Negative skin friction on end bearing piles.

[connect2]

In the case of an end bearing pile founded on 'bedrock', placed thru a compressible layer called 'clay' with a fill material placed over the clay, just prior to driving, and the clays resultant consolidation causing down drag-negative skin friction = additional load on the pile. Is this additional load a permanent load or does it dissipate with time? Assuming the pile tip and the rock can carry the load.

 

[BigH]

connect2 - I bring to your attention that there were a number of papers written where piles founded on bedrock subjected to dragdown forces like you described failed. These were described in papers back in the 60s and 70s I believe - ISSMFE. (eg. Johannessen and Bjerrum "Measurement of the compression of a steel pile to rock due to settlement of the surrounding clay" Proc 6th ISSMFE Montreal, 1965). Piles on bedrock will not 'move' and therefore, in my view, more load will be dragging down the piles where piles in compressible stratum will have some give so that the dragdown forces are not as high. You may see good explanation in Tomlinson's piling books (Pile Design and Construction Practice) - I have an older edition with me (1977) - see Section 4.8

Note that much has been written on dragdown on the recent years (notably Fellenius) but these are piles in soil on soil.

 

[moe333]

The neutral plane concept discussed by Fellinius basically states that provided the capacity of the pile tip and lower portion of the pile exceeds the drag load from the settling soil above, then the drag load would tend to compress the pile through elastic shortening rather than cause downdrag of the pile. If the structural capacity of the pile is sufficient to carry the drag load, it actually turns out to be beneficial since the pile is pre-compressed prior to application of dead load, resulting in smaller structure settlement. The big question is where is the location of the neutral plane? as this will determine whether the bottom of the pile has sufficient capacity to resist the drag load from the settling soil.

The location of the neutral plane is a function of the load distribution in the pile which only includes the dead load.

You can download many of Fellinius's papers:

http://www.fellenius.net/papers/258 Fellenius Unified Design-Goble volume.pdf

 

[BigH]

The stumbling point as I see it is that the structural capacity of the pile must be able to withstand the loads - and many case histories in the Scandanavian countries showed that this many times did not happen. You must be able to address the load and ensure capacity.

 

[connect2]

Yes to the above.
Assuming the load can be adresses by the pile, is this a permanent, sustained, long term load on the pile? Or does it dissapate with time?

 

[moe333]

It takes only very small strains to mobilize the full drag load, and consolidation will usually occur over many years, so I would consider the drag load to be a long term load.

 

[DRC1]

Generally, but not always, a pile founded on rock has sufficent geotechnical capacity through end bearing, thus the neutral plane is located at the toe. The dead load what ever percentage of the liveload is typically applied and the drag load are in general permenant loads. Should larger loads, such as those froma storm event or full live load occur, the pile will shorten relative to the soil and the drag load wil dissapear. Once the load is removed the pile will eventually end up where it started.

So for dead load, and maybe a portion of live load, yes the drag load is permenant. Note that drag loads occur in all soils and not only soft or compressible soils.

As Big H noted the piles must be structurally designed to carry the drag loads, or coated to reduce drag loads.