Silver Lake Low Level Outlet and Tower Remediation

After an underwater inspection and analysis of the outlet facilities at Silver Lake was completed in June 2008, it was concluded that the structural steel outlet tower was failing.

The tower, which supports the gate stem and provides a platform from which an operator can operate the gate, underwent a structural evaluation and was found to be susceptible to failure due to corrosion in the bracing as well as other critical steel support members. Accordingly, the supporting structure was deemed unsafe and unreliable for continued gate operation. The recommended remediation measures consisted of replacing the steel tower, gate, gate frame, gate stem, trash rack, and mechanical operator in-kind with new materials, and complete miscellaneous concrete repairs, if necessary, in order to restore safe and reliable operation to the outlet control system.

Following an emergency declaration, Syblon Reid was awarded an emergency design/build contract for modifications and repairs which included developing plans to lower and store reservoir water, assisting in obtaining permits with seven government resource agencies, and replacing the existing low level outlet gate and severely deteriorated operating tower and platform.

The major challenge was to make all the necessary repairs without prematurely drawing down the reservoir that sits at elevation 7,300 feet in the High Sierra. The normal period when the reservoir is at its lowest is in the winter time (February-March) prior to spring runoff. Syblon Reid accepted the challenge to perform this work in the dead of winter.

Understanding the challenges associated with extreme cold weather and short construction windows, Syblon Reid prefabricated as many items as possible before transporting them to the site. Onsite snow removal was necessary to set up construction equipment and provide access throughout the project site. Water -filled bladder dams, stream flow bypass system piping, pumps with electrical units, and a filter/settling tank were subsequently delivered and set up onsite. Spill containment best management practices (BMPs) were implemented to prevent fluid spills and/or leakage. Other BMPs were employed, as necessary, to prevent erosion. To the extent possible, a small amount of snow was left on the ground to minimize soil erosion.

After dewatering and demolition of the existing facilities, is was discovered that no concrete foundation existed below the tower structure other than an unreinforced 3 to 6-inch thick concrete leveling slab, which was not adequate for the new tower design. However, contingency plans were already in place to deal with the discovery, including prefabricated metal components.

Despite an onslaught of winter storms and temperatures as low as minus 5 degrees, construction was completed in 18 days.

All work was performed without a single first aid injury, OSHA recordable incident, or time loss injury.

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