Screw Piles 3

[DataAB]

Is there a comprehensive book or manual available on screw (helical) pile design?

I have some design literature on screw piles based on:
1) capacity due to bearing capacity of the helices only, and
2) capacity due to a combination of bearing capacity of the bottom helix (pile is in compression) and the cylindrical shear resistance of the soil between helices (assuming there are two or more).
I get different values using both approaches (about 20% difference in calculated capacity).

Any comments would be appreciated.

 

[ishvaaag]

One value may refer to the maximum load in bearing able to be taken (within code assumptions) by the helices, and the other to the expected service level capacity within a specific soil. One must use the lower. Yet be attent it the loads are both service level or else.

 

[csd72]

Its mostly proprietary information, look up

http://abchance.com/helical/

There are others too.

Structure mag also has some articles:

http://www.structuremag.org/article.aspx?articleID=489

http://www.structuremag.org/OldArchives/2004/june/Deep Foundations.indd DLS.pdf

http://www.structuremag.org/OldArchives/2004/june/ScrewPileFigures.pdf

 

[PSlem]

I would be curious data you used. The loss of strength due to remolding for cohesive soils would have an effect on the upper helices. A paper by Liu, Zubeck and Schubert with two 10" helices found the load distribution to be 75%/25%. Also some take helix spacing as 3 times diameter of lower helix so would be less than 3D if different sized helices used. Alan Lutenegger presented a paper on spacing this Spring and found ideal spacing to vary from 3D for soft clays to 1.5D for hard. If one just set the end bearing equal to the cylindrical shear ignoring shaft diameter with Nc = 9 the spacing would be 2.25D.

 

[csd72]

PSlem,

Generally for initial design the helical piling manufacturer has their own empirical formulii, or they generally use only the bottom helix in bearing and then the friction on the cylinder of soil between the lower and upper helix.

These values are then verified on site by comparing installation torques with the expected torque for the required strength and soil type. (once again empirically based).

I have seen references in manufacturer documentation that is similar to the criteria you mentioned above.

 

[Jalthi]

Here is a comprehenive list of references.