To address continuing leakage at its Gem Lake Dam, Southern California Edison commissioned installation of a geomembrane on the upstream surface. Results indicate seepage through the dam has been reduced by as much as 90 percent since the geomembrane liner was installed three years ago.
Like many other dam owners, Southern California Edison (SCE) is faced with aging infrastructure that has experienced decades of harsh environmental conditions. SCE owns and operates 37 dams with an average age of 80 years. Concrete deterioration, seepage, and degradation of facing materials are just a few of the issues that need to be addressed to maintain safe operation of dams. SCE has a comprehensive plan and schedule for critical dam rehabilitation that will allow these structures to continue to be valuable resources well into the new century.
One aspect of that plan is the installation of geomembrane liners on the upstream faces of four of SCE's concrete dams and all four of SCE's wood-faced dams. In 2006, SCE completed placement of the first geomembrane liner on Sabrina Lake Dam, a wood-faced rockfill dam on the Middle Fork of Bishop Creek.1 The liner reduced seepage through the dam by 90 percent.
In the summer of 2007, Gem Lake Dam became the second SCE dam fitted with a geomembrane liner. This was needed to address freeze-thaw conditions that had led to concrete deterioration and subsequent seepage.
Structural deficiencies and complicating circumstances
SCE's 13-MW Rush Creek Power Project is on Rush Creek in Mono County, on the eastern slope of the Sierra Nevada mountains. The project consists of three dams (including Gem Lake Dam), three storage reservoirs, and one two-unit powerhouse.
Gem Lake Dam is a multiple arch concrete dam that impounds a reservoir with a capacity of 17,288 acre-feet. The dam is located in the Ansel Adams Wilderness Area. Gem Lake Dam is one of the first examples of multi-arch concrete dam design in the U.S. The dam is composed of 16 complete arches, each of 40-foot span between the centers of the buttresses. In addition, a partial arch at each end of the dam ties into the rock abutments. The dam is 688 feet long, with a maximum arch height of about 84 feet. The arches are 1 foot thick at the top and 3.95 feet thick at the bottom and are reinforced with concrete gravity sections up to elevation 9,027.5 feet. The buttresses range in thickness from 1.85 feet at the top to 4.25 feet at the lowest point. The buttresses are braced by counterforts ranging in width from 4.5 feet at the top (15 feet below the crest of the dam) to 11 feet at the lowest point. Double 12-inch by 18-inch concrete struts are placed between the buttresses.
The dam was built in 1915 and 1916 using aggregate found in the streambed of Rush Creek and adjacent rock. Because of the construction techniques of the time and the design of the concrete mix, the concrete is somewhat porous, which allows water to fill voids in the structure. In the winter, this water freezes, causing expansion and spalling of the concrete surface. Water migrating through the dam causes deterioration of the cement/aggregate matrix as cementitous material is leeched out, leaving pockets of loose aggregate. In several places, the concrete to a depth of 1 foot or more can be removed using a geologist's hand pick.
In 1966, a program was undertaken to cover the entire upstream face with about 3 inches of gunite reinforced with heavy steel wire mesh. The gunite was sealed with polysulfide, placed in two coats over a primer. In 1969, a third coat of polysulfide was placed on the upstream face. The polysulfide coatings initially were successful in preventing water migration. However, their long-term performance proved less than desirable, as the coatings peeled away from the dam, leaving the gunite exposed.
In 2006, SCE conducted an extensive site investigation to quantify the extent of concrete deterioration. Dean White, a concrete consultant, observed the concrete condition, obtained corings, and took Schmidt hammer measurements. The conclusion of this investigation was that decades of uninterrupted freeze-thaw cycles had left substantial pockets of degraded concrete in two of the arches. Any attempt to repair those areas would be fruitless until the seepage through the dam was stopped.
Determining how to repair the dam
SCE identified and investigated three alternatives for dealing with the leakage at Gem Lake Dam:
– Remove the degraded polysulfide coating and replace with like in kind;
– Apply another gunite layer to the upstream face; and
– Apply a geomembrane liner over the upstream face.
With regard to the first two alternatives, removing the peeling polysulfide would require a method for collecting and disposing of the debris. In addition, the polysulfide coating already demonstrated a relatively short life-span (about 20 years). SCE had applied a layer of reinforced gunite in 1966 and, while it did significantly reduce seepage through the dam (by more than 50 percent), large cracks developed after about 20 years, allowing water to reach the original concrete and initiating the freeze-thaw cycles once again.
