An answer from the panel

May 17, 2002

MKK Constructors faced the usual pressures when entering a design-build project in Colorado—time and money. Frustration could have set in, but the prime contractor wasn’t banging his head against the walls. In fact, the retaining wall system used provided peace of mind.

MKK Constructors faced the usual pressures when entering a design-build project in Colorado—time and money. Frustration could have set in, but the prime contractor wasn’t banging his head against the walls. In fact, the retaining wall system used provided peace of mind.

The E-470 toll road in Brighton, Colo., has involved the construction of over 40 bridges in a 21/2-year time frame. The first phase, installing 102,000 sq ft of full-height panel  mechanically stabilized earth (MSE) retaining wall, was smooth and quick.

According to Joe Kerrigan, project manager for Tensar Earth Technologies Inc., Atlanta, Ga., full-height panel MSE walls offer significant advantages over the standard cantilever concrete retaining wall.

“One of the primary benefits with the MSE wall is there are no form costs,” he told ROADS & BRIDGES. “You’re not forming up the concrete like you would have to with a cantilever retaining wall.”

Flexibility also comes into play, which is critical during a design-build job. With full-height panel MSE walls, the facing installation and backfill operations are separate. Other methods require the two to run concurrently, which forces the contractor to run a rigid schedule.

“When the two don’t rely on one another the contractor can more efficiently schedule his manpower,” said Kerrigan.

The job, however, did require a little extra effort. Early on it was discovered some of the subgrade soils had bearing capacity problems.

“In the design-build process not a lot is known up front and everything is a moving picture,” Scott Sothen, business development manager for Tensar, told ROADS & BRIDGES. “We had some challenges in terms of the soils and that required us to be flexible in our design process.”

The solution required the use of chemical grouting, Tensar’s biaxial grid and a more suitable fill.

Sema Precast, Denver, fabricated the panels about a mile from the start of the project. The design process took about three weeks, and after receiving approval from the toll authority casting commenced. Some of the 7-in.-thick panels reached a maximum height of 42 ft. Sema had four beds 32-ft in length, so to remedy this situation a smaller panel was cast and bolted on to the top of the lower piece after the filling of the first section was completed. Cure times for the panels ran 14 days. The concrete had a minimum compressive strength of 5500 psi.

When it came time to ship, pieces were delivered on flatbed trailers with an A-frame. The maximum number delivered to the site was four.

Prior to arrival at the wall location the site was excavated and a leveling pad was poured. The purpose of the pad was to facilitate establishing the line and grade of the panels.

Manitowoc cranes were used to set the panels on the pad. Traffic barriers were used as deadmen to brace the panels prior to releasing the crane. Standard height panels were braced at the 3/3 point while the taller ones were braced at two locations. As the fill came in, the contractor monitored the braces and either tightened or loosened to maintain positive batter—the deviation from the vertical position. In order for the panels to be perfectly vertical when backfilling was complete, the pieces were set slightly back toward the fill. When fill came in the panels moved into the vertical position. A geocomposite joint material was placed over a 3/4-in. joint between the panels to retain backfill. Geogrid tabs were cast into the panel, and as the fill reached the level of a tab the prescribed length and type of Tensar geogrid was attached to the tab using Tensar’s patented Bodkin bar.

Some walls on the project contained as many as 87 panels, and cost was about $12.50 per sq ft.

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