The Firm

Lime, fly ash blend into Pennsylvania’s mix of soil characters for stronger subgrade support

Larry Cole, Contributing Author / November 08, 2004

But when, after many years of good performance, complete pavement reconstruction is needed, it makes sense to rebuild the roadway with the best performing, longest lasting and most economical solutions that are available today. To that end, the Pennsylvania Department of Transportation (PennDOT) recently added soil stabilization using lime and fly ash to its repertoire of construction techniques.

As with many states, Pennsylvania has many miles of roadways requiring reconstruction. While roadway maintenance and rehabilitation efforts have extended roadway service well beyond the original design number of traffic loads, such roadways have reached the condition where complete reconstruction is warranted.

Complete roadway reconstruction involves removing the existing asphalt or concrete pavement and underlying base course material down to the natural soil—the pavement subgrade. Among other advantages, pavement reconstruction exposes the pavement subgrade, offering the opportunity to repair or improve the pavement foundation.

From the ground up

“The underlying subgrade is a major factor on many of our roadways that require reconstruction,” explained Dan Dawood, chief of PennDOT’s Pavement Design and Analysis Section. “In the past, we didn’t pay as much attention to the pavement subgrade during initial construction as we should have. Through reconstruction we have the chance to improve the pavement’s foundation. Subgrade improvement, coupled with the advances in pavement drainage and new asphalt and concrete pavement technologies, should make our reconstructed highways perform much better than the original construction.”

However, the varying geology and topography of the Pennsylvania landscape adds a level of complexity to PennDOT’s roadway reconstruction challenge. Even within a single project, the pavement design engineer and road-building contractor are likely to encounter a number of subgrade conditions.

“We see a lot of different subgrade soil types and soil conditions when we reconstruct or build a new roadway in Pennsylvania,” said Kerry Petrasic, PennDOT’s chief geotechnical engineer. “Sometimes we’re cutting through solid rock; other times the subgrade is soft and wet, with a high content of silts and some clays. It’s not uncommon to encounter a wide variety of soil types within a relatively short section of roadway.”

The fine-grained materials (silts and clays) can be problematic when used as pavement subgrade. Petrasic explained that, “due to their greater sensitivity to moisture and freezing, fine-grained soils do not provide the same long-term support to the pavement structure as coarser-sized sands and gravels. Fine-grained soils are also more challenging to work with during construction, but are unfortunately also the materials most commonly available for subgrade construction in Pennsylvania.”

Pavement reconstruction allows PennDOT to address these conditions once the old pavement and base course are removed and the subgrade is exposed. Soft, poor-quality subgrade can be improved or replaced.

Contractor’s choice

Recognizing the need and opportunity to improve the subgrade during pavement reconstruction led PennDOT to another decision—how to do it. Soft soil could be removed and replaced with more stable, granular material, or the existing soil could be chemically stabilized in place. On several recent pilot projects, including reconstruction of I-81 in Lebanon County, PennDOT left the options open by allowing the successful low bidder to select the subgrade improvement technique. The contractor could remove and replace the soft subgrade soil with a geotextile and 1 ft of stone or treat the top 12 in. of subgrade with lime and fly ash to stabilize the soil.

“Pennsylvania road builders have told us they like options, when possible,” explained Chris Cepko, PennDOT’s quality assurance team leader, Bureau of Construction and Materials, and chairman of the joint PennDOT-industry subcommittee that developed the pilot specification for subgrade treatment. “By allowing the contractors to choose between cut-and-replace and in-place soil stabilization, they can select the best option for the job conditions, the weather and their equipment and capabilities. The decision is usually based on cost, but the work schedule and maintenance of traffic issues can also be factors. The contractor is in the best position to decide which option is best, and the tool box concept gives them the choice.”

For the I-81 project, contractors were given the choice of removing and replacing the soft subgrade or chemically stabilizing it with lime and fly ash.

“Because we have quite a bit of variability in our subgrade material, we often find three or four soil types within a short stretch of roadway,” said Cepko. “For instance, on the I-81 job, we encountered four soil types within a 1-mile section. Most of it was a mix of clay, silt and some sand—not the best foundation for a long-lasting road. We can vary the amount of lime and fly ash for each soil type. On some soil we used straight lime, about 4% by dry weight of soil. On other sections we added fly ash with lime. We engineer the subgrade improvement by selecting the proper amount of lime and fly ash based on lab tests of the actual subgrade soil. A certified geotechnical lab samples and tests the subgrade soil after it’s been rough graded to determine the amount of lime and fly ash that’s needed.”

Sense and dollars

While it makes sense to improve the pavement foundation during roadway reconstruction, dollars also were a key factor on the I-81 project. A review of PennDOT’s publicly available bid tab shows the project low bidder, Allan A. Myers Inc., a company of American Infrastructure, submitted a bid of $5.22 per sq yd for subgrade treatment. For the same pay item, the average of the three low bidders was $4.59 per sq yd.

Allan A. Myers Inc. chose the lime-fly ash option and engaged E.J. Breneman LP of West Lawn, Pa., to spread and mix the stabilizing agents. E.J. Breneman specializes in soil stabilization and full-depth pavement reclamation and was well equipped to handle the job. E.J. Breneman retained the services of American Geotech Inc., Reading, Pa., to sample and test the subgrade soil to determine the proper amount of lime or lime and fly ash needed for soil stabilization. As expected, American Geotech encountered several types of soil along the length of the project, including low-plasticity clay, mixtures of silt and sand and mixtures of clay and sand. American Geotech customized the mixture of lime and fly ash for each soil, recommending lime-only for some sections and adding fly ash to the mix when needed to obtain the minimum strength requirements per PennDOT’s job specifications.

The lime and fly ash were spread onto the soil surface in separate operations, then thoroughly mixed with E.J Breneman’s CMI-Terex Model 650 reclaimer/stabilizer, with water added as needed. Following compaction and final grading operation by Allan A. Myers Inc., the subgrade is ready for aggregate base and concrete pavement.

“The I-81 project is a good example of the pavement reconstruction projects we’re undertaking at PennDOT,” concluded Kerry Petrasic. “Using in-place soil stabilization with lime and fly ash, we are stabilizing the subgrade material to provide greater strength, uniformity and stability and improved long-term performance. We expect the stabilized subgrade will extend the new pavement’s service life, being much less sensitive to changing moisture conditions and freeze-thaw action.

About the Author

Cole is a technical marketing manager, construction, for Carmeuse North America Services, Pittsburgh, Pa.

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