Prediction of the resilient modulus of two tropical subgrade soils considering unsaturated conditions

The resilient modulus (M-R) is essential for the understanding of pavement response to climate effects, and to seasonal fluctuations of water content and matric suction. Unfortunately, only few studies have been directed towards the unsaturated resilient behavior of tropical soils and the influence of weathering and degree of laterization. This paper presents a study of the unsaturated resilient behavior of two tropical soils with distinct degrees of laterization, used as subgrades of highways in Brazil. The experimental program involved the measurement of the soil-water characteristic curve (SWCC), and the M-R at different water contents and matric suctions. Eight models for M-R, taken from the literature, were evaluated, and a new family of equations is proposed. The obtained SWCCs of the lateritic soil presented bimodal behavior, while the non-lateritic soil is unimodal. The bimodal behavior was attributed to clay aggregates produced by the laterization process. The observed relationship between M-R and the stress-state variables depended on the degree of laterization. The lateritic subgrade soil showed significantly higher values of M-R, but the resilient behavior was more sensitive to the water content when compared to the non-lateritic soil. The analysis of the literature models for M-R indicated limitations in the manner how the unsaturated behavior was incorporated, producing adjusted coefficients of determination (Adj. R-2) between 0.48 and 0.77. The newly proposed M-R model was based on the statistical analyses of seven equations. The final proposed model includes two independent stress-state variable and a third state variable, that accounts for the water content deviation from optimum conditions. Finally, the proposed model was validated using M-R data from the literature. The proposed model produced superior results when comparing to the previously proposed equations, resulting in values of Adj. R-2 varying between 0.93 and 0.99. The newly proposed model proved to be flexible, being capable of modeling both the lateritic and non-lateritic soils in an accurate manner. These results are expected to offer significant improvements to the Brazilian mechanistic-empirical pavement design guide, and for similar guides in other countries where tropical soils are found.