Cantilevered Sheet Pile Design

[tq3610]

Hello Group,

I'm new to the forum, but have used advice from you guys for various projects in the past. Thanks for all your insight.

I have been tasked with designing a sheet pile wall for a 20-foot excavation (temporary, as it will be backfilled). The soil behind the sheet pile wall is organics/peat. Below 20 feet is a low-end medium dense sand. Static ground water is at about ground surface.

Are there any immediate concerns you have about this? Based on the organics behind the wall I don't see anchors as an option, so it will have to be entirely cantilevered. Do you see any concern with a cantilevered wall for a 20-foot excavation with ground water at ground surface on the back side and 22 feet +/- deep on the front side? I've read somewhere that cantilevered walls shouldn't be used for deeper than 15-foot excavations. Is this true?

Also, what do you think is the best way for me to design this wall? Should I design it all by hand calcs, or purchase software and do it both ways? Any recommendations for software I should purchase?

Thanks for all your help.

Brian

 

[tq3610]

- regarding the comment about cantilevered walls not being used for deeper than 15-foot excavations, I assume if this is true it's based on the sheet pile section not being able to withstand the moments without addition bracing? Obviously I can still balance the moments with embedment depth.

 

[PEinc]

Higher cantilevered wall can be designed but usually the sheet pile deflection and resulting ground movement will be excessive or the SSP will be large, long, and expensive.

Your wall probably could be tiedback with steeper tiebacks bonded into the sand below the peat. The problem would be cutting holes in the sheet piling below the water table. Can you internally brace the sheeting with either wales and cross braces or with wales and raker braces? I assume that the sheeting is a closed structure that forms a cofferdam. Otherwise, how will you control the water in front of the wall?

http://www.PeirceEngineering.com

 

[tq3610]

Thanks PEinc

Some additional information: I just completed a round of test borings. Soil conditions (at their worst) are about 18.5 feet of ORGANIC SILT/ORGANIC FIBERS (i.e., peat with 50/50 fine fibers and organic silt), underlain by loose (Nave ~7) fine sand to 35 feet, elastic silt to 40 feet, and gravelly sand (glacial till) to 49 feet over bedrock. We completed 6 borings with a very consistent soil profile, spaced at about 150 apart along the proposed sheeting line.

So, my task is to excavate the peat, and I'm confined to the excavation so I can't slope. Previous geotech engineer's recommendation was to sheet and excavate, so the owner is stuck on this idea that we need to excavate the peat (rather than stabilize the peat via CMCs and grid-reinforced soil mat).

Here is the major issue: excessive depth of excavation for cantilevered sheet piling, peat has poor strength and groundwater is at ground level, so I have poor soil strength and hydrostatic pressure, soils at the toe are essentially loose saturated sand that has minimal passive support. Can't tie back because the soil behind the excavation is peat, and can't install cross-beams because the excavation width is 200+ feet.

So, in the end it appears to me that this is not a doable situation with sheet piling. The only reasonable solution is to rock socket H-piles, which is going to be extremely costly.

Does this seem like a doable situation, or do I just bite the bullet and tell the client this is just not possible with this method/within any reasonable cost?

I'm also brainstorming some kind of happy medium where we could sheet pile a less wide area, cross-support, and grid-reinforce outside the sheet piling (less critical area).

FUN PROJECT! But some hurdles.

 

[PEinc]

Install tieback high up on the sheet piling, near original grade. Use a fairly steep tieback angle - maybe 35 - 45 degrees below horizontal. The tie backs will be longer than usual in order to be bonded into loose sand below the peat. You probably should figure on using regroutable tie back anchors. Cantilevered,rock-socketed H-Piles are not practical.

http://www.PeirceEngineering.com

 

[PEinc]

If this is a soil remediation project, another option is to install sheet piling for smaller, braceable sections. Sheet and excavate smaller areas. However, tie backs will be your best option.

Drilling into bedrock below the water table in loose sand would be expensive. The holes would need to be cased and you would still need to use steel sheet piling that interlocks with the drilled-in king piles.

http://www.PeirceEngineering.com

 

[GeoPaveTraffic]

You could also look at using rakers instead of tiebacks.

Mike Lambert