Quantification and description of rock breakdown for experimental weathering studies

Nicholson, DT (2002) Quantification and description of rock breakdown for experimental weathering studies. In: Stone Weathering and Atmospheric Pollution NETwork (SWAPNET) 2001, 07 May 2001 - 11 May 2001, Czech Republic.

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Abstract

Determination of the severity of rock breakdown due to experimental weathering is strongly influenced by the properties quantified. Traditionally, percentage weight loss has been used to quantify breakdown but this method does not reflect in situ breakdown, nor hidden, internal breakdown which may occur at the microscale. In this paper, results of some experimental rock weathering studies are presented to illustrate the relative merits associated with a range of different methods for quantifying rock breakdown. A range of sedimentary rocks were subjected to accelerated freezing and thawing, salt weathering and wetting and drying tests. The amount of deterioration was measured using techniques selected to provide insight into different facets of rock breakdown. For instance, percentage weight loss was measured to indicate the extent of material detachment; change in fracture density was measured to indicate visible, in situ weakening and fracturing; and fracture porosity (based on change in ultrasonic velocity) was calculated to indicate change in void space induced by weathering. Percentage change in Young's dynamic modulus was also recorded to indicate changes in elastic and mechanical properties. The results show that in some cases there is good agreement between different methods, in terms of the relative magnitude of breakdown and temporal trends, while in other cases there is little or no agreement. For cases in which there is good agreement between methods, it can be inferred that a range of breakdown processes are operating and that the quantification of breakdown is therefore independent of the method used. However, the converse is true for rocks where there is poor agreement between methods. For these, it can be inferred that a much narrower range of breakdown processes are operating and that the results of quantification of breakdown are therefore dependent on the method used. This may bring into question the validity of breakdown assessments for some rocks where the results are based on a single measurement method. The paper includes a description of the theoretical basis of the measurement methods listed. Selected results from experimental weathering are used to illustrate their relative merits. It is concluded that rock weathering studies would benefit from a more holistic approach to the quantification of breakdown which utilises several complementary indicators.