ISO 14689-1:2003 Geotechnical investigation and testing — Identification and classification of rock — Part 1: Identification and description

4.1 Rock identification

The identification of rocks is based on the determination of the following:

• a) genetic group:
  • sedimentary: clastic, chemical, organic;
  • metamorphic;
  • igneous: plutonic, volcanic;
• b) structure:
  • bedded, foliated or massive (without sharp genetic discontinuities);
• c) grain size:
  • descriptive terms (for various sizes) are given in Table A.1 in correlation to rock types;
• d) mineralogical composition:
  • quartz, feldspars and related silicate minerals;
  • dark coloured minerals (e.g. biotite, amphibole, pyroxene);
  • clay minerals;
  • carbonate minerals (e.g. calcite and dolomite);
  • siliceous amorphous material (e.g. glass);
  • carbonaceous material (e.g. coal and graphite);
  • salts (e.g. halite [rock salt]), gypsum);
  • swelling minerals (e.g. anhydrite and clay minerals)
  • sulfide minerals (e.g. pyrite);
• e) void content:
  • primary voids (e.g. gas bubbles in volcanic rocks);
  • secondary voids (e.g. solution voids).

NOTE Lithological identification of rock is necessary to appreciate the geology of an area, to correlate geological profiles seen in boreholes or to distinguish boulders from bedrock. It is also important when rock material is required for construction purposes. Engineering properties can only partially be inferred from the identification of rock type.

The names of the more common rock types are given in Table A.1, which presents an aid to rock identification for engineering purposes.

Rock names are given particular combinations of features in this subclause and correct naming requires recognition of the attributes listed. The rock shall be correctly identified within geological science.

Geological maps related to the project shall be used for the designation of rocks.

4.2 Description of rock material

4.2.1 Colour

Rock material colour may be described using Colour Charts of an approved type. As an alternative, the following simple system should be used, which serves to limit the subjectivity of an estimation. One term is selected as required from each column (see Table 1) and combined as a colour assessment.

Examples of use are: yellow, light yellowish brown, dark reddish brown, dark brown, etc. If necessary, colour differences can be emphasised separately by the use of terms such as spotted, dappled, mottled, streaked; for example, light yellowish brown spotted with dark brown.

A colour chart provides a useful aid, particularly to improve the consistency between descriptions by different persons.

Table 1 — Terms for lightness, chroma and hue which may be used in combination for colour description (examples)

4.2.2 Grain size

A descriptive scheme is given in Table A.1. Grain size refers to the average dimension of the predominant mineral or rock fragments comprising the rock material. It is usually sufficient to estimate the size by eye, which may be aided by a hand lens in the assessment of fine-grained or amorphous rocks, but separate descriptions of the grains and the matrix may be appropriate.

4.2.3 Matrix

The particle size and the mineralogical composition of the matrix shall be determined. Mineralogical composition shall be described using the terms given in Table A.1 (siliceous, calcareous, carbonaceous, etc.) but may be amplified, as appropriate, with such standard geological terms as ferruginous, argillaceous (containing clay minerals), quartzose and others.

4.2.4 Weathering and alteration effects

The results of weathering/alteration of rock material are given in Table 2. Any or all of the descriptive terms can be used to describe weathering/alteration

Table 2 — Terms to describe weathering/alteration of rock materials

The weathering terms given in Table 2 may be subdivided using qualifying terms, for example “partially discoloured”, “wholly discoloured” and “slightly discoloured”, as this will aid the description of the material being examined. The last three terms may be used in combination, for example, “wholly discoloured and slightly decomposed”.

4.2.5 Carbonate content

The carbonate content is determined by the application of droplets of dilute hydrochloric acid (HCl) (3:1 or 10 %). The following characteristics could be distinguished:

• a) carbonate-free (O) if the addition of HCl produces no effervescence;
• b) calcareous (+) if the addition of HCl produces clear, but not sustained, effervescence;
• c) highly calcareous (++) if the addition of HCl produces strong and sustained effervescence. It should be noted that, in wet or moist rocks, the effervescence usually occurs with some delay.

