STEEL SHEETPILE RETAINING WALL IN CLAY

[gator123]

The majority of my sheetpile design experience has been in fine to medium coarse sand with traces of clay and silt. I am designing a 30' x 30' sheetpile box with 28 foot deep excavation for a lift station. The soil profile is very soft clay (N=1 to 2)from ground level to 45 feet and fine to medium coarse sand (N-30) from 45' to 60' below land surface. Groundwater table is 2 feet below land surface and I am requiring dewatering inside and outside the box to lower grounwater table to 30 feet below land surface (2 feet below dredge line). I have a 4 tiered waler system with diagonal bracing at the corners. Length of sheetpile is 55 feet. I am getting very high maximum bending moment on the sheetpile using the Pile Buck SPW911 software that require PZ40 sheets whereas if the excavation was in sand PZ27's would be more than adequate. Does clay create that much more bending moment or am I miscalculating?

 

[PEinc]

Without my running any design calcs, it seems to me that a 28' deep excavation with 4 tiers of internal bracing should not need PZ40 sheets. Check your soil properties. Are you using an empirical earth pressure envelope such as one by Terzaghi & Peck? I am not a fan of SPW911. I own a copy but do not use it. I have found it often gives much higher moments and brace loads than other analysis methods or computer programs - especially when there is soft soil, which you have.

http://www.PeirceEngineering.com

 

[bridgebuster]

PEinc -

Are you familiar with the software Prosheet from Skyline Steel? Presently, I rarely do sheeting design but I recall that about 10+ years ago they had software that gave very conservative results.

Thanks

 

[PEinc]

I have ProSheet. In fact, I believe that Skyline is just getting ready to present an on-line seminar for experienced ProSheet users. I don't often use ProSheet. It is OK for sheet piling (hence the name) but not good for soldier beams.

The default earth pressure diagram for ProSheet is triangular. You can enter in other diagram shapes but not easily. There are other easier programs for that. I rarely use triangular earth pressure diagrams except for cantilevered sheeting and walls where the retained soils are very soft or for single tier braced or tiedback walls where the brace or tieback level is unusually close to the top of the wall.

Also, for the Boussinesq surcharges, I believe it multiplies the calculated surcharge by Ka which then gives less lateral surcharge than what some agencies (railroads) are requiring.

You can't beat the price though.

http://www.PeirceEngineering.com

 

[bridgebuster]

Thanks PE

 

[PilingEngr]

I also utilize Pile Buck's SPW911 but just purchased another software package worth considering is Civil Soft's Shoring Suite....sometimes provides a check to hand calcs.

 

[PEinc]

That's Shoring Suite from CivilTech. IMHO, it is a much better program than SPW911.

http://www.PeirceEngineering.com

 

[mudman54]

PEinc, thanks for the comment about surcharges multiplied by Ka which bothers me, but perhaps that is a new thread.

 

[wmta]

Be very careful with the soft clay soil. The shaft loads may be very very high. I would never depend on a computer program to design a cofferdam like this. There have been a number of failures of cofferdams in soft clay. It is my experience that dewatering of soft clay is not possible, but the sand below need to have pressure relief. Just be very very careful with this..

 

[anzineer]

I suspect that the heavier section is due to the lessened passive resistance (very soft clay vs silty sand) on the embedded portion of the sheet piling, resulting in larger deflections and bending moments. That's been my (very limited) experience when analyzing sheet piling in soft clays with the SPW911 software.

I also think dewatering the soft clay will be difficult. Would it be possible to brace the piling across the excavation, and 'seal-in' the excavation bottom by over-excavating and replacing with a thickness of concrete equivalent (with a proper safety factor) to the hydrostatic head difference?