Geomembranes have been installed on more than 85 dams worldwide, constituting an area of more than 6.2 million square feet of dam face and a service life exceeding 850 years. Given this performance record, a life expectancy of more than 40 years was easily justified.
While the three alternatives had similar initial costs, they varied significantly in the estimated amortized annual costs (see Table 1). In addition, the geomembrane liner had a longer projected life (in excess of 40 years, compared with 20 years for coatings or gunite) and fewer environmental issues related to installation.
Design of the rehabilitation
The dam's location in a designated wilderness area provided some challenges. The primary means of transportation is a combination of two cabled tramways and a boat. Permits were required from several agencies, including the U.S. Forest Service (USFS), U.S. Fish and Wildlife (FWS), U.S. Army Corps of Engineers, and Regional Water Board. Activities in the wilderness require primitive methods to be used unless a compelling case can be made for mechanization. SCE showed that the liner could only be installed in a single season through the use of a mechanical excavator and four-wheeled vehicle. The effect on the wilderness environment, in the form of a drained reservoir, that would be incurred by extending the work to two seasons was greater than the effect of using mechanical equipment.
SCE negotiated a design and construction contract with CARPI USA in 2008. The contract included terms for ACE Restoration Company to perform grouting on the two arches suffering from significant damage. Before installing the geomembrane, CARPI provided a detailed design of the system, plans, and specifications. This included a constructability review with USFS and a pre-construction meeting of representatives from SCE, CARPI, ACE Restoration, and USFS to ensure all components of the installation were properly planned.
The design and execution of the Gem Lake Dam geomembrane system was complicated because of:
– Logistics involved with accessing the dam;
– Complicated geometry of the dam structure;
– Short window for construction due to high altitude; and
– Environmental constraints of working within a wilderness area.
The unique geometry of Gem Lake Dam required new design features and installation techniques to cost-effectively access the front face of the dam. On previous similar installations, CARPI's installation crew accessed the front face of the dam using scaffolding. For Gem Lake Dam, the cost to transport the scaffolding up the mountain and assemble it would be prohibitive (estimates were in excess of $200,000). In fact, if scaffolding was used it is unlikely the installation could have been completed in one season, a major goal of the installation.
In 2006, CARPI installed a geomembrane liner on Linach Dam in Germany. For this installation, CARPI personnel used swing stages to access the upstream face of the dam. A swing stage is a two-point adjustable suspension scaffold that is hung by ropes or cables connected to stirrups at each end of the platform. Swing stages typically are used by window washers but also are used in the construction industry. Although there would still be small areas of Gem Lake Dam that required scaffolding, the swing stages would allow access to more than 90 percent of the surface.
Because of their reduced transportation costs, use of the swing stage platforms would allow a significant reduction in potential costs to the project.
During this design phase, a careful preliminary design and analysis of the structure led to the conclusion that the typical method of installing tensioning profiles would not work at the intersection of adjacent arches. The geometry of the arches at Gem Lake Dam (relatively short arch spans) created an area at that intersection that was simply too tight. CARPI developed a new multi-layer system design and then constructed a full-size mockup, documenting each stage of the installation. This provided assurance to SCE and the California Department of Safety of Dams (DSOD) that the installation could be completed successfully.
Installing the geomembrane
Work to install the geomembrane liner at Gem Lake Dam began in June 2007 and involved many steps. In rough spots (about 3 percent of the surface), CARPI installed a 2,000-gram-per-square-meter geotextile directly on the surface to smooth irregularities and thus decrease surface preparation costs. Next, over the entire face, CARPI installed a Tenax Tendrain geonet (triplanar) for a drainage layer, with a thin geotextile to contain the existing polysulfide coating. CARPI then installed submersible watertight perimeter (stainless steel) seals along the foundation, crest of the two spillway arches, and both abutments. A non-submersible watertight perimeter (stainless steel) seal was installed along the crest of the 16 non-spillway arches. At three locations, CARPI installed drainage plates, with a drilled hole through the face at each location to allow discharge of water through the dam body. Stainless steel batten strips then were installed at the spring of the arches vertically, and stainless steel tensioning profiles were installed on the center of each arch vertically to hold the geomembrane to the dam.
