Define joint; classification and importance

A joint is a fracture or crack in a rock along which there has been no (or negligible) displacement of the two sides.

Geometric classification of joints

Based on the relation of the joint with the attitude of the host rock (strike and dip of beds):

1. Strike joints - run parallel to the strike of the beds.
2. Dip joints - run parallel to the dip direction of the beds (at right angles to the strike).
3. Oblique (diagonal) joints - cross the bed at an angle to both strike and dip.
4. Bedding joints - lie along the bedding planes.

Based on the relation to folds, longitudinal, transverse and oblique joints are also distinguished. Joints that occur in parallel groups form a joint set, and two or more sets form a joint system.

       strike joint   dip joint   oblique joint
     ----------------  |  |  |    /  /  /
     ----------------  |  |  |   /  /  /
     (parallel to      (along      (at an angle)
      strike)           dip)

Engineering importance

• Reduce rock strength and bearing capacity; blocks bounded by joints can slide or fall.
• Slope stability: joints dipping out of the slope or intersecting each other give wedge, plane and toppling failures.
• Seepage: joints are paths for ground water, causing dam leakage and tunnel inflow.
• Weathering starts along joints, so the rock deteriorates to a greater depth.
• Quarrying: joints help in extracting blocks, and close jointing reduces block size.
• Tunnelling: joint orientation controls overbreak and roof falls; the tunnel axis should be at a high angle to the strike of the joints.
• Grouting and rock bolting are needed in highly jointed rock.