Dissecting a 107-Year-Old Dam with a Barrier Wall

Antero Dam is an earthen embankment about 45 feet high and about 4,200 feet long located on the South Platte River in Park County, Colorado, at an elevation of about 9,000 feet. The dam and reservoir are owned and operated by Denver Water. Embankment construction was completed between 1908 and 1909 with a hydraulic fill core in a wide, shallow valley with variable alluvial soils over highly variable sedimentary and volcanic geology. Concerns related to seepage stability began shortly after construction of the dam and prevented complete filling of the reservoir. Reservoir storage has been restricted to less than 1/4 of the total storage. The area downstream of the dam generally along the entire length of the toe has always been wet and marshy, with areas of standing water. No evidence of movement of fines at the current restricted storage level have been previously documented. Antero Dam has undergone a significant rehabilitation that occurred over several phases and construction seasons. Rehabilitation has included a downstream filter trench installed using the bio-polymer method, a toe and chimney drain, flattening the upstream and downstream slopes, additional upstream slope protection, installation of a soil-bentonite barrier wall through the center of the embankment, hardening of the spillway, and raising the outlet works tower. This paper will focus on the evaluation, design, and construction of the soil-bentonite barrier wall installed through the center of the dam. Extensive evaluations occurred during design to select the installation method and backfill material type appropriate for this 107-year-old embankment. Significant consideration was given to the risk for damage to the embankment during construction related to hydraulic fracturing or trench instability that could lead to additional dam safety concerns. Regular quality control and quality assurance testing occurred during construction to verify the slurry properties, backfill properties, trench stability, and that the total excavation depth met the requirements and intent of the design. Additionally, daily dam safety inspections occurred in the vicinity of the open trench to confirm dam safety incidents were not developing. Construction was successfully completed in fall 2015 without incident, under budget, and on schedule.