Describe the role of pore pressure changes in triggering liquefaction during earthquakes.

• A. Shaking increases pore water pressure, reduces effective stress, causing saturated soils to lose strength and behave like a liquid
• B. Pore pressure decreases causing soils to strengthen
• C. Liquefaction occurs without affecting pore pressure
• D. Pore pressure changes only affect rocks

Answer: (C)

Pore pressure changes during earthquakes drive liquefaction by lowering the effective stress in saturated soils. When shaking occurs, pore water pressure rises inside loose, saturated sediments. The effective stress that holds soil grains together is the total stress minus this pore pressure (sigma' = sigma − u). As u increases, sigma' falls, causing the soil to lose strength. If the shear stresses from the shaking exceed this reduced strength, the material behaves like a liquid and can flow, a phenomenon we call liquefaction. This most often happens in loose, saturated sands and silts near the surface where drainage is limited.

The other ideas don’t fit the mechanism: reducing pore pressure would not trigger liquefaction (it would tend to stiffen or strengthen soil), and liquefaction is fundamentally tied to changes in pore pressure, not to processes in rocks.