Driven Piles for foundation onto Rock

[swimfastalex]

I am currently reviewing a design from another company and was wondering if anyone could help with finding a good Foundation Book on Rocks.

I am a Structural Engineer, but we design our own foundations, using the AASHTO LRFD code, so I'm well versed when it comes to Driven Piles and Drilled Shafts in soil (plus I love geotechnical engineering as well). Also Drilled Shafts in Rock, usually using a rock socket for drilled shafts. However, AASHTO doesn't seem to have a whole lot of rocks when it comes to Driven Piles. They have two ways to determine the geotechnical resistance when there is rock, either assuming a granular material when it's soft rock or the pile material as the pile will fail before the rock when it's hard rock. The explanation on the difference between soft and hard rock isn't the best. I've also referenced the FHWA Driven Pile Design and Construction document, but that doesn't have much when it comes to driven piles and rock.

Usually I've seen driven piles bearing on rock, and this will satisfy the bearing resistance for the piles. However, in this case the piles will be driven through rock. The soil above the rock is very poor (SPT between 2 to 20 for Silty Sands) and some of the piles are in tension. My only assumption is the fact they assumed that driving the piles through rock they will obtain enough skin resistance from the rock for the uplift of the pile (HP Piles by the way).

So is there a static analysis method of determining if the rock can provide a skin resistance? Also, is there any good books when it comes to Rock and Foundations? If no static analysis method, I'm assuming then they would have to perform a dynamic testing on the pile to provide enough uplift resistance? The contractor will be performing a CAPWAP Analysis on test piles anyways.

Thanks. Any help would be appreciated.

 

[oldestguy]

A pretty broad question. Perhaps clarify by stating the area of work so that the geology factor comes in (driving through rock?), types of structures commonly involved. Have you done or will you do any load testing? In your review of anther company design, why? and did an experienced geotech engineer prepare part of that?

 

[swimfastalex]

Sorry, let me clarify a little bit. The project that is being designed is a Bridge, and the Bridge abutment will be on HP piles currently. This is just the design phase of the project. I'm not sure if it was a geotechnical engineer or not. Trying to be a little vague, but mainly we are reviewing the final design calculations (as an independent reviewer instead of someone in house). From the plans and information from talking with them, the piles will be driven, not to the rock but basically through the rock (as in the estimated tip elevation will be below the top layer of the rock). The rock is a schisctose (sp?) and an unconfined compression strength test was performed on core samples. Hopefully this helps some.

So, I did a design check by just calculating everything on my own (the loads, pile structural resistance and geotechnical resistance). Based on my calculations (and the assumption that the pile material will fail before the rock does) the axial load on the piles will be under the toe resistance of the rock (or steel pile in this case). However, my calculations also determined that some of the piles will be in tension (uplift). Looking at the boring logs, the soil itself will not be sufficient enough to sustain the uplift from the piles. So I'm wondering, is there a case where piles can truly be driven through a softer rock than normal and that rock provide any sort of skin resistance? If so, is there a static analysis to determine this, or does this uplift resistance need to be determined in the field by CAPWAP?

 

[SlideRuleEra]

...the piles will be driven, not to the rock but basically through the rock...
...assumption that the pile material will fail before the rock does...

There may be an unpleasant surprise... steel H-piles are not indestructible. When rock is encountered they can easily be overdriven:

At a minimum, consider using H-pile points when driving in rocky conditions:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

It would seem that these questions should be posed to the author of that report, since that person may have much more experience as to what actually happens in that situation. For instance I did a Google search "piles in soft rock" and many papers come up,with one, in particular, by a DOT engineer who likely has dealt with many bridges and support thereof. As to load tests, when I have had any significant question as to what to expect out there, load tests, both down and up, reveal a lot. Even pulling some piles is very educational as to what goes on down there. Those H piles can take on a major change in appearance!!! I doubt that any book will fully answer your question, but technical papers may help.

 

[swimfastalex]

SlideRuleEra, yea I know that will definetely not be a great idea. It's a comment I already added though to them but thanks.

Oldestguy - It's more we will make comments to them. So I've already put a list of comments together to send to them, and it's their choice of what they want to do with them. I was really looking for if there was an possibility as to that. Seeing as I assumed there really wasn't I'll just make a comment and see if I get a response as to what they did.

The book was also more for my reference. We don't do much with rock, usually I've come across clay or sand material. So I was seeing if there was. But thanks.

 

[oldestguy]

You may not be happy with the results of written back and forth comments when a person to person conversation with the data in front of each can clear up things much more rapidly. Remember who is responsible for what. I'd expect the geotech report to fully cover all aspects of the performance of the project foundations, so asking for an explanation won't hurt, but don't take over the foundation basic input support data without taking over the responsibility for what happens later.

 

[cvg]

for an ITR, I would fully expect that the designer would provide any and all information requested by the reviewer necessary to document and justify the design. If it is not in the report, it should be. typically, we require all comments to be addressed to the satisfaction of both the reviewer and the designer before the ITR can be completed.

I don't know why the independent reviewer would take on any responsibility if he is not actually approving anything?

 

[EireChch]

From Section 7.12 of Tomlinsons-Foundation Design and Construction (Seventh Edition), piles driven through moderately weak to moderately strong rocks will likely be shattered by driving and may only provide the frictional resistance of a loose to medium dense gravel.

Not good news if you are relying on a sort of rock socket for uplift forces. You may also need to consider the potential for the pile buckling as the softer soils above rock will not provide much lateral support if they are closer to an N of 2

 

[oldestguy]

By discussing these questions with the geotech of record, they may or may not affect the product. H piles shattering??? perhaps distorting.

 

[Okiryu]

Since you will be driving piles thru rock, I would also recommend to do some Pile dynamic testing to check stresses during driving, so you can avoid damages as shown in the pics posted above. You can get some benefits on doing dynamic testing as lower ring the FOS and therefore increasing the pile capacities.[pre][/pre]

 

[dik]

SRE... I usually spec Pruyn points... used them for 4 decades and no problems. How does the geotech stipulate 'refusal'? and does he provide a specified load for refusal?

Dik

 

[swimfastalex]

For driving through rock or just skin resistance in general, I made a comment about it and will wait to see the response. Dynamic Testing will be performed. The plans show a minimum penetration elevation, based on scour, settlement, fixity and lateral movement (I think that's all). Then there is a design load and a driving load, in which the driving load needs to be met (by dynamic testing) to determine the length of Pile.

So the contractor will need to drive the Pile past the minimum penetration elevation, then drive it in order to meet the geotechnical resistance.