The resilient modulus has been used to characterise the stress-strain non-linear behaviour of subgrade soils and is an important parameter for pavement design and analysis. This study aims at developing an accurate and efficient methodology to estimate the resilient modulus of subgrade soils. Therefore, repeated load triaxial tests were carried out in this study. The soil matric suction was measured by the pressure plate test and the soil-water characteristic curve (SWCC) was described using the Fredlund & Xing model. Then, the influence of stress state, matric suction and relative compaction on the resilient modulus were analysed. Subsequently, a new resilient modulus model was proposed to incorporate the relative compaction and matric suction in addition to the stress state. The new model matched 7 sets of experimental data well and the coefficients of determination were high, which indicates that this new model is reasonable and widely applicable. Finally, the correlations between the basic properties variables of soil samples and the regression coefficients of the new model were established.

The cumulative permanent strain of subgrade soils plays an important role in the pavement structure design and long-term pavement performance maintenance. In Southern China, the hot and humid climate increases the risk of becoming a saturated condition for subgrade soils. The saturated subgrade soils will generate significant cumulative permanent strain under long-term cyclic loading compared with unsaturated soils. This study investigated the impact of the moisture content, number of cyclic loading, deviatoric stress and confining pressure on the evolution of the cumulative permanent strain for subgrade soils by using long-term cyclic (10,000 cycles) triaxial tests. The test results indicated that the cumulative axial permanent strain increase with the increasing moisture content, as well as the increasing number of cyclic loading and deviatoric stress but decreases with the increasing confining pressure. In order to predict the permanent deformation of subgrade soils in Southern China, a new model for predicting the permanent deformation of subgrade soil, which contains number of cyclic loading, initial bulk stress, octahedral shear stress and saturation, was proposed and validated.

This study aims at developing an efficient and accurate methodology to estimate the resilient modulus of subgrade soils. First, a new resilient modulus model incorporating stress dependence and moisture dependence was proposed. Second, prediction models were developed to conveniently and accurately determine model parameters of SWCC and resilient modulus model. In order to characterise the moisture dependence of subgrade soils, the matric suction was added into the proposed model. The matric suction was measured by the pressure plate test and the soil-water characteristic curve (SWCC) was used to determine the matric suction value at any given moisture contents. In order to develop prediction models for model parameters of SWCC and resilient modulus model, the laboratory experiments and multiple regression analysis were conducted on 22 soil samples. A series of performance-related soil properties were measured and used to develop the coefficients prediction models. The developed coefficients prediction models using the performance-related soil properties have high R-squared values and were validated by comparing the measured and predicted values of resilient modulus. Therefore, when the basic physical properties of soils were obtained, the resilient modulus can be predicted for the subgrade soils at any given matric suctions and stress states.

In order to predict the resilient modulus of compacted clays, the material indicator tests, repeated load triaxial test, and pressure plate test for compacted clays from South China were carried out. The soil–water characteristic curve (SWCC) was described using the Fredlund and Xing’s model. And a logarithmic function between the soil suction and resilient modulus was built. Then, a new variable named the minimum bulk stress was defined to separate the shear effect of soil samples from the bulk stress, which avoids the bulk stress reflects two contrary effects, namely hardening effect and softening effect. Subsequently, the influences of the degree of compaction, soil suction, minimum bulk stress, and octahedral shear stress on the resilient modulus were analysed. In the following, a new resilient modulus prediction model of compacted clays, which took the soil suction, minimum bulk stress, and octahedral shear stress as the model variables, was developed and verified using the data of different cohesive soils from this study and existing literatures. The results show that the new model matched these data well and have a high prediction accuracy, which indicates that this new model is reasonable and widely applicable.

The resilient modulus has been used to characterize the stress–strain nonlinear behavior of subgrade soils and is the primary property needed for pavement design and analysis. The degrees of stress and moisture content have a significant impact on the resilient modulus of compacted cohesive soils. Due to the hot, humid climate in South China, the moisture content in embankments will increase gradually from the optimum moisture content (OMC)—the design water content for embankments—to the equilibrium moisture content, which relates to soil properties and the surrounding climate. In this process, the resilient modulus of embankments will obviously decrease. In order to predict the resilient modulus of typical compacted cohesive soils in South China, repeated triaxial tests were carried out in this study. The soil matric suction was measured by the pressure plate test and the soil–water characteristic curve (SWCC) was described using the Van Genuchten model with a relatively high coefficient of determination. Then, the effect of the moisture content, degree of compaction, and stress state including the deviator stress, confining pressure, and octahedral shear stress to the resilient modulus were analyzed. In addition, a logarithmic function was utilized to build the relationship between the matric suction and resilient modulus. Subsequently, a new resilient modulus estimation model of compacted soils in South China, which took the bulk stress, octahedral shear, and matric suction as the model variables, was developed and verified using the data of different cohesive soils from different studies. The results show that the new model matches their data well, and the coefficients of determination are high, which indicates that this new model is reasonable and widely applicable. Finally, the correlations between the physical parameters of soil samples, such as the liquid limit, dry density, the maximum dry density, plasticity index, percentage passing through a 0.075-mm sieve, and the regression coefficients of the new model, were established. The resilient modulus can be predicted much more easily with these physical parameters of compacted cohesive soils rather than conducting triaxial tests.