Joints and JointingPublished on February 15 2015
The figure below shows the main features occurring in rock masses, which are divided into two main groups: Detailed jointing and Weakness zones.
Joint features, like joints characteristics and the degree of jointing are important parameters applied as input to classification systems, as described in the tag ‘Classification systems’.
The main characteristics of joints include:
- joint plane planarity or waviness,
- joint surface smoothness, and
- condition (alteration) of the joint wall (whether it is weathered/altered or has coating or the joint has some sort of filling
- joint size (length) and continuity
These are indicated in the figure below:
You can find more on the joint features in the tag ‘Field Observations’
There is a great variety of joints, from small cracks to long shears or seams, as seen below:
Joint characteristics are used as input parameters to many classification systems, see the tag ‘Classification systems’.
You will find examples and applications of joint characteristics in several papers presented in other locations on this web page and also in the Rock Engineering book.
Jointing here means the assemblage of joints. It can be measured and characterized in different ways, manly from:
- field observations at terrain surface or in the underground excavation (tunnel, cavern, shaft)
- drill core logging
- seismic or sound velocities
The main jointing features are:
1. Degree of jointing.
This property can be measured as rock quality designation (RQD), volumetric joint count (Jv), block volume Vb), and joint spacing (S), as shortly presented in the paper Block sizes and block size measurements
The volumetric joint count (Jv) is a measurement of the degree of jointing. It is given as the number of joints in a volume of rockmass (of 1m3 size) The following papers deal with the Jv, which was first presented (in Norwegian) by Palmstrom in 1974, later presented in 1982, 1985, 1986, and 1996.
The paper Application of seismic refraction survey in assessment of jointing shows how the seismic velocities can be used to estimate the Jv.
The paper Measurements of and correlations between block size and rock quality designation (RQD) shows the difficulties in establishing a good comparison between RQD and Vb (block size).
2. Orientation of joints and joint sets.
This has special interest when the joint set is unfavourably orientated parallel or at a small angle to a tunnel or cavern. This feature is used as input to the RMR system and the RMi rock support method.
3. Pattern of joints, which is used as input in the Q-system and the RMi support method.
Joint pattern is the occurrence of joint sets in an area. It is manifested in the shape of the rock blocks as shoen in the figure below:.
The Q and the RMi classification systems use a simplified jointing pattern given as ‘number of joint sets’ as an input parameter. See also the paper on ‘Observation of jointing features’.
A more comprehensive description of various measurements and observations is given in the paper Measurement and characterization of rock mass jointing.
More can be found in the Rock Engineering book.