Key geological issues - Geotechnical Engineering of Dams - page 37

The authors consider that the chart can be useful if used in this way. As pointed out
above, they consider that in most cases systems for weathered rock mass should be site-
specific. Unfortunately the Geological Society chart is included as Figure 19 in British
Standards Institution (1999), without the above note. However, in Section 44.4.2.2 the
Standard notes that “ Formal classification may often not be appropriate, and so is not
mandatory”.

2.8.1 Recommended system for classification of weathered rock substance

Any system should be used in conjunction with appropriate geological and engineering
descriptions, and should fulfill the following requirements:

1. It should provide a “shorthand” of descriptive terms and abbreviations, to facilitate
recording (during logging and mapping) of the distribution of rock substance at vari-
ous recognisable stages of change due to weathering;
2. This distribution can then be used to provide an understanding of the local (site) rela-
tionships between the intensity of weathering effects and the common controlling fac-
tors discussed in Sections 2.3 to 2.7, in particular
-the distribution of rock substance types, and
- the pattern of defects in the mass.
Such understanding should assist the site investigator to make soundly based correla-
tions, and predictions about the distribution of rock in various weathered conditions in
other parts of the site, not directly explored;
3. It must allow unambiguous communication of the descriptions and predictions, ver-
bally, and in logs, drawings and reports;
4. It should enable determination of any site relationships which may exist between the
weathered conditions of the rock substances present, and their strengths and other
engineering properties. If at a particular site such relationships can be adequately
proven, it may be possible for them to be used as part of a system of acceptance crite-
ria e.g. for rockfill or for foundation levels.

The classification system for weathered rock substance (Table 2.3), when used in con-
junction with the ISRM substance strength classification ( Table 2.4 ) has been shown to
meet these requirements, for many common rock types, and is recommended by the
authors. Using this approach, rock substance is classified as in the examples in Table 2.5 .

Table 2.3. Recommended descriptive terms for weathered condition of rock substance. Modified from

McMahon, Douglas & Burgess (1975).

Term Symbol Definition

Residual soil RS Soil developed on extremely weathered rock; the mass structure

and substance fabric are no longer evident; there is a large change
in volume but the soil has not been significantly transported.

Extremely weathered XW Rock is weathered to such an extent that it has soil properties,

i.e. it either disintegrates or can be remoulded, in water.

Distinctly weathered DW Rock strength usually changed by weathering. The rock may be

highly discoloured, usually by iron-staining. Porosity may be
increased by leaching, or may be decreased due to deposition of
weathering products in pores.

Slightly weathered SW Rock is slightly discoloured but shows little or no change of

strength from fresh rock.

Fresh rock FR Rock shows no sign of decomposition or staining.