Phi=0 Analysis Saturated Clay

[moe333]

In performing a pseudo-static slope stability analysis for a medium stiff saturated clay (fill or natural) I would think that the undrained strength would increase with depth. But many authors (Duncan & Wright, 2005) recommend a phi=0 analysis with constant undrained shear strength for large thicknesses (50 feet+).

Any thoughts on this?

 

[dgillette]

Good question. Haven't actually read it. Do they mean constant for large thicknesses, or just constant at a given location, meaning that the strength is assumed constant irrespective of changes in the stress state or PWP at that location? Seems to me if Su varies with depth (according to CPT, VST, or whatever), you should treat it that way (by layers in the model). If you use a strength typical of the upper part of the deposit and it actually increases with depth, you would be throwing away some usable resistance.

I had Duncan reviewing a project of mine a few years ago. Since we couldn't actually do complete SHANSEP calculations with Slope/w, we faked it with a phi-cu calculation (which gives undrained shear strength as a function of effective normal stress on the failure surface), which he was willing to accept, since we explained our rationale and tweaks. We lucked out, because you can't always do that - see CC Ladd's 1986 Terzaghi lecture for explanation of why phi-cu goes bad with some geometries, like steep exit of slide surface. (At least I think that's where I saw it. I'm home now and don't keep duplicate references.) When we used FLAC, we were able to program in the SHANSEP-type strength ratio and apply it to sigma'1c (pre-earthquake major principal stress). Unfortunately the strength ratio or the phi-cu you fake it with varies with stress path.

Let me know if you find an answer to the original question.

 

[moe333]

Dave,

I know SlopeW has a SHANSEP strength model. I have tried to use it previously without much luck-it uses a B-Bar method for the piezometric surface-which was difficult to figure out. I ended up back calculating the corresponding strength increase with depth and used a different SlopeW strength model...I think it was an increase with depth using a datum and a ap on max. strength.

Duncan and Wright (2005) don't actually state that strength increase with depth should be ignored but their examples show thick (50' +)layers with constant Su and they state Su can be determined with UU tests (for existing slopes). I am not a fan of UU tests...the philosophy that reasonable results are obtained throuh a coincidence of compensating errors does not give me much confidence. I also think unsaturated conditions exist in these tests since there is no back pressure saturation.

Generally I think there would be strength gain in a sat. clay layers that thick (which is in consolidated equilibrium with static loads). You can see this in total stress phi and c from CU tests at different confining pressures. I think total stress phi and c from CU tests may be a more appropriate strength for this condition.

Thanks for the input, I'm still trying to come to a conclusion on this.....

 

[fattdad]

I'd take it as for any given layer, the undrained shear strength of the clay remains constant (given the friction angle is zero). If you have data showing that the shear strength increases with depth, then use sublayers to model that.

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

I tend to do my analysis much as fattdad indicates. If I don't have data to support a change in strength, I leave the strength alone.

 

[fattdad]

you can also jam a cone or dilatometer in the ground and see what you get.

f-d

¡papá gordo ain’t no madre flaca!