ROCK WEATHERING

This research is funded by the Nuffield Foundation NAL programme

The aim of this research is to examine the extent to which rock mass deterioration is controlled by rock properties and their modification due to weathering. Specific objectives are to:

1. Determine if the spatial distribution and characteristics of microcracks and pores are reflected in macrofracture properties.
2. Infer the nature of weathering processes operating and establish the relative importance of climatic controls compared with rock properties.
3. Examine variations in rock properties in relation to small scale weathering forms.
4. Refine preliminary spatial and temporal models of fracture modification due to weathering (developed in earlier research).
5. Consider temporal variation in properties of rocks subjected to experimental weathering and model the development of fractures from microcracks and pores.

The research will be conducted with reference to natural rockslopes in Norway and the UK to obtain data from similar lithological rock masses in different environments.

A fundamental hypothesis of the proposed research is that rock mass breakdown by fracturing and fragmentation is preceded by modification at the micro scale. Such modification includes microcracking, pore coalescence and enlargement, both of which lead directly to the generation and propagation of macrofractures and large-scale fragmentation. In granular rocks such as granites and sandstones, rock mass deterioration occurs via disintegration of the material (Nicholson and Nicholson 2000) around grain boundary microcracks and also via development of macrofractures visible at the rock surface. If sufficiently persistent, these macrofractures isolate blocks which are then available for removal from the rock mass. It is reasonable to expect, therefore, that macroscale evidence of deterioration will be reflected in variations in the spatial distribution of micro-scale rock properties. In experimental weathering of limestones conducted previously (Nicholson 2001) evidence was provided that changes to the internal pore structure of rock was a precursor to more substantial deterioration at the macro scale. A two stage process was proposed linking micro- and macro-scale deterioration in which increased pore connectivity by modification of the existing pore structure precedes an increase in the total volume of new void due to microcracking and pore coalescence. Earlier work demonstrated the importance of textural, lithological and structural flaws in controlling the nature and severity of breakdown (Nicholson and Nicholson 2000). Several models describing the influence of pre-existing flaws on deterioration were proposed. These described the relationship between rock strength, porosity and the presence of small scale structural weaknesses.

Field investigations

• Surface morphological mapping of macro-fractures characterised on the basis of spacing, trace length, aperture, infilling and type.
• Characterisation of in situ rock properties using Schmidt hammer rebound, intact rock strength, field moisture content, surface roughness and morphology.
• Sampling for laboratory analysis.
• Variations in rock properties in relation to small scale weathering forms (eg tafoni, spalls and flutes).

Laboratory testing of rock properties

• Lithological and structural description (including petrological analysis)
• Morphological description (including visual description and Fracture Density)
• Mechanical properties (eg compressive and tensile strength, sonic velocity)
• Void dependent properties (eg saturation porosity and water absorption, mercury intrusion porosity and pore size distribution, capillarity, SEM image analysis of pore structure and microcrack type and distribution)
• Chemical and magnetic properties (eg magnetic susceptibility, chemical composition of intact rock and weathering rind using XRF).

Nicholson, D. T. 2009. Holocene microweathering rates and processes on ice-eroded bedrock, Røldal area, Hardangervidda, Norway. In: J. Knight and S. Harrison (eds). Periglacial and Paraglacial Processes and Environments. Geological Society Special Publication 320, 29-49.

Nicholson, D. T. 2008. Rock control on microweathering of bedrock surfaces in a periglacial environment. Geomorphology 101(4), 655-665.

Nicholson, D. T., Nicholson, F. H. and Webber, E. 2004. Micro-weathering processes, rates and morphology: Evidence from a high mountain plateau. Poster presented at the Binghamton Geomorphology Symposium, Kentucky, October 1-3 2004. Abstract.

Nicholson, D. T., Nicholson, F. H., Booth, F. and Wildman, J. J. 2004. Post-glacial weathering and blockstream development in a periglacial environment, Hardangervidda, southern Norway. Poster presented at the Binghamton Geomorphology Symposium, Kentucky, October 1-3 2004. Abstract.

Nicholson, D. T. 2002. Quantification of rock breakdown for experimental weathering studies. In: R. Prikryl and H. Viles (eds) Understanding and Managing Stone Decay, Karolinum Press, Prague, 59-74.

Nicholson, D. T. 2001. Pore properties as indicators of breakdown mechanisms in experimentally weathered limestones. Earth Surface Processes and Landforms 26, 819-838.

Nicholson, D. T. and Nicholson, F. H. 2000. Physical deterioration of sedimentary rocks subjected to experimental freezing and thawing. Earth Surface Processes and Landforms 25, 1295-1307.