--- title: "Engineering significance of fault" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:engineering-significance-of-fault" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/engineering-significance-of-fault site: BCT Engineering --- # Engineering significance of fault Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2081 Chaitra (new course), 2 marks; 2080 Bhadra, 3 marks; 2078 Kartik, 3 marks; 2076 Asoj, 3.5 marks; 2061 Baisakh (old course), 3 marks; 2065 Shrawan (old course), 2 marks (asked 6 times) - Video: [Effect & Significance of Fault](https://www.youtube.com/watch?v=0aCIqT9XTfM) — S Barad - 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-fault - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question Describe the engineering significance (effects, problems) of fault in civil engineering. ## Short answer Faults create weak, crushed zones causing foundation settlement, seepage under dams, tunnel collapses and slope failures. Active faults pose seismic risk. They must be avoided or treated with excavation, concrete plugs and grouting after careful mapping. ## Answer A **fault** is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres. #### Engineering significance (effects and problems) - **Weak zone**: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail. - **Leakage**: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage. - **Tunnelling**: sudden inflow of water, roof collapse, squeezing and heavy support requirements when a tunnel crosses a fault. - **Slope instability**: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides. - **Differential settlement**: a structure built across a fault, with different rock properties on the two sides, can settle unequally. - **Seismic hazard**: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must be kept away from them. - **Uncertainty in exploration**: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable. #### Precautions - Avoid building dams, big bridges and tunnels across active faults, or provide a special design for the expected displacement. - Treat the fault zone with excavation, concrete plugs, grouting and drainage. - Map faults carefully by surface mapping, trenching, drilling and geophysical survey before design. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a fault zone poor for foundations? A. Crushed rock has low strength and high compressibility B. Strong and impermeable C. Unbroken D. Cemented Answer: A. Breccia and gouge. (Key point: Weak zone) 2. Fault zone effect on water? A. Barrier B. Channel causing seepage and leakage C. No effect D. Seals reservoir Answer: B. High permeability. (Key point: Leakage) 3. Why avoid active faults for dams? A. Always dry B. Weathered C. They can move again, causing earthquakes and rupture D. No reason Answer: C. Seismic hazard. (Key point: Seismic) 4. A precaution for fault zones? A. Ignore B. Build directly C. Fill with sand only D. Excavation, concrete plugs, grouting and drainage Answer: D. Treat the zone. 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