Calculate aprox. Caisson dia. with added skin friction

[Rohrscheib]

I'm trying to figure out a recommended caisson diameter and depth for a structural load of 400 kips. I attached a table with recommended lateral load design.

Below is what I came up with.

450 #/sf (12.57SF) = 5,655 #/FT
5,655 lb/ft (57 ft) = 322,327.41 lb
=322.33 kips
400kip - 322k
=78kip

qu = 12 ksf
p = 78

qu=p/a

a = p/qu = 3.14d2/4

d=2.88

Any ideas?

 

[jayrod12]

Both square footage for skin friction and end bearing are related to the diameter of your pile. you don't give any pile dimensions. If you are going to 57 ft you didn't account for the fact that your first 18 feet are in crappy soil.

Or are you going to the top of the dense till and neglecting the first 18'?

Either way. Your Pr=Prs+Prb where Prs is contribution from skin friction and Prb is the contribution from the base.

Prs = (pi)*d*s*l s is skin friction

Prb = (pi)*[(d^2)/4]*bearing pressure at tip

If you know your length you can solve the equation (it will be a second degree polynomial)

If you are attempting to optimize the design then you've got to modify both length and diameter to find the best solution.

 

[Rohrscheib]

I am trying to optimize the design. My best solution would be a 3.5' dia. shaft with a 8ft. bell bearing at 45'.

qu = 6 KSF
Skin friction = 450 #/ft
Laffective = 37-18 = 19ft (no skin friction above the bell).
P total = 400 kip

sk = 94 kip
P = 306 kip

D= square root(4p/piQU) = 8.05 ft (Bell)

qu = 6 ksf @ 45ft

 

[jayrod12]

That would be one scary ass bell to go and hand clean. But it seems you have the right of it.

 

[Rohrscheib]

Thanks for the input jayrod!

 

[EireChch]

A kind of related question that may need a new thread but anyways I'll shot here first- I presume that the pile would be cased with some structural casing the prevent collapse of the pile wall while a man is down hand cleaning the bell? Would this be correct? Even though the soils at 35 are stiff to very stiff it would still be cased?

 

[jayrod12]

I would say so. I would never climb down a hole that deep without some form of steel casing protecting me. I'll be honest, it would literally take a dumptruck full of $100 dollar bills for me to climb down into that hole, sleeved or not.