Investigation of factors affecting the behaviour of subgrade soils resilient modulus using robust statistical methods

When designing pavements, there are three fundamental external design parameters to evaluate (1) the characteristics of the subgrade upon which the pavement is placed, (2) the applied loads and (3) the environment. This study is based on laboratory testing of cohesive subgrade materials to characterise their resilient modulus properties and to identify the main controlling factors of this property. Experimental results showed a slight increase in resilient modulus values with an increase in confining pressure and a noteworthy decrease in resilient modulus accompanied by an increase in deviatoric stress. This finding was verified statistically. Based on the independent samples t-test analysis, it was revealed that soil type and water content caused statistically significant difference in resilient modulus values. According to one-way ANOVA, deviatoric stress turned out to be influential while confining pressure did not. In addition, it was demonstrated using the two-way ANOVA that none of the two-factor combinations was significant in changing resilient modulus values of A-4a and A-6a soils. Finally, three-way ANOVA showed that changing the deviatoric stress, soil type and water content all together had a noticeable effect on resilient modulus behaviour. Furthermore, all predictive models were found statistically significant at the 0.05 significance level with high predictive power. Confining pressure and deviatoric stress are the two predictors whereas bulk stress was not included by the stepwise regression method. The non-linear regression relatively enhanced the predictive power of all the models.