Designing and Constructing Spillway and Stability Improvements for Loch Raven Dam

Loch Raven Dam, located just north of Baltimore, Maryland, is a 100-foot high, 650-foot long concrete gravity dam with an ungated 288-foot wide spillway. The 93-year-old structure impounds Loch Raven Reservoir, a primary water supply source owned and operated by the City of Baltimore. The dam has undergone several modifications during its life; however, the structural height and configuration remained unchanged from 1922 until 2002 when the current modifications were initiated. Inadequacies regarding safety of the dam were first noted in the Phase I inspection report prepared in 1978. During the 1980s and 1990s, additional inspections and investigations were performed to evaluate the extent of the safety deficiencies, which consisted primarily of inadequate spillway capacity and structural stability. The studies ultimately led to preparation of final design documents and construction of a $30 million rehabilitation project. Construction began in 2002 and was completed in mid-2005.

A key requirement faced during design and construction was the need to maintain a full pool throughout the project. This constraint combined with the runoff from a 300 square mile drainage area significantly impacted design and construction of the dam rehabilitation. Two key features of the selected rehabilitation measures were installation of high-capacity post-tensioned anchors and placement of approximately 60,000 cubic yards of roller-compacted concrete. The anchors served primarily to stabilize the structure during excavation, while the RCC was used to raise the non-overflow sections and provide a permanent buttress to improve stability. Other key aspects of the project included sequencing construction to maintain flow on half of the spillway at all times, performing large-scale construction activities including supplying two concrete plants with very limited staging and working areas, and maintaining public relations during the 3-year construction duration with the project embedded in a suburban location. 9 pp.