A $29-million rehabilitation project is well under way at the Loch Raven Dam at Gunpowder Falls in Baltimore, MD.
The structure is a gravity dam, meaning that it relies on its own weight for stability. The rehabilitation project will add significant mass to the existing dam as a public safety improvement.
According to Baltimore’s Department of Public Works (DPW), state and federal regulations require that the dam be rehabilitated and modified to withstand future significant storm events — 31 in. (79 cm) of rain within 72 hours. The dam was originally built in 1912 to a height of roughly 20 ft. (6.1 m) above the streambed. In 1922, it was raised approximately 60 ft. (18.3 m) to its current elevation of 288 ft. (87.8 m) above sea level. In the early 1970s, structural improvements to the face of the dam and mechanical improvements to the intakes were completed.
A joint venture between Cianbro Corp. Inc. and ASI RCC Inc. (ASI/C), the job is funded by both the city of Baltimore (63 percent) and Baltimore County (37 percent). The changes were designed by project engineer Gannett Fleming Inc.
Rachel Ellis, community and public relations coordinator of the DPW, noted that the greatest challenge in this project is completing the construction while maintaining the reservoir level.
“Loch Raven reservoir provides drinking water for approximately one million residents in Baltimore City, Baltimore County, Anne Arundel County and Howard County,” she explained. ’This means that flow from the reservoir cannot be interrupted, and more importantly, the reservoir level cannot be lowered.”
Mitchell Rubin, project manager of ASI/C, added, ’The biggest challenge to do a job of this size and this type is that we have to maintain the water source at all times. It cannot be jeopardized, so when the processing plants demand the water, we can’t stop the water flows. We have to make sure that those flows make it down there. We’re basically rebuilding this dam while maintaining the water supply.”
Ellis further explained that water has been cresting the dam since February 2003. “During the design,” she noted, “it was agreed that only one half of the dam could be constructed at a time, as lowering the reservoir level was not an option. At times, the entire work area has been completely flooded due to this flow.”
The DPWs’ Web site notes, “Steel anchors will be installed through the dam and into the underlying bedrock. Silt and debris that have accumulated at the base in front of the dam will be removed, and roller compacted concrete [RCC] set to stabilize the toe of the dam. Then, RCC will be added to the face of the dam.
“The height of the spillway will not change. However, it will be tapered to extend further into the lower reservoir at the base. The sidewalls, or non-overflow sections, will be raised 20 feet on each side of the spillway, and a steel gate, much like those used in canal locks, will be installed at Loch Raven Drive to seal the non-overflow section during major storm events.”
Work began in August 2002 and is currently on schedule at the halfway mark. Completion is set for August 2005.
Installation of the rock anchors also has proved challenging. Ellis said, “Drilling up to 58 feet into the face of a dam that is holding back billions of gallons of water is, to say the least, a bit intimidating. At this point, nearly all of the 57 anchors have been installed.”
Rubin noted, “The rock anchors get installed along the downstream face of the existing dam. This is to anchor the existing dam to the bedrock. Right now, the existing dam is just gravity-held down, so we’re anchoring the existing dam down. Then we come back in two phases: Phase 1 would be the east side, and Phase 2 would be the west side. We’ve just basically split the dam down the middle because we have to maintain the water flows.”
Rubin noted that the group is currently working on Phase 1 and has placed approximately 30,000 to 35,000 cu. yds. (22,937 to 26,759 cu m) of RCC. “There are two different processes on the spillway face,” he explained. “We placed the RCC and then we come back and pour a concrete facing over it. On the non-spillway area, we’ve introduced a value engineering proposal, which the city has accepted, and that is that we’re placing the facing and the RCC concurrently as we go.”
The group placed two concrete plants on site — one to make the RCC and the other to make conventional concrete. Rubin explained, “We stockpile about a third of the materials at a time, so that way we can work when the temperatures allow us — the RCC is temperature-sensitive.” The RCC plant is a Johnson-Ross Bandit 12 with a capacity of up to 200 cu. yds. (153 cu m) per hour, while the conventional plant is a Johnson-Ross Rustler III capable of up to 160 cu. yds. (122 cu m) per hour.
Also included in the contract are two conduit lines that supply water and take it to the processing facilities. Rubin noted that one of the lines is a 120-in. (305 cm) steel line that was built when the dam was first put in. The group also cleaned approximately a mile of concrete pipeline and inspected it for the integrity of the pipe.
“One other thing that’s kind of unique to this job is the fact that there is a road that comes alongside the existing dam,” Rubin said. “When we’re finished, the spillway will stay at the same elevations, but the two wings will go about 20 feet higher, and we’re also doubling the size as to the thickness of the dam. That pushes the new structure of the dam over the top of this roadway, so what we’ll be doing is putting in a roadway. It will be going through part of the wing wall of the dam, and there’s a water-tight gate that gets installed. If the mother of all storms comes through here, they would shut that gate, and it would hold that amount of water back.”
Rubin noted that there are approximately 30 homeowner’s associations in the area, and someone meets with them regularly to keep them informed about the current schedule.
In addition to the homeowner’s associations, the group also must coordinate with police and rescue personnel from both Baltimore City and Baltimore County.
Rubin noted that the Joint Venture was formed with the idea that ASI RCC had experience with RCC, while Cianbro had experience with excavations and an existing relationship with the city. He said, “What we’ve done is just mixed both company’s strengths on the job. For example, I work for Cianbro Corporation, but the project superintendent works for ASI. Then we have one engineer from each company surveying, and we just try to pick and choose people on both sides to strengthen the group.”
Depending on activities, 25 to 50 people are assigned to the job at one time. Besides Rubin, key leaders for the project include Mike Schultz, chief, construction management division, Bureau of Water and Wastewater, DPW, Baltimore and David B. Smyth, engineer in charge, Gannett Fleming Inc.
The project includes 65,000 cu. yds. (49,696 cu m) of roller compacted concrete, 7,000 cu. yds. (5,352 cu m) of conventional concrete, and 50,000 cu. yds. (38,228 cu m) of excavated material.
Major subcontractors include Brayman Construction Corporation, PA, rock anchors; MoreTrench American Corporation, NJ, dewatering; McCall Trucking Inc., MD, off-site hauling; and D&G Brice Construction, MD, on/off-site hauling.
The equipment list for the rehabilitation includes: Manitowoc 4000, 4100 cranes, and a Grove 28-ton (25.2 t) crane; Cat D3, D5, and D8 dozers; Komatsu 250, Volvo L70 and L90 loaders; John Deere 312, 450 and 992 excavators; Johnson Ross Bandit and Rustler 3 concrete plants; one Creter crane 200; two GMC 10-yd. (9.1 m) concrete mixers; three generators; and two Dynapac CC122 vibratory rollers.