Slippery Slope Mitigated by Owner/Engineer Partnership

Ralston Reservoir is located northwest of Denver Colorado in the foothills of the Rocky Mountains. A reservoir side slope failure occurred in 2009 and threatened one of the two inlet channels for the reservoir. The landslide was the third landslide to occur on the rim of this reservoir since filling. Reservoir landslides occur along many reservoir side slopes, causing impacts ranging from degradation of water quality, to structural damage, to wave generation that can impact the dam and spillway operation.
The Ralston Dam was completed in 1937 as a storage reservoir for domestic water supply. The reservoir is on a small stream and is principally filled from a transmission canal and pipeline. The canal discharges into the reservoir by either a submerged box culvert flume in a small, steep sloped valley, or by a baffled chute drop structure. The box culvert discharges into a concrete channel that extends down the small valley to the main body of the reservoir, and is typically submerged.

The massive landslide showed potential to undercut the box inlet flume, and reduce Denver Water’s means to fill the reservoir and indicated a potential to impact water supply to portions of the City of Denver. Monitoring points were established and surveyed to develop an understanding of ongoing movement and preparations were made to perform field geologic and geotechnical investigation. Investigations, including geologic investigation and mapping, three borings/piezometers, and detailed inspection of the box conduit were made and summarized to prepare a model of the slide mass and the extent of movement. Stability analyses were performed and alternative methods of stabilization were developed and analyzed for benefits and costs.

The data and analyses were used to develop a remediation approach that utilized Denver Water’s internal forces and equipment, and took advantage of an onsite quarry for supply of buttress stabilization material. The partnering effort between the Owner and the Engineer developed a remediation plan that was (1) technically sound, (2) constructible by the Owner’s own forces, and (3) cost effective.

This project is an illustration of how an owner and engineering firm can work closely together to evaluate a failure, consider many options and implement a successful repair. 16 pp., 3 references.