Drilled Pier Depth

[aseeng]

I review many soils reports with recommendations for drilled piers that include a minimum depth of penetration. Based on various soil layers it is sometimes obvious why the soils engineer selects a particular depth. Other times it is not so obvious and I am curious if this is determined by calculations or judgement or just picking a number because it has been used before.

For a specific case, I am looking at a site where there is a thick layer of sandy clay. We are looking at shallow piers here and the report recommends a minimum depth of 10 ft. No expansive soils, frost depth is shallow - say 2 ft. I will be designing a number of piers for pipe supports at a gas facility. I need 10 ft for some of the taller ones but many of them will be short lightly loaded supports and it seems we could use shorter 6-8 ft piers to handle these light vertical and lateral loads. I understand that when the piers get really short they act more like a shallow footing but my question goes back to the determination of the minimum depth.

Thanks for your opinions.

 

[Ron]

I calculate the depth needed to resist the applied loads...vertical down, uplift and lateral, and based on the soil conditions encountered.

 

[GeoPaveTraffic]

aseeng, in short minimum depths are determined by all of the methods you list. For the specific conditions you list in the post, it is difficult to know for sure why the 10 ft value was stated. Could be that the design is based on 9c/3 in which case Df/B would need to be greater than about 3.5. Or it could be that the near surface soils are somewhat weaker.

Mike Lambert

 

[jayrod12]

2 feet of frost can still cause frost jacking issues. Especially since the uplift capacity of a pile is conservatively 50% of the vertical down capacity. so really it's saying you've got 4 feet of resistance versus 2 feet of frost adfreezing. You'd be surprised how strong the frost can be, the Canadian Foundation Engineering Manual gives a range of 65kPa for fine grained soils frozen to wood or concrete up to 150kPa for steel.

That's a hell of a lot of uplift!