The geocomposite, which consists of a polyvinylchloride (PVC) geomembrane 3 millimeters thick with 500 grams per square meter of geotextile, was installed in 1.05-meter widths horizontally on the face of each arch. Finally, CARPI personnel installed 3-millimeter-thick PVC geomembrane welding strips to cover the stainless steel tensioning profiles.
The key element of this system is the PVC geocomposite that waterproofs the entire face of the dam. This geocomposite consists of a geomembrane heat-coupled during extrusion to a non-woven geotextile. The geocomposite is attached to the face vertical profiles. This anchorage technique enables the geocomposite to elongate over large areas, minimizing stress to the material. The anchorage system also allows a drainage layer to be attached to the face of the dam because there is no adhesive layer to clog the drainage layer. This liner anchorage allows for two key benefits:
– Provides the possibility of drainage and subsequent discharge by gravity of seepage water that infiltrates the waterproofing liner or dam body. The water is collected and discharged downstream by a pipe installed through the dam; and
– Allows tensioning of the geomembrane to prevent the formation of wrinkles and sagging that can reduce the longevity of the installation.
Benefits from dam rehabilitation
Many benefits were achieved from installation of the CARPI geomembrane system at Gem Lake Dam:
– Seepage control. Seepage measurements taken since the geomembrane installation was completed in September 2007 show reductions of 50 percent to 90 percent and no visible seepage in the unbuttressed sections (upper 30 feet of arches).
– Eliminating freeze-thaw deterioration. Stopping the passage of water through the dam will dry the downstream face, eliminating the cause of freeze-thaw deterioration.
– Longevity. The CARPI system has a successful track record on more than 85 dams. Dam rehabilitation installations have been in operation for more than 30 years. It is expected that these installations will exceed 50 years, as documented in a geomembrane sampling study from six existing projects.2
– Cost effectiveness. The geomembrane system and the other options had effectively the same installation price.
However, with the proven longevity of the geomembrane system, the life-cycle cost analysis shows the
CARPI geomembrane to clearly be the most cost-effective. The geomembrane system had minimal environmental effects, while both shotcrete and polysulfide would have required significantly more environmental safeguards. Lastly, the volume and weight of materials required for the shotcrete option made it unattractive because of the additional cost of tram transport and likely maintenance issues from heavy use. The polysulfide and geomembrane system had similar transportation costs. However, the geomembrane system was significantly safer. Almost all the geomembrane system materials were solids so that if a load was lost during transit, there was no significant hazard. The polysulfide system involved liquids and paste that would have represented a significant clean-up risk in the event of an accident.
– Fewer environmental effects. The geomembrane system had significantly fewer project environmental effects than the other options. The geotextile on the geonet drainage layer enabled the existing polysulfide coating to be captured in place without any need to remove. Both the shotcrete and polysulfide options would have required this layer to be removed, which would have been expensive and difficult. The geomembrane installation at Gem Lake Dam was completed without any construction or alteration of the site. No sediment was released, as the reservoir shoreline was more than 100 feet upstream of the dam face. Minimum instream releases were maintained throughout the installation process.
– Greater aesthetic value. The geomembrane installation at Gem Lake Dam will remain mostly underwater more than 90 percent of the time, leaving the dam appearance essentially unaltered. When the geomembrane is exposed, either during low water years or drained reservoir conditions, the geomembrane is a neutral gray color that gives the dam a clean, uniform appearance that blends well with the surrounding terrain.
– Minimal maintenance. The CARPI geomembrane system is a passive system that requires no maintenance.
SCE is investigating the benefits of installing similar geomembrane liners on several other dams, including Agnew Lake (concrete multi-arch), Hillside and Saddlebag (wood-faced rockfill), Shaver Lake (concrete gravity), and Tioga Lake (wood-faced rockfill main dam and concrete arch auxiliary dam).
John Stoessel, P.E., a senior engineer with Southern California Edison's Dam Safety Group, was project manager for the geomembrane installation work. John Wilkes, P.E., is president of CARPI USA, the company that manufactured and installed the geomembrane liner.
- Tech Briefs, Hydro Review, Volume 26, No. 4, August 2007.
- Cazzuffi, D., "Long Term Performance of Exposed Geomembranes on Dams in Italian Alps," International Conference on Geosynthetics Conference Proceedings, Industrial Fabrics Association International, Roseville, Minn., 1998.