Joints and Jointing

Published on March 12 2011

Joint features, like joints characteristics and the degree of jointing are important parameters applied as input to classification systems, as described in Classification systems under the Classification and Design tag.

 

 Joint characteristics

The main joint characteristics are indicated in the figure below

 

There is a great variety of joints, from small cracks to long shears or seams, as seen below

The 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

Joint characteristics are used as input parameters to many classification systems, see Classification systems in the Classification and design tag:

You will find examples and applications of joint characteristics in several papers presented in other locations on the rockmass.net page

 

Jointing

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 tunnel
  • 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 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.

A more comprehensive description of various measurements and observations is given in the paper Measurement and characterization of rock mass jointing.

 

Quotation

Important quotations will be presented here from time to time

A. M. Muir Wood, 1979:  
"Soil mechanics is the study of the strength of soils while rock mechanics is the study of the weaknesses of rock"