Minimum Pile Length

[ktwannabe]

Does anyone have a good reference for minimum pile lengths for HP steel piles driven to refusal on limestone bedrock? The piles are only loaded axially. For PADOT, the minimum allowable pile length is 10 feet. I have used 6 feet in the past with success but that was based on my "gut" instinct and not "hard" science. I am looking for a reference that will explain why there is a minimum pile length. I am assuming it is based on pile driving stresses or lateral loading but am not sure.

Thanks

 

[ashjun]

I am not exactly sure of the answer since it has been a long time that pile designs have actually been given to me. However, in foundation engineering, I did study that if the piles are designed as friction piles, then the length should be at least 5 times the pile diameter - any length less than that the soils tend to move upwards and the entire skin friction is not generated in this length.

For end bearing, the socket length should be at least 2 to 3 times the pile diameter. I think both the conclusions are based on field tests.
Rgds

 

[PEinc]

ashjun, what is the "socket length" of an end bearing pile?

 

[GeoPaveTraffic]

If the piles would be that short, why not just use a footing?

 

[boo1]

I would consider using poured in place concrete plies.

 

[ashjun]

hi, socket length is the length of the pile embedded in rock or refusal strata.
Rgds

 

[PEinc]

I'm not sure how you can get a socket length in rock for a driven pile. You may be able to drive the pile slightly into rock, but probably not enough to consider it a socket.

I would not be surprised if the minimum pile length was based on economics in comparing the cost of additional driven pile length versus extra excavation, concrete, and backfill required to build a spread footing at a deeper elevation. Maybe someone calculated that a footing with 10 feet maximum of extra depth would be cheaper than driving piles for a shallower footing.

 

[BigH]

It seems to me that the "minimum" length in the original post is not a theoretical minimum based on soil/rock mechanics principals but based on common sense construction considerations. They must figure that anything less than 3 m could have had another system used that would make more economic/construction sense. Seems expensive to bring in a pile rig for a lot of 7 footers! Excavation to the 7 or 8 ft, set footing, etc. might be a whole lot more appropriate.

 

[PEinc]

BigH,

I guess great minds think alike. We responded within 11 seconds of each other with essentially the same answer.

 

[BigH]

OPEinc - and from across the world!!

 

[ashjun]

Hi again,
Yes, it all depends upon the minimum cost, the availability of equipment and the experience of the local engineers amongst other factors.