Prospect Lake is located in Memorial Park in south-central Colorado Springs, Colorado. The high profile, 52-acre recreational lake has continually experienced seepage losses throughout its 100-year history. The treated water used to fill the lake quickly drained away when Colorado Springs Parks and Recreation (CSPR) stopped filling it to conserve water in response to several years of drought.

CSPR retained K+A to explore alternative liners for reducing seepage losses. Analyses indicated that a geomembrane liner system was preferable to a compacted clay liner. K+A recommended lining the lake with a 30-mil PVC liner overlain by an 8-ounce geotextile separator fabric and 1 to 2 feet of protective soil cover.

K+A was the principal designer responsible for the liner system and teamed with the civil engineering firm Associated Design Professionals (ADP) of Colorado Springs to complete the project. The lake is impounded by two jurisdictional dams, so proposed modifications to the dams and reservoir required the approval of the State Engineer’s Office (SEO). K+A and ADP worked closely with the SEO to reduce a review process that can frequently take up to several months and was able to prepare final designs, obtain SEO approval, and get the project out to bid in just over two months.

The project was constructed from early April to mid-July 2005. Besides spring weather concerns, the greatest challenge was stabilizing the 2- to 4-foot thick highly plastic clay in the middle of the lake for the support of the liner and the equipment used to install it. Step one was to quickly drain the lake to allow the clay time to dry out before the summer monsoon rains. Although large inflows in the flowing sands underlying the clay were initially encountered, the lake was successfully dewatered using trenches and a central sump.

Approximately 6 ½ acres of the soft subgrade was successfully stabilized using biaxial geogrid placed directly on the clayey subgrade, and overlain by 12 to 18 inches of on-site material. Approximately 224 tons of liner material, 8 miles of field seams, and 72 tons of geotextile separator fabric were placed. The project was completed slightly over the contractor’s bid price, but below the engineer’s anticipated cost given the significant unknowns regarding weather and subgrade stabilization. The project was completed on-schedule to allow for lake filling by early September and the annual Balloon Day Festival at Prospect Lake.