--- title: "Define fault with diagram and its importance" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:define-fault-with-diagram-and" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/define-fault-with-diagram-and site: BCT Engineering --- # Define fault with diagram and its importance Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2075 Asoj, 4 marks - Video: [Overview of Geologic Structures Part 2: Faults and Folds](https://www.youtube.com/watch?v=aodnQWXYkZ0) — Professor Dave Explains - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/define-fault-with-diagram-and - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question Define fault with a neat diagram and discuss its importance in civil engineering. ## Short answer A fault is a fracture with relative displacement of rock blocks, consisting of a fault plane, hanging wall, and foot wall. It creates weak crushed zones causing foundation settlement, water leakage, tunnel collapses, slope failures, and seismic risks. Active faults may rupture the surface, and displaced strata complicate exploration. ## 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 range from a few millimetres to hundreds of kilometres. ```text fault plane \ FOOT WALL \ HANGING WALL ===== bed A ======\ \ ----- bed B --------\ ===== bed A ===== \ \ ----- bed B ----- \ The hanging wall (above the fault plane) has moved down: a normal fault. Throw = vertical offset of a bed; heave = horizontal offset. ``` Parts: fault plane, hanging wall (the block above the plane), foot wall (the block below the plane), dip, throw and heave. #### Importance in civil engineering - **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 may settle unevenly, because the rock properties differ on the two sides. - **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which block lies above the fault plane? A. Hanging wall B. Foot wall C. Heave D. Throw Answer: A. The hanging wall is above the plane, the foot wall below. (Key point: Parts) 2. Why is a fault zone a poor foundation? A. It is made of strong massive rock B. Crushed rock (breccia, gouge) has low strength and high compressibility C. It is impermeable and dense D. It never moves again Answer: B. Crushed sheared rock is weak and compressible. (Key point: Weak zone) 3. Why must dams and bridges keep away from active faults? A. Faults are always inactive B. Faults raise bearing capacity C. They can move again, causing earthquakes and surface rupture D. Faults make water flow stop Answer: C. Active faults can rupture the surface. (Key point: Seismic hazard) 4. What can happen when a tunnel crosses a fault? A. Dry stable rock needing no support B. Lower rock pressure C. Reduced seepage D. Sudden water inflow, roof collapse and squeezing Answer: D. Fault zones carry water and are crushed, causing inflow and collapse. 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