For the fourth time since it was put into service in 1936, Lock and Dam 6, near Trempealeau, Wis., is being dewatered. The lock was closed Dec. 3, and the repairs are scheduled to be completed by March 11.
The 13 lock and dams within the district are dewatered for maintenance on a schedule of every 15 to 20 years. The work is scheduled around the navigation season, so maintenance takes place during some of the coldest weather of the year. At least 35 people make up the dewatering team; they are drawn from lock and dam personnel, maintenance and repair crews and personnel from the dredging team.
Traditionally, temperatures during the operation are seasonably cold with daily highs averaging around 30 degrees. This is mild compared to the cold temperatures endured during previous dewaterings. When the lock chamber was dewatered in 1994, workers faced temperatures from 25 to 35 degrees below zero. Comparatively, this year’s dewatering took place during very good weather. “Before maintenance operations could even begin, the crew removed four barges of mussels, sand and other debris that had accumulated at the bottom of the lock,” said Rojean LeSeure, Lock and Dam 6 lockmaster.
“Before dewatering begins, the cranes at either end set the bulkheads into slots in the concrete, they bulkhead the Tainter valves and check all the seals - then the lock can be pumped down. We sink an equipment barge with heavy machinery on it to allow access for the crew.”
During this scheduled maintenance, work includes repairing concrete, sandblasting and painting the miter gates, and replacing most of the bubbler system. The bubbler system assists in dislodging debris that may disrupt the operation of the gates. Workers expect to replace 80 percent of this system with stainless steel tubes that have a longer service life.
Sandblasting of the miter gates takes place during the night and painting is done during daylight hours. “The sandblasters can see what they’re doing better at night, in low light, and the painters work better during the sunlit hours” says Scott Uhl, operations.
Uhl said the team is also repairing concrete along the lock chamber walls. The Corps is using a wet-cut system that minimizes airborne dust and debris. This method is more difficult to use in near-freezing conditions but is more environmentally friendly, he said.
Gate repair also entails removing the diagonals that criss-cross the gates by heating them with a blow-torch. The larger diagonals weigh close to 1,000 pounds each, while the smaller ones weigh in at about 600 pounds each. The lower gates are 25 feet high and weigh 92 tons, while the upper gates are 23 feet high and weigh 85 tons each. Uhl said the team needed to put in a little extra work on one of the gates, because it had bent and deformed diagonals. The diagonals are tension bars that normally keep the gate plumb when stationary and prevent the gate from twisting while in motion.
Lock and Dam 6 is also the first site in St. Paul District to deploy a greaseless pintle ball. The pintle is the bottom hinge point for the gates. The greaseless ball will require much less resources and maintenance.