--- title: "Importance of fold; define thrust" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:importance-of-fold-define-thrust" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/importance-of-fold-define-thrust site: BCT Engineering --- # Importance of fold; define thrust Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2081 Baisakh (new course), 2 marks; 2081 Bhadra, 1.5+2.5 marks (asked 2 times) - Video: [Himalaya's Fold mountains and Thrust boundaries explained with animation (part 2/3) | UPSC](https://www.youtube.com/watch?v=mF58L2J0FK4) — Zeta Axis (UPSC-SSC) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/importance-of-fold-define-thrust - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question Define thrust. What is the importance of fold in civil engineering? ## Short answer A thrust is a low-angle reverse fault (dip <45°) bringing older rocks over younger. Folds affect dam foundations (dip upstream favourable), tunnels (anticline drains, syncline traps water), slope stability, and groundwater/oil traps. ## Answer #### Thrust A **thrust** is a low-angle reverse fault (dip generally less than 45°) in which the hanging wall has moved up and over the foot wall under compression, bringing older rocks over younger rocks. Examples in Nepal are the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT). #### Importance of folds in civil engineering - **Foundation**: fold limbs dip in different directions, so a dam or building on a fold must take the dip of the beds into account. Dip upstream is favourable for dams; dip downstream with weak beds is dangerous. - **Tunnelling**: in an anticline the roof is in tension (more joints and loose blocks, but water drains away); in a syncline water collects and rock pressure is high. The crest and trough zones are more fractured. - **Reservoir**: a syncline can hold ground water, so leakage is low; an anticline cut by a valley may let water escape along the dipping beds. - **Slope stability**: beds dipping in the slope direction at an angle less than the slope can slide. - **Water supply**: synclines form artesian basins; anticlines form petroleum and gas traps. - **Quarrying and exploration**: beds repeat on the two limbs, so the outcrop pattern must be understood to estimate rock depth and bed thickness. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which description fits a thrust? A. A high-angle normal fault where the hanging wall slides down B. A low-angle reverse fault (dip under 45°) where the hanging wall moves up over the foot wall C. A fracture with no displacement of the two sides D. A vertical fault with only sideways movement Answer: B. A thrust is a low-angle reverse fault formed by compression, with the hanging wall pushed up and over the foot wall. (Key point: Definition of thrust) 2. What is the usual age relation across a thrust? A. Younger rocks are brought over older rocks B. Rocks of equal age are placed side by side C. Older rocks are brought over younger rocks D. Older rocks are removed completely Answer: C. Thrusting under compression carries older rocks up and over younger rocks. (Key point: Rock age relation in thrust) 3. Why does an anticline matter for tunnelling according to the answer? A. The roof is in tension, with more joints and loose blocks, though water drains away B. Water collects there and rock pressure is highest C. It has no joints, so no support is needed D. It always gives artesian water Answer: A. In an anticline the roof is in tension and more jointed, while a syncline collects water and has high rock pressure. (Key point: Folds and tunnelling) 4. Which statement about fold type and reservoir leakage is correct? A. An anticline always makes a watertight reservoir B. A syncline always leaks along its limbs C. Fold type has no effect on leakage D. A syncline can hold ground water so leakage is low, while a valley cutting an anticline may let water escape along dipping beds Answer: D. Syncline structure retains water, whereas dipping beds exposed in an anticline valley can leak. 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