Dickerson, Maryland-based Comus Construction has been at work since December 2006 building four storm water management ponds for Lehigh Cement Company in Union Bridge, Maryland. Comus is the general contractor on the project and is self-performing all of the work. The four ponds range in size from 19,000 square feet to 50,000 square feet and are at various stages of completion.
Each pond has essentially been constructed the same way. Using two excavators, a Hitachi 350 LC and 330 LC, a Caterpillar D6 dozer equipped with a Leica GPS system, and Terex articulated trucks, the soil is excavated and removed. The D6 dozer then cuts the slopes and contours for the pond based on a 3-D model that has been created from the project plans.
A 12-inch perforated HDPE pipe wrapped in geotextile fabric and then covered with #67 stone collects ground water and carries it to an under-drain sump. The pond itself is constructed of filter fabric, a heavy duty geogrid material, another layer of filter fabric, and then a layer of #67 stone. Another layer of geotextile material is placed above the stone followed by a 40-millimeter liner and shotcrete. The shotcrete is 6 inches thick on the bottom and 4 inches thick on the sides. All of this creates a very slow filtering process for the storm water.
According to Justin Fanning, Comus project manager, Pond 3 was constructed first and has been the easiest so far because of its location on the site. Pond 1, which was actually an existing pond that was redesigned and enlarged, followed and presented some challenges because of ground water and rock issues.
"The water table was actually above the bottom of the pond," says Fanning. If the ground water rises, it could put pressure on the bottom of the pond and cause problems with the lining. Therefore, in addition to the under-drain perforated pipe and the sump, Pond 1 also has a permanent pump in place. "There's a check valve on the sump pump so when the water gets to a certain level it turns on to pump the water out." A discharge line carries the ground water into a riprap channel.
During the excavation of Pond 1, Comus encountered rock that had to be blasted, hammered and excavated. None of the soil was able to be used for back fill or structural fill.
Pond 4 proved to be a challenge due to its location at a former building site. Comus encountered buried footings and a concrete pad that had to be removed, but the most difficult obstacle was a water line running through the middle of the pond site.
"Unfortunately, when they installed this line they worked around the buildings, so the lines (zigzagged) through the site. We had to first locate and survey the water lines so that the engineer could design the relocation. The lines are the main water supply for the entire plant. If we were to hit these and rupture them, it could be a very expensive mistake," says Fanning.
Comus has relocated and installed the 320 feet of water line. However, they will have to wait for a plant shutdown to tie into the existing system.
Pond 5 is still in the design phase, and Comus has also been tasked with installing a new storm drain that is an addition to the project. The $5.5-million project is scheduled for completion in 2008.