--- title: "Fault: causes, classification, slope effects" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:fault-causes-classification-slope-effects" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fault-causes-classification-slope-effects site: BCT Engineering --- # Fault: causes, classification, slope effects Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2065 Shrawan (old course), 2+4+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fault-causes-classification-slope-effects - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question What is fault? Write down its causes. Classify fault with suitable diagram. State the effects of fault on the strength of the slope as a whole. ## Short answer A fault is a fracture with relative displacement. Caused by tectonic stress, plate movements, isostatic adjustment, volcanic activity, and gravity sliding. Classification: normal (tension), reverse/thrust (compression), strike-slip (shear), oblique-slip, rotational, horst/graben. Faults reduce slope strength by creating weak crushed zones, providing sliding surfaces when dipping out of slope, increasing water pressure, accelerating weathering, and enabling earthquake-triggered movement. ## 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. #### Causes of faulting - Tectonic stress (compression, tension, shear) exceeding the rock strength. - Plate movements, and uplift and subsidence of crustal blocks. - Isostatic adjustment, volcanic activity and magma intrusion. - Release of strain energy during earthquakes; also gravity sliding. #### Classification **By relative movement (genetic classification)** 1. **Normal fault** - the hanging wall moves down relative to the foot wall; caused by tension. 2. **Reverse fault** - the hanging wall moves up relative to the foot wall; caused by compression. A reverse fault with a dip of less than 45 degrees is a **thrust fault**. 3. **Strike-slip (lateral) fault** - the blocks move horizontally along the strike of the fault; caused by shearing. It may be right-lateral (dextral) or left-lateral (sinistral). 4. **Oblique-slip fault** - the movement has both dip-slip and strike-slip components. 5. **Rotational (hinge/pivotal) fault** - one block rotates about a point on the fault. 6. **Horst and graben** - a horst is a block raised between two normal faults; a graben is a block dropped between them. ```text NORMAL FAULT (hanging wall moves down) foot wall \ hanging wall ==== A ========\ \ ---- B ----------\ ==== A ==== \ \ ---- B ---- REVERSE FAULT (hanging wall moves up) foot wall \ ==== A ==== hanging wall \ ==== A =========\ ---- B ---- \ ---- B -----------\ (\ = fault plane dipping to the right; the right-hand block is the hanging wall) ``` #### Effects of a fault on the strength of a slope - The rock in the fault zone is crushed (gouge, breccia) and weak, so the strength of the slope is reduced. - A fault plane dipping out of the slope is a ready-made sliding surface, leading to planar or wedge failure. - Fault zones carry water, which increases pore pressure and reduces shear strength. - Differential weathering along the fault produces deep weathered zones. - Movement may be renewed by earthquakes, triggering landslides. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the basic cause of faulting? A. Weathering of surface soil B. Wind erosion C. Tectonic stress exceeding the rock strength D. Growth of vegetation Answer: C. Faults form when stress exceeds strength. (Key point: Cause) 2. In a strike-slip fault how do blocks move? A. Vertically down along the dip B. By rotating about a point C. Upward along a low angle D. Horizontally along the strike, due to shear Answer: D. Strike-slip is horizontal along strike. (Key point: Strike-slip) 3. Why is a fault dipping out of a slope dangerous? A. It acts as a ready sliding surface for planar or wedge failure B. It strengthens the slope C. It blocks all water D. It prevents landslides Answer: A. The fault plane is a plane of weakness. (Key point: Slope with fault dip) 4. How does water in a fault zone affect slope strength? A. It increases shear strength B. It raises pore pressure and reduces shear strength C. It has no effect D. It cements the rock Answer: B. Pore pressure lowers effective stress. 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