DETAILED EVALUATION ALLOWS CONTINUED OPERATION OF DAM WITH HIGH SETTLEMENT RATES

Highline Dam is a zoned earthfill dam located in Western Colorado, with a height of 80 feet, length of 1,350 feet. and capacity of 3,400 acre feet. The Dam was constructed in 1967. During the initial filling, seepage rates in the left abutment area were serious enough to require lowering of the reservoir and implementation of remedial measures. The dam continued to exhibit high seepage rates and movements. During the period from 1981 to 1986 instrumentation was added to the project. In 1986, tension cracks appeared on the downstream right abutment ground surface which exhibited vertical and horizontal displacements of several inches. An investigation was commissioned to address several serious issues in addition to the cracking. These issues included continued high settlement rates, relatively high seepage rates, erratic seepage characteristics, apparent bulging of the downstream slope and spillway distress due to foundation movements. The investigation included identifying embankment and foundation material characteristics, conducting field permeability tests, installation of additional piezometers, installation of additional inclinometers, measuring seepage at additional locations, seepage analyses, stability analyses and estimating settlement due to soil consolidation. At that time the settlement was attributed predominantly to consolidation of low density soils and the cracking to creep in the downstream direction of shallow soils. Monitoring since 1986 has provided a more complete picture of the cause and extent of movements. The settlement has continued with characteristics which cannot be attributed to soil consolidation. The dam foundation consists of a marine shale formation containing a high percentage of soluble material. More recent analysis of the monitoring data has indicated that settlement can be mainly attributed to solution of these materials with the estimated solution rate under the dam matching the annual settlement quite closely. Movement monitoring has identified locations of historic maximum solution, changes in solution patterns and locations of potential maximum future solution paths. The evaluations have also indicated that solution is accelerated in the vicinity of subsurface drains and is generally progressing in the downstream direction.