Discontinuity characterization

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Discontinuity type

The type of discontinuity present at a site often vary from project to project, and the terminology can also vary based on the background of those involved in the work. A common set of types used in geotechnical investigations and engineering geology studies is provided in Table 1. This suggested breakdown is simple and can be expanded for site-specific requirements, but when combined with the other discontinuity observations, should cover most cases.

Table 1: Common discontinuity types, definitions, and possible codes

Group Type Definition Code
Separations Fault -Displacement evident
-Zones or fillings of brecciated rock, soil-like gouge, mylonitic, and or cataclastic fabrics are typical
Shear -Displacement evident
-Discrete plane which is polished or slickensided, no infill
Joint -Little to no displacement
-May be infilled with weathered material or have mineral coatings
Structures Bedding -Layering due to sedimentary deposition
-May be open or closed
Foliation -Surface of easy splitting in a metamorphic rock defined by the preferred orientation of metamorphic minerals O
Vein -Geological discontinuity with mineral infilling V
Contact -Boundary between two geological units C

These simple codes can be combined into compound codes that include one separation type and one structure type. For example, F-B can represent “fault // to bedding” while J-O can indicate “joint along foliation.” These compound codes are extremely useful when determining the important discontinuity sets for a project site, and for assigning shear strengths for kinematic analysis.

Discontinuity aperture

Aperture should be noted as a single value and be measured perpendicular to the plane defining the discontinuity (Figure 1). If the aperture is variable, use the aperture along the core axis and note the variability as a comment. Wherever possible, aperture is best measured in the splits, as it will likely increase when the core is disturbed by being placed into the core box. Aperture is an important parameter for estimating the hydraulic conductivity of the rock mass and the shear strength of the discontinuities. The aperture can more accurately be measured from televiewer surveys. Televiewer survey can also be used to confirm measurements in the core.

Closed discontinuity.jpg
Open discontinuity.jpg Filled discontinuity.jpg

Figure 1: Definitions of aperture / infilling thickness for open discontinuities

Infilling type and thickness

When logging from the core box, it can be difficult to accurately estimate infilling type and thickness. However, the presence and type of infilling or coating, in conjunction with the aperture, can determine the shear strength of the rock block-to-block contacts. Infillings and coatings can have a large impact on the engineering behaviour of the rock mass. For the discontinuities logged, the presence or absence (just as important) of infillings should be noted. Types of infillings or coatings may include soil materials (clay, silts, sand), minerals (talc, chlorite, calcite), or disintegrated/decomposed rock products (gouge, rock fragments).

The types of infillings should be noted using project-appropriate codes and the thickness should be recorded using the standard units of the project. It is important that care is taken with the thickness estimates. Default values should be avoided and where no infill is observed, a zero thickness (0) should be recorded.

Joint roughness coefficient

The surface roughness of the logged discontinuities should be estimated. The roughness of these surfaces can influence the rock block-to-bock shear strength and therefore the engineering behaviour of the rock mass. A simple roughness index, the Joint Roughness Coefficient (JRC) has been developed based on the maximum amplitude of the peaks and valleys (asperities) of the discontinuity surface at the drill core scale. Qualitative descriptions (Figure 2) equivalent to the JRC index numbers can also be included in the geological description completed by the geologist.

Joint roughness values.jpg

Figure 2: Qualitative descriptions for JRC values

Joint condition

The joint condition (JC) describes the surface properties of each discontinuity and can be an indication of the shear strength of each joint. The JC can be logged based on the table proposed by Bieniawski (1976) shown here as Table 2. Note that intermediate readings can be applied to better capture the range of joint conditions.

Table 2: Joint condition description

Rating Condition of discontinuity
25 Very rough surface; not continuous; no separation; unweathered (hard) wall rock
20 Slightly rough surfaces; separation <1 mm; slightly weathered (hard) walls
12 Slightly rough surfaces; separation <1 mm; highly weathered (soft) walls
6 Slickensided surfaces or gouge < 5 mm thick or separation 1 to 5 mm; continuous
0 Soft gouge >5mm or separation > 5 mm; continuous joints

It is noteworthy that the JC parameter also has a hardness component to it. For the purpose of this classification system "soft" wall rock is defined as rock having a hardness less than R3 (Field strength grades - Table 4).

Joint wall compressive strength modifier

The joint wall compressive strength (JWCS) modifier is a measure of how the strength of the joint wall compares to the intact strength of the rock and is based on the intact hardness grade (Field strength grades - Table 4). The JWCS modifier is positive if the strength of the wall rock is stronger that the intact rock and negative if the strength is weaker. For instance, if the intact rock is medium strong (R3) and the joint wall is weak to medium strong (R2.5) the JWCS modifier would be -0.5. Note that the JWCS modifier is assigned a value of 0 if the joint wall and intact strength are the same.

See also

Geotechnical logging techniques
Geotechnical drilling
Geotechnical site investigation

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