4.2.6 Stability of rock material

The degradation of rock material when it is exposed to a new water or atmospheric environment should be assessed where the relevant conditions shall be determined (see Table 3).

Table 3 — Stability of rock material

Its behaviour when exposed to water should be described using the terms in Table 4, together with a description of the test undertaken. Some weak rocks do not show disintegration in water straight away, but only after being dried.

Table 4 — Rock material stability in water

4.2.7 Unconfined compressive strength

The unconfined compressive strength of rock material can be estimated according to Table 5.

Table 5 — Field identification of the unconfined compressive strength

NOTE Unconfined compressive strength cannot always be defined in the field, thus point load tests are often used as indices for strength of rock material.

Any report describing compressive strength tests shall mention the specimen size, the test procedure, the anisotropy of the specimen and its water content.

4.3 Rock mass

4.3.1 General

The description of rock mass shall include:

• a) types of rocks;
• b) structure;
• c) discontinuities;
• d) weathering;
• e) groundwater.

4.3.2 Structure

The structure of the rock mass should be described with relation to the larger scale interrelations of geological features and the associations between rock types in the mass.

Examples of common terms which may be used and which are defined in standard geological practice are as follows (see Table 6).

Table 6 — Examples of terms which may be used in the description of rock mass structure

4.3.3 Discontinuities

4.3.3.1 Introduction

The tensile or shear strength across or along the surface is lower than that of the intact rock material. They have many modes of origin, for example, bedding planes, joints, shears, faults, cleavages or foliations and may occur locally in sets of relatively uniform characteristics or individually.

In addition to describing the type of discontinuities, their orientations, spacing, persistence, roughness, aperture and filling, and seepage characteristics, the number of sets and the rock block size resulting from their combination should be recorded. It may often be necessary to describe large or important discontinuities individually. Full information on rock mass discontinuities can only be obtained from suitable exposures of the rock mass.

4.3.3.2 Measurement of dip and dip direction

The maximum declination (dip) of the mean plane of the discontinuity is measured with the clinometer, and should be expressed in degrees as a two digit number, e.g. 50 (00 to 90). The azimuth of the dip (dip direction) is measured in degrees counted clockwise from true north, and expressed as a three digit number, e.g. 240 (000 to 360). The dip direction and dip should be recorded in that order, with the three digit and two digit numbers separated by a slash, e.g. 240/50. The pair of numbers represents the dip vector. The relationship between dip, strike and dip direction is given in Figure 1.

Key

• 1 dip direction
• 2 strike
• α dip direction (dip azimuth) = 240°
• β dip (dip angle) = 50°

plane of discontinuity 240/50

Figure 1 — Diagram indicating, dip, dip direction and strike

4.3.3.3 Discontinuity spacing and block shape

The term “spacing” refers to the mean or modal spacing of a set of discontinuities and is the perpendicular distance between adjacent discontinuities. The terms to be used to describe bedding thickness are given in Table 7 and discontinuity spacing in Table 8.

Table 7 — Terms to describe bedding thickness

Table 8 — Terms to describe discontinuity spacing

In drilled cores, it is usually difficult to measure the true discontinuity spacing: measurements are commonly made along the core axis. The method of measurement shall be reported.

Discontinuity spacing in three dimensions should be described with reference to the size and shape of rock blocks bounded by the discontinuities. For the rock block size, the following scheme should be used (see Table 9).

Table 9 — Dimensions of rock blocks

The rock block shape should be described according to the terms in Table 10. The rock block shape shall be correlated to the spacing of discontinuities.

