‘Weathering is the process of modification and degradation of rocks and soil materials on and near the Earth’s surface by chemical decomposition and physical disintegration’ (Geological Society Engineering Group Working Party Report, 1995).
Of particular note in this definition is that weathering is described as a method. It’s not a method for describing the technical properties of a rock mass. As such, describing the weathering state should never be a substitute for an adequate description of the intrinsic physical/mechanical properties of a rock mass using the standard descriptors set out in this guideline (e.g. strength, defects, tissue, rock type). If the degree of weathering cannot be determined, for example because the unweathered version of a certain type of stone has not been seen, it is best left out of the description.
It is recognized that the weathering state of all rock masses (and ‘soil’ masses such as coarse alluvial and pyroclastic deposits) cannot be accurately described using a general weathering scale, and this should not be attempted if ambiguity arises. It is important to fully describe the effects of weathering.
There are many methods and scales to describe the weathering of rock masses.
This post is focused on the general description of a rock mass, a description of weathering that addresses changes in the rock material as well as in the discontinuities is preferred.
If there remains a significant difference in the weathering of the defects compared to the weathering category for the total rock mass, this should be stated as part of the description (e.g. 200mm bottom seam within a slightly weathered rock mass).
TYPES OF ROCK WEATHER
The effects of weathering should be described using standard soil and rock description terminology in terms of:
- Color and color changes;
- Strength and reduction of strength;
- State of discontinuities and their interpretation; and
- Weathering products.
While it applies primarily to rock masses, the effects of weathering on other geological materials such as alluvial and volcanic deposits are also planned to be routinely included in descriptions.
Based on the descriptions of weathering effects and products, the rock mass can be classified according to a general weathering scale (see table below). Other scales can be used to facilitate classification and communication, especially if weather conditions are unusual (eg desert/alpine).
In very general terms, the boundary between a rock mass that is more ‘rocky’ than ‘soily’ is the boundary between moderately weathered and heavily weathered.
Different types of rock mass weathering are given in the following table.