36 Description and classification of rocks

36.1 The scope of rock description


As for soils, rock descriptions are made on samples recovered from boreholes and excavations and/or from examination of the in-situ materials. In the following clauses "material characteristics of rocks" refers to those visible in an intact block free from discontinuities, and "mass characteristics of rocks" refers to the overall structure including, particularly, the discontinuities. Thus, only material characteristics can be described on a hand specimen, but an in-situ exposure would permit description of material and mass characteristics. Samples or cores from boreholes normally only allow a limited description of mass characteristics. The quality of the observed sample or exposure is reflected in the level of detail in the description; it is essential that any doubts as to the representativeness or reliability of the sample be stated. An accompanying report gives the origin, type and quality of each sample. In this regard, particularly for rocks, it is often the 5% not recovered that might be more critical than the 95% actually recovered.

36.1.1 General

The characteristics of a rock of engineering significance should be described; these include the strength, weathering effects and the discontinuities. The discontinuities are the most significant of these (unless the discontinuity spacing is wide with respect to the engineering structure) and so particular attention should be paid to their description.

The geological aspects of a rock should be described using terms from geological science.

NOTE 1 These (often detailed) considerations of aspects such as mineralogy and petrography are not directly applicable to most engineering problems but the use of particular mineral or rock names can often indicate a range of typical engineering characteristics.

Geological classification of rock materials should be made to appreciate the geological origin and structure of an area, to establish geological correlation between boreholes, and to distinguish boulders from bedrock.

NOTE 2 This knowledge of the geology of the rock is also of importance when rock material is required for construction purposes, for example as building stone, concrete aggregate or roadstone.

NOTE 3 The characteristics of rock material and rock mass can be inferred from natural outcrops, excavations and rock cores. The amount of information that can be obtained from cores is usually limited, compared to in-situ exposures, unless special techniques are employed.

36.1.2 Description

Rocks seen in natural outcrops, cores and excavations should be described in the following sequence:

  • a) material characteristics (see 36.2):
    • 1) strength;
    • 2) structure;
    • 3) colour;
    • 4) texture;
    • 5) grain size;
    • 6) rock name (in capitals, e.g. "GRANITE");
  • b) general information (see 36.3):
    • 1) additional information and minor constituents;
    • 2) geological formation;
  • c) mass characteristics (see 36.4):
    • 1) state of weathering;
    • 2) discontinuities;
    • 3) fracture state.

NOTE Further information on rock identification is given in BS EN ISO 14689-1.

36.2 Description of rock materials

36.2.1 Strength of rock material

Strength should be described as follows (see Table 25), in terms related to the rock material's unconfined compressive strength, as recommended in BS EN ISO 14689-1.

Table 25 Terms for description of rock strength
Term for use in field or based on measurement Definition for field use Definition on basis of Unconfined Compressive Strength measurements MPa
Extremely weak Can be indented by thumbnail. Gravel sized lumps crush between finger and thumb. 0,6 – 1,0
Very weak Crumbles under firm blows with point of geological hammer. Can be peeled by a pocket knife. 1 – 5
Weak Can be peeled by a pocket knife with difficulty. Shallow indentations made by firm blow with the point of geological hammer. 5 – 25
Medium strong Cannot be scraped with pocket knife. Can be fractured with a single firm blow of geological hammer. 25 – 50
Strong Requires more than one blow of geological hammer to fracture. 50 – 100
Very strong Requires many blows of geological hammer to fracture. 100 – 250
Extremely strong Can only be chipped with geological hammer. > 250
NOTE Based on BS EN ISO 14689-1:2003 4.2.7, Table 5.

The strength of a rock material determined in the uniaxial compression is dependent on the water content of the specimen, anisotropy and the test procedure adopted, all of which should be reported. Simple index tests should be used in the field to provide additional data and as a check on the manually assessed strengths.

NOTE The point load test (see the ISRM suggested methods [55]) and Schmidt Hammer are amongst the more commonly used field tests. Calibration of the test results with the unconfined compressive strength is required.

Each logger should ensure their descriptions are calibrated by strength determinations as the size and shape of lumps, strength of operator, weight of hammer and surface on which lumps rest affect the field assessment of the strength.

36.2.2 Structure


The structure of the rock is concerned with the larger-scale inter-relationship of textural features and lithology. Terms frequently used to describe sedimentary rocks include "bedded", "laminated"; metamorphic rocks can be "foliated", "banded"; igneous rocks can be "flow-banded". Note that structure features are not synonymous with mechanical discontinuities, but they can include potential planes of weakness or incipient discontinuities in the rock mass (see 36.4.3).

Terms to describe the thickness and spacing of these structures are given in Table 26 and should be used where possible.

NOTE More detailed terms for the description of structure are provided in BS EN ISO 14689-1:2003, 4.3.2.

Table 26 Terms for description of thickness and spacing of structure
Thickness term Spacing term Thickness or spacing
Very thickly Extremely wide >6 m
Very thickly Very wide 2 m — 6 m
Thickly Wide 600 mm — 2 m
Medium Medium 200 mm — 600 mm
Thinly Close 60 mm — 200 mm
Very thinly Very close 20 mm — 60 mm
Thickly laminated (Sedimentary)
Narrowly (Metamorphic and Igneous)
Extremely close 6 mm — 20 mm
Thinly laminated (Sedimentary)
Very narrowly (Metamorphic and Igneous)
Extremely close <6 mm
NOTE A spacing of less than 20 mm or a bed thinner than 6 mm is still large in some deposits. Where finer features are observed, their spacing or thickness can also be given in mm, or additional terms used. For example, a paring is a bed which is only one or two grains thick.

For sedimentary rocks, structures such as bedding may be described as "thick beds" or "thickly bedded", for example, "a thickly bedded SANDSTONE". For igneous and metamorphic rocks, the appropriate descriptive term for the structure should be used, for example, "narrowly foliated GNEISS", "very thinly flow-banded DIORITE".