H Pile Driving into Shale

[jamoca67]

I'm working on a site that has used 12 x 53 steel H piles for end bearing piles. I'd like to use these piles again (Grade 50), but I also need the pile for lateral capacity purposes. I'd like to embed the pile a minimum of 1 ft into a shale grading highly to moderately weathered, +/- 5,000 psi unconfined strength. I need this embedment because the soils above the rock are pretty crappy. I'm planning to use pile points, and I'll be specifying a wave equation analysis prior to the start of work for the Contractor.

I was thinking of writing the spec so that it gives a minimum penetration requirement into the shale, and let the contractor figure out how to do it. Maybe, require the Contractor to submit a pile driving plan with contingencies should they be unable to get 1 foot of penetration with an impact hammer.

Does anyone see any issues with this approach? Thank you for your time.

 

[SlideRuleEra]

jamoca67 - I don't believe that 12 x 53 HP, even with pile points, can withstand impact driving needed to do accomplish that 12" penetration... but that is just my opinion. Could drive a few index piling before bidding the contract, or shortly after contract award? This would give you a definite answer. Is the 12" penetration a required number or a goal?

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[oldestguy]

Does your lateral capacity requirement mean a "toe hold"? On what basis is this figured? Using bedding plane shear strength? How would you test that capability if you had to?

 

[oldestguy]

A Google search for "rock socket design" comes up with plenty of info. Do these fit with your design method?

 

[Panars]

5,000 psi is still pretty good shale. Consider that it is stronger than typical concrete (3-4 ksi). The problem you will run into is that a pile hammer big enough to drive the pile into this rock a significant distance will also be too big to drive the pile without damaging it. In order to put that much energy into the pile, you will end up exceeding the yield strength of the steel. If you really need that much of embedment, you will have to drill and socket the piles into the shale.
But what about using battered piles? Will this be able to provide your lateral resistance?

 

[BigH]

The OP stated that the shale was grading heavily to moderately weathered. I find that statement at odds with an unconfined compressive strength of 5000 psi which is about 350 kg/cm2 (or 360 US t/ft2). As a quick check against unconfined compressive strengths for shales, Roy Hunt's Geotechnical Engineering Investigation Manual provides a range of 7 to 350 kg/cm2 for shales (Table 3.26). I would think that with a "standard" hammer (not too big) that you should be able to drive further than 1 ft into the shale - please check your unconfined compressive strength.