--- title: "Engineering significance of three rock types" subject: "Engineering Geology I" chapter: 4 key: "semester_3/engineering_geology_i:engineering-significance-of-three-rock" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/engineering-significance-of-three-rock site: BCT Engineering --- # Engineering significance of three rock types Engineering Geology I · Chapter 4: Petrology - Asked in: 2081 Kartik (new course), 3 marks; 2068 Chaitra, 4 marks; 2063 Baisakh (old course), 3 marks; 2059 Chaitra (old course), 3 marks; 2076 Chaitra, 4 marks (asked 5 times) - Video: [Petrology, Rock Cycle & Rock Types in Civil Engineering | Engineering Geology](https://www.youtube.com/watch?v=wuToPOh72V0) — CiviL TechnO BrickS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/engineering-significance-of-three-rock - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4 ## Question Discuss the engineering significance of the three rock classes (igneous, sedimentary, metamorphic). ## Short answer Igneous rocks (granite, basalt) are strong, dense, good foundations and aggregate but may have joints and deep weathering. Sedimentary rocks vary: well-cemented sandstone and limestone are good building stones; shale is weak, swells, and causes landslides; limestone may contain cavities. Metamorphic rocks: massive types (quartzite, marble) are durable; foliated types (schist, slate) are anisotropic and weak along foliation, affecting slope stability and tunnels. ## Answer The type of rock affects its strength, durability, permeability and behaviour, so it is a basic consideration in selecting sites and materials. #### Igneous rocks - Generally hard, strong, dense, massive and low in porosity; granite and basalt are good foundation rocks for dams, bridges and heavy buildings. - Used as crushed aggregate, road metal, building and dressing stone. - Problems: fractures, joints and weathering in humid areas; sheet jointing; columnar jointing in basalt causes leakage; weathering of granite to clayey soil. #### Sedimentary rocks - Properties vary widely with cementation and layering. - **Sandstone** (silica-cemented) and **limestone** are good building materials and aggregate; limestone is the raw material for cement. - Bedding planes are planes of weakness, so slope and tunnel stability depend on the dip direction. - Shale is weak, slakes in water and swells, and causes landslides. - Limestone has solution cavities, which cause leakage in reservoirs. #### Metamorphic rocks - Massive types (quartzite, marble, gneiss) are hard and durable; marble and slate are used for flooring, roofing and decoration. - Foliated rocks (schist, phyllite, slate) split along foliation, are weak, and cause slope failure and tunnel squeezing; mica minerals reduce strength. - Some are anisotropic: strength depends on direction of loading. Hence the engineer needs to study the type, structure and weathering of rocks before choosing foundation level, slope angle and support. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are granite and basalt good foundation rocks for dams and bridges? A. They are hard, strong, dense and low in porosity B. They are layered with weak bedding planes C. They are foliated and split easily D. They are soft and swell when wet Answer: A. Igneous rocks are generally hard, strong, dense and low in porosity. (Key point: Igneous rocks as foundations) 2. What problem is caused by columnar jointing in basalt? A. Slaking B. Leakage C. Tunnel squeezing along foliation D. Swelling of clay Answer: B. Columnar joints open paths for water, causing leakage. (Key point: Columnar jointing) 3. Why does dip direction matter for slope and tunnel stability in sedimentary rocks? A. Bedding planes are the strongest part of the rock B. Dip changes the mineral composition C. Bedding planes are planes of weakness D. Dip decides the type of cement Answer: C. Bedding planes are weak surfaces, so stability depends on how they dip relative to the cut. (Key point: Bedding planes) 4. Why do schist and phyllite cause slope failure and tunnel squeezing? A. They are massive and brittle B. They dissolve in water forming cavities C. They are glassy and vesicular D. They split along foliation and are weak, and mica reduces strength Answer: D. Foliated rocks part along foliation planes and mica lowers their strength. (Key point: Foliated metamorphic rocks) ## Related in Engineering Geology I - [Metamorphic rock, cleavage; igneous rock significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ametamorphic-rock-cleavage-igneous-rock) — Engineering Geology I, chapter 4 - [Petrology; engineering significance of rock classes](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrology-engineering-significance-of-rock) — Engineering Geology I, chapter 4 - [Recognising sedimentary, metamorphic, igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arecognising-sedimentary-metamorphic-igneous-rocks) — Engineering Geology I, chapter 4 - [Metamorphic rock formation; igneous rock texture](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ametamorphic-rock-formation-igneous-rock) — Engineering Geology I, chapter 4 - [Petrogenesis, igneous rock identification, rock significance](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrogenesis-igneous-rock-identification-rock) — Engineering Geology I, chapter 4, asked 2× - [Identify the three rock types in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-the-three-rock-types) — Engineering Geology I, chapter 4, asked 5× ## Related in other subjects - [Sedimentary versus igneous stones](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Asedimentary-igneous-stones) — Civil Engineering Materials, chapter 2 - [Rock classes needing steel rib and shotcrete](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-steel-rib-classes) — Engineering Geology II, chapter 6 - [Rock forming minerals and significance](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-forming-minerals) — Engineering Geology II, chapter 1, asked 6×