Rehabilitation of North Lake Dam to Address Uplift and Unfiltered Seepage

North Lake Dam is a high hazard embankment dam built in 1964 in Las Animas County, Colorado. The normal maximum pool elevation is 8586.5, which is about 70 feet above the downstream toe. The original dam did not have filters, an internal drainage system, or a grout curtain. Concentrated seepage through and around the right abutment has been observed since the 1970s. Rehabilitation of the dam was completed in stages, involving an author of this paper since 1998. This paper presents the final stage of rehabilitation, which was completed in 2014 and addressed concentrated seepage, high foundation pore pressure in the bedrock, and slope stability issues. The 2014 rehabilitation modifications included a two-stage filter and seepage collection system, a stability berm, outlet works pipe replacement, and instrumentation. Seepage modeling was calibrated based on piezometer measurements. Slope stability analysis considered artesian pore pressures in the foundation, three-dimensional influences from the irregular downstream geometry, and residual shear strength of the shale bedrock. Potential deflection and elongation of the existing outlet works pipe below the stability berm was evaluated using two-dimensional settlement analysis. Also discussed are seepage control challenges encountered during construction and presents post-rehabilitation instrumentation data. Hydraulic aspects of the rehabilitation are not presented in this paper.