By: Larry Cole, Contributing Author
Defying common construction limitations, the Indiana
Department of Transportation (INDOT) started a large excavation and embankment
project in the dead of winter. With 24-hour operations and by chemically
treating the embankment soil, the construction team, led by Walsh Construction,
Chicago, successfully placed over 1 million cu yd of fill, often in sub-freezing
temperatures. This remarkable effort kept a key project on schedule, improving
both the highway and air transportation system around the Indianapolis
International Airport.
Freeze-thaw holdup
Winter's freezing temperatures usually mean the end of a
season for earthwork and embankment projects. Typically, roadway embankments
and other engineered fills are built by moving earth to the embankment
location, placing the material in layers of controlled thickness, then
compacting the soil to the specified density. Layer upon layer of soil is
placed and compacted until the embankment reaches the desired elevation. Soil
moisture is monitored and controlled to ensure proper compaction.
Most highway construction specifications do not allow for
soil to be placed and compacted when the underlying soil is frozen. In northern
states, where the winter air temperature drops below freezing for many
consecutive nights, embankment work must stop until warmer weather arrives.
Months may be lost while the contractor waits for the soil to thaw. Spring's
warmer weather often brings rain, further delaying earthwork as the embankment
material becomes too wet for proper compaction. More weeks pass until earthwork
can finally get under way.
Lime aid
In late 2002, INDOT began construction on a significant
project to relocate a section of I-70 near the Indianapolis International
Airport. INDOT, the Indianapolis Airport Authority and the city of Indianapolis
jointly fund the project. Because the project has such a significant impact on
the region's transportation system, the stakeholders opted to accelerate the
construction process so that the project can be opened to traffic in December
2004.
After considerable planning and constructability review, an
overall project schedule was established to meet the desired completion date.
One of the first steps was getting the necessary drainage and grading work done
so that construction could begin on certain highway structures. Much of the
roadway embankment material needed to be placed by May 2003. This meant that a
large amount of excavation and embankment construction had to be done during
the winter months. In total, over 1 million cu yd of soil needed to be
excavated, placed into new embankments and properly compacted during a time period
when the air temperature routinely fell well below freezing. Additionally, much
of the embankment material was overly wet and could not meet the moisture
content requirements for proper compaction.
After evaluating several options INDOT, in cooperation with
consulting geotechnical engineers Professional Service Industries Inc. (PSI),
Indianapolis, determined that the required embankments could be built during
the winter months using soil excavated from other locations within the project
boundaries. To do this it would be necessary to:
* Remove frozen soil from the surface of the excavation,
exposing unfrozen soil;
* Excavate unfrozen soil;
* Transport the unfrozen soil to the embankment site;
* Spread and dry the soil to acceptable moisture levels;
* Compact the soil; and
* Cover the soil with another layer of soil before freezing
could occur.
This would require two construction techniques:
* Continuous earthwork operations 24 hours per day, seven
days per week; and
* Use of chemical agent to lower the soil moisture content
when the soil was overly wet.
A variety of chemical agents, including quicklime, hydrated
lime, lime kiln dust, fly ash and cement, can be added to soil to lower its
moisture content. Based on PSI's recommendation and after consulting with
earthwork contractors and soil stabilization specialists in the area, INDOT
chose lime kiln dust for this project. Lime kiln dust (LKD) is a lime product
collected by filtering the exhaust gases from rotary kilns used to manufacture
lime.
As would be expected, a variety of soils with different
characteristics and in-situ moisture contents were encountered during I-70
embankment construction. Tests showed that, in most cases, 3% LKD by dry weight
of soil would do the job, with more lime applied to areas that were
particularly wet.
As with all chemical agents used for improving soil in
embankments and other engineered fills, lime products should be evaluated on a
case-by-case basis to ensure they will give the desired results.
Quicklime and LKD cause a number of things to happen that
affect soil moisture and compaction. Water is absorbed as CaO becomes Ca(OH)2.
This process is called "hydration" and occurs quite rapidly.
Hydration lets off heat, driving off water through increased evaporation. On
the I-70 project, the heat of hydration had the additional benefit of warming
the soil, allowing more time for the contractor to compact the treated soil and
cover the layer with more soil before it could freeze.
Additionally, lime and LKD change the soil characteristics
so that more moisture is needed for compaction--the optimum moisture increases.
Figure 1 illustrates this effect for one soil sample from the I-70 project, a
lean clay loam composed of 27% clay, 44% silt and 27% sand.
Lime and LKD also will reduce the plasticity of soil
containing significant amounts of clay. Reducing the plasticity of clay soil
makes it easier to compact.
Still on 35
Construction began on Dec. 26, 2002, and continued 24 hours
per day, seven days per week, with only a few stops when snow or rain forced a
temporary shutdown. Continuous construction lasted from January until early
April. By early April, the freshly placed layers of soil would not freeze
during the night.
Walsh Construction led the construction effort, serving as
general contractor and doing much of the earthwork. Walsh teamed with Koester
Contracting Corp., Evansville, Ind., to excavate the embankment material, haul
it to the embankment location and spread the soil. Two soil stabilization
specialists--Mt. Carmel Sand and Gravel, Mt. Carmel, Ill., and Specialties Co.,
Anderson, Ind.--spread and mixed the lime material into the soil. Walsh
Construction or Koester Contracting then compacted the soil, grading as
necessary for the next lift. INDOT technicians were on hand to ensure the
compacted soil met project specifications and to monitor soil temperatures to
see that frozen soil was not incorporated into the embankments.
The construction team worked throughout the winter, facing
sub-freezing temperatures much of the time. One night, the air temperature
dropped to -17°F, and on many days the air temperature did not rise above
freezing.
INDOT established reasonable but stringent standards to
ensure that the embankment material did not freeze during construction. In the
excavation area, any frozen soil was removed to a depth where the soil
temperature was at least 35°F. The frozen soil was stockpiled for later
use. The unfrozen material was then excavated, hauled to the embankment site
and spread to a thickness of not more than 12 in. The lime material was quickly
applied and mixed into the soil. Lime hydration caused the soil temperature to
rise and reduced the soil moisture content to within specified limits. The
treated soil was compacted and the process was repeated before the treated
layer dropped below 35°F.
INDOT technicians monitored the soil temperature using
hand-held noncontact temperature sensing devices, manufactured by the Gilson
Co. On some occasions, snow caused the construction to stop--in one case for
three days. When construction could resume, the snow and frozen soil was
removed until soil at 35°F or greater was uncovered. Construction then
began again.
The earthwork contractors and stabilization specialists carefully
coordinated their work to ensure that the soil temperature did not fall below
35°F. This meant limiting the amount of exposed soil, particularly when
very low temperatures were expected. In the most severe conditions, the team
found that lifts had to be completed within three hours, limiting the work area
to 2,000-4,000 sq ft.
By May, over 1 million cu yd of embankment had been built to
support key highway structures. A review of INDOT's bid tabulations shows the
following construction costs:
* Common Excavation: $3.76 per cu yd. This included soil
removal, hauling to embankment location, soil spreading and compaction; and
* Lime: $60 per ton. This included the cost of the delivered
material, lime spreading and soil mixing.
About The Author: Cole is technical marketing manager, construction, for Carmeuse North America Services, Pittsburgh, Pa.
