36.4.3 Discontinuities
36.4.3.1 General
COMMENTARY ON 36.4.3.1
Discontinuities are breaks, fractures or planes of weakness in the rock mass; the discontinuities present in the ground (natural) are the most important, but discontinuities can also be induced by the creation of the exposure. The various types are detailed in Table 29.
Discontinuities within the rock mass are, in most cases, of primary importance to the rock's overall engineering properties and the maximum possible amount of information from the investigation should be identified and reported as shown in Table 30. A number of different types of discontinuity can be recognized as given in Table 29. Full and accurate description of recovered cores should be carried out and more frequent use should be made of the borehole itself with downhole logging (geophysical, scanning, see Section 5) or cameras. In addition, exposures, whether existing or created for the investigation, should be used wherever practicable to inspect the in-situ mass.
NOTE 1 A distinction can be drawn between "mechanical discontinuities", which are already open and present in the rock, and "incipient discontinuities", which are inherent potential planes of weakness.
NOTE 2 Full recommendations for the recording of discontinuities are given in DIN 4022-1, ISRM, 1978 [65], Quarterly Journal of Engineering Geology, 1977 [66] and ASTM D4879-89. Additional recommendations for describing discontinuities are given in BS EN ISO 14689-1:2003, 4.3.3.
Free moisture or water flow visible at individual spots or from discontinuities should be described as recommended in BS EN ISO 14689-1:2003.
| Type of discontinuity | Description |
| Joint | A discontinuity in the body of rock along which there has been no visible displacement. Joints are synonymous with fissures in soils. |
| Fault | A fracture or fracture zone along which there has been recognizable displacement. |
| Bedding fracture | A fracture along the bedding (bedding is a surface parallel to the plane of deposition). |
| Cleavage fracture | A fracture along a cleavage (cleavage is a set of parallel planes of weakness often associated with mineral realignment). |
| Induced fracture | A discontinuity of non-geological origin, e.g. brought about by coring, blasting, ripping, etc. |
| Incipient fracture | A discontinuity which retains some tensile strength, which might not be fully developed or which might be partially cemented. Many incipient fractures are along bedding or cleavage. |
The recording of induced and incipient discontinuities is important as they can indicate weakness within the mass, but they should not be included within the assessment of fracture state, see 36.4.4. If incipient or induced fractures are included in the fracture state, this should be clearly stated on the borehole log.
NOTE 3 The conventional exclusion of such integral discontinuities from reported indices is conservative, but only for foundation studies; for bulk excavation studies, for instance, it might be preferable to include them.
NOTE 4 Discontinuities usually occur in more than one direction in a rock mass, and might be present as distinct sets. Borehole cores provide essentially one dimensional data on discontinuity spacing; exposures or orientated cores are usually needed for full evaluation of the discontinuity pattern.
The following features of discontinuities should be described (see 36.4.3.2 to 36.4.3.9). The amount of detail included depends on the quality of the exposure or core, whether it is representative and the requirements of the problem in hand. The descriptive terms are summarized in Table 29, which defines the terms in the order in which they should be used in a description.
| Orientation | Discontinuity spacing | Persistence | Type of termination | Roughness | Wall strength | Aperture | Filling | Seepage | No. of sets |
| Dip amount only in cores | Discontinuous | Cannot normally be described in | Intermediate scale (cm) and small scale (mm) | Schmidt hammer | Cannot normally be described in cores | Cannot be described in cores | Can be described or summarized in cores where sets of different dip are present | ||
| Stepped | |||||||||
| Continuous in cores | Rough | ||||||||
| Smooth | Point load test | ||||||||
| Striated | |||||||||
| Extremely wide > 6 m | Termination | Clean | |||||||
| Very wide 2 m — 6 m | Undulating | Extremely wide > 1 m | Surface staining (colour) | ||||||
| Wide 600 mm — 2 m | Very high > 20 m | Rough | Very wide 0,1 m — 1 m | Moisture on rock surface | |||||
| Take number of readings of dip direction / dip, e.g. 015/18° | Medium 200 mm — 600 mm | High 10 m — 20 m | x (outside exposure) | Smooth | Other index tests | Wide 0,01 m — 0,1 m | Soil infilling (describe in accordance with Clause 33) | Dripping water | |
| Striated | |||||||||
| Close 60 mm — 200 mm | Medium 3 m — 10 m | Planar | Moderately wide 2,5 mm — 10 mm | Water flow measured per time unit on and individual discontinuity or set of discontinuities | |||||
| r (within rock) | Rough | Mineral coatings (e.g. calcite, chlorite, gypsum, etc.) | Record orientation and spacing of individual sets and all details for each set | ||||||
| Very close 20 mm — 60 mm | Low 1 m — 3 m | Smooth | Open 0,5 mm — 2,5 mm | ||||||
| Striated | |||||||||
| Report as ranges and on stereo net if appropriate | Extremely close < 20 mm | Very low < 1 m | d (against discontinuity) | Large scale (m) | Partly open 0,25 mm — 0,5 mm | Other (specify) | Small flow 0,5 l/s — 5,0 l/s | ||
| Waviness | |||||||||
| Take number of readings and state min., average and max. | Curvature | Tight 0,1 mm — 0,25 mm | Medium flow 0,05 l/s — 0,5 l/s | ||||||
| Record also size of discontinuity | Straightness | Visual assessment | Record width and continuity of infill | ||||||
| Measure amplitude and wavelength of feature | Very tight < 0,1 mm | Large flow > 5 l/s |
36.4.3.2 Orientation
The convention of three-digit dip direction/two-digit dip should be used, e.g. 015º/26°. In cores, only dip related to the normal to the core axis should normally be determined unless core orientation or downhole measurement methods have been used.
36.4.3.3 Spacing
The descriptive terms in Table 29 should be used for discontinuity spacing in one dimension. The spacing should be measured for each joint set; the convention is to measure discontinuity spacings perpendicular to the discontinuities. In cores with steeply dipping discontinuities, it might only be possible to measure "spacing" along the core axis; if so, this should be stated.
NOTE The spacing of discontinuities in three dimensions may be described with reference to the size and shape of rock blocks bounded by discontinuities using the terms defined in BS EN ISO 14689-1. The use of these terms requires an understanding of the distribution of discontinuities in three dimensions, and so cannot be used in description of drill core. Where there is sufficient exposure for such terms to be used, the measurement of discontinuity spacings along orthogonal scan lines might be more appropriate.
36.4.3.4 Persistence
The descriptive terminology should be applied to sets; actual measurements are preferred for individual discontinuities. Very limited information is available from cores.
36.4.3.5 Termination
The nature of the discontinuity termination should be recorded in the context of the size of the exposure. A discontinuity might start and end within or beyond the limits of the exposure. Very limited information is available from cores.
36.4.3.6 Roughness
Descriptions should be made at three scales, where possible (see DIN 4022-1). The intermediate scale (several cm) should be divided into stepped, undulating or planar. The small scale (several mm) of rough, smooth or striated should be superimposed on the intermediate scale. Smooth surfaces can be matt or polished; the degree of polish should be qualitatively described. The term striated should only be used where there is clear evidence of shear displacement. There might also be a large scale (several m) which may be reported as measured wavelength and amplitude; the smaller scales may be reported similarly. An individual joint can, therefore, be described as wavy (wavelength 2 m, amplitude 1 m), stepped (wavelength 500 mm, amplitude 200 mm) and smooth. If more precise detail is required, roughness should be measured quantitatively (see DIN 4022-1).
36.4.3.7 Wall strength
Index tests should be used to measure wall strength. Numerical results should be reported, and can be summarized using the terms in 36.2.1.
Wall weathering and alteration should be described in accordance with 36.4.2.
36.4.3.8 Aperture and infilling
Where possible, measurements of aperture should be reported. Full description of rock, soil or mineral infill should be provided. Care should be taken when reporting aperture in rock cores; the observer should comment on whether the reported apertures are present in the intact rock mass, or a consequence of geomorphological/weathering agencies, or whether due to engineering activities or creation of the exposure. The thickness and type of infill should be reported using standard terms, e.g. 1 mm surface film of calcite, 10 mm cemented breccia.
36.4.3.9 Number of sets
The descriptive terminology may be applied to individual discontinuities, or summarized to sets or to zones of uniform character.
Where extensive detail of the rock mass is required, systematic record sheets should be used and numerical data for use in rock mass rating schemes should be recorded to facilitate data handling, see for example IRSM, 1978 [65].