Table 10 — Terms to describe the main rock mass structures and block shapes

Term	Figure	Description
a) Polyhedral blocks		Irregular discontinuities without arrangement into distinct sets, and of small persistence.
b) Tabular blocks		One dominant set of parallel discontinuities (1), for example bedding planes, with other non-continuous joints; thickness of blocks much less than length or width.
c) Prismatic blocks		Two dominant sets of discontinuities (1 and 2), approximately orthogonal and parallel, with a third irregular set; thickness of blocks much less than length or width.
d) Equidimensional blocks		Three dominant sets of discontinuities (1, 2 and 3), approximately orthogonal, with occasional irregular joints, giving equidimensional blocks.
e) Rhomboidal blocks		Three (ore more) dominant, mutually oblique, sets of joints (1, 2 and 3), giving oblique-shaped, equidimensional blocks.
f) Columnar blocks		Several, usually more than three, sets of continuous, parallel joints (1, 2, 3, 4, 5) usually crossed by irregular joints; length much greater than other dimensions.

4.3.3.4 Persistence of discontinuities

The linear extent of discontinuities from their inception to their termination in solid rock mass or against other discontinuities has to be measured in metres. The size of the exposure shall be recorded and, if possible and appropriate, measurements should be made in two or preferably three orthogonal directions.

4.3.3.5 Roughness

The surface of discontinuities has to be described on the basis of three scales of observation:

• a) small scale (several millimetres) — rough or smooth;
• b) medium scale (several centimetres) — planar, stepped or undulating;
• c) large scale (several metres) — wavy, curved or straight;

and using terms as indicated in Figure 2. A discontinuity surface could thus be described combining large or medium and small scale terms to give descriptions such as “stepped smooth” or “planar rough”. For total clarity of description, it may be necessary to give wavelength and amplitude measurements of the larger scale features. Note that all “smooth” discontinuities may be slickensided. Slickensides are striations on a discontinuity surface as a consequence of movement and pressure. Slickensided surfaces may be polished and reflect light.

“Slickensided” should only be used with clear evidence of shear displacement along the discontinuity. The vertical and horizontal scales are equal.

	Rough (irregular)	Smooth
Stepped	1	2
Undulating	3	4
Planar	5	6

Key

• 1 stepped rough surface
• 2 stepped smooth surface
• 3 undulating rough surface
• 4 undulating smooth surface
• 5 planar rough surface
• 6 planar smooth surface

Figure 2 — Terms for discontinuity roughness (not to scale)

4.3.3.6 Aperture

The perpendicular distance between the two surfaces of a discontinuity is referred to as the aperture. The origin of the exposure, such as natural outcrop, artificial excavated surface, etc., shall be reported. The separation has to be described using the terms in Table 11.

Table 11 — Terms for the description of discontinuity aperture

4.3.3.7 Infilling

The infilling material between discontinuity surfaces shall be identified and described (e. g. soil, minerals such as calcite quartz, epidote, chlorite clay gouge or breccia). The shear strength of infilling and the potential for infilling to swell shall be described, when relevant.

4.3.3.8 Seepage

Free moisture or water flow visible at individual spots or from discontinuities should be described using the terms “moisture on rock surface” and “dripping water” respectively. If the rate of flow can be estimated or measured then the rate of flow may be described using the terms given in Table 12.

Table 12 — Terms to describe seepage rates from discontinuities

4.3.4 Weathering of the rock mass

The weathering of the rock mass should be described in terms of the distribution and relative proportions of fresh rock and discoloured, decomposed or disintegrated rock, and the effects of weathering on discontinuities.

Weathering eventually converts rock to a soil and the weathering profile shall be described in terms of three basic units: rock, rock-and-soil, soil.

To subdivide the different weathering units, a scale with six distinct grades applicable to a profile in rock is given in Table 13.

Table 13 — Scale of weathering stages of rock mass

Table 13 gives a typical classification which is unlikely to apply to all rock types. More specific local classifications may be available and can be used where they are useful and unambiguous.

In logging cores, the distribution of weathering grades of rock material may be recorded; distribution of weathering grades of the rock mass from which the cores were obtained has to be inferred from this type of evidence.

Distribution of weathering grades in a rock mass may be determined by mapping natural and artificial exposures. It should be borne in mind, however, that isolated natural exposures of rock and excavations of limited extent are not necessarily representative of the whole rock mass.

4.3.5 Rock mass permeability

The rock mass permeability shall be measured by appropriate tests (e.g. pump test, Lugeon test).