Contending with Louisiana's silty soils

LTRC research aids DOTD operations

By Mark Morvant

Silty soils are a fact of life in Louisiana. As a result, the state's Department of Transportation and Development (DOTD) engineers are constantly dealing with construction and performance problems associated with wet sub-grades.

However, an extensive laboratory testing program sponsored by the Louisiana Transportation Research Center (LTRC) has provided a means to better identify and stabilize soils with high silt percentages. The University of New Orleans (UNO) conducted the laboratory testing, and LTRC performed the field verification program. UNO's Kenneth McManis, Ph.D., P.E., and Mysore Nataraj, Ph.D., P.E., were the principal investigators.

advertisement

When these high-silt soils are wet, they exhibit low strength and minimal bearing capacity, and construction traffic on these soils can cause detrimental pumping action. DOTD has addressed this problem in two distinctly different situations. In the past, identification of these silty soils, whether under existing pavements or in locations where pavement would be constructed on natural ground, has not been effective. Therefore, contract provisions have often included sub-grade lime treatment at the discretion of the project engineer.

In some cases, expensive plan changes for excavation and replacement have also been necessary. DOTD has also attempted to minimize the problem silts' effects on pavement constructed on embankments by prohibiting soils containing more than 65 percent silt in its standard specifications, but this requirement has not effectively solved the problem either.

In addition to documenting the field experiences of DOTD, an elaborate testing program investigated the nature of the pumping problem, the character of silt materials, and their performance with stabilizing agents. Soil samples from current projects experiencing pumping problems were gathered from four DOTD districts for the laboratory program.

Standard laboratory tests identified the basic characteristic-parameters of the natural samples. The response and stability of the silts under compaction and loading with various moisture levels and compaction efforts were also tested. Susceptibility to pumping was reviewed in terms of the physical characteristics of the samples. In addition to the silt content percentage, the plasticity character was noted as significant during testing.

Anomalies were also found between DOTD's earthwork specifications and the physical properties of the high silt-content soils. Results indicate that soils containing greater than 50 percent silt which also have a plastic index less than 10 have a high potential to pump under high moisture conditions.

The potential for stabilization of the problem silt soils was also studied. The laboratory tests were selected with construction needs and possible post-construction conditions in mind. A limited number of specific additives were proposed based on their ability to dry the sub-grade silts sufficiently so that the silts could be compacted with the strength to support the construction of the base and pavement. Limited tests were also conducted to evaluate long-term stability of the stabilized silt sub-grade exposed to accelerated curing followed by vacuum-saturation conditions.

Cyclic triaxial testing was performed on wet soil samples stabilized with varying percentages of Portland cement, Portland cement/slag mixture, lime, and lime/fly ash mixture. While results indicate that lime stabilization reduced pumping potential, Portland cement and Portland cement/slag stabilization provided the greatest strength increase with negligible pumping.

DOTD standard specifications for usable soils have been changed to eliminate the high-pumping-potential silt soils from its roadway embankments. New guidelines established to better identify pumping potential will enhance design capabilities and result in more accurate plans, thereby reducing costly plan changes and project cost overruns.

These research results have been incorporated into the pavement sections of several projects including LA 182 in St. Landry Parish and US 171 in Beauregard Parish. LTRC is continuing to work with DOTD field personnel to further refine the design and stabilization process.

Test sections will be constructed with different chemical stabilizers in a variety of field conditions in order to monitor pavement performance. The cost benefits of providing stronger foundations will be further quantified with the next experiment to be conducted at the LTRC Pavement Research Facility. A cement-treated sub-base section will be tested under accelerated loading conditions.

The final results of this research are expected to provide significant savings to the taxpayer by providing longer-lasting and more durable roads.

Editor's Note: Mark Morvant has 22 years of experience with DOTD in the area of pavement and geotechnical engineering. His last seven years have been at the Louisiana Transportation Research Center in Baton Rouge. He is currently the LTRC pavement & geotechnical research administrator.

Resource Info

For more info about LTRC research initiatives, go to:
http://www.ltrc.lsu.edu/research.html