Lock 6 dam no longer in danger of collapse, Corps officials say
KITTANNING — While Army Corps of Engineers officials declared the dam at Lock 6 on the Allegheny River is “stable and no longer in danger of collapse,” they said there is still work to be done at the structure.
Emergency repairs began at the facility, which stretches 992 feet between the banks of the river between Kelly Station, Bethel Township and Clinton, South Buffalo Township, in November, after an underwater survey of the dam showed a void under the base of the dam that traveled about 200 feet across the dam, was about 20 feet deep and undercut the dam by nearly two thirds of its length.
The $2.9 million project is being completed by crews from Brayman Construction of Saxonburg, who also built the new bridge across the river at East Brady in 2007.
Project manager Brian Greene said Thursday during a stakeholders meeting that a bulk of the repair work was able to be completed between mid-November and mid-January, and that the remainder of the project would be completed in April or May.
“On two dates, in December and January, we were able to pour 750 cubic yards of tremie concrete to fill the void under the dam,” he said. “It was a very labor intensive project between making sure the concrete pouring was continuous and having to drill holes into the dam itself to allow the concrete to be poured into all areas of the void.”
Weather conditions along the river were the main reason for the project not being completed by the end of the year as Corps officials had hoped.
“Mother Nature just did not want to cooperate with us,” Greene said. “We had very challenging work conditions where we saw low water flows where there was no water flowing over the dam to high water flows where there was several feet of water coming over the face of the dam.”
Corps officials said they decided to wait until spring when water levels tend to be more stable to complete the task of placing various layers of rock below the dam to help prevent further erosion.
“At that time, we will place a layer of bedding material, which is grapefruit-sized stones and then place a layer of riprap, which are roughly four-foot square rocks,” he said. “and on top of those layers, we will place derrick stones, which are on the order of about five tons each.”
Greene said the material will be placed for about 70 feet on the downstream side of the dam. Workers also will fill the remaining portion of the repaired dam with a concrete cap to seal the areas filled with concrete.
Despite the success of the project, Greene said that a similar problem has been noticed downstream at Lock 5 near Freeport.
“The problem at Lock 5 is similar in nature, but nowhere near the size and scope of the problem at Lock 6,” he said. “Lock 5 is not in a crisis situation to where we would have to come in and fix the problem like we had to at Lock 6.”
Greene said the Corps is developing a plan to fix the problem at Lock 5, including the use of civil engineering students at the University of Pittsburgh.
According to Greene, the problem at both dams occurred near the part of the dam where the dam connects to the hydroelectric plant.
Mark Jones, chief of technical services for the Corps, said that since the discovery of the erosion problem at Lock 6, the Corps has changed its policy to require underwater surveys on all eight dams along the Allegheny River.
“We originally completed surveys every five years, but then changed it to every 10 years,” Jones said. “Had the Corps not done that, we would have noticed the problem at Lock 6 five years sooner and might not have been put in the position where we were forced into fixing it before it potentially failed.”
During meetings in early November, the Corps said that if the dam at Lock 6 were to have collapsed, the upstream river pool, which runs roughly 11 miles to Lock 7 in Kittanning, could have dropped to levels rivaling a small stream, affecting municipal water supply intakes and aquatic life, including endangered mussel species, and dewatering one of the region’s largest wetlands, the Cogley’s Island Complex, across from Manorville.
The 80-year-old dam has a height of 12.4 feet and a single 56-by-360-foot lock chamber. It features a 9.5 megawatt hydroelectric plant on the Bethel side, operated by Sithe Energy. The company also operates hydroelectric plants at Locks 5, 8 and 9.
The dam, according to engineers, was built in 1928 on wood pilings driven into the river bottom. A steel frame was placed on top of the pilings, and the concrete dam was poured and shaped on top of the frame. Additional wooden baskets, filled with stone, were placed along the downstream side to prevent erosion.