Saturation effects on the resilient response of fine-grained soil

The 1993 AASHTO Guide for the Design of Pavement Structures uses resilient modulus to characterize the response of subgrade soils under vehicular loading. In order to determine a design resilient modulus, or effective roadbed resilient modulus, the pavement designer must be able to estimate variations in resilient modulus due to seasonal moisture change. A laboratory investigation was performed on eight different Tennessee soils to determine which types of soils are most susceptible to decreases in resilient modulus with increasing moisture content, and to obtain relationships describing these decreases. The soils were tested in a cyclic triaxial testing device using the SHRP Protocol P46 procedure. Samples were prepared using a kneading compaction techniques at optimum moisture content and maximum dry density. In addition, some of the samples were later subjected to post-compaction saturation. The decrease in resilient modulus with increasing saturation was shown to follow a somewhat linear relationship. Methods to predict reductions in resilient modulus were proposed using the average slopes of these relationships. The same eight soils were tested using the existing Alternative Test Method (Drumm et al. 1993). Samples were prepared at optimum moisture content and maximum dry density using the Standard Proctor procedure, with slight modifications. The ATM test results were compared with cyclic triaxial test results for the same soils at similar moisture-density conditions. For most of the soils, the difference in ATM test results relative to triaxial results was less than 25%.