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.

 

[oldestguy]

Unlikely your new lightly loaded pier will affect the adjacency pier as long as you don't fuond below that pier base elevation.

 

[JAE]

Drilled piers typically don't have the same confining hooping or stirrups that columns have.
So if you drill real close and reduce the confining action of the soil around the existing pier, there could possibly be an issue.
I doubt there'd be an issue but something to keep in mind.

 

[dauwerda]

Your geotechincal engineer will be able to provide you the best answer for this. In my past experience, I have typically seen 3-5 pier diameters used as minimum spacing in which interaction effects are negligible and don't need to be considered.

 

[r13]

The minimum allowable spacing of piles is usually stipulated in building codes. The spacings for straight uniform diameter piles may vary from 2 to 6 times the diameter of the shaft. For friction piles, the minimum spacing recommended is 3d where d is the diameter of the pile. For end bearing piles passing through relatively compressible strata, the spacing of piles shall not be less than 2.5d.

For end bearing piles passing through compressible strata and resting in stiff clay, the spacing may be increased to 3.5d. For compaction piles, the spacing may be Id.

 

[JAE]

Well, like azcats said, the second, new pier, is not really taking any significant axial load - only lateral load, so the 3x diameter rules really don't apply.

 

[dauwerda]

Yeah, but for a sign I imagine there is a decent amount of lateral load/ground line moment. So lateral interaction will still need to be considered.

 

[KootK]

1) Do you know for a fact that your piers are not, in part, skin friction? In my area, it's not uncommon for both mechanisms to be developed. Perhaps this is clear on the existing drawings etc.

2) If the piers would be very close, I'd definitely not want the new piers deeper than the existing without geotech guidance. I'd not want bearing material sloughing out of the existing shaft bearing strata and into the void temporarily created by the auger when it's backed out.

3) Given that this is a free standing sign, I'm surprised at the depth. But, perhaps, this is just the only depth you can go to for any vertical load where the geotech can guarantee limited settlement.

4) Given that this is a free standing sign, I'd suspect the governing condition to be the top of the pier/soil interface resisting lateral loads resulting from wind on the sign. Give some thought to what the pile would be pushing against when the wind would tend to push it towards it's neighboring piles. That, because that likely will not be "free field" soil. You might consider a grade beam etc back to the main building. Is the sign parallel or perpendicular to the building face?

 

[r13]

In order to resist lateral load, I think there shall be a minimum space to allow passive soil pressure to develop, geotechnical engineer should be tasked to perform the evaluation.

 

[BridgeSmith]

Given the scale of the garage vs. the sign foundation, the lateral pressure from the sign foundation isn't going to move the garage. The only potential issue I see is during construction, when you'll have a loss of lateral resistance for the nearby shaft(s) because of the adjacent open hole. Once again, given the scale of the garage foundation, the reduction should be negligible.

The only real effect is that the lateral resistance of soil around the sign foundation will be slightly higher, due to the presence of other concrete shafts nearby.

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

 

[KootK]

The only real effect is that the lateral resistance of soil around the sign foundation will be slightly higher, due to the presence of other concrete shafts nearby.

I'm not so sure. I feel as though, if the piles are too close together, the material between them may wind up being crap. Sure, the sign isn't going to move the parking garage but, if it's loose soil and McDonald's wrappers between the piles, the sign pile will be moving anyhow. At the least, I'd be temped to excavate a little between piles and maybe fill that back in with lean concrete etc. This all depends on the actual spacing though. If it's 3D, you're surely fine. If it's 1D, maybe not. Make sure there's something solid at the top for load transfer.

 

[r13]

Pier/pile cap can assists in develop required lateral load resistance.

 

[BridgeSmith]

I agree, KootK - much depends on 1) If the soil between the existing and new shafts sloughs off, and 2) what it gets backfilled with. I forget that sometimes the QC for building construction isn't what it typically is for bridges. Were this a bridge foundation, we could be fairly confident that any voids would be just left open and get backfilled with concrete when the new shaft was pumped full.

