Importance of fold; define thrust

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.