasibility of some modification alternatives. Field investigations to define the extent and configuration of the rock toe became a priority.