Permanent Deformation Behavior of Coarse-Grained Residual Subsoil Under Large Amplitude Loading Cycles

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Traffic-load-induced deformations in a subgrade occur due to long-term loading and unloading cycles. Residual soils with coarse-grained gradation have been extensively used as subgrade soil in the mountainous region of China. It is of practical value to study the permanent deformation behavior of coarse-grained residual soils subjected to different cyclic stress ratios (CSRs) under their optimal moisture content conditions. This paper presents an experimental investigation of the effect of the CSR on the accumulative deformation behavior of a residual soil as well as field tests on the traffic-load-influenced depths in a case subgrade. A series of large-scale monotonic triaxial (TX) tests and large-scale cyclic TX tests, wherein 50,000 loading cycles were applied, were conducted under optimal moisture content conditions. An increasing CSR increases the breakage of the coarse-grained particles. The values of the particle breakage under monotonic loading conditions are compared to corresponding values under cyclic loading conditions using the Hardin relative breakage index. For the cyclic TX tests, the long-term cyclic loading produces a >1.5-times greater Hardin breakage index compared to the values obtained from monotonic TX tests. The obtained experimental results show that an increasing CSR increases the cumulative strain but, in contrast, decreases the resilient modulus. The CSR primarily shapes the strain–stress curves obtained from the cyclic TX tests. Based on the laboratory results, a cumulative strain prediction formula, as well as a formula for the prediction of the steady resilient modulus, was established. The field tests were also involved in the case study on accumulative deformation prediction. Consequently, the traffic-load-influenced depths were identified, and an empirical method for predicting traffic-load-induced deformation was established for this residual soil subgrade. The magnitude of the CSR, the number of traffic-loading cycles, and the in situ confining pressure are considered in the empirical method. Good agreement between the predicted deformation and the measured deformation indicates the feasibility of this method. The intensity of dynamic stresses on the subgrade surface and the attenuation law of the dynamic stresses along with the subgrade depth were also discussed. The findings are expected to improve our understanding of the permanent deformation behavior of coarse-grained residual soil under optimal moisture content conditions when subjected to long-term traffic loading.