Shear strength inputs for slope stability software

[Raglangeo]

Hi. I am new here

While I have worked with slope stability analysis through direct calcs for some time I have just started to look at available software. So far I don't see any that include measured shear strength as an input parameter. Only internal angle of friction which is kind of guessing.

Is there a software package that uses actual measured shear strength out there somewhere? To me this is the most important insitu data I collect.

Thanks for any help

 

[geomane]

What shear strength are you measuring and how are you measuring it? Couldn’t you just back-calculate to get your shear strength parameters?

 

[Raglangeo]

I measure using a shear vane that is inserted vertically at 300 mm intervals as a borehole is being drilled. The result is in KPa

This to my mind is the most important data we can get from field testing. It is a direct measurement of the soils resistance to shear. Back analysis to establish a theoretical shear strength is putting the cart before the horse IMO. This approach using a rough guesstimate on angle of friction is prone to big inaccuracies.

Are there any slope stability formula that have shear strength as a factor? This would be a start

 

[geomane]

If you’re using a shear vane I assume you are measuring undrained shear strength of fine grained soils. In that case, you would just input your undrained shear strength value for an undrained analysis... or you could input that value as cohesion and use a phi=0 approach.

This would give you your short term strength. If you are analyzing long term analysis of a slope, you will need drained/effective shear strength parameters.

 

[Raglangeo]

I am relatively new to using software. I was wondering about using the cohesion. How am I going to establish a relationship between the two? I have searched for research on this matter but have not found anything yet.

The point I am making here is that I can measure shear strength in the field but to the best of my knowledge one can't directly measure Phi or cohesion in the field. In the commercial world a large portion of geotechnical investigations do not include lab analysis. The scale of the projects do not justify it.

I am thinking that if I was to do a heap of back analysis on existing slopes, including ones that have failed I may be able to establish a conversion factor for shear strength:cohesion.

I am really surprised that all the available SS formula do not use shear strength as a factor as this is how slopes fail. Soils shear! Undisturbed shear vane testing is the nearest we are ever going to get to real field conditions IMO.

I think some programs can use CPT data? However CPT data is not that accurate either, especially in soft-firm clays with high moisture content.

Thanks for the feedback

Michael C

 

[geomane]

You should read through soil strength and slope stability by Duncan. It will probably answer a lot of your questions.

 

[jdonville]

Raglangeo,

Relation between plasticity index and phi':

https://www.issmge.org/uploads/publications/1/2/423-428.pdf

I use Table 3-1 and Figure 3-1 from USACOE EM1110-2-2504 (March 1994) based on Teng 1962 for estimates of unit weights and drained strength parameters from SPT blowcounts and visual-manual classifications.

As you say, "a large portion of geotechnical investigations do not include lab analysis". If you would like this to change, then when *you* are the investigation firm's client, specifically ask for and pay for more lab analysis.

 

[GeoPaveTraffic]

Raglangeo,

Undrained shear strength, such as that measured by a vane shear test, is the same thing as phi=0 and cohesion=shear strength. So if you want to do an undrained slope stability analysis using vane shear test data, you could use a phi/cohesion strength model. You could also develop a c/p ratio for the soil strengths based on your field measurements. If this doesn't make sense to you, then you will need to do some reading or find someone with more experience to work with you.

Also, don't forget about drained slope stability analysis. Some slopes are more prone to fail in drained conditions that undrained.

Best of luck to you.

Mike Lambert

 

[Raglangeo]

Thanks folks. I now see what I need to do.

 

[GeoEnvGuy]

Hi Raglangeo, based on your discussion I believe you require guidance from a senior engineer in your firm. From what I gather in your posts you want to take field vane tests and determine the shear strength to use in a slope stability program.

Field vanes are corrected two ways the first is the size and shape the second is the plasticity index commonly referred to as the bjerrum correction. Once you complete the corrections you can get a value for cohesion to use for undrained shear strength. I work in a place where all my Clay's are saturated and normally consolidated and have undrained shear strengths well below 100kpa and follow the tau/sigma ratio of 0.22. Other areas have overconsolidated Clay's where the undrained strength is much higher than the drained strength. Which needs to be used as upper boundary in stability modelling.