Characteristics of Permanent Strain Accumulation and Resilient Modulus of Unsaturated and Structured Soil under Repeated Loads

Intact soils commonly exist in pavements constructed in cuts or on-grade (i.e., without filling or cutting). Its inherent structure can significantly influence engineering behavior under repeated loads. So far, previous studies on structure effects in this context have focused on saturated conditions, although subgrade soils are often unsaturated in the field. This study investigated the permanent strain accumulation and resilient modulus of compacted and intact loess under repeated loads, considering various suctions and confining pressures. The results show that the effects of soil structure on permanent vertical strain can be categorized into two types based on the strain increment at the first cycle, δϵz,1p. Type I represents that the intact specimen exhibits a smaller δϵz,1p than the compacted specimen, and Type II refers to a larger δϵz,1p in the former. As the number of cycles increases, the intact specimen in Type I may have a larger strain increment due to structure degradation. In Type II, the strain increment of the intact specimen may become smaller because of failure in the compacted specimen. Moreover, the resilient modulus of the intact specimen is consistently larger than that of the compacted specimen, with the difference diminishing at higher confining pressures and deviatoric stresses and lower suctions, where the intact specimen experiences more significant structure degradation.