Pile Group Efficiency

[EI12345]

Hello,

I am working on a problem for extrapolating a single pile load test to a pile group. I have not had much luck finding methods for determining pile efficiency. Some resources I have found that use Feld's rule indicate a 1/16 reduction in capacity for each neighboring pile. Yet, this method is irrespective of pile spacing.

The pile spacing I am working with is 5' O.C., with approx. 1' diameter piles. If pile spacing is >5*diameter, are group effects negligible? If not, what is an appropriate method to extrapolate a single load test to a pile group.

Thanks,

EL

 

[oldestguy]

There likely are more formulas but go to AASHTO files and "group action". One formula is Converse-Labarre. Others are Feld method; Masters; Seiler-Keeney method; These are from an old text by Chellis.

 

[SlideRuleEra]

If pile spacing is >5*diameter, are group effects negligible?

Much reduced, compared to 3*diameter, but not negligible. Spacing of 8*diameter is considered by some to be spacing where group effect becomes negligible.

Are you considering friction piling? Group efficiency does not apply to point bearing piling.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[EI12345]

I am considering a friction piling. Thank you oldestguy for the recommendations.

 

[LRJ]

I think most of the simplified methods are quite conservative. I like the Lehane et al. (2004) method as it simple and flexible in that you can have any pile layout.

I know of references which hstate 8 pile diameters as a limit for pile group effects. However, isn't this just for lateral group effects, with axial groups effects limited to something like 3 pile diameters? Are there references to support the spacings I've stated, or have I misremembered/misinterpreted something along the way?

Group efficiency does not apply to point bearing piling.

What is the basis for this? I would've thought the pressure bulbs of closely spaced piles will interact and you would need to consider an equivalent pier-type approach? Of course, the results of that analysis might suggest a single pile mechanism is more critical, in which case group efficiency on the friction between individual piles should be considered.

Genuinely I'm interested to know if there is a reference for this common assumption. I also only consider group efficiency on the shaft, but that's because of the piles I'm more familiar with designing (i.e. long, slender, effectively friction-only piles).

 

[SlideRuleEra]

(SlideRuleEra) Group efficiency does not apply to point bearing piling.

(LRJ) What is the basis for this?

Section 9 (Pages 33 -36) of "Bethlehem Steel H-Piles" (this page of my website) discusses the differences and similarities between friction and point bearing steel H piling. For point bearing on rock, minimum spacing is addressed with calculations based on the rock's properties - not pile efficiency (other than a minimum spacing recommendation).

Of course, as you know, most piling are a combination of friction and point bearing. As a 1970's bridge contractor, I had opportunities to drive wood, steel and prestressed concrete piling at both extremes, friction vs. point bearing, and at various combinations.
Driving "pure" point bearing... easy, get to "hard" soil or design elevation and stop (stop quickly with wood or the point will "broom").
Driving "pure" friction piling... all "art", not much science involved (surprise...wood is best - it's tapered & "swells" in our high ground water conditions).

For the majority of piling that are a combination of friction and point bearing, yes, I'll agree that pile efficiency does involve point bearing. But pile plan configuration and soil properties are so important that mathematical efficiency equations have limited value, IMHO.

I believe you will find the 2016 paper "A Simplified Formular For Analysis Group Efficiency of Piles in Granular Soil" interesting.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BridgeSmith]

Quote:
(SlideRuleEra) Group efficiency does not apply to point bearing piling.

(LRJ) What is the basis for this?

For piles designed as end bearing (driven to refusal), it is assumed that the bedrock is thick enough that the overlap of pressure will have a negligible effect on the capacity. If anything, the pressure from one pile would serve to increase the resistance of adjacent piles, through increased 'confinement' of the rock under the tip.

 

[bdbd]

LRJ,

Can you give full reference for Lehane et. al. (2004) method?

 

[LRJ]

Thanks SlideRuleEra and HotRod10. Valid points you've raised. Having thought about it more, for piles with friction and end bearing components, the working load should be sufficiently below the ultimate load that the end bearing is pretty much not mobilised at all, so all the interaction effects are on the friction.

What about the 3 diameter spacing for axial effects and 8 diameter spacing for lateral effects? Have you heard of this, or is it something that I've got wrong? I could've sworn I'd seen some evidence for this, but I can't find where.

The full Lehane et al. (2004) reference is: Lehane, B.M., Jardine, R.J., and McCabe, B.A., 2004. Response of a pile group in clay to first-time one-way cyclic tension loading. Proceedings of the Skempton Memorial Conference, Advances in Geotechnical Engineering, London, UK, 29-31 March, pp. 700-709.

Despite reference to cyclic loading in the title, the paper does actually detail a general 'load shedding' approach based on an earlier paper by McCabe (2002), who is one of the co-authors.

 

[SlideRuleEra]

What about the 3 diameter spacing for axial effects and 8 diameter spacing for lateral effects?

Bethlehem Steel recommends 2.5 diameter spacing for point-bearing HP (page 34). Other references (that I would have to find) say 3 diameter minimum for both point-bearing and friction piling. Three diameters is my design minimum because of construction tolerances. Consider a nominal 12" piling... 36" center to center spacing"

1) A driven displacement pile (wood or concrete) takes up a lot of volume. Without earth auger predrilling, soil compression can cause adjacent previous driven piling to heave. Three diameter spacing is about the minimum where heave is not a problem... as long as the Contractor has an "open" driving plan. Of course, non-displacement HP and cast-in-place piling don 't have this concern.

2) Typical specifications allow a 3" horizontal tolerance from design location. This is for the pile head... but could apply to the tip, also.

3) Piles that are driven plumb or battered have a typical tolerance of up to 1/4" per foot from design. For, say a 30' long pile, the tip could be 7.5" from intended location.

I won't figure the "worst case", but my point is that even with due care by the Contractor, a good many piling can end up with tips that are spaced 2.5 diameter, or so (when design is 3 diameter spacing).

The 8 diameter spacing for friction piling is well supported in the literature. I expect this applies to lateral loading, too. This chart is from the link provided:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net