| This investigation consisted of a laboratory study designed to predict the effect on the performance of flexible airfield pavements of reducing current density criteria for compacted subgrade soils. Laboratory prepared specimens of three subgrade soils, a silty clay, a plastic clay, and a silty sand, compacted at three different densities, were subjected to repetitive axial loading in a triaxial chamber. Maximum repetitive axial stresses were 12.5, 9.5, and 7.0 psi. Permanent axial strain response was observed and a multiple strain model was developed from the laboratory data. The multiple model involves a spline function relating the permanent axial strain and number of load repetitions and other regression equations relating the coefficients of the spline function to certain soil properties and stress conditions. Independent variables used in the regression analyses on the coefficients were: soil density; ratio of soil water content to density; ratio of maximum repetitive axial stress to compressive strength; slope of a plot of maximum dry density versus compaction energy used to obtain that density; and percent by weight of soil (as indicated by hydrometer analysis) having a grain size smaller than 2 microns.;Using the multiple model, predicted values of total subgrade deformation at 70,000 load repetitions were made for various density combinations of the soils tested. Based on a limiting subgrade deformation of 0.5 in., it generally appeared that the minimum allowable density range (based on the maximum ACTM D-1557 density) for the silty clay, plastic clay, and silty sand would be about 85-87, 79-80, and 92-94 percent, respectively. However, there were indications that, due to the use of low confining pressures with the sand specimens, the response of this soil may have been biased unfavorably and that, in an actual subgrade environment, this material would perform better than indicated. |