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

 

[BridgeSmith]

That must be one biiiig sign. The foundations for our 120' highmast light towers are 4' in diameter and typically around 20' deep.

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

 

[KootK]

we could be fairly confident that any voids would be just left open and get backfilled with concrete when the new shaft was pumped full.

That may well be the case here too and I'm just paranoid. For piers that large, I'd have to think it would be the same folks that usually bid transportation foundation work in the region. Although, if it's just two piers, perhaps not.

 

[azcats]

1) Do you know for a fact that your piers are not, in part, skin friction? In my area, it's not uncommon for both mechanisms to be developed. Perhaps this is clear on the existing drawings etc.

This assumption was made b/c the garage geotech report listed end bearing values only and gave no recommendations for skin friction. I don't have access to the structural calcs at this time. However - now I'm doubting this assumption. The drawings listed design allowable loads for different diameters and only end bearing of those is 4x higher than the 20 ksf indicated in the soils report. Drawings also mention some Geotech report addenda that I don't have ATM. I need to figure this out.

2) If the piers would be very close, I'd definitely not want the new piers deeper than the existing without geotech guidance. I'd not want bearing material sloughing out of the existing shaft bearing strata and into the void temporarily created by the auger when it's backed out.

Agree. IMO, this is the biggest concern.

3) Given that this is a free standing sign, I'm surprised at the depth. But, perhaps, this is just the only depth you can go to for any vertical load where the geotech can guarantee limited settlement.

That must be one biiiig sign. The foundations for our 120' highmast light towers are 4' in diameter and typically around 20' deep.

It's got about 800 sq-ft of wind loaded area and is around 50' high. There's also some dead load moment. I believe the depth was calculated per the IBC lateral pole formula with an assumed soil value.

4) Given that this is a free standing sign, I'd suspect the governing condition to be the top of the pier/soil interface resisting lateral loads resulting from wind on the sign. Give some thought to what the pile would be pushing against when the wind would tend to push it towards it's neighboring piles. That, because that likely will not be "free field" soil. You might consider a grade beam etc back to the main building. Is the sign parallel or perpendicular to the building face?

Sign will be ~45° to the building. In the direction of maximum load, it's really pushing between two existing piers. My gut is to not be concerned about the interaction given the scale of the two structures provided the material is decent near the top as mentioned below.

I'm not so sure. I feel as though, if the piles are too close together, the material between them may wind up being crap. Sure, the sign isn't going to move the parking garage but, if it's loose soil and McDonald's wrappers between the piles, the sign pile will be moving anyhow. At the least, I'd be temped to excavate a little between piles and maybe fill that back in with lean concrete etc. This all depends on the actual spacing though. If it's 3D, you're surely fine. If it's 1D, maybe not. Make sure there's something solid at the top for load transfer.

Valid concern. I think a some exploratory excavation near the top would be a good idea while they're drilling. Remove anything loose and fill back up with concrete upon pouring the new shaft. Looking at the proposed locations - it's in the 2D range.

So my next task it to indeed figure out if the existing piers are indeed using friction or not. I sure as heck don't want to be making recommendations that will induce settlement in the existing structure.

For my reference, when you guys mention 3D (etc) spacings, are those on-center, or clear between?

Your input is much appreciated.

 

[JAE]

3D for pier spacings typically are center to center. That is for shared gravity loading on end bearing piers.
For skin friction based piers, not sure if 3D is applicable - maybe our geotech friends here can chime in.

 

[r13]

Unlike pile group, friction for single pier/pile is not reliable. I think the Geotech will not recommend it.

 

[BridgeSmith]

For our sign structures, the axial (skin friction/end bearing) load is relatively so small, that it's barely a consideration. I'd be surprised if it wasn't the same for your sign structure. I'd suggest checking your skin friction/end bearing demand vs. capacity before getting too worked up about interaction/downdrag between the shafts.

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

 

[azcats]

Rod - I'm really not worried about the new sign at all. Like you said - new axial loads in the sign are so small we usually don't even consider them.
More worried about eroding soil capacity of the existing shaft that's potentially using skin friction and somehow causing a problem with the existing building. Unwarranted?