Swelling stress effects on shear strength resistance of subgrades

Abstract

ABSTRACT Pavements across Free State in South Africa deteriorate as a result of swelling and shear failure of the subgrade materials, and cyclic loads associated with moving vehicles. This study investigated the causes of distresses and cracks identified on the pavements across the province. Material samples were collected from three representative sites by core drilling. Series of consolidated undrained (CU) triaxial tests, unconfined compressive strength (UCS) tests, free swell index (FSI) tests, zero swelling tests (ZSTs) and California bearing ratio (CBR) tests were performed to determine shear strength resistance and swelling index of the subgrade material, respectively. X-Ray Diffraction (XRD) tests were employed to identify the minerals responsible for swelling activities in the subgrades. The results revealed that the investigated pavements deteriorate by swelling of the subgrades and were significantly affected by traffic loads that triggered resistance failure to moisture from the asphalt layer. The asphalt thickness layer failed to provide enough surcharge pressure to confine swelling stress from the subgrades. In comparison, swelling stress values from the CBR mould were 1.6% lower than that of the swelling stress values obtained from oedometer ring cell on the average. 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 of the subgrades.