Pile Axial Flexibility -Difference between subgrade reaction modulus and soil modulus of elasticity?

[theshearstud]

1. What is the difference between the subgrade reaction modulus and soil modulus of elasticity when calculating axial pile flexibility?

2. Say you have a pile foundation system and the soil type varies roughly 4 times throughout the pile length. Subgrade moduli are vastly different among soil types. They geotech has given each subgrade moduli, but has only given one pressiometric modulus, E. Would it be appropriate to use Gazetas (1991) formula for axial pier stiffness?

Thanks geotechs!

 

[ATSE]

For pile settlement, modulus of subgrade rxn (usually the variable "k") is NOT used. The units are force/length^3 (like kN/m^3 or lbs/in^3). Think pavement and lightly loaded slabs.
If you are using E (Youngs Modulus), then you need load distribution information (and layer thicknesses). If you haven't done this since college, you will probably want some help from your project geotech.
Your best bet for pile stiffness is to review full scale load test results for similar piles in similar soils.

Even without load test results, you can bound your solution. Your service settlement is going to be 10mm to 20mm (3/8" to 3/4"). Let's say your working (service) capacity = 700 kN (157 kips).
k = P/delta = 700 kN / 10mm = 70 kN/mm high stiffness, 35 kN/mm low stiffness.
That should be a good start for structural modeling.