Permanent Deformation Characteristics of Coarse Grained Subgrade Soils Using Repeated Load Triaxial Tests

The resilient modulus (M_R) is used to represent the subgrade soil stiffness and is one of the key parameters for modeling pavement permanent deformation and subgrade rutting in the mechanistic-empirical pavement design guide (MEPDG). However, soils having good resilient modulus may or may not have small permanent strains under repeated loading. Therefore, it is necessary to study both the resilient and permanent strain characteristics of subgrade soils under repeated loading. In this study, repeated load triaxial tests were performed following AASHTO T307 on remolded soil samples collected from different regions of South Carolina. The samples were prepared at optimum moisture contents (w_opt) and ±2% w_opt. Resilient modulus and permanent strains of subgrade soils were measured under different repeated deviatoric loads and confining pressures. Statistical models were developed to correlate resilient modulus model parameters (k₁, k₂, k₃) and permanent strain model parameters (α₁, α₂, α₃, α₄) with soil index properties. Results showed that the compaction and optimum moisture content (w and w_opt), percent passing No. 4 sieve (P₄), maximum dry density (${\gamma }_{d_{max}}$ ), uniformity coefficient (C_u), liquidity index (LI), and specific gravity of soil (G_s) have statistically significant effects on the resilient modulus model parameters and the permanent strain model parameters for South Carolina coarse grained soils. The correlation between M_R and permanent strain suggests that M_R is a satisfactory soil property to explain permanent deformation or rutting characteristics for the South Carolina soils studied herein. Thus, permanent deformation for these soils can be predicted from index properties using MEDPG with the developed resilient modulus model, or directly using the developed permanent strain model.