Effects of Suction and Drying–Wetting Cycles on Shearing Response of Adobe

Abstract

Adobe has been extensively used as a viable construction material in various hot and arid climatic regions throughout the world for several centuries. Adobe bricks, primarily made out of clay, remain in an unsaturated state for the majority of their life span. These unsaturated bricks are vulnerable to weathering, erosion, cracking, and strength degradation due to changes in moisture and drying–wetting cycles imposed by climatic events and seasonal changes. However, limited information exists in the literature regarding the effect of suction and degree of saturation on the mechanical response of unsaturated adobe. The main objective of this study was to experimentally examine the effect of suction on the shear strength and dilatancy of the adobe used for the repair of a 1,500-year-old adobe castle, the Izad Khast Castle, in Central Iran. We tested the index, physical, mineralogical, and microstructural characteristics of the adobe. After determining the drying and wetting paths of the soil water retention curve (SWRC), we built and employed an osmotic direct shear device to evaluate the shear strength extensively and dilatancy of the adobe under various matric suctions (0, 200, 400, and 800 kPa) and vertical stresses (39, 66, and 122 kPa) without and with the application of drying–wetting cycles. The tested adobe exhibited a significant suction hardening with a brittle behavior along with the shear-induced dilation. Increasing suction prompted a notable rise in the effective angle of friction and a linear increase in dilatancy of the adobe. Applying drying–wetting cycles led to a less brittle response accompanied by a decrease in the peak shear strength. The findings offer new insight into the mechanical behavior of unsaturated adobe, which can contribute toward the preservation and maintenance of historical and contemporary adobe structures.