36.3 General information

36.3.1 Additional information and minor constituents

Any other information or observations on the rock material should be included (mineralogy, other tertiary constituents, presence of large voids) as for soils (see Clause 32). Similar information relating to the rock mass should be included at the end of the description (e.g. face stability, voids). The size, spacing or proportion of any qualitative terms should also be given.

Stability of rock material when it is exposed to a new water or atmospheric environment should be assessed where the relevant conditions might be important (see Table 28) together with a description of the test undertaken. Some weak rocks do not show disintegration in water straight away, but only after being dried.

36.3.2 Geological unit

A guide to the name of a geological unit is given on the maps, memoirs and sheet explanations of the British Geological Survey or its antecedents, and the name should be written with at least capital initial letters, e.g. "Wilmslow Sandstone Formation", "Middle Chalk", "Lower Lias". Alternatively, the unit may be given in brackets and/or in upper case letters for clarity. The geological unit should be named where this can be done with confidence, but it might not be easy to tell to which unit a core or exposure belongs, or to locate unit boundaries in a borehole or exposure; conjecture should be avoided but degrees of uncertainty may be indicated. The logger should make this identification, being best placed while carrying out the logging of the exposure(s), and as a first step to identifying the correct geological code.

NOTE The comments in 33.5 are also pertinent in applying unit names to rocks.

36.4 Description of rock masses

36.4.1 General

The description of rock masses should include information, additional to and following the description of the rock material, about discontinuities and other features of engineering significance. Such additional information should include:

• a) details of the weathering profile;
• b) a full description of the discontinuities or sets of discontinuities;
• c) evaluation of the discontinuity state.

36.4.2 Weathering

36.4.2.1 State of weathering

COMMENTARY ON 36.4.2.1

The description of the weathering of rocks is of particular importance in ground investigations as most construction on or in a rock mass is undertaken at shallow depth within the zone of surface weathering. Many attempts have been made to devise weathering grade scales for particular rock masses. Scales have been devised for granite (see Moye, 1955 [56]); Southern Province White Chalk subgroup (see Ward, Burland and Gallois [57], CIRIA Project Report 11 [58] and Spink and Norbury, 1990 [59]); mudstones (see Spink and Norbury 1993 [60]) and Mercia mudstone (see Chandler, 1969 [61]). Working parties of the Engineering Group of the Geological Society have also devised general scales (see Quarterly Journal of Engineering Geology, 1972 [62], Anon, 1995 [63] and Martin and Hencher, 1986 [64]). The relative merits of previous schemes are discussed in Anon, 1995 [63].

A full factual description should be given of the degree, extent and nature of weathering (Approach 1 as shown in Figure 9) as described in Anon, 1995 [63]. In any rock description, full details of the degree, extent and nature of weathering effects should be included so that readers can appreciate their influence on engineering properties.

NOTE Prescriptive classification might be inappropriate in many cases, whereas factual description of weathering:

• is a mandatory part of the full description;
• is of use for subsequent classification;
• is often the only possible way of dealing with weathering where the full profile is not seen;
• is carried out at material and mass scales as appropriate;
• assists interpretation of how the rock has reached its observed condition; and
• provides information for separating rock into zones of similar engineering character.

Changes in engineering properties caused by weathering should be highlighted. In the event that the cause is uncertain, then terms such as "probably" or "possibly" should be used. In addition to the standard terminology, a typical description should include "non-standard English" descriptors commenting on whether features are due to weathering or not, or which weathering processes or combinations of processes might have resulted in the observed state of the rock. All standard terms should be used according to their defined meanings. To avoid confusion, terms that are used in the prescriptive classifications, such as "slightly", "highly", "completely" should not be used in this description. The features most commonly to be examined and reported on should include the following:

• Strength and reduction of strength should be reported using defined terminology. The inclusion of any direct or indirect strength measurements made should be encouraged, whether the test used is "standard" or not. Where it is thought that the change is due to weathering, this information should be provided, for example "very weak within weathered zones", or "generally strong but weak adjacent to weathered discontinuities" (the extent of any such feature should also be reported as a measurement wherever possible).
• The degree of colour change should be described using terms such as "faintly discoloured", "discoloured" or "strongly discoloured". The extent of colour change should be described using terms such as "locally discoloured" or "pervasively discoloured". These terms are not quantitatively defined for general use, although specific criteria may be applied if appropriate. Additional information should be provided, for example, on the extent of colour change by reporting measurements of inward penetration from discontinuities. Comment should include the nature of the colour changes, and whether they are thought to be as a result of weathering, alteration or some other process. Standard colour charts should be used where appropriate.
• The nature and extent of weathering products should always be described using the appropriate rock or soil descriptive terminology and measurement and should be quantified wherever possible.
• Fracture state and changes should be reported using defined terminology. Actual measurements should be reported because these are more precise, although the terms provide a useful shorthand. Where it is thought that changes are attributable to weathering, this information should be provided, for example, "closely spaced, becoming very closely spaced due to weathering between 15.00 m and 15.75 m".

36.4.2.2 Weathering classification

COMMENTARY ON 36.4.2.2

The weathering classifications given in BS EN ISO 14689-1 mean that previous Approaches 2 and 3 (see Anon, 1995 [63]) are no longer to be used for geotechnical category 2 projects due to conflict for strict compliance with that standard. However, these approaches might still be appropriate for geotechnical category 3 projects.

For the classification of weathering in rocks as diverse as, for example, karstic limestones, granites and shales, affected by different weathering processes, a variety of approaches for different situations and scales should be used. Formal classification might often not be appropriate, and should not be used in these cases. Classifications are often useful but should only be applied where well established, where there is sufficient information to classify unambiguously and to do so would be clearly beneficial. Where classification is used, the particular system adopted should be reported.

The approach to description and classification is given in the flow chart in Figure 9 (Anon, 1995 [63], Figure 19); within the various classifications presented as Approaches 4 and 5, definitions of sub-classes in terms of typical characteristics should be broad. Classes may be more rigorously defined by following local experience, site-specific studies or through reference to established schemes. The unit specific weathering classification scheme for the Southern Province White Chalk Subgroup given by CIRIA Project Report 11 [58] can be used.

In logging cores, the distribution of weathering classes of rock material should be recorded wherever possible (see Martin and Hencher, 1986 [64]).

NOTE 1 Distribution of weathering classes of the rock mass from which the cores were obtained has to be inferred from the available evidence and this is not always possible. The weathering of a rock mass cannot be deduced from individual boreholes.

Distribution of weathering classes in a rock mass may be determined by mapping natural and artificial exposures.

NOTE 2 Isolated natural exposures of rock and excavations of limited extent are not necessarily representative of the whole rock mass influenced by the project.

36.4.2.3 State of alteration

Common terms should be used where appropriate, e.g. kaolinized, mineralized. The terms used for the description of weathering of rock material should be used where appropriate, because in many instances the effects of alteration are not easily distinguished from those brought about by weathering.

NOTE A full petrographic determination involving microscopic examination of thin slices of the weathered or altered rock may be used to determine the suitability of the rock for particular purposes, for example as a concrete aggregate.