Sandy clay - Does it have a cu value and Internal friction?

[ac4u2nv]

I have a firm sandy clay, ive said it has a Cu of 40kn/m2, would it have an angle of internal friction value as well, or in undrained conditions it only has shear strength?

I have no lab data just a description to try to model off.

 

[Retrograde]

Generally the friction angle is taken as zero for undrained conditions.

Good discussion here:

http://www.eng-tips.com/viewthread.cfm?qid=292575

 

[Okiryu]

I was looking at fattdad's replies in the previous thread that Retrograde mentioned and I remember the attached graph for unsaturated and saturated conditions for UU tests. The graph is from Holtz and Kovacs book. Also, without any supporting lab data, for undrained conditions (short-term), I would assume only the contribution of cohesion and phi=zero for shear strength. For drained conditions (long-term), I would assume the opposite (c= zero and some value for phi).

 

[fattdad]

Not sure anybody can really answer your question as we don't know where you are located and we don't know whether your sample is from near or below the water table (i.e., unlikely to be influence by dry strength). If you are not dealing with dry strength and if the sample is close to saturated, then I'd use undrained conditions to represent the short-term, "Undrained" strength. For this condition, I'd use C=Su and phi=0. As Okiryou cites Holtz and Kovacs correctly, there is some likely undrained friction angle if the soil is unsaturated and as confinement increases, the air would go into solution and the sample ultimately saturated by pressure. To take any advantage of undrained friction angle, you'd need more testing, but you'd also need to know how the state of stresses by the engineering design would change the saturation. You'd also need to know whether saturation is temporal, which is almost the case in USA.

I use phi=0, C=Su for all short-term undrained analyses.

I always do drained design assuming c=0 as highway work considers a design life of 75 years and cohesion has been shown to attenuate over decades of performance - this is often just within the upper 10 or 20 ft, so for me that means for all slope stability analyses.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

My take - for a deposit with depth, one does not expect the undrained shear strength in the deposit to be uniformly the same throughout the depth of the deposit. Su/pv' ~ 0.25, say (with a range). Hence at 5 ft depth, assuming no desiccated crust, you have an undrained shear strength Su5. At 15 ft depth you have Su15. I do not see why these initial undrained strengths would be the same. There is going to be an increase with depth. So, if one is to model you have a choice - having a "uniform" Su for a layer (as many use a uniform Eo for the deformation modulus) or to have an increasing Su with depth (as there is an increasing Eo). As the Su, phi is a model, I typically use phi=0 - but will develop several substrata with different Su values - or I would think that you could use a small phiu value of 5 or so degrees if you start with the initial Su at the surface of the layer. Tradition is phi=0.

 

[fattdad]

I also agree with the bigger point that BigH is making.

f-d


ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Good point, BigH. I also agree. Just for my own knowledge, if I remember correctly the Su/pv' approach was developed by Prof. Mesri at Urbana-Champaign. Can we say that the Su/pv'pv approach is similar to the Gibson model (increase of Eo with depth)?. Thanks!

 

[BigH]

I would think so - remember you need Su at the surface and then the increase of Su with depth. One such graph is below - Matich and Brownbridge Soil Investigation for Rainy Lake Causeway, Highway Research Record, 1964. Might want to be a bit more conservative than 0.25

 

[bdbd]

BigH is right in this argument, but, we should be careful with this formula.

I am aware of the works of Mesri's and Ladd's. However, I have experienced opposite behavior in many sites. For example, it is not uncommon to observe 20m same SPT around 10 and same su values from UU test. These sites usually located near seas and rivers. I think Mesri's and Ladd's works are for undisturbed geological conditions. So, the clay has time to consolidate under upper layers.

I have seen engineers that they use these formulas with only one UU. You have to prove and develop your own formula with 3 or more UU test and of course with SPT-CPT analyses.