A load-deformation model for subgrade soils where total cumulative axial strains are correlated with applied stresses and number of repetitions is used to study the behavior of subgrade soils under repeated loads and to investigate the load repetition effects on subgrade modulus and shear strength. Predictions of allowable stresses for different limiting strains are also established, and limiting criteria in terms of applied stresses and number of repetitions to failure are compared with other subgrade criteria available in the literature. Model predictions indicate that the subgrade modulus increasesfor repeated stresses lower than a "threshold stress" and assumes aconstant value with increased number of load repetitions. For higher stresses, the modulus decreases, indicating a strain-softening behavior and a corresponding increase in the rate of accumulation of axial strains. On the other hand, cohesion and friction associated with failure under repeated loads decrease with increase in number of repetitions to failure. The variation of repeated stress level with number of repetitions to failure determined by the proposed model is compared with subgrade criteria suggested by other investigators. Stress predictions by using those criteria are generally conservative for number of repetitions greater than about 1, 000. For smaller number of repetitions, those criteria could be nonconservative. Moreover, permanent strain predictions by those criteria for number of load repetitions greater than 10, 000 could range from 20% to 25% of the permanent strains at failure as determined by the proposed model. This paper appears in transportation research record no. 1278, Dynamic testing of aggregates and soils and lateral stress measurements 1990.
Abstract