H-Piles: Minimum Embedment?

[mike80]

For H-piles bearing on bedrock, is there a minimum length which should be provided between the bottom of footing and top of rock, other than for cost considerations? The overburden for this particular project is mostly clay material.

 

[SlideRuleEra]

Many DOT specifications require 10 feet (minimum) - a reasonable number that has provide satisfactory service.

http://www.SlideRuleEra.net

 

[emmgjld]

I have typically recommended a socket depth of 5 to 10 feet unless Lateral or Uplift Forces is a concern. In most instances, I have found the geology of the bearing material to have a lot of relevence. An erratic weathered surface, a steeply dipping bedding or an surface bedrock surface tends to increase the socket depth. In some very special instances, I have allowed as little as 2 feet, but you had better know the geology and the design requirements VERY WELL.

 

[Qshake]

The fact that many DOTs use 10' is due to the AASHTO requirement.

Additionally some DOTs require that pile groups are designed such that under no load condition does a pile experience uplift. I'm sure others allow some uplift.

Most geotechs will recommend that the upper 5 feet of disturbed soil be disregarded anyway which doesn't leave a lot of resistance.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[emmgjld]

mike80,

I am embarrassed. I misread your question and answered wrong.

The 10 foot requirement between the rock and the bottom of footing has a lot to do with how the pile behaves under load. Much less than 10 feet and the element does not behave as a column. Different methods of analysis come into play. The same situation can exist with cast-in-place concrete piers.

 

[mike80]

Thanks for the quick responses. This particular question applies to a railroad bridge abutment under construction. The abutment was designed so that there is no uplift on the piles. The bedrock elevation varies, so in most cases there is fifteen feet or more from the bottom of footing to top of bedrock. However, due to the variation, there are also a few piles (4 out of 24)which hit refusal at 5 to 10 feet below the footing. The railroad is requiring the contractor to place additional piles adjacent to the short piles, preboring holes into the bedrock so that a minmum of 15 feet of length is provided.

Based on emmgjld's statement that the 10 foot minimum is to allow the pile to behave as a column, this seems to be reasonable.

Would shorter piles change the distribution of load from the footing to the piles?

 

[emmgjld]

I admit that I have not extensively studied the issue, except in a couple of cases where my driven piling where only about 6 feet long. In that case the single pile acted like an extended footing/pad, while the others were definately piles. I did not require a longer pile or more piles as I was dealing with expansive shales and more piles would have been wrong from a minimum bearing standpoint and no advantage was seen to 'forcing' the longer pile into the shale.

I have always recommended to make the foundation a pile foundation or a footing/pad foundation. The in between state does not lend itself to straight forward computations and it is not desirable to mix foundation types. I try to avoid the 'neither fish nor fowl' condition.

 

[Qshake]

What has been proposed is exactly what I've seen done over the years in the same situation. I thought you anticipated the short piles but from your last message they've been driven already.

When you anticipate them, it is most often dealt with by pre-boring the pile in place.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[BigH]

Scratching my 'ead on this one, what is the extra piles going to provide in the small area where the piles are just a few feet short? Normally foundations are judged deep if D/b >5 . You have this with normal "H" piles. Boring and embedding up to 10 ft into the shale for the four piles seems overkill to me and gives just as much of a "non-uniform" design as if the short piles were left alone. It is quite doubtful that the short piles will be 'overloaded' due to vertical loads nor would their uplift capacity be reduced considerably from that of the longer piles driven into rock embedded in clay. The only reason, in my view, to take the piles more into the shale is if the slope of the shale is steep and you want to ensure that the pile will not 'kick-out' in practice.

 

[emmgjld]

BigH,

Your question/analysis is very similar to my reasoning when I have anticipated very short piling. As I generally have very large pile capacity (90+ tons S.F.>2.5) for fairly small loads (<40 tons) I usually disregarded the so called problem unless I had a concern with subsurface topography (similar as you mentioned) or geology.

Qshake,
The few cases where I had to really think about the problem were when I was surprised by the short depth and either loads (V & H) were high or the short pile was caused by unanticipated subsurface topography.