36.4.4 Discontinuity state
COMMENTARY ON 36.4.4
Various criteria can be used for quantitative description of the fracture state of rock cores; these are the total core recovery (TCR), solid core recovery (SCR), fracture index and rock quality designation (RQD). The definitions of these terms are given in BS EN ISO 22475-1. The fundamental definition is that solid core has a full diameter, uninterrupted by natural discontinuities, but not necessarily a full circumference and is commonly measured along the core axis or other scan line (see DIN 4022-1 and Laubscher, 1990 [67]). By this definition, core is solid unless intersected by more than one joint set with different strike directions.
The measurement of the discontinuity state should be made using the indices defined in Table 31. The measurement of discontinuity state of rotary cores and in-situ exposures should follow the general procedures in this clause; RQD and fracture index can be determined from scanlines where appropriate.
| TCR (%) | Length of core recovered (solid and non-intact) expressed as a ratio of the length of core run. |
| SCR (%) | Length of solid core recovered expressed as a ratio of the length of core run. Solid core has a full diameter, uninterrupted by natural discontinuities, but not necessarily a full circumference and is commonly measured along the core axis or other scan line. |
| RQD (%) | Length of solid core each pieces longer than 100 mm expressed as a ratio of the length of core run. |
| Fracture index | Count of the number or spacing of fractures over an arbitrary length of core of similar intensity of fracturing recorded as minimum/mode/maximum. Commonly reported as Fracture Spacing (If, mm) or as Fracture Index (Fl, number of fractures per metre). Where core is non-intact in the ground, the abbreviation Nl may be used. |
| NOTE The total core recovery (TCR) records the proportion of core recovered and is read with the description, solid core recovery (SCR) and rock quality designation (RQD). The TCR of itself gives little information on the character of the core or the rock from which it was recovered. This measurement is required to ensure that all depth related records such as boundaries, markers and samples are correct. | |
Where a section of core contains no fractures, between two sections of fractured core, a single fracture spacing (If) equal to the length of non-fractured core should be reported. Non-intact zones of about 300 mm or greater extent can usefully be identified as a separate unit within the discontinuity log to enable their ready identification in a borehole record.
Where core loss is identified, the logger (in consultation with the driller) should identify the amount of loss and, wherever practicable, the depth at which it occurs. There are practical difficulties in recording the TCR whenever the recovery is not 100% (Valentine and Norbury, 2011 [68]), which occurs where:
- the core recovery is less than complete as core has been lost; or
- there is more recovery than there should be with core being gained; this can either result from core being lost from one core run and recovered in the subsequent run, or from core swelling after recovery.
NOTE 1 Assignation of a depth range to all zones of core loss enables corrections to be made to the actual depths of the recovered core and thus the true depth of any logging observations and sub-samples taken.
NOTE 2 Apparent core gains can occur where core is dropped from one run by being left down the hole but picked up on the subsequent core run. This gives core recovery of less than 100% in the first run, but might well result in measurements of recovery ostensibly exceeding 100% in the subsequent run or runs.
The logger should correct the recorded depths to take account of core losses and gains, which might extend over several core runs, preferably before recording any depth related remarks and test and sample depths.
NOTE 3 The application of these terms is illustrated in Figure 10 (see Bienawski, 1984 [69]).
It is conventional to include only natural fractures in determining these indices; departure from this convention should be stated on the log, as should any uncertainties. The treatment of incipient discontinuities should be reported.
NOTE 4 Useful guidance on the interpretation of natural and induced fractures is provided in Deere and Deere, 1988 [70] and Kulander et al., 1990 [71]).
NOTE 5 It is not usually appropriate to record these indices, other than TCR, in soils or other materials, such as concrete or brickwork, recovered by rotary core drilling.
Alternative definitions or applications of these indices, and particularly of RQD, have been put forward widely in the literature; if any alternative definitions are used this should be indicated on the borehole or exposure log.
Key
| 1 Drilling induced fractures 2 At least one full diameter 3 No single full diameter |
4 At least one full diameter 5 Non-intact 6 No recovery |
| NOTE All features shown are natural discontinuities unless stated otherwise | |
36.4.5 Example rock material and rock mass descriptions
The overall description should match the style and coverage as appropriate of the examples of rock descriptions given in Table 32.
| An example of a description of a rock mass seen in a section of drill core might be "Very strong thinly flow banded dark greyish green fine-grained quartz DOLERITE. Joints dipping 5 degrees very widely spaced with red penetrative staining to 10 mm and locally weathered to moderately strong to 5 mm penetration". The borehole log also includes the indices giving the fracture state. |
| An example of the description of a rock mass seen in a trial pit might be "Very stiff fissured thickly laminated to very thinly bedded brown mottled grey CLAY varying to very weak grey mottled brown MUDSTONE. Occasional gypsum crystals up to 5 mm, and rare pyritized wood fragments. Fissures very closely spaced (20 mm to 40 mm) with brown oxidation penetrating up to 3 mm. (Class B London Clay Formation)". |
| An example of the description of a rock mass seen in a quarry face might be "Medium strong very thinly bedded reddish brown fine and medium grained SANDSTONE. Rare moderately weak light green siltstone elliptical inclusions up to 20 mm by 5 mm. (Weathered Sherwood Sandstone Group). Small blocky jointing. Joint set 1 — 045/75, medium spaced, medium persistence, terminations outside exposure, curved planar rough, weak friable up to 5 mm penetration moderately wide open, clean. Joint set 2 — 110 – 130/80 – 90, closely spaced, low persistence, terminations outside exposure and against discontinuity, planar smooth weak friable up to 3 mm penetration, tight, clean. Bedding fracture set 3 — 180 – 190/0 – 10, medium spaced, high persistence, no termination seen, straight stepped smooth, slightly polished, moderately open up to 1 mm infilled with firm grey clay. Joints generally dry, local small flows". If this is from a cored borehole, the log should also include the indices giving the fracture state; comparable information can also be obtained from scanlines on an exposure. Such a description could refer to the rock mass in a specified location in a quarry. An account of the whole quarry would require many such descriptions perhaps displayed on engineering geological maps, plans and sections. Data on the orientation of discontinuities can be displayed and analysed using stereonets (see Norbury, Child and Spink, 1986 [72]) or rosette diagrams. |
NOTE In addition to classification on a geological basis, the assessment of rock mass conditions can be summarized and assessed for engineering purposes using one of the many available rating systems. By ascribing weighted scores to certain pertinent characteristics of the rock mass, a classification or zonation of the rock in the area influenced by the engineering works can be derived. A useful summary of these ratings is given in Bienawski, 1989 [73].
