Controls on the geotechnical response of sedimentary rocks to weathering

Abstract

Weathering reduces the strength of rocks and so is a key control on the stability of rock slopes. Recent research suggests that the geotechnical response of rocks to weathering varies with ambient stress conditions resulting from overburden loading and/or stress concentrations driven by near‐surface topography. In addition, the stress history experienced by the rock can influence the degree to which current weathering processes cause rock breakdown. To address the combined effect of these potential controls, we conducted a set of weathering experiments on two sedimentary lithologies in laboratory and field conditions. We firstly defined the baseline geotechnical behaviour of each lithology, characterising surface hardness and stress–strain behaviour in unconfined compression. Weathering significantly reduced intact rock strength, but this was not evident in measurements of surface hardness. The ambient compressive stress applied to samples throughout the experiments did not cause any observable differences in the geotechnical behaviour of the samples. We created a stress history effect in sub‐sets of samples by generating a population of microcracks that could be exploited by weathering processes. We also geometrically modified groups of samples to cause near‐surface stress concentrations that may allow greater weathering efficacy. However, even these pronounced sample modifications resulted in insignificant changes in geotechnical behaviour when compared to unmodified samples. The observed reduction in rock strength changed the nature of failure of the samples, which developed post‐peak strength and underwent multiple stages of brittle failure. Although weakened, these samples could sustain greater stress and strain following exceedance of peak strength. On this basis, the multi‐stage failure style exhibited by weaker weathered rock may permit smaller‐magnitude, higher‐frequency events to trigger fracture through intact rock bridges as well as influencing the characteristics of pre‐failure deformation. These findings are consistent with patterns of behaviour observed in field monitoring results. © 2019 John Wiley & Sons, Ltd.

Profiles of Near‐Surface Rock Mass Strength Across Gradients in Burial, Erosion, and Time

“…When erosion rates are sufficiently rapid, the extent and rate of weathering becomes limited by the kinetics of weathering reactions, and the decrease in chemical weathering leads to a decrease in soil residence time and soil thickness as weatherable minerals are eroded before they have sufficient time to weather completely (Dixon et al., 2012; West et al., 2005). The length of time that a particular parcel of rock resides in the CZ likely has a direct influence on the degree of weathering, and therefore, the strength of the rock mass once it reaches the surface (de Vilder et al., 2019; Moon & Jayawardane, 2004).…”

“…Using short geophysical arrays (<80 m in length), we measure subsurface velocities of S‐waves, which are a measure of the stiffness of rock masses (Barton, 2006). Stiffness is influenced by porosity, density, and elastic moduli, which vary with both weathering extent and degree of lithification of sedimentary rocks (Barton, 2006; de Vilder et al., 2019; Leung & Radhakrishnan, 1990; Von Voigtlander et al., 2018), allowing us to use seismic velocities to explore these variables across environmental gradients. At outcrop sites, we use a Schmidt hammer to quantify hardness of intact rock blocks between fractures, and we assess the density, orientation, and surface conditions (i.e., weathering) of fractures with Geological Strength Index (GSI) observations (Hoek & Marinos, 2000).…”