Dynamic Load Testing of Pile on Hard Granite

[tq3610]

I'm looking for some input on dynamic load testing of an end-bearing steel pile in hard granite bedrock.

Consider a PDA test was carried out on a steel pile acting in end-bearing (~90% EB, loose granular soil above rock) that results in 1,000 tons ultimate capacity at a given set blow-count. The acheived 1,000 tons capacity is limited by the pile driving stresses (45 ksi for 50 ksi steel). The structural capacity of the pile under static load is higher (say, 1,200 tons based on the IBC allowable load capacity).

If this pile is end-bearing in very hard rock, and the limiting capacity of the pile as deterimined by PDA was the upper limit of driving stresses, then what is the fault in using the ultimate pile capacity of 1,200 tons?

From a practical perspective, if the pile was installed to an ultimate capacity of 1,000 tons based on a set blow count and a limiting driving stress, once the pile has been installed and it is then loaded (statically by the weight of the structure) the increase in load from 1,000 tons to 1,200 tons would seemingly only result in the pile penetrating some very minor amount into the very hard to rock. Maybe the pile is set an additional 1/8 or 1/4-inch into the very hard rock? Understandably the pile couldn't be driven to a capacity of 1,200 tons due to the driving stresses, but application of the structure loads being very different in nature to the pile driving stresses, it seems that this situation would be much underestimating the capacity of the pile. Once the pile is confirmed to be well-set into the hard granite bedrock, loading the pile to the maximum allowed by IBC per allowable stresses seems reasonable.

Thoughts on this? If such a pile were to be tested with a SLT I would expect it would yield a result allowing the 1,200 tons acceptable per the structural capacity of the pile.

 

[MTNClimber]

The question I would have is why you are only getting 1,000 tons on the PDA if the allowable structural capacity is 1,200 tons? Did the pile get damaged during driving? Speaking of, the other thing to consider is staying well away from from the structural capacity while driving because as soon as you hit hard rock you will have a short time period to stop the hammer before damaging the pile. Although its considerably more difficult to damage a steel pile, compared to timber or concrete, it can still happen if you have a bunch of numskulls running the show.

When it comes down to it, if the extra 200 tons is going to save you a lot of money then run a SLT so you can use 1,200 tons. If you don't have many piles and therefore a SLT isn't economical see if using a bigger steel section to get 1,200 or adding another pile or two at 1,000 tons is cheaper. In the end, it's what's best for the client's purse.

 

[tq3610]

The reason for the 1,000T versus 1,200T is based on the driving stresses in the pile. Hitting the hard rock, as you stated, results in a quick increase in pile stress. It may only take an increase of 1 blow to go from 900T to 1,000T to 1,100T to 1,200T. 1,000T results in a stress of 45ksi (limiting driving stress) while 1,200T results in a stress closer to 48ksi, which isn't acceptable for driving.

Part of this goes into selection of pile hammer. There could be a hammer that delivers a reduced force that could get closer to 1,200T at 45ksi, and that could be evaluated with a WEAP analysis.

But - choosing a different hammer doesn't really to get to the actual question. That being, if the pile is end-bearing in hard bedrock (and we know it based on the PDA testing and numerous test borings), then why not run the allowable capacity up to that allowed by IBC for the structural material?

I would suggest in this case the PDA be used to verify the pile is well-seated into the top of the rock (and close to the ultimate capacity), but not to specifically determine the ultimate capacity.

 

[MTNClimber]

For structural allowable capacity it's 0.9Fy for LRFD and Fy/1.67 for ASD. I'm not understanding what's stopping you from driving it to those stresses? I must be missing something here.

 

[SlideRuleEra]

...why not run the allowable capacity up to that allowed by IBC for the structural material?

If design value is 1200 tons, can the actual load on each pile be both precisely and accurately calculated to not to exceed 1200 tons for all loading conditions for the life of the structure?

IMHO, reserve capacity is important since building code loading combinations are only ideal, simplified mathematical simulations of loads that really occur.

http://www.SlideRuleEra.net

 

[tq3610]

The pile is driven to 45ksi during driving. At 45ksi (driving stress) the calculated ultimate capacity per CAPWAP is 1,000T. That driven capacity is not equal to the allowable load given by static analysis of the pile section. This is due to the particular hammer, soil conditions during driving, etc. I think you are not accounting for the difference between the stresses during install versus a static design capacity.

 

[tq3610]

The basis of this question isn’t in an active project where I’m trying to increase a pile capacity. It was a years old project where the calculated static capacity was 1200, but the foundation contractor’s engineer said the pile could only achieve 1000 based on their WEAP analysis and their particular hammer. The design didn’t change much (added a handful of piles), but my thought had always been if the structural capacity is 1200 and it’s on very hard rock, even if the CAPWAP says 1000 is the ultimate, the actual consequence of using the 1200 design would be a minor additional penetration into rock if the foundations ever actually saw that extreme state.

 

[MTNClimber]

I understand. So in this case it just added some piles, not a big deal. But if your gut is telling you that you can get the extra capacity and save a significant amount of money on piles, pile cap size, ect. then go for the SLT if the PDA is coming up short (which in my experience it always has). The building inspector won't let you just assume that you can get that extra 200 tons without proving it.

 

[tq3610]

Thanks MTN. I agree the building inspector isn’t going to approve a foundation based on gut. I’m more just asking if there is a fatal flaw in the premise that the additional load from 1000T to 1200T in this scenario is essentially inconsequential as far as whether it puts the foundation system below acceptable factor of safety. In actuality it’s not really based on gut at all - it’s a capacity of steel for that section amounting to 1200T per IBC allowable design loads. The only leap is that there is a 200T increase on the pile that couldn’t be “verified” by PDA, but that can be easily accounted for as I’m suggesting.

I could have pushed back and said use a different hammer so we can get to 1200 and not change the design, but as was suggested the cost of a few extra piles didn’t warrant it.

Mostly it was just personal frustration on my part that the contractor suggested the design wasn’t achievable.

Do you agree the consequence of going from 1000T to 1200T would be only negligible penetration into the rock?

 

[Settingsun]

What are the pile details? Is 1200 tonnes equal to pile cross-section area multiplied by 45ksi? In my area, it's conventional wisdom that a pile driven to refusal on sandstone will achieve (approximately) the structural capacity of the steel, but that's on the basis that the axial capacity is somewhat reduced by bending moment (from analysis or the code minimum).

 

[HENRYZAU]

Any horizontal load to the pile? The bearing capacity of the pile is governed by its structural capacity, not geotechnical. If it stops on top of the hard rock, there will be no tip fixity expected in terms of sliding or rotation.

 

[MTNClimber]

Do you agree the consequence of going from 1000T to 1200T would be only negligible penetration into the rock?

I don't have all the information so its hard to tell. It all depends on how solid the rock is. If its not weathered then it may not penetrate at all and you'll only see elastic deformation in the pile. If the top couple of inches or feet are weathered, it could plunge. You can't tell until you get a SLT on it. That being said, the pile won't ever see 1200 tons since IBC would only allow a design load of 600 tons.

If it stops on top of the hard rock, there will be no tip fixity expected in terms of sliding or rotation.

I don't have the loading or subsurface information but if the pile stops on hard rock 55 feet below bottom of cutoff, I'd assume that the pile tip would be relatively fixed. Fixity is based on subsurface conditions and depth... but at a certain point it doesn't matter what the subsurface conditions are. You'll get fixity with enough penetration.