Liquefaction-Pore water pressure 1

[Dakka]

Hi
Can someone please explain how and why does pore water pressure increase during an earthquake?
Thanks in advance.

 

[EireChch]

Porewater pressure increases because the loading is applied so quickly

 

[TigerGuy]

The motion generated in the earthquake effectively pumps additional water into the pore space of the soil being analyzed. If enough water is pumped into the soil mass, it will cause the individual particles to separate and eliminate the strength produced by the particles pressing against one another.

 

[geotechguy1]

Liquefiable soils want to contract when sheared (that is, the soil particles want to move closer together). In order for a saturated soil to contract, the water has to be pushed out of the pore spaces. However, the earthquake shaking induces the shear loads faster than the water can be squeezed out. This generates increasingly high water pressures which eventually become so large that the effective stress drops to zero.

 

[EireChch]

Good descriptions provided above by TG and GG. To add to that, for a soil to "technically" liquefy, the pore pressure has to be larger that the effective stress. Liquefiable soils, when tested by cyclic triaxial tests are deemed to liquefy when Ru coefficient (pore pressure / effective stress) is greater than 1.

 

[le99]

Can someone please explain how and why does pore water pressure increase during an earthquake?

I think it is mainly due to the hydrodynamic effect that moves the water from one cell to the other due to shaking. On top of the usual hydrostatic pressure, the cells downstream (in direction of earthquake movement) will receive additional pressure from the upstream cells.