Soldier pile retaining wall in clay - undrained or drained?

[demayeng]

Hi,

I am designing a maximum 2m high soldier pile (steel H sections) in clay. It is a replacement for a similar wall that has rotted out. I don't know anything about the clay except that it is 'good and firm', and that the retained soil has water delivered to it from an uphill property. I am casting the piles into 300mm diameter concrete below the excavation.

I am trying to do an 'undrained' analysis using this document:

http://vlee.utpl.edu.ec/VLEE/LinkClick.aspx?fileticket=piymgNPD1d4=&tabid=142&mid=520

There is a formula for the horizontal pressure at the depth of excavation: pressure = density*height - 2*Cu
As this has to be greater than 0, I have to limit the Cu I use depending on what height I am designing for (1m up to 2m). I end up getting an embedment depth of around 1.3m, independent of the retained height.


Am I going about this the right way? Should I be using undrained or drained analysis, given that the excavation has been in place for many years?

Thanks very much

 

[PEinc]

If the undrained cohesion is giving you little or no active earth pressure, design for the drained case without cohesion. I probably would not use less than 35H as my earth pressure. Be sure to install drainage to prevent hydrostatic pressure. The type of drainage depends on how you are planning to build the wall.

http://www.PeirceEngineering.com

 

[demayeng]

Thanks for your reply.
How do I justify that drained with no cohesion is suitable?
Do I use the same properties for the retained soil as the soil below the excavation depth?
Also, is 35H a US measurement ratio?
Thanks

 

[PEinc]

It will give a more conservative answer and is suitable for longer term applications. Yes, use the soil's unit weight and a drained phi angle of about 24 to 26 degrees and no cohesion. 35H is a triangular active earth pressure in pounds force per cubic feet per vertical foot. Sorry for not using SI units. SI is almost dead for construction in the US.

http://www.PeirceEngineering.com

 

[demayeng]

Great, thanks very much

I'll try that and see how I go

 

[AussieGeoEng]

You should always check both.

Re; "How do I justify that drained with no cohesion is suitable?"

Using critical state OCC theory soil has no cohesion - only apparent cohesion due to dilation.
Check out

http://www2.eng.cam.ac.uk/~ans/