Evaluation of Cougar Dam Embankment Performance

Cougar Dam is a 445-ft-high rockfill embankment completed in 1964, having a volume of 12,700,000 cu yd. It has a narrow impervious core upstream of centerline, gravel transition zones, and outer zones of basalt rockfill which were spread in 18-in. to 36-in. layers and compacted by tractors and vibratory rollers. Construction pore-water pressures developed in the predominately ML core material placed at about 1% over optimum ranged from 20% to 50% of the fill pressure. Maximum settlement of the cross arms during construction of the rockfill occured at mid-height and amounted to 4.7 ft. The total compression of the rockfill at the end of construction was 8.5 ft. The downstream shoulder of the crest of the dam at maximum height has settled 1.4 ft. and deflected downstream 1.75 ft in 3 yr since completion. Most of this deformation occured during the first filling to full pool and 158 ft. of drawdown. Differential settlement between zones caused longitudinal cracks in the crest rock surfacing and one shallow transverse crack occured near the left abutment. Provided keywords: compression, deflection, deformation, pore-water pressure, settlements, soil mechanics.