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Kevinkdb (Structural) (OP)
5 Oct 10 21:36
Need help from experienced or knowledgeable pile guys.
Piles in my project have vertical compression loads of about 2000 kN (say 500 kips), and traction loads of 1000 kN.
Depth to rock, which is not very good rock, is about 50 feet, and the piles will probably be 12 inch round tubes filled with concrete.
If the piles are inclined, will this increase the tension capacity wrt vertical piles ?  Like an inclined nail is harder to pull ...
Each winter I anchor down my temporary garage with inclined 1" dia. pipes driven about 3 feet in the ground, and when I try to remove them in spring, I need to pull exactly along the axis or there is no way they will come out.
 
ishvaaag (Structural)
18 Oct 10 12:27
There is a way to ascertain the gain by doing FEM analyses with three materials soil(s), pile and cap and then subjecting the cap to the forces to see what happens. In fact it will be maybe even more dubious (locally) what one would have to model for the pile interface in a pure axial pull than one case where the force pushes pile and cap against the soil.

Quite likely the behaviour is favorable as you think, for the normal forces induced along the piles' surfaces should lead to bigger friction, and you are also adding passive push to overcome, the sum of which may lead -depending on the configuration- to a higher limit or allowable load.

In a prestressed works' book combinations of inverted V pair of piles are said to be used to support alternate directions of tension-compression. It also warns of that raker or batter (inclined) piles (prestressed variety at least) are being discontinued for use in seismic areas due to poor performance when receiving the brunt of the seismic load at their tip, being now preferred sets of vertical piles able to develop swaying hysteretic behaviour.
cvg (Civil/Environmental)
18 Oct 10 12:59
if you incline them too much, the verticle tension on the inclined pile will produce a bending moment in the pile. Are you going to design for that?  
paddingtongreen (Structural)
18 Oct 10 15:31
Could you use reinforced, belled, caissons instead?
 

Michael.
Timing has a lot to do with the outcome of a rain dance.

BigH (Geotechnical)
18 Oct 10 18:16
The "increase" you are getting with inclined is due to the passive earth pressure on the "nail" when you pull it so the "nail" moves into the soil.  Watch the use of raked piles in seismic zones.  paddingtongreen has a good suggestion of using belled caissons - 250 US Tons on a 12 inch steel pipe pile (closed ended?) seems a bit high from my recollection of driving them into shales and the like. You will need to ensure that you aren't exceeding the structural capacity of the pile.
DRC1 (Civil/Environmental)
24 Oct 10 23:38
Tension is resited by soil friction. 125 kips capacity is a lt to develop. (if you are sizing by static analysis your FS is generally 3). You can reliably develop tension capacity by installing a rock anchor in the center of the pile. You can use strand if you feel the rock can take 750 kips in compression (6.5 ksf). If not use bar anchors, wich will not add any additional compressive force. I would install the piles plumb if you are using anchors.

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