On Site

Construction Begins With Concrete Mat Placement

January, 2002 – Construction work on the Greenville Bridge reached into the waters of the Mississippi River on Monday, December 10, 2001, as the U.S. Army Corps of Engineers began laying prefabricated ribbons of concrete called Articulated Concrete Mattress (ACM) on the bottom of the river. ACM was laid in the areas where support caissons and piers for the new bridge will eventually be located. The concrete mattress stabilizes the river bottom and will prevent scouring — the erosion and excavation of soil caused by river current — when caissons and other structures needed for construction are sunk through the mattresses and into the river floor.

Historically, bridge builders have used a basket-like mattress of woven wood planks to control scouring during caisson work. Timber mattresses are built on shore and floated into position, then piled with enough crushed stone to sink them to the river floor — a time consuming process that requires expert control. The placement of timber mattress would have taken about three months; laying of articulated concrete mattresses took less than 5 days.

Articulated concrete panels are typically used by the Corps of Engineers in river bank stabilization or "revetment" operations. The Greenville project represents the first use of ACM in bridge construction — another footnote in the history of Mississippi River bridges.

December, 2001: The U.S. Army Corps of Engineers lays concrete mat on the bottom of the Mississippi River.

THE MAT LAYING PROCESS
Concrete mats are laid by a vessel conveniently called the Concrete Mat Boat, assigned to the U.S. Army Corps of Engineers, Vicksburg District. The mat boat is the only vessel of its kind in the world, a floating assembly floor where prefabricated concrete panels are laid out and stitched together.

In the process:

• Stacks of panels are ferried to the back of the mat boat by barge; each panel consists of 16 concrete blocks that are wired together to form a panel roughly 25 feet long, 4 feet wide and 4 inches thick (7.6m x 1.2m x 10.2cm).
• Panels are lifted from the barge and placed on the assembly floor. Workers with wire tying tools stitch the panels together. When the tying is done, the panel is edged forward into the water. More panels are tied onto the trailing edge, the process repeats and the ribbon of concrete gets longer.
• The concrete mattress that slides into the water is 35 panels wide, or 140 feet (43m). Workers lay three strips on each side of the river, starting approximately 300 feet (91m) from shore. Strips are overlapped by about 10 feet (3m); each strip is roughly 450 feet (43m) long. The concrete mats will provide more than 175,000 feet (16,000 square meters) of coverage on each side of the river.

Photos of the mat boat and the ACM assembly process are available on the Construction Photo Album page dated December 13, 2001.

LAYING THE FOUNDATION
Construction activity during 2002 will take place largely under water as concrete support caissons for the two main bridge towers are poured and sunk into the bottom of the river. Pouring of caissons is scheduled to begin in January.

• To prepare for pouring, the contractor will build chevron-shaped breakwaters at the two caisson sites, located approximately 596 feet (181.5m) from shore on both sides of the river. Breakwaters are built with walls of steel fastened to 6-foot (1.8m) diameter steel pipe that is driven into the river bottom. The river is 70 to 80 feet (21 to 24m) deep at both locations. Breakwaters will protect the caisson sites from the strong river current and support a caisson guide — a track also made of steel pipe — that will assist in the accurate placement of the caissons as they are sunk.
• To support the breakwater and the caisson guide, a series of 6-foot (1.8m) diameter steel pipes will be driven into the river bottom.
• Once the breakwaters are in place, a steel box form called a cutting edge will be floated out to both sites and positioned in the caisson guide. Concrete will be poured to fill the cutting edge, which will eventually become the bottom of the caisson after it is sunk into the river bed. Subsequent pours into forms atop the cutting edge will create a long, hollow concrete column.
• As concrete is added, the cutting edge will slowly descend down the caisson guide until it reaches the river bottom.