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Rock cleavage: types and significance

Short answer

Rock cleavage is the tendency of metamorphic rocks to split along closely spaced parallel planes due to pressure. Types are slaty cleavage, fracture cleavage, and flow cleavage (schistosity). In civil engineering, cleavage causes directional weakness, instability of slopes dipping into excavations, water seepage, tunnel overbreak, but enables easy slate quarrying.

Define rock cleavage. Describe the types of cleavage. Mention the civil engineering significance of rock cleavage.

Rock cleavage

Rock cleavage is the tendency of a rock to split along closely spaced, parallel, planar surfaces that are not related to the original bedding. It is a secondary structure formed in metamorphic rocks (especially slate, phyllite and schist) by pressure.

Types of cleavage

  1. Slaty cleavage - very fine, parallel planes; the rock splits into thin, flat sheets. Developed by parallel orientation of mica and chlorite flakes at right angles to the maximum compressive stress (slate).
  2. Fracture cleavage - numerous closely spaced parallel fractures with small displacement; usually in brittle rocks and in the crests of folds. Also called shear or strain-slip cleavage.
  3. Flow cleavage - formed by plastic flow and the parallel arrangement of flattened grains and recrystallised minerals; seen in phyllite and schist, where it is also called schistosity.

Civil engineering significance

  • Cleavage planes are zones of weakness, so the rock strength is low parallel to them.
  • They make cut slopes unstable when cleavage dips towards the free face.
  • They give leakage paths for water in dams, reservoirs and tunnels.
  • In tunnels they lead to overbreak and roof falls.
  • On the positive side, slates are cut easily into roofing and paving slabs.

✓ Answer checked by Claude Opus 5.5 (high effort)

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