Hydrostatic Pressure on Drilled Shafts 1

[JR55]

The geotechnical report for the drilled shafts (approx 3'-6" dia) I'm working on says to add hydrostatic pressure when below the groundwater table. I'm hoping for guidance on how this load should be applied. As far as I understand, the 62.4 pcf value should be multiplied by the depth of the pier below the GWT and the resulting value applied as a psf uplift load to the end area of the pier with a factor of 1.6.

Am I missing anything there?

 

[SlideRuleEra]

As long as the drilled shaft has constant diameter below the water table, you've got it.

If the shaft has a belled end, the calc would be different.

 

[humanengr]

Are you also applying a load factor (reduction factor) to the Dead Weight of the drilled shaft / concrete?
If so, applying 1.6 load factor to the hydrostatic pressure may be "double-dipping" on safety factors.

 

[ARPASEVAN]

Hello, maybe I am misunderstanding the question, but the caisson will be pushed up with the weight of displaced water. So for every foot below GWT, the force will be 3.14x1.75x1.75x62.5=600 lb uplift.

 

[BridgeSmith]

We usually just reduce the weight of the shaft to the net weight (150-62.4=87.6 pcf) below the highest expected water table for the uplift load case, apply the load and resistance factors, and move ahead. No reason to get too precise with the buoyancy when combining with soil friction resistances which are highly variable.

 

[jerseyshore]

I also take the net weight approach.

 

[HTURKAK]

-If the question is calculation uplift of the GW, you may calculate the volume of shaft below GW and multiply with γw=62.4 pcf. or use the approach discussed by BS.

- If you are in a position to calculate the pile carrying capacity, the submerged soil friction resistance will change.








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