Secant Wall (Design as Soldier Pile, Unreinforced Pile Capacity)

[MJ23struct]

I found a few threads somewhat related which I will post links for convenience.

http://www.eng-tips.com/viewthread.cfm?qid=360274

http://www.eng-tips.com/viewthread.cfm?qid=252351

For a secant pile wall:
1.) How is the wall typically analyzed?
2.) How is the unreinforced section checked?

Question 1: How is the wall typically analyzed?

My thought was to evaluate the wall like soldier pile wall. The active pressure width would be the spacing between the reinforced full length soldier piles. The passive pressure width would be based upon the reinforced full length soldier piles. My thought was that the unreinforced piles would essentially be treated like lagging of soldier pile walls (and theoretically or practically the unreinforced piles could just be partial depth, say a few feet below mudline).
My thought was the reinforced full length pile reinforcing would be sized using the corresponding soldier pile forces as described.

Or, is it more common practice to treat the secant wall as a continuous wall and evaluate on a per foot basis?



Question 2: How is the unreinforced section checked?

I have the impression that a tolerance of 1:100 or 2:100 is specified for plumb tolerance so that an excessive gap between piles doesn't develop. Is the overlap of the reinforced and unreinforced secant pile generally checked or is it like timber lagging where soil arching effects come into play? When I made an attempt to check the shear strength of the unreinforced section the capacity was not sufficient so I was thinking soil arching must be considered. (For this check I used the calculated pressure to determine the shear on the concrete section and checked the shear capacity of that unreinforced section)


Thanks for any insight.

 

[FixedEarth]

1- For secant piles, you may decide to reinforce every other pier. In such a case take your spacing from center to center of the reinforced piles and treat the unreinforced middle pier as lagging like you suggested.

2- The bending will govern, so calculate your required section modulus by assuming a low modulus of rupture of the plain concrete. Then provide Sx greater than required. For reinforcement, factor your maximum bending moment by 1.6 to get Mu (plus any Pu) and then come up with your longitudinal reinforcement and tie spacing. You will see that the column would be tension controlled and shear is negligible.

http://www.soilstructure.com/

 

[hokie66]

As to the "soft" pile, the unreinforced one, I think arching would apply. The concrete arches, not the soil.

 

[dcarr82775]

I look at the pier as if it is a soldier beam, and the unreinf pier I have never put numbers on. It acts like an arch. I do not agree that shear is negligible. I have always had to provide shear steel as the concrete alone is normally not enough.

 

[hokie66]

MJ23struct,
The first thread you referenced addresses most of your issues, although there is some disagreement there with terminology.

 

[MJ23struct]

Thanks for replying.

Could you confirm that the depth of the soft pile is irrelevant? If the soft pile is the same depth as the reinforced pile the soft pile is treated as lagging and the reinforced pile evaluated for spacing center to center of reinforced piles.

I have someone stating secant walls can be designed like sheet pile walls on a per foot basis which I am reluctant to accept.

 

[hokie66]

Not sure what you are asking, but the usual way of constructing secant pile walls is to use the same size auger for both the soft and reinforced piles. The tributary area to each reinforced pile is based on the spacing center to center of the reinforced piles.

 

[dcarr82775]

The secant wall will be significantly stiffer than a sheet pile wall. You could use the basic design premise of a sheet pile wall particularly with how the embedded portion is treated, but you only have real bending resistance in the structural piers.