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Full-Depth Reclamation Solves Construction
Problems in Wet and Weak Soils
by Katherine M. Dingman, Harbor Consulting Engineers, Incorporated
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| Failed pavement surfacing due to excessive
loading. |
Terminal 115, located just south of Seattle, Washington, is occupied
by Northland Services, Incorporated – the Port of Seattle’s
fourth largest tenant. On average, 57,000 tons of cargo is pushed
through the Terminal each month. When rehabilitation became necessary
for this busy yard, special consideration was given to the alternatives
assessed before deciding on a paving and pavement foundation method.
A method of pavement rehabilitation known as full-depth reclamation,
or FDR, proved to be the foundation method of choice as it provided
greater strength and durability than most of the other pavement alternatives.
FDR is a process in which the existing asphalt surface is pulverized
in place and blended with the underlying base, subbase, and/or subgrade
materials, mixed with portland cement, and compacted to provide
a new stabilized base. A new surface course is then applied, which
completes the FDR process, providing a new pavement structure using
recycled materials from the failed pavement. For Terminal 115, FDR
was found to be the most cost efficient solution – especially
when working through Seattle’s naturally wet environment.
Terminal 115 has a total yard space of 65 acres, with heavy lifting
throughout and nearly 60 of those acres in need of repaving. A 3.5-acre
FDR trial repaving project took place in March and April of 2005.
During the six weeks of construction, eight inches of rain fell
in the Seattle area. Because of Seattle’s heavy precipitation,
it is important that marine terminals carrying heavy loads be properly
drained and resist pavement deflection. A properly designed FDR
pavement was selected for this project as it could distribute large
wheel loads to weak subgrade soils, maintaining a stable platform
for many years. At Terminal 115, heavy equipment had destroyed the
bituminous pavement surface and caused foundation deterioration.
These failures led to safety and operating concerns as well as damage
to equipment.
A section of the existing asphalt surfacing, stone base, and subgrade
soils were pulverized and then blended together. Samples of this
material were taken to a local materials laboratory where their
maximum dry density and optimum moisture content were determined.
Test specimens at 8, 10, and 12% portland cement by dry weight of
this material were prepared, cured, and tested for unconfined compressive
strength. This laboratory testing showed that 9% cement, incorporated
into the pulverized and blended material, produced a consolidated
base with a maximum dry density of 144.5 pcf at 9.0% moisture, exceeding
the 3-day design strength requirement of 650 psi.
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| New pavement next to FDR at Terminal 115. |
Using information supplied by the Portland Cement Association,
pavement design engineers recommended the use of 16 inches of FDR
covered with six inches of new asphalt surfacing. M & M Road
Recycle, Incorporated out of Redmond, Washington performed the pulverization
and incorporation of cement while S.C.I. Infrastructure, LLC out
of Pacific, Washington completed the compaction and grading. An
initial trial area used approximately 1,300 tons of cement –
including some for wet weather modification of the subgrade soils.
Because all design strength requirements were met or exceeded, an
additional 15-acre site was proposed for rehabilitation using FDR
and is anticipated to use approximately 6,000 tons of cement for
FDR including allowance for adverse weather construction.
In anticipation of wet weather, additional laboratory testing was
conducted to ensure proper strength with increased cement content
and higher soil moisture. This effort proved valuable during the
spring of 2005. Testing determined that adding cement controlled
the excess moisture and increased foundation strength, thereby allowing
construction to continue during the adverse weather conditions.
Exposing small areas to rainy weather prior to constructing the
FDR helped to control foundation saturation and optimized cement
utilization.
Minimal tenant disruption was essential. FDR saved both time and
money. It solidified and stabilized the pulverized asphalt surfacing,
stone base, and subgrade soils, and eliminated the need to export
unusable materials. A reconstruction process that would have taken
months was reduced to mere weeks, and the tenant resumed a new efficient
operation quickly. By phasing and pre-staging construction, only
small portions of the Terminal yard were removed from the tenant’s
operation at any given time.
The key to success at Terminal 115 was full time inspection and
ongoing laboratory analysis. By combining the laboratory results
from previous years including in-place testing and preparing additional
laboratory specimens throughout construction proved much larger
areas could be reconstructed than other options would allow. There
can be little doubt that the use of FDR accelerated this project
and reduced the downtime of the Terminal.
For further information, please contact:
Katherine M. Dingman
Harbor Consulting Engineers, Incorporated
3006 Fuhrman Avenue East
Seattle, Washington 98102
(206) 709-2397 PHONE
(206) 709-2398 FAX
kmdingman@harborengineers.com
E-MAIL
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