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Cement-Treated Base Case Study
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Cement Sails to the Rescue
Hurricane Isabel caused a significant amount of damage to the
mid-Atlantic states when it roared ashore in mid-September 2003.
The Outer Banks of North Carolina were hit with a severe tidal surge
and many hours of relentless storm pounding. Particularly vulnerable
was Ocracoke Island, which lies just east of the path taken by the
hurricane’s eye.
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| NOAA satellite image of Ocracoke Island showing
areas of road destruction following Hurricane Isabel. |
NC 12 is the North Carolina state highway that runs the length
of Ocracoke, with NCDOT ferry service being the only way for public
transportation to reach the island. On the morning of September
19, it was clear that NC 12 had sustained major damage, and in certain
locations was completely undercut and washed away.
With NC 12 being the vital lifeline of the Outer Banks, NCDOT needed
to act fast to get the highway operational. Making this task more
daunting were the logistics of getting construction materials and
equipment there by ferry. A plan was needed where a stable road
could be constructed by using a minimum amount of material hauled
to the site. Asphalt plants and sources of crushed stone were too
far away, and would involve too much hauling, which would not only
cause logistical problems, but would be extremely expensive.
NCDOT decided to use the material they had plenty of on site –
beach sand – and use portland cement to stabilize it into
a sturdy roadbed. They did not feel they were taking a risk in making
this choice, since a soil-cement base had been used when NC 12 was
previously reconstructed in 1991. The road had performed well over
the past 12 years, and survived several hurricanes (Emily in 1993,
Bertha in 1996, and Dennis and Floyd in 1999).
 Kevin
Sebold, a NCDOT Project Engineering Geologist based in Raleigh,
was on-site during the construction of the original soil-cement
base on Ocracoke in 1991. The decision to use soil-cement again
was “primarily based on economics, logistics, and the good
performance we’ve had from the old soil-cement base”
said Sebold. In fact, the destroyed sections of NC 12 had broken
up under Isabel because the entire roadway had been undercut and
fractured, not because of erosion of the soil-cement. This fact
is evident in viewing pictures of the road debris piles, where the
soil-cement is still intact and attached to the asphalt surface.
The road reconstruction project involved cement stabilization of
approximately 15,000 square yards of road base, built seven inches
thick. Seventy-two pounds of portland cement per square yard was
selected for construction, the same soil-cement treatment used when
the road was reconstructed in 1991. Of course, this necessitated
the hauling of nearly 525 tons of cement over the ferries, not to
mention the construction equipment.
Site Prep Inc. was contracted to perform the cement stabilization
work, but logistics was not the only obstacle.
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| The beach sand was difficult to work with, since
it was very unstable before being treated with cement, and water
encroached from both sides. |
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| Fresh water (available from the village of Ocracoke)
was used to moisten the sand before the application of cement.
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| The soil-cement was mixed with a CMI RS-425 (Reclaimer/Stabilizer),
followed by grading and compacting. |
A larger CMI RS-800 was used in those locations where sections
of the old pavement were still intact. In those sections, the old
soil-cement base and asphalt surface had to be pulverized with full-depth
reclamation, and re-treated with cement. That operation was slow
going, even for the 800-horsepower reclaimer.
The soil-cement base was topped with a bituminous surface treatment,
which temporarily completed the pavement structure. NCDOT will wait
until spring to pave an asphalt surface course, when better supply
routes will be operational. NC 12 will be perfectly functional until
then, providing the lifeline needed for the Outer Banks of North
Carolina.
For an additional CTB case study in Tampa Bay, Florida,
click here.
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