SOIL-WATER POTENTIAL AND RESILIENT BEHAVIOR OF SUBGRADE SOILS

The special significance water has for pavement systems is that such structures are generally associated with soils close to ground surface, are subject to large moisture content variations, and are strongly influenced by environmental conditions. The object of this study is to establish experimentally the relationship between the strength and deformation characteristics under highway loading conditions and the soil moisture regime in terms of the soil-water potential (or soil suction). To do this, laboratory specimens of a mixture of a typical Wisconsin subgrade soil and 25 percent sand were compacted at optimum and plus or minus 2 percent of optimum moisture content. These were subsequently equilibrated to a range of suction values up to 1500 kPa (217.4 lbf/sq in) in a soil moisture extractor. The specimens were subjected to a repetitive uniaxial compression loading of 80 kPa (11.6 lbf/sq in) at a frequency of 0.5 cycle/s up to 10,000 loading repetitions. After the computed values of resilient modulus and residual strain were studied as a function of the moisture parameters, it was concluded that soil suction is the fundamental parameter in characterizing the moisture state and its effects on the mechanical behavior of soils. For the silt loam soils (low-plasticity clays) considered, compaction at dry-op-optimum moisture content results in subgrades more susceptible to moisture changes. There appear to be significant advantages in taking suction-related improvements in the mechanical properties into account if the regional climatic conditions are to ensure a suitable range of moisture index. However, there seems to be a limit to the increases in resilient modulus with increasing soil suction because the resilient moduli decrease for suction values greater than a critical value. /Authors/