Overburden Correction for Shear Wave Velocity

[nbr1]

Determination of Liquefaction Susceptibility/Potential may involve utilization of shear wave velocity which is corrected for overburden stress.

My question is why/how is shear wave velocity impacted by overburden stress and why is a correction necessary (I cannot rationalize this in my mind)? Seems like if the shear wave velocity is higher at depth, then the soil/sands would be stiffer (more dense).

 

[dgillette]

This is because the stiffness and shear-wave velocity for a sand AT A GIVEN DENSITY increase with increasing confining pressure. Hence, FOR A GIVEN SHEAR-WAVE VELOCITY, the sand is looser at greater depth. That's why SWV is divided by (sigma'v/atm)^0.25 before going to the chart, analogous to Cn for SPT.

(Empirically, shear modulus is roughly proportional to (sigma'v)^0.5 By theory of elasticity, SWV is proportional to (shear modulus)^0.5. Therefore, SWV is roughly proportionals to (sigma'v)^0.25.)

I think I got that right, without taking time to look it up.