The resilient modulus value (3, 000 psi) used to represent the AASHO Road Test subgrade in the AASHTO flexible pavement design equation is examined. The subgrade modulus for pavements designed by the AASHTO Guide for Design of Pavement Structures must be consistent with this value for the guide to be used correctly. For cohesive soils, it is shown that this requires that laboratory values be based on tests conducted unconfined using a deviator stress of 6 psi. For backcalculated modulus values from cohesive soils to be consistent with the guide, the backcalculated modulus must be multiplied by a factor no greater than 0.33. Unmodified backcalculated values are unconservative. Laboratory values are also unconservative when the tests are conducted with a confining pressure and deviator stresses less than6 psi. However, if the subgrade soils are noncohesive, it is not clear what the laboratory test conditions need to be or whether backcalculated Mr-values need to be adjusted to be consistent with the AASHO subgrade value. Appropriate values for noncohesive soils need further study. (Author/publisher)
Abstract