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# Can i use undrained value for slope stability?(application of UU test)2

## Can i use undrained value for slope stability?(application of UU test)

(OP)
Hi,

Can i use the cu value (phi=0) obtained from the UU test for slope stability analysis instead of using c' value from CIU test?

If no, why it is not possible to use the undrained strength for slope stability calculation.

I have road embankment failure/road foundation