By: Bill Wilson
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.
