Pore pressure (or pore water pressure, 𝑢) -TS

 

Pore pressure (or pore water pressure, 𝑢)

Soil is the pressure exerted by the fluid (usually water) within the spaces (pores) between soil particles, acting equally in all directions. It's crucial in geotechnical engineering because it reduces the effective stress (the force carried by the soil skeleton, between particles, impacting soil strength, stability, and deformation, governed by the principle 

 (Total Stress - Pore Pressure). This pressure can be positive (below the water table, hydrostatic), zero (dry soil), or negative/tension (unsaturated soil, known as suction). 

• Three-Phase System: Soil has solids, water, and air; pore pressure relates to the fluid phase.
• Neutral Stress: Pore water pressure is also called neutral stress because it acts equally in all directions, like hydrostatic pressure.
• Effective Stress: The actual stress carried by the solid particles, calculated as Total Stress minus Pore Pressure, which dictates soil behavior.
• Hydrostatic Conditions: Below the water table, pore pressure generally increases with depth
• Unsaturated Conditions: In the "vadose" zone (above the water table), pore pressure is negative (suction) due to capillarity, measured by tensiometers. 

Importance in Geotechnics 

• Shear Strength: Higher pore pressure reduces effective stress, weakening the soil and decreasing its shear strength (resistance to sliding).
• Consolidation: Changes in pore pressure drive consolidation (settlement) as water is squeezed out of fine-grained soils.
• Liquefaction: Rapid increases in pore pressure (e.g., during earthquakes) can cause soil to lose strength and behave like a liquid.
• Stability: Critical for assessing slope stability, bearing capacity, and potential for landslides.