Bearing Capacity of Timber Piles 1

[JohnnnyBoy]

Is there a bearing capacity difference between a timber pile with a driving shoe compared to a driven pile with a flat bottom and does the soil matter. Normally driving shoes are used when needing to go through boulders or other material although I have seen them be used by contractors on many occasions for easier installations.

Generally looking at it the bearing surface area of both would be the same, thus the bearing capacity should be the same. When I look more into it the bearing capacity is normally driven by a settlement of 20-25mm normally. Rationally thinking, it seems like using the same driving energy on a driving shoe would yield more settlement per blow although I am not sure if that assumption is true. I would imagine that the point of the driving shoe pushes the soil out of the way much more efficient than with the flat bottom pile.

Any thoughts/comments would be much appreciated.

 

[SlideRuleEra]

1) Is there a bearing capacity difference between a timber pile with a driving shoe compared to a driven pile with a flat bottom...

2) ...and does the soil matter.

3) Normally driving shoes are used when needing to go through boulders or other material although I have seen them be used by contractors on many occasions for easier installations.

4) ...bearing capacity is normally driven by a settlement of 20-25mm normally.

5) Rationally thinking, it seems like using the same driving energy on a driving shoe would yield more settlement per blow although I am not sure if that assumption is true.

6) I would imagine that the point of the driving shoe pushes the soil out of the way much more efficient than with the flat bottom pile.

Good questions, but the answers are complex. I'll try to put my spin on it.

1) Probably, yes, but there are so many other factors that the difference is too small to measure and varies from one pile to the next.

2) Yes, absolutely. A pile and soil work together and when properly designed and constructed, available bearing is greater than either the pile or the soil could accomplish alone.

3) Shoes are also used on timber piles to keep the points from "brooming" in hard driving condition. A broomed pile tip looks like the end of a giant toothpick that has been chewed until it is "mush". Practical point bearing value is zero, and if driving continues, brooming continues until the entire pile disappears below the ground.

4) In this case, skin friction is a major factor. Point bearing does not amount to much, with or without a shoe. Compared to concrete and steel piles, timber piles are unique:
[ul]
[li]Total allowable timber pile loading is much lower than a similar size concrete of steel pile... but the surface area (available for skin friction) is about the same. Therefore, skin friction unit loading (lb / in[sup]2[/sup]) is lower... skin friction is a greater factor.[/li]
[/ul]

[ul]
[li]Timber piles are tapered (often about 1" per 10' of length). Timber piles tend to get "wedged" in the soil... not much, but it does tend to increase bearing capacity because...[/li]
[/ul]

[ul]
[li]If there is soil moisture (or groundwater) timber (wood) piles "swell" (a little bit) because of the water... increasing both the wedging effect and skin friction.[/li]
[/ul]

5) If piles are mindlessly driven to a specific tip elevation... and driving stops, this may be true.
Instead, if they the piles are driven to specific bearing value (based on energy delivered by the hammer to the pile) this should not be a factor. Of course, a minimum pile embedment should be specified.

6) Maybe true, but trivial because of Items 1) through 5) above. Also, some timber pile points are not "points" at all, they are blunt.

http://www.SlideRuleEra.net

 

[JohnnnyBoy]

Thanks for the response and I appreciate your spin on everything.

 

[jdonville]

The American Wood Preservers Institute have design guidance for timber piles. Here's the 2002 version:

http://www.piledrivers.org/files/99...a-402f-b252-e7e8f85df9f8/timberpilemanual.pdf