Pile Capacity In Different Subsurface Conditions

[geodoc]

Two identical pipe piles are driven close-ended at a site to the same driving criteria using the same pile driving hammer. Based on a review of test borings at the site, one pile is founded on bedrock whereas the other pile is founded on a dense sand layer located significantly above the bedrock. Assume that the pile lengths are essentially the same and that the capacity from side friction is essentially the same for both piles and is small in comparison to end bearing. Here's my question: Will these piles have the same capacity? Here's my answer: For the same load placed on each pile, I would expect the pile founded in the sand layer to experience more settlement than the pile founded on rock because the sand is more compressible. However, doesn't the fact that I installed the piles to the same driving criteria act to reduce the compressibility of the sand thereby giving the same capacity for each pile? If anyone knows of a good reference, or some good case histories for this, I would appreciate it.

 

[VAD]

Hello geodoc:

There are many ways to look at your problem. However, first I am curious to know the lengths of piles and the thickness of sand stratum and also why piles were stopped in sand and others went to bedrock, if I interpret your information correctly.

When you speak of capacity are you thinking ultimate under a pile load test. If so one would expect higher capacity with pile resting on bedrock. If you are taking about non-failure loads then it is possible depending on the load magnitude and transfer that both piles can sustain this load and in a sense behave similarly.

The fact that you used the same driving criteria may suggest to you that the energy imparted to each pile is the same. However, damping characteristics of the sand and bedrock would be different and would account for differences in capacity if these piles are subjected to a PDA test. This is my projection.

While shaft resistance may be thought to be small, this feature my account for significant load capacity if piles are long. Remember also that full toe resistanc is called into play only when the base of the pile is moved a certain amount - some say 5 to 10 pile diameters. The pile resting on competent bedrock could be designed as a structural member. The pile in sand would likely move down to accommodate a similar load, but not fail except if settlement is a criterion which should be the case.

I would prefer to look at pile design with an understanding of the loads to be transferred to the pile with allowance for future increase in loadings due to increased superstructure loads as in the case of a building for example. As well, we should be looking at the load transfer mechanism, drag loads, neutral plane etc.

In terms of references I would suggest that you look at the work by Dr. Bengt Fellinius on pile design. However, some aspects are still confusing which is moreso promoted when one - foundation or structural engineer asks for pile capacity to undertake a design. What is needed is a cooperative approach by both the geotechnical and structural engineer. While this is slightly off track from your questions, lack of this approach fuels some degree of uncertainty and promotes the old comfort factor of driving to refusal or to hard ground. This could be expensive in some situations.

 

[Focht3]

Good job, [blue]VAD[/blue]. Given the very approximate conditions described by [blue]geodoc[/blue], I would expect the "ultimate" bearing capacity of the sands to be reached with a tip deflection roughly equal to only about 1/3 of the pile diameter. Resistance would continue to increase with increasing deflection, but would provide no real additional benefit. And remember that the crushing strength of silica sands and gravels is usually reached when the bearing pressure approaches 100 ksf. The use of design ultimate bearing pressures that exceed 100 ksf is not a good idea...

If 'reasonable' design values and factors of safety were used, I would expect the performance of the two foundations to be quite similar because most of the working loads would be carried in skin friction in the upper 2/3 of the pile.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DRC1]

I would also expect the short term performance of the piles to be the same. Once you achive a certian set, you have obtained a capacity. If there is a significant difference in the soil, you will drive further in one condition to btain the same set. However set only measures the capacity of the pile during driving. Relaxtion or set up can alter the soil capacity (either up or down) after driving. If the granular soil gives you a significant increase in set up, and the bedrock is a weak sedimetary rock that is somewhat crushed by the driving, it is possible that the bedrock may show a little more settlement. Enough for us to debate but probably not enough to worry about. Thanks for an interesting question.