Assessment of the Permanent Strain Responses of Subgrade Materials

Pavement foundation stiffness is a critical parameter for pavement design and improving pavement durability. When the subgrade soil is subjected to repeated traffic loading, the initial nonlinearity behavior and permanent strain of subgrade per load repetition are high. As the number of load repetitions increases, the subgrade strain decreases, and it begins to behave more elastically. This condition allows for a more accurate measurement of the resilient modulus (M_R) value of subgrade soils. The stress-strain behavior of the soil depends on several factors such as the index properties, stress history, and confining pressure. In this study, the stress-strain responses of sand and fine soils are thoroughly investigated via laboratory-resilient modulus tests, and the shakedown theory was adopted to evaluate the stress-strain response of four subgrade soils. A total of four different subgrade soils were tested. The results showed that the sandy soil exhibited a quasi-brittle behavior characterized by sudden failure when the applied stress exceeded the recoverable strain. The resilient behavior of this material was highly dependent on the confining pressure. The fine materials showed stress-softening behavior and were less dependent on confining pressure. The fine soil exhibited the behavior of over-consolidated soils when the material was subjected to cyclic loading at low confining pressure. The analysis of permanent strain revealed that the summary resilient modulus (SM_R) measured for the soils with the lowest fines contents did not accurately reflect the SM_R of the material. This material continued to exhibit high permanent strain when the SM_R was measured. This suggests that the initially calculated SM_R may not be indicative of the material’s stiffness.