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# Need help to determine Section Modulus of FSPIII Sheetpile wall when sandwiched together.

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## Need help to determine Section Modulus of FSPIII Sheetpile wall when sandwiched together.

(OP)
Hi all,

New member here. Hope to get some help with a problem I am having. I am working on a project to design a temporary shoring for an embankment using FSP III sheet pile wall. The wall is 5m exposed facing.
Using Deepex software to anaylse I managed to get the design to work except the bending moment on the sheet pile wall exceeds the bending capacity of the sheet pile wall.
Since I am limited to the use of only FSP III sheet pile as the contractor has lots of this, I plan on using a double layer of the sheet pile sandwich together to increase the
moment capacity. The two sheet pile will only be retrained via tieback pipe micropile at 3m aparts (2 layers).

Will this method work to increase the moment capacity? If it can, how do I calculate the section modulus of the section seeing its an irregular shape section to begin with. Thanks.

Alex

### RE: Need help to determine Section Modulus of FSPIII Sheetpile wall when sandwiched together.

A double thickness of driven SSP with no fastening between the two individual SSP walls? I would just dust double the section modulus and moment of inertia. As they bend, they will slide against each other. Ignore any friction or shear between the sheets.

www.PeirceEngineering.com

### RE: Need help to determine Section Modulus of FSPIII Sheetpile wall when sandwiched together.

(OP)
Hi PEinc,

To response to your first question, yes. That's what I was thinking of doing. I did think about the two sliding against each other. I will have the tieback going through both sheets but its only a few points so this might end up being detrimental and snap the bolt holding the tieback to the sheet piles. Worst case scenario would be to bolt both sheets together at several points to ensure the will act as one sheet pile. This can be done if this method is reasonable.

"I would just dust double the section modulus and moment of inertia. As they bend, they will slide against each other. Ignore any friction or shear between the sheets." Sorry but could you explain a little more? I don't quite understand your comment. Not sure whether its good or bad. :)

### RE: Need help to determine Section Modulus of FSPIII Sheetpile wall when sandwiched together.

Example 1, bending a deck of cards is easy if the cards can slide across each other (little or no friction between cards). If the cards are glued together, it is much harder to bend the deck.
Example 2, a 2" thick steel plate, 12" wide, has a section modulus of 8 in^3 while a double 1" plate, 12" wide, has a section modulus of only 4 in^3. Both total 2" thick but the solid 2" plate has twice the section modulus of the thinner, double plates and 4 times the moment of inertia.
Two, unconnected sheet pile walls will bend easier than two walls connected by bolts or welding. If the two walls are driven very close to each other, I would ignore the soil between them. Therefore total section modulus = 2 x one section modulus.

When you refer to "bolts," do you mean tieback anchors? If the holes through the SSP are big enough, they should not hit, bend, or damage the tieback anchor tendons when the sheets deflect. Also, because tiebacks are installed at an angle below horizontal, and because each sheet pile usually does not get its own tieback anchor, some sort of tieback wale is used to support the SSP between tieback anchors. These wales are most often inclined at the tieback angle and are connected to the SSP with welded, angled, wale seats. Wales are not usually bolted to the SSP. There should be pictures of these tieback wales on my web site.

### RE: Need help to determine Section Modulus of FSPIII Sheetpile wall when sandwiched together.

(OP)
Hi PEinc,

Thanks for the explanation. It's much clearer now. For my case I think sandwiching two fspiii should be more than sufficient for me. Thanks.

Tie back I mean 168mm diameter pipe which is spaces at 3m c/c. One at the base of the 5m wall and the other layer at 2m from finish platform level. Not sure how this can be connected to the waler beam. Cutting a big hole to insert the pipe through might not be a good option as this would damage the pile for future use. Will study your website when I can today.

Thanks once again.

Alex

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