Cofferdams Control Water While Trenching Under a River

June 30, 2015

In the summer of 2014, Midwestern Contractors was involved with laying two high-pressure gas transmission lines across the DuPage River, located in the western suburbs of Chicago.

“In this project, we had to lay two 18- or 20-inch lines across the river,” explains Mike Purpura, the company’s director of business development. “To do this, we had to control the surface water. And before that, we had to bring the pipe to the edge of the river.

In the summer of 2014, Midwestern Contractors was involved with laying two high-pressure gas transmission lines across the DuPage River, located in the western suburbs of Chicago. “In this project, we had to lay two 18- or 20-inch lines across the river,” explains Mike Purpura, the company’s director of business development. “To do this, we had to control the surface water. And before that, we had to bring the pipe to the edge of the river. [text_ad] “We established cofferdams out into the river to temporarily control the flow of water into the area that we would ultimately excavate. The cofferdams were set almost as a set of brackets, with the open end of the brackets connecting into the shore, and the outer, long edge of the bracket parallel with the flow of the river. “Once that was established, we used pumps to dewater the inside of this bracketed area. Once it was dewatered, we used an excavator to go in and expose a trench for the pipe, to get the pipe down to a suitable depth. You don’t want the pipe too shallow, where someone can drop something on the pipe and damage it. So it might be five or six feet below the bed of the river. “We then fabricated the pipe and laid it into this trench. We welded it to its adjoining piece that was protruding from the shore. This area was then backfilled, the bottom profile was re-established, and the cofferdam was removed. “The ‘bracket’ then essentially flipped, with the open end now facing the opposite side of the river. The same process was then used on this opposite side, the only difference being that the first piece of pipe protruded into the new dewatered cofferdam area, so when we excavated, we had the stub of the first piece sticking into the excavation. “We laid the new pipe in, made a weld out into the river, and then made a weld onto the shore, so we had a unified piece of pipe going all the way under the river.” Crews did the process twice, for two pieces of pipe, using the same cofferdam method. “I’m making it sound easy,” says Purpura, “but in the process there was a lot of sandbagging, adjusting, sealing, and pumping, with 24-hour-a-day work because once the pumps are actually started, you can’t shut them off. So we had to go into shift work to make this whole thing happen. “In addition, we had traffic to control. There is a road that parallels the river, not too far away from it. We had to keep the traffic flowing while we were moving excavated spoils across this road, both during the excavation, when we were taking spoils out, and then when we were putting spoils back in. They all had to be moved back and forth across this road, and down to where the backhoe could get at them and use them to backfill the excavation.” The “spoils” consisted of dirt, rocks, and mud excavated out of the river. It had to be stored temporarily until the pipe was in place. The pipe was first covered with sand, to protect it, and then the native spoils were returned to the riverbed. The cofferdams were created on the 130-foot-wide river from a series of bladder dams from Dam-It Dams. “They were about 22 to 25 feet wide and 8 feet tall when they were filled with water,” says Purpura. “The project didn’t require water filtration. But it did require that we minimize the turbidity by taking whatever water was coming into the excavation, and if it was clean, pumping it out. If it was dirty, then we had to route it through a filter bag and hay bales. We also had silt fences and some geotextile fabric. We didn’t do anything really exotic there. The most exotic thing we did, I suppose, was filling up these bladder dams to control the surface water of that river, so we could excavate. “Once we excavated and moved the spoils out, we were more or less in a contained area, with everything contained with silt fencing. So any runoff from our spoil piles—and we did get some heavy rain—would be caught by the silt fencing.” Purpura notes crews were careful not to do any undue damage to the area. “Obviously, when you get construction equipment in there, it kind of tears things up. We used crane mats for our access roads, so we had vehicles traveling on crane mats. In those areas immediately around where this work was taking place, on the shore, we had silt fencing and various erosion control methods set up to control anything that might wash off from our activities on the shore. “The crane mats are 8 inches thick, 16 feet long, and 4 feet wide. We had to lay a series of those, for over a quarter-mile, making a road out of timbers, essentially. This got us inside our silt fence work area and enabled us to get materials and vehicles in and out without doing any permanent damage to the area. It killed the grass, but with a little reseeding and straw, we can bring all that back pretty quickly. “On the river bottom, we were limited to working within a fairly confined right of way. We had to stay within about 50 or 60 feet to minimize disturbance of the river bottom.”  

“We established cofferdams out into the river to temporarily control the flow of water into the area that we would ultimately excavate. The cofferdams were set almost as a set of brackets, with the open end of the brackets connecting into the shore, and the outer, long edge of the bracket parallel with the flow of the river.

“Once that was established, we used pumps to dewater the inside of this bracketed area. Once it was dewatered, we used an excavator to go in and expose a trench for the pipe, to get the pipe down to a suitable depth. You don’t want the pipe too shallow, where someone can drop something on the pipe and damage it. So it might be five or six feet below the bed of the river.

“We then fabricated the pipe and laid it into this trench. We welded it to its adjoining piece that was protruding from the shore. This area was then backfilled, the bottom profile was re-established, and the cofferdam was removed.

“The ‘bracket’ then essentially flipped, with the open end now facing the opposite side of the river. The same process was then used on this opposite side, the only difference being that the first piece of pipe protruded into the new dewatered cofferdam area, so when we excavated, we had the stub of the first piece sticking into the excavation.

“We laid the new pipe in, made a weld out into the river, and then made a weld onto the shore, so we had a unified piece of pipe going all the way under the river.”

Crews did the process twice, for two pieces of pipe, using the same cofferdam method. “I’m making it sound easy,” says Purpura, “but in the process there was a lot of sandbagging, adjusting, sealing, and pumping, with 24-hour-a-day work because once the pumps are actually started, you can’t shut them off. So we had to go into shift work to make this whole thing happen.

“In addition, we had traffic to control. There is a road that parallels the river, not too far away from it. We had to keep the traffic flowing while we were moving excavated spoils across this road, both during the excavation, when we were taking spoils out, and then when we were putting spoils back in. They all had to be moved back and forth across this road, and down to where the backhoe could get at them and use them to backfill the excavation.”

The “spoils” consisted of dirt, rocks, and mud excavated out of the river. It had to be stored temporarily until the pipe was in place. The pipe was first covered with sand, to protect it, and then the native spoils were returned to the riverbed.

The cofferdams were created on the 130-foot-wide river from a series of bladder dams from Dam-It Dams. “They were about 22 to 25 feet wide and 8 feet tall when they were filled with water,” says Purpura.

“The project didn’t require water filtration. But it did require that we minimize the turbidity by taking whatever water was coming into the excavation, and if it was clean, pumping it out. If it was dirty, then we had to route it through a filter bag and hay bales. We also had silt fences and some geotextile fabric. We didn’t do anything really exotic there. The most exotic thing we did, I suppose, was filling up these bladder dams to
control the surface water of that river, so we could excavate.

“Once we excavated and moved the spoils out, we were more or less in a contained area, with everything contained with silt fencing. So any runoff from our spoil piles—and we did get some heavy rain—would be caught by the silt fencing.”

Purpura notes crews were careful not to do any undue damage to the area.

“Obviously, when you get construction equipment in there, it kind of tears things up. We used crane mats for our access roads, so we had vehicles traveling on crane mats. In those areas immediately around where this work was taking place, on the shore, we had silt fencing and various erosion control methods set up to control anything that might wash off from our activities on the shore.

“The crane mats are 8 inches thick, 16 feet long, and 4 feet wide. We had to lay a series of those, for over a quarter-mile, making a road out of timbers, essentially. This got us inside our silt fence work area and enabled us to get materials and vehicles in and out without doing any permanent damage to the area. It killed the grass, but with a little reseeding and straw, we can bring all that back pretty quickly.

“On the river bottom, we were limited to working within a fairly confined right of way. We had to stay within about 50 or 60 feet to minimize disturbance of the river bottom.”

About the Author

Steve Goldberg

Steve Goldberg writes on issues related to erosion control and the environment.