Drilled Pier Adjacent to Another

[azcats]

Existing multi-story (6-ish) CIP PT concrete garage with steel framed stair cores at the corners. Land owner wants to erect a free-standing single pole sign structure adjacent to the garage. The garage is really big (250' x 600').

At the sign location, the nearby garage building and stair core are founded on 4' & 5' diameter, end bearing, drilled piers (no bells) on dense sand/gravel/cobble at about 30' deep. We'll likely be closest to a 4' stair core pier.

Preliminary engineering for the sign structure shows that a 5' laterally loaded drilled pier will also need to be about 30' deep. After reviewing the geotech report for the garage construction, I believe the depth is conservative and can likely be reduced a few feet

They're looking for some preliminary guidance for how close the sign structure foundation can be to the existing drilled piers.
1. Since it's end bearing I needn't be concerned about area of influence at for skin friction, correct?
2. Any general guidance for this situation?

I've not dealt with this before and don't want to overlook anything.

Input and questions welcome.

 

[BridgeSmith]

I'd say concern about losing skin friction capacity due to soil sloughing away from the existing shafts could be warranted. This could likely be mitigated by the use of temporary (or permanent) casing for the new shaft.

I was thinking of the interaction effects of the shafts putting pressure on the same soil where the zones of influence overlap. That was the effect that I expected would likely be negligible due to the minor amount of pressure exerted by the sign foundation.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

More in concern is the lateral pile movement, without adequate distance between piles with good soil medium, the influence can be undesirable.

 

[BridgeSmith]

I'm not sure I understand your concern, retired13. You believe it's possible that lateral movement by the sign foundation shaft could overstress the adjacent shafts?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

I think the whole idea of limiting pile spacing is to minimize the stress interaction between piles, OP mentioned the new shaft is about 2D to the existing, that is less than the usually required/suggested 3D - 4D range, so the concern. Also, without good soil and not utilizing a pile cap, the lateral displacement could be un-tolerable, another concern.

Actually I do have a question for you, as for the large sign subjects to wind, how is the twist handled by the pier, shear friction? Or the twist is somehow handled internally by the pier, and has negligible effect on surrounding soil?

 

[BridgeSmith]

Limiting pile spacing has 2 purposes that I know of:

1) Limiting the overlap of soil 'blocks' resisting settlement (attempting to support adjacent shafts on the same soil mass). There are reductions of skin friction capacity in AASHTO for shafts at spacing less than 5D.

2) Similarly, for lateral loading if the shafts are too close and bending in the same direction (as with a single cap supported by multiple closely spaced shafts), there is a reduction in the capacity of the shafts bending in the direction of an adjacent shaft.

In the case under consideration, the vertical load of the sign structure is relatively small, so it's effect on the garage foundations should be negligible.

My assumption is that the lateral loading on the garage foundation due to wind will be negligible. Typically, any direct shear on a building foundation like that is resisted by footings, grade beams, etc. and is not resisted by bending the drilled shafts. Moment transferred to the footing is resolved into force couples, producing mostly uplift and bearing loads on the shafts, rather than bending. Therefore the sign foundation should be largely unaffected by the presence of the other shafts, other than a possible increase in the stiffness of the surrounding soil, due to presence of a stiffer element within the resisting soil mass.

Rod Smith, P.E., The artist formerly known as HotRod10