Assessing the Extent of Pavement Deterioration Caused by Subgrade Volumetric Movement Through Moisture Infiltration

Abstract

Pavements across Free State in South Africa often experience seasonal moisture variations that trigger, volumetric movement of the subgrade soil. The interaction of moisture-suction relationship and associated cyclic loads also contribute to pavement failure across this region. The current study investigated the failure cause of existing pavement structures constructed on expansive subgrades in the Free State province of South Africa. Material samples were collected from three representative sites by core drilling. Series of consolidated undrained triaxial tests (CU), free swell index (FSI) tests, zero swelling tests (ZST), filter paper tests, and unsaturated California bearing ratio ( $${\mathrm{CBR}}_{\mathrm{u}}$$ ) tests were performed to investigate causes of pavement failure through the principles of unsaturated soil mechanics. The results revealed that the investigated pavements deteriorate by swelling of the subgrades. The findings also evaluated that traffic loads significantly affected pavement structure as resistance failure to moisture were triggered from the asphalt layer. The asphalt thickness layer failed to provide sufficient surcharge pressure to confine swelling stress from the subgrades. The result further revealed that the $${\mathrm{CBR}}_{\mathrm{u}}$$ and matric suction decreased by 52.33% on average, with an increase in swelling stress as moisture content increases. The study suggested that seasonal moisture variations associated with excessive traffic load could be the main cause of pavement failure, as its pavement was designed for low to medium volume road users. In comparison, swelling stress values from the dry side were 8.63% higher than that of the swelling stress values obtained from the wet-side. The result further revealed that swelling stress was the major cause of the pavements' failure. Thus, as swelling stress increased with an increase in moisture; this consequently led to a considerable decrease in shear resistance and bearing strength capacity of the subgrades.