Key Lime

Sept. 25, 2003

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.

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.