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Petrology; engineering significance of rock classes

Short answer

Petrology studies rock origin, composition, and classification. Metamorphic massive rocks are strong, but foliated ones are weak along planes; sedimentary sandstone strength depends on cement, limestone may leak, shale is weak and swells, and bedding dip controls stability.

What is petrology? Describe the engineering significance of the metamorphic and sedimentary rock classes.

Petrology is the branch of geology that deals with the origin, occurrence, composition, structure, texture and classification of rocks.

Engineering significance of metamorphic rocks

  • Massive types: quartzite, marble and gneiss are strong and durable; they are used as aggregate and as building and decorative stones, and are good foundation rocks.
  • Foliated types: slate, phyllite and schist split along foliation planes; strength is low along these planes, so slopes, tunnels and foundations can fail. Slate is used for roofing.
  • Mica- and talc-rich rocks are slippery and weak.
  • Weathering of schists produces thick, weak soils, which are common in the Nepal Lesser Himalaya.

Engineering significance of sedimentary rocks

  • Sandstone: building stone, aggregate; silica-cemented sandstone is strong, clay-cemented sandstone is weak.
  • Limestone: raw material for cement and lime, and aggregate, but solution cavities cause reservoir leakage and settlement.
  • Shale, mudstone: weak; they swell and slake, cause slope failures and make poor foundations.
  • Conglomerate: may be strong or weak depending on the cement.
  • Bedding planes control stability: an unfavourable dip toward the cut face causes sliding.
  • The Siwalik sedimentary rocks of Nepal are weak and erodible.

✓ Answer checked by Claude Opus 5.5 (high effort)

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