Moment of Inertia - Secant and Contiguous Piled Walls

[UP830]

Hello,

I am looking at how to design contiguous and secant piled walls. Rest assured, I wont be designing these walls anytime soon. I am just trying to learn for my own benefit, so please excuse me if my question is stupid / easy. I have a basic understanding of retaining walls, how to calculate earth pressures etc, these are from my old university notes, from a long time ago!

Can someone provide me with a reference to calculate the moment of inertial for a secant or contiguous piled wall. I have an example design report (which i stole from the internet). I have been working my way through it and trying to understand the steps involved. I have gotten stuck on the MoI.

Wallap gives some guidance on their software of how to do it. See below, again I found these in a manual online.

I dont know if this is the correct way to do it. I havent got Wallap so cant interrogate it any more. The reason I ask is because the above explanations dont match up with the values used in my example?

 

[ukbridge]

For a contiguous pile wall i.e. no overlapping piles I use the following formula to work out the inertia of the wall per metre run. It depends on what you are modelling; if the provided loads are given for 3m worth of bridge or what have you, then multiply Iyy by 3, or alternatively divide the loads by 3.

S = Pile Spacing (c/c)
D = Pile Diameter

Iyy = (Pi/64)*D^4 * 1000/(S) (mm4/metre width)

For a secant pile wall i.e. overlapping piles I'd use superposition to work out the inertia of the piled wall per m run.

1) Rectangle Area, Iyy = 1000*D^3/12
2) "Empty bits", Iyy = ?? (you would have to look up the formula for a fillet or similar. Its probably a lot simpler to conservatively treat the empty space as an equivalent triangle which is simpler to calculate section properties for. I can't see it making that much difference as long as the overlap is quite large.

Iyy,net = Iyy rectangle - Iyy empty bits. (mm4/metre width

I generally use section property calculators for this type of thing in FE packages, or you can use CAD and do a sanity check on the answer (i.e. it should be a bit or fair bit less than Iyy for an equivalent rectangle depending on the overlap.)

 

[Retrograde]

For secant pile walls I normally ignore the contribution of the soft piles. You should also make some allowance for stiffness reduction due to cracking and creep.