## Dynamic Load Testing of Pile on Hard Granite

## Dynamic Load Testing of Pile on Hard Granite

(OP)

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.

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.

## RE: Dynamic Load Testing of Pile on Hard Granite

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.

## RE: Dynamic Load Testing of Pile on Hard Granite

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.

## RE: Dynamic Load Testing of Pile on Hard Granite

## RE: Dynamic Load Testing of Pile on Hard Granite

If design value is 1200 tons, can the actual load on

eachpile 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.

www.SlideRuleEra.net

## RE: Dynamic Load Testing of Pile on Hard Granite

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.## RE: Dynamic Load Testing of Pile on Hard Granite

## RE: Dynamic Load Testing of Pile on Hard Granite

## RE: Dynamic Load Testing of Pile on Hard Granite

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?

## RE: Dynamic Load Testing of Pile on Hard Granite

## RE: Dynamic Load Testing of Pile on Hard Granite

## RE: Dynamic Load Testing of Pile on Hard Granite

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.

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.