ASI Constructors Inc. Gives 19th Century Dam a 21st Century Fix
Construction of a new dam is nearing completion in the wilderness of West Central Maine to replace a water control dam that has been part of the Androscoggin River Storage System for more than a century.
ASI Constructors Inc. is building the new structure at the Upper Dam site on Mooselookmeguntic Lake, which straddles the borders of Franklin and Oxford Counties at nearly 1,500 feet above sea level. This body of water is more than 130 feet deep in places and has a surface area of almost 26 square miles - making it the state's fourth largest lake. The lake is well known for its aesthetic natural scenery and recreational activities, with a number of protected land parcels open to the public for camping, hiking, and wildlife viewing, and the Appalachian Trail located nearby.
Upper Dam is one of several dams in western Maine and parts of eastern New Hampshire comprising the Androscoggin River Storage System that has regulated flows in the Androscoggin River for power and other uses. Upper Dam controls water levels in Mooselookmeguntic Lake and the Richardson Lakes. When it was built by the Union Water Power Company sometime during the 1850s, Upper Dam created a pool that feeds the major inlet stream to the Richardson Lakes. The Upper Dam pool has a surface area of about 3 acres and mean and maximum depths of 12 feet and 20 feet, respectively, and has been a favorite fishing spot for generations of anglers. Fish congregating in the pool include landlocked salmon, brook trout, and the Upper Dam tailrace is an important spawning tributary for rainbow smelt, the principal prey species of predator fish in the lakes.
The new spillway structures will not harm the lake ecology and its abundance of fish.
Upgrade to Meet FERC Standards
ASI Constructors is performing the work at the Upper Dam for Brookfield Renewable Energy Partners. The project's goal is to meet design requirements established by the Federal Energy Regulatory Commission (FERC), the independent agency responsible for licensing hydroelectric facilities. Based in Colorado, ASI Constructors is a self-performing, heavy civil contractor specializing in construction and rehabilitation of dams, hydroelectric and renewable energy facilities and pipelines. Work on this project is covered in a contract, "Renewal of Embankments and Spillway Structures at Upper Dam."
The planned modifications include replacing the existing spillways and bridges to allow for the design flood flow rates required by FERC to be safely passed downstream through the structures. Furthermore, the contractor is re-grading the downstream slope of the embankments to meet new design requirements; adding material along the upstream embankment slope to prevent erosion; and raising the elevation of the saddle dike at Upper Dam in the area of the canoe portage to the new design flood elevation. This latter work was designed to maintain the general character of the portage.
Labyrinth Spillway Boosts Capacity
Preliminary work on Upper Dam began in 2009, with major construction getting underway in 2012. An ASI supervisory team including Tristan Proffet, Blake Bennetts and Bill Callaway have been overseeing the project, which is subject to time-sensitive conditions. Long winters in this wilderness area usually limit construction from "ice out" in late May, to early November.
Designed by Schnabel Engineering, the new Upper Dam spillway incorporates the latest developments in labyrinth weirs. These weirs, which are "V" shaped in plan view, add to the effective length of a dam crest and thus increase the safe discharge capacity by a factor of three to four times that of traditional linear weir structures. This makes labyrinth design especially appropriate for dam sites that require increased spillway capacity, but have insufficient building area to linearly expand the dam's crest. In addition, in a labyrinth weir design, excess water passes through the structure, instead of flowing over the top of the dam, preventing downstream "wash-out" conditions.
Controlling Waters During Work
Upper Dam is reached by using an old camp road running from Route 16. The dirt road leads to a steel bridge over the existing dam. However, the maximum load posted for the bridge is 30 tons, significantly lower than the weight of the 120-ton capacity Manitowoc crane needed at the site, so ASI had to barge the machine to the working area.
The existing dam site had to be encased in a cofferdam for construction to proceed. But prior to this, an auxiliary spillway had to be constructed on the east end of the dam to control waters from the lake.
Once the 24-foot-wide concrete auxiliary spillway was built, the contractor began to install sheeting for the main cofferdam. This proved to be exceptionally challenging, according to ASI's Callaway, citing the preponderance of rocks and boulders under the stream bed.
Callaway said they had to use a vibratory driver mounted on the Manitowoc to install the steel sheet pilings. The cofferdam included a bypass road on the north side of the dam, formed by driving two rows of sheet piling approximately 30 feet apart.
Cement Treated Aggregate Provides Stable Base
With the cofferdam and roadway in place, ASI started demolishing the existing dam and its foundation.
The old dam was covered by a timber-frame, metal-sided building, and was built on a foundation composed of large timber cribs, stone, giant logs and large pieces of concrete. Demolition was a relatively slow, tedious task for the excavator operators. After the superstructure and foundation were demolished, the site contactor had to remove a substantial amount of unsuitable material ranging from 2 feet to 16-feet-deep.
Unsuitable material was replaced with cement treated aggregate. This is an extremely dry concrete mix containing portland cement, stone, and just 12 percent moisture which forms a stable subgrade for the new spillway structure.
Due to the remoteness of the job site and the large quantities of concrete required, ASI set up a concrete batch plant on the camp road not far from the dam. This plant produced some 8,000 cubic yards of cement-treated aggregate for the subgrade and about 6,000 cubic yards of 4,500 psi ready mix concrete for the new spillway structures.
Harris Forms for Massive Structures
The overall footprint of the dam is approximately 184 feet by 89 feet. Principal structural elements include, from west to east, four V-shaped labyrinth spillways with a total width of 112 feet, a 24-foot-wide radial gate, a 12-foot-wide turbine bay, and two 10-foot-wide split-leaf gates.
The concrete wall separating the labyrinth spillway from the radial gate, and the walls enclosing the turbine bay and the split-leaf gate bay, are massive structures, measuring 4-feet-thick, almost 70-feet-long, and about 25-feet-tall. They are set on a base slab averaging 3-feet-thick, supported in turn by a 7-foot-tall cutoff wall.
To handle the forming of these huge walls, as well as all other vertical concrete on the project, ASI selected the 1500 Clamp System of A.H. Harris & Sons Inc. Harris' Portsmouth, New Hampshire office supplied the forms, which are manufactured by Harmac Rebar & Steel, a Harris division. The forms are made with galvanized steel frames and a resin-coated, 1/2-inch, 9-ply birch veneer face, and are assembled together using handset clamps. According to Mike Griffin, New England Regional Vice President of A.H. Harris, Tad Spiller played a key role in form erection and technical field service, with early field service by Scott Coffin, while Harris personnel Mark Fernandes, Russ Tooker, and Robin Nichols provided technical field support and formwork layouts and drawings.
Barring unforeseen developments, this Upper Dam project will be completed in 2016, according to Callaway.