In Uxbridge in south central Massachusetts, the West Hill Dam is part of a 557-acre park of pine and oak forest offering swimming, hiking, biking and horseback riding.
Besides anchoring the scenic park, however, the dam provides vital flood control for Massachusetts and Rhode Island communities on the West and Blackstone rivers.
The 2,400-ft.-long (731 m) dam can impound a 12,440-acre reservoir, which is equivalent to four billion gallons of water, or 8.3 in. (21 cm) of runoff from the 27.9-sq.-mi. (72.2 sq km) upstream drainage area.
Built in 1960, the dam included stone slope protection, gated concrete conduits, an ogee spillway and earthen dikes.
To date, the 41-year-old structure has prevented an estimated $30 million in damage, according to the Army Corps of Engineers New England District.
Even so, planners came to realize that the dam embankment and foundation were inadequate to prevent seepage when the reservoir level exceeds a certain stage. The dam was built of random and impervious fill, with limited upstream impervious blankets, and without any significant foundation seepage control features.
Excessive seepage occurred during flood events in 1979, 1987, and 1998. A shallow downstream toe drain was installed in 1979, and extended in 1989. Piezometers were installed the following year, and in 1998, a review of the structure’s condition including sample borings and field tests suggested remediation was needed.
Engineers evaluated various options, such as installing an impervious blanket upstream, enhancing drainage downstream, or employing some type of cut-off wall.
They decided on building a concrete panel cut-off wall tied to bedrock along the full length of the embankment. This would help maintain future flood storage pools, and prevent seepage.
With Soletanche Inc., which has a Maryland office, as prime contractor, the $12-million project that began in September 2001 consists of a 48-ft. high (14.6 m) earthfill dam with stone slope protection.
Features include three 5-by-3-by-39 ft. (1.5 by .9 by 11.8 m) gated rectangular concrete conduits, and a 50-ft.-long (15.2 m) concrete gravity-type ogee spillway section at an elevation 18 ft. (5.4 m) below the dam.
Four earthfill dikes up to 14 ft. high (4.2 m) will be built with stone or gravel slope protection.
Building the 2-ft. (.6 m) thick concrete cut-off wall involves digging a trench through the dam’s center. Crews are using two Link-Belt cranes rented from Wood’s CRW, which has a location in nearby Worcester and is a specialist in lattice boom crawler cranes.
The 308HII, a model introduced at ConExpo, features a boom constructed of angle steel rather than tubular steel, and strong enough to drop a 32,000-lb. (14,515 kg) chisel to help create the series of holes used for pouring the concrete for the wall.
The LS-218HII, equipped with a large clamshell bucket, was used to bring up the debris broken loose by the 308HII.
“The Link-Belts performed flawlessly, especially the 308, which had to perform the crane-punishing task of repeatedly dropping a 16-ton chisel for several hours each day, due to the crane’s duty-cycle capabilities,” said Greg Dolan, contractor quality control manager, of Soletanche.
The plan is to work in 25-ft. (7.6 m) long alternating segments along the dam’s length. As one length undergoes improvement, another is left to cure.
Subcontractor McManus Excavating, Northborough, MA, was hired to lower the dam embankment by 10 ft. (3 m) to allow sufficient staging area for the rest of the work. Material removed will be used to rebuild the embankment when the cut-off-wall is done.
The lowered dam will have more than 14 in. (35 cm) of runoff for flood control, and 18 ft. (5.4 M) spillway surcharge.
During the project, which is slated for November completion, West Hill Dam is remaining partly operational so it can store water in case of major flooding.
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