Dynamic Properties of Subgrade Soils, Including Environmental Effects (164-3)

Three fine-grained soils, varying in clay content between 20% and 70%, were tested in a unique repetittve loading apparatus to determine how soil suctionl temperature, and stress state affect the restlient modulus and residual strains expected under highway and railroad loadings. In developing equations to predict these dynamic properttes, three values of soil suction, stress intensity, and temperature were used in tests of each of the three soils in a statistically designed experiment. A fundamental change in the behavior of the tested soils from "effectively saturated" to effectively unsaturated" occurs at a soil suction corresponding to two percent dry of the optimum moisture content. The critical soil suction is directly related to the clay content of the soils. This relation has important implications for the climatic design and stablization of highway pavements and railroad grade crossings. Equations are developed for the resilient modulus and the residual strain for fine grained soils with clay contents within the range tested. The most important terms in the equations are the number of load cycles and the soil suction, although the other factors that enter·.into~the:'.,equations are degree~Df saturation, volumetric moisture content, volumetric soil content, deviator stress, and mean principal stress. Changes in the dynamic properties due to temperature variations from 72°F (22°C) are determined by a temperature correction factor for which another equation has been developed for each of the two dynamic properties. The powers and the constants of the equation for the resilient modulus temperature correction factor are related to the c1ay content and the plastic limit of the soil. The powers and the constants of the equati'on for the residual strain temperature correction factor are related to the clay content and the percent passing the #200 sieve. All of the equations have coefficients of determination above 0.50, which'is better than any other published results on these qynamic properties. The equations developed are to help the destgn engineer to incorporate the resilient modulus, residual strain and climatic conditions into design procedures for highway pavements and railroads, and especially in areas where the dynamic loading is important as, for example, in the case of intersections, railroad grade crossings, and bridge approaches.