Vinyl sheetpile over existing concrete seawall

[struct_eeyore]

I'm working on remediating concrete seawalls. The contractor wants to drive vinyl corrugated panels in front of the existing, backfill a bit with concrete and cap with a concrete cap with new tiebacks to deadmen. (Most of the existing tiebacks are rusted, and are near the end of life) My original approach was the standard analysis, assuming the existing concrete wall was not present, and full saturated soil load acts on the sheetpile directly (obviously there are several design load combinations that we check for, but that's beside the point). I ended up with a pretty heavy section just to control deflection - which is on the order of 2.5" for 14' total height (7' exposed). The client was not too happy, and his engineer made a point about how the existing wall with the added concrete in front will take out most of the deflection - I don't necessarily disagree with this. So the question that stands now is how safe of an assumption is that? If the concrete backfill only goes to the mudline, with the lower half of the sheetpile not directly braced by the concrete, are we introducing any sharp corners where shear/moment might be magnified? FYI, I'm using the free earth method with 50% sheet embedment. Also, as a second question, I could not find specified deflection limit for vinyl sheet piles - not in USACE or anywhere. I was thinking keeping it to L/180, with full height for the L. Any input is greatly appreciated.

 

[CRQUT]

Im no expert but depending on the height of the wet concrete wouldnt this be a higher load than saturated soil? Also if its a seawall, does it natter if it deflects as long as the vinyl sheetpile doesnt yield?

 

[Settingsun]

There may be some funny business but mostly I'd see this as a case of adding the stiffnesses above dredge level, and taking just the existing concrete below at worst. The vinyl's flexibility should keep it safe by shying away from load it can't handle. I'm assuming similar toe levels for both walls.

If the concrete wall is ok, is the vinyl just to tidy up appearance?

 

[MTNClimber]

When you say vinyl corrugated panels, do you mean vinyl sheetpiles similar to steel sheetpiles or do you mean something else?

Pouring concrete between the existing wall and the "panels" will cause issues with drainage. You could develop large hydrostatic pressure behind the wall during and after storms. If this panel is a vinyl sheetpile wall, I would recommend backfilling with crushed stone between the walls to prevent this issue. This is assuming the concrete seawall has adequate drainage provisions too. We do this all the time when oversheeting existing SSP bulkheads.

2.5" for a 7' tall anchored wall is a lot. We typical limit deflection of anchored sheetpile walls to around 1".

They could be correct that the concrete seawall will reduce the deflection on your vinyl wall if the wall is in good shape and will be dependable for the anticipated service life of the remediation. But I'm assuming there is something wrong with the existing wall which is why you're addressing the issue. If there's a structural issue with the existing wall beyond the simple fix of replacing the anchors, I would personally ignore it like you have.

 

[GC_Hopi]

For deflection, I have seen/used 2% of the exposed height with a limit of 2-3 inches for structures/development in the backfill zone or a limit of 6 inches if no structures/development in the backfill zone.

As far as the analysis, your approach is reasonable and conservative. The whole point of the "replacement" is to support the soils without having to rely on the existing concrete. I guess I would point that out to the owner and maybe that is not their understanding.

The other approach is to evaluate the existing concrete and "repair" it. If you can determine the concrete can still take the bending moment and still have x amount of design life left but the tiebacks are no good. Then you could analysis this as replacing the tiebacks and the vinyl sheet piles are just aesthetics. Maybe this is the owners understanding.

 

[struct_eeyore]

To clarify.

Corrugated vinyl sheet piles with interlock. SG-750 or similar. The existing seawalls are reaching the end of their life, with spalling and rusting tiebacks - but they are still standing. In a perfect world, I'd design the new sheetpiles as completely independent, but being asked for VE taking into consideration the existing panels. My concern is with deflection - I'm still well below my allowable moment for the design. My thought is that if the existing concrete wall start to fail and tilt out, it will rotate rather than bow-out as a beam - by doing so, it will "lean in" to the vinyl panel uniformly, thus limiting deflection. Let me know if I left out any critical details. Thanks!

 

[PEinc]

Check the sheet piling for the load from the wet, fluid concrete. Consider whether the concrete is being placed in multiple, short lifts (less concrete pressure) or in one full height lift (maximum concrete pressure).

http://www.PeirceEngineering.com

 

[GC_Hopi]

I thought your new design had tiebacks. They would prevent rotation

 

[Settingsun]

You could give them a range of deflections assuming the lighter profile they want: first assuming the existing wall contributes to stiffness; and second assuming the existing wall fails in future due to deterioration so the new wall is all that's left.

I wouldn't do this for a non-technical client who's just looking to save a buck but, if they have an engineer looking over your shoulder, they have the ability to process this and make a decision.

 

[phamENG]

Do not take the existing piles into account. If you do, then the new bulkhead will have the same service life as the remaining life in the existing one. If they are already spalling and coming apart, then water is getting the rebar. Even if you design a decent drainage system, 8 out of 10 of them are never maintained. Add that to the probably difficulty in maintaining it in this arrangement and your drainage system is going to have a relatively short life. Water is already getting to the rebar of the existing wall. When your drainage inevitably fails (or just gets a little less efficient), those bars will continue to corrode, the concrete will crumble, and any "help" it gave you in stiffness will decay away.

If deflection is an issue, you can add a second wale and tie back lower down on the wall. Don't go lower than about 10% of the tide range above MLW if you can avoid it (that should give a good 2 hours of working time at low tide). You may have to core drill through the existing, but hopefully you're doing that anyway for the drainage issues mentioned above.

Be careful, though. Vinyl sheet piles are great when used properly, but if the contractor is going into this thinking they'll be just like steel sheet piles, they'll be sorely mistaken. Lighter vinyl sheet piles don't always mean lower installation costs.