Predicting Weathering Characteristics of Carbonate Rocks Under Different Geo-Environmental Conditions

Abstract

Rock moisture content, temperature and physical and chemical conditions of water in the rock mass vary under different geoenvironmental conditions. These factors are critical in the mechanical and chemical disintegration of rocks with time. Changes in weathering characteristics of a rock mass correlate with major changes in shear strength and deformability of rocks. Therefore, durability is an important parameter for rocks and weathered rocks in predicting and assessing long term problems in their use for engineering purposes. Carbonate rocks are generally considered as durable strong rocks. It is important to test this claim in the UAE, where carbonate rocks are the main available rock materials in many locations e.g. in Al Ain city. The slake durability index (SDI), with tests involving several cycles of wetting and drying, is widely recognized and has been used to measure the durability of weathered rocks. Carbonate rocks in the Jabel Hafit mountain, Al Ain region, have highly fractured and cavernous surface and near surface structures. In this study, degradability of samples from three different carbonate sequences: the Rus, Dammam, and Asmari Formations, were studied petrographically, mineralogically and texturally before and after slaking. A number of slaking tests were performed on twenty-one rock samples, seven from each rock formation, in order to investigate the effects of various geoenvironmental conditions on carbonate rocks weatherability at various pH levels 2, 4, 6, 8, 10, 12 and 14. Detailed mineralogical examinations, rock thin sections, Scanning Electronic Microscope (SEM) and X-ray diffractometry (XRD) were employed to reveal the relationship between mineralogy and weathering characteristics of rocks on the representative rock samples, before and after the slake durability tests. Furthermore, the index properties of all samples such as unit weights (dry, γd and saturated, γsat), the water absorption (Wa), the porosity (n), void ratio (e) and the specific gravities (GS,(OD), GS,(SSD), GS,(A)) were measured. The study reveals that the weathering characteristics of carbonate rocks under different geoenvironmental conditions is strongly controlled by the mineralogical composition and textural features of the rocks. The impact of slaking fluid in the degradability of limestone is very limited due to chemical reaction of CaCO3 especially in lower pH level acidic fluid, which reduced the bonding strength between grains. The findings will certainly help to resolve durability problems associated with engineering applications such as tunnels, slope stability, and etc. in the region and elsewhere.