Driven Piles for foundation onto Rock
Driven Piles for foundation onto Rock
(OP)
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.
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.
RE: Driven Piles for foundation onto Rock
RE: Driven Piles for foundation onto Rock
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?
RE: Driven Piles for foundation onto Rock
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:
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RE: Driven Piles for foundation onto Rock
RE: Driven Piles for foundation onto Rock
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.
RE: Driven Piles for foundation onto Rock
RE: Driven Piles for foundation onto Rock
I don't know why the independent reviewer would take on any responsibility if he is not actually approving anything?
RE: Driven Piles for foundation onto Rock
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
RE: Driven Piles for foundation onto Rock
RE: Driven Piles for foundation onto Rock
RE: Driven Piles for foundation onto Rock
Dik
RE: Driven Piles for foundation onto Rock
So the contractor will need to drive the Pile past the minimum penetration elevation, then drive it in order to meet the geotechnical resistance.