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Interpretation of CPT data for slope stability purposes.

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CEMAB

CEMAB

Geotechnical
Joined
Jan 22, 2014
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Hello,

I would appreciate if you could share your experience on processing CPT data for the slope stability purposes. To be more specific, I have a record of interpreted undrained shear strength obtained from CPT. The Su values fluctuate substantially with the depth (see the attached file). I guess soil stratigraphy is responsible for this pattern, but for the modeling purposes it seems to be impractical to get into such a level of details defining a new layer with different soil properties every 5-10 cm. I would appreciate your advise on this.

Thank you in advance,
CEMAB
 
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I'm sure someone will argue differently, but drill a boring to determine the soil types and properties. CPTs are useful, but by themselves they are not sufficient for stability analysis.

Mike Lambert
 
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To: GeoPaveTraffic

Thanks for the reply, but it is not practical in current circumstances.
 
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Two layers: one with su = 20kPa and other with su = 50kPa.
There. Problem solved.
And please: drill a borehole.
 
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Not practial by for what reason and by whom?

If your analysis is incorrect, what are the consequences?

If the interpretation of the CPT is incorrect because the infered soil type(s) are wrong, who do you think will be held responsible? Who will die or what will be the property damage?

Mike Lambert
 
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To: GeoPaveTraffic
Thanks for the professional ethic lesson, but I’ve been through it and have a really good idea of what the responsibility means. If I feel that I need more data I’ll get it, but that’s not the case. CPT is widely recognized as a standard field test to collect data on substrata, including bearing capacity and frictional resistance of soil. Why should I ignore it? But again, that’s not the question I asked originally.
 
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You are right it is not the question you asked originally. However, you asked how to use it for slope stability. CPTs have little use for slope stability. Therefore you need borings and laboratory testing.

Mike Lambert
 
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This is the price of more detailed testing methods; we learn that the world is more complex than we knew. If you had conventional borings with samples every 1 or 2 meters, it would look much simpler.

I would tend to discount the sharp peaks, thinking they may be too brittle or otherwise incompatible to truly contribute to the mass strength. I would divide the profile by eye into 3 to 5 layers and use the average strength in each layer without including the sharp peaks.

What causes the zero values? I thought they might be where the rods are released to add another rod section, but they do not appear at a regular spacing. If there really are thin very weak layers, they may control the slope stability.
 
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To: GeoPaveTraffic
I assume you base your statement "CPTs have little use for slope stability" on some sources, textbooks etc. Could you please refer me to a few of them. I am just curious to see what they are based on.

BTW, my original question was on how to process CPT data for the slope stability purposes and not whether it is sufficient or not for stability analysis.
 
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