Pile Capacity

[swimfastalex]

I had a question about determining Pile Capacity. I'm using the Nordlund Method for Driven Piles to determine the skin and tip resistance. In most cases when designing a bridge Abutment, as a conservative approach I usually take the bottom of footing as the top of the ground (usually the ground elevation in front of the abutment is only a few feet above the top of footing). Therefore the Total and Effective Stress start at 0. However, in this case I am designing driven piles for a buried structure. The structure is a culvert type in which the proposed bottom of footing will be 10 feet below the existing ground line and 20 feet below the proposed ground line. When determining the Total and Effective Stress (for skin and tip resistance), would I take the stress starting from the proposed ground line, existing ground line, or from the bottom of footing?

 

[oldestguy]

To get an idea as to how precise your estimating is, keep a record of pile penetration records. After a significant number of jobs, you will have an answer to your question. I had this luxury when it was my responsibility to estimate what might occur out on the job. Simple soil friction and end bearing estimates came out amazingly correct with some 90 percent of driven lengths within a 5 foot bracket. No computer program back then. I suggest shooting for that precision, which probably says either assumption you might make will be OK. Hopefully the pile driving formula is as precise.

 

[Ron]

Whether or not and when the pile "feels" the additional lateral pressure from the increased surcharge of the new ground level will depend on the soil conditions. If sandy, it won't take long. If clayey, then it could be a while before significant increases in lateral pressure are noted.

You might consider before and after pressuremeter readings or cone penetrometer readings to check the changes. For design, I would conservatively use the bottom of the footing level for your calcs.

 

[geotech_ben]

Good Day Alex -

There are about 1,282 different parameters that can affect pile design. Type of pile, length, closed end/open ended, plugged/unplugged, concrete, timber, steel, stresses during driving, setup time. Then you have to consider all of the different ways to calculate the capacity: Nordlund, alpha, beta, modified beta, USACE, API, FDOT, Nottingham and Schmertmann, Brown methods. I would recommend a good read of the new FHWA LRFD Design Manual. It's only around 1,000 pages and it will give you a quick peak into the world of pile design. There is a lot more if you want to d(r)ive deeper... Don't forget the Nordlund is not as applicable for piles over 24 inches in diameter and it does generally overpredicts the capacities at greater depths. Consider downdrag too if you are applying a surcharge above the piles. I believe that the new FHWA has a revised criteria for downdrag.

Have fun!
Ben

 

[swimfastalex]

Ben,

Thanks. Oh the puns lol.

Yea I was finally happy that FHWA came out with a new Driven Pile manual, took them a while. I'm using the Nordlund method for an HP 14x73. I did look at settlement and I'm good there, so no Downdrag. I guess for me, I'm usually designing Bridge Abutments and for Driven Piles we call out a minimum Pile penetration based on fixity, settlement, lateral movement and scour. Then I put a driving load on there that will need to be meet in the field based (luckily lately) on dynamic testing (CAPWAP). I only usually determine a Pile tip elevation (from geotechnical analysis) because the client wants this for estimating purposes and also to make sure that it works in general without being crazy lengths.

I know being conservative would be to take determine the effective stress as if the bottom of footing is the top of ground. When designing piles for bridge abutments this isn't too conservative as the proposed ground at the toe is usually a few feet above the bottom of footing. However, this is a buried structure (concrete arch) with the crown being 5 feet. Slow the proposed ground line and the rise being 17'. So I wasn't sure how a buried structure would affect the vertical stress of the soil and in turn increaSe the Pile capacity.

I've always loved soils so I keep up with geotechnical Engineering, and I make sure I know what I'm designing and such. I've just never come across designing piles for a buried structure, in which the structure itself has it's own soil-structure interaction.