Dawn T. Nicholson

Determination of the severity and nature of rock breakdown due to experimental weathering is strongly influenced by the properties measured. 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 and describing rock breakdown. A range of sedimentary rocks were subjected to accelerated freezing and thawing, salt weathering, wetting and drying and slake durability tests. The amount of deterioration was measured using a range of techniques selected to provide insight into distinct aspects of rock breakdown. For instance, percentage weight loss was measured to indicate the extent of material detachment; change in fracture density was used to indicate visible, in situ weakening and fracturing; fracture porosity (based on change in ultrasonic velocity) was measured to indicate change in void space induced by weathering; and percentage change in Young's dynamic modulus was used to indicate changes in elastic and mechanical rock 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 quantification of breakdown is therefore dependent on the method used. This may bring into question the validity of breakdown assessments for some rocks where based on a single measurement method.

The paper includes a discussion of the theoretical basis of the measurement methods listed and results of the experimental weathering studies are used to illustrate some problems and errors which can arise. Consideration is also given to the value of keeping a written, photographic or pictorial record of the nature of breakdown, an area often neglected in rock weathering studies. It is concluded that rock weathering studies would benefit from a more holistic approach to the quantification and description of breakdown which utilises several complementary indicators.