Civil Engineering Association

IDENTIFICATION AND STABILIZATION METHODS FOR PROBLEMATIC SILT SOILS

Many areas of Louisiana consist of soils with high silt contents, low strengths, and minimal bearing capacity. Construction traffic in these soils can cause detrimental pumping action in areas with a high water table. These wet subgrades under Louisiana pavements cause both construction and performance problems. Common solutions to the problem include excavation and replacement, lime treatment, or cement stabilization. Special provisions are often included in the contract for chemical additives in lieu of undercutting. The research emphasis of this study was placed on efforts to refine the pumping problem and on the development of guidelines for identifying the problem silt-soils. Secondary importance was given to the identification of alternate methods for stabilization. The study consisted of two phases. Phase 1 documented the field experiences of the Louisiana Department of Transportation and Development (DOTD) districts. Phase 2 consisted of a testing program to investigate the nature of the problem, the character of the silt materials, and their performance with modifying and stabilizing agents. Eight soil samples from four of the DOTD districts were used in the laboratory program. The soils were typical examples of those commonly encountered with a high-silt content. Several were acquired from current projects in which pumping problems were occurring. The basic characteristic-parameters of the natural samples were determined with standard laboratory tests. The response and stability of the silts under compaction and loading with various moisture levels and compaction efforts were also tested. The susceptibility to pumping of the different samples was reviewed in terms of their physical characteristics. In addition to the silt content percentage, the plasticity character was noted as significant during testing. Anomalies were also found to exist between the DOTD's earthwork specifications and the physical properties of the high silt-content soils. The potential for the modification and stabilization of the problem silt soils was also studied. The laboratory tests were selected with respect to construction needs and possible post construction conditions. A limited number of specific additives were proposed with consideration for their ability to dry the subgrade silts sufficiently in order that they be compacted and with the strength to provide a working table for the construction of the base and pavement. The additives selected included hydrated lime, portland cement, and Class C fly ash. Limited tests for evaluating long-period stability of the stabilized silt-subgrade subjected to accelerated curing followed by vacuum-saturation conditions were also conducted.