Characterisation of weathering propagation from joint surfaces in different rock types

Abstract

The propagation of weathering into adjacent rock material from joint surfaces represents a significant gap in the Engineering Geology field, requiring further investigation and detailed analysis. The impact of weathering on the engineering properties of rocks stands as an indispensable consideration in geotechnical designs, as it intricately alters the quality of the rock mass across temporal scales. This also influences the engineering lifetime of structures (slopes or tunnels) and the appropriate design parameters for these structures. This study encompassed a thorough analysis of various properties, including the physical, mineralogical, and textural attributes of the rock. This investigation sought to reveal how these properties undergo transformation as the process of weathering from a joint surface extends into the adjacent parent material. Physical properties were assessed using the Schmidt hammer test to derive rebound numbers (R-values), while mineralogical and textural analyses were conducted through Polarized Light Microscopy, Scanning Electron Microscopy (SEM), X-Ray Fluorescence (XRF), and X-Ray Diffraction (XRD) techniques. Weathering indices, including the Chemical Index of Alteration (CIA), Mineralogical Index of Alteration (MIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA), were used to assess the degree of weathering across each sample. Indications of weathering were evident at the periphery and adjoining rock material of the samples. Fracturing was deemed as being one of the main drivers during the process of propagating weathering. The thin sections indicated that the process of weathering propagated further into the unexposed portion of the rock, unnoticeable to the naked eye. Weathering indices revealed granite as the least weathered rock sample, followed by quartz-diorite and shale. Samples with lower quartz content experienced a greater reduction in hardness (strength), indicating the role that mineralogy plays in the ability of a rock to resist the weakening effects of weathering. The chemical composition of a rock, as determined by X ray Fluorescence (XRF) analysis, provides valuable insights into both the degree of weathering the rock has experienced and its potential for future weathering. This relationship is primarily influenced by the mineralogical composition of the rock, as different minerals weather at varying rates. Igneous rock samples exhibited a linear correlation between weathering degree and visible weathering propagation depth. The relationship between the decrease in hardness (strength) and the depth of weathering propagation revealed that as the depth of weathering propagation increased, the greater the decrease in hardness of the rock material adjacent to the joint. The joint weathering classification scheme adaptation, in which the methods of Amin et al. (2000) and Barton et al. (1977) were combined and used as a basis for the adaptation proposed, indicated that the igneous rock samples classifies as intermediately weathered, whereas the sedimentary rock sample (shale) classifies as slightly to intermediate based on the percentage decrease in hardness (strength) from the unexposed regions of the samples to the exposed regions of the samples as well as the zoning and depth of discolouration observed within the material adjacent to the joint opening. Each rock type showed distinct joint zoning patterns resulting from weathering propagation. This indicated that each rock responded differently to the process of propagating weathering. This highlights the importance of recognising that not all rocks react to weathering in the same way and emphasises the inadequacy of a single classification system for diverse rock types.