What is Excess Pore Pressure 1

[DougHole]

Can someone explain to me using a simple analogy what excess porewater pressure is in the context of:

1) preloading

2) pile driving

 

[ItsOnlyMud]

Excess pore pressure is pore water pressure generated by loading the soil. Imagine a sponge soaked with water. If you load it (push down on it) water comes out, exactly the same sort of thing happens when you load soil, you try and squeeze the water out of it. However, clay soil is not very permeable, so the water can't get out quickly, so the water pressures increase, and gradually decrease as the excess pressure is dissipated by seepage through the soil. Sands and gravels are more permeable so rarely generate significant excess pore pressures, the excess pressure gererated by the load dissipates almost immediately.

Pile driving imparts force onto soil, this also generates excess pore pressures.

 

[fattdad]

excess pore pressure is when soil water pressure is greater than atmospheric pressure (at the phreatic surface) or greater than gammaW*Z below the phreatic surface.

Is this a take-home quiz from some class?

f-d

¡papá gordo ain’t no madre flaca!

 

[DougHole]

Thanks Fat Dad and Mr Ratty. I particularly like the sponge analogy from Mr. Ratty - makes good sense.

This is not a school quiz. I am just trying to learn about the wonderful world of soils engineering!

Any other comments would be appreciated.

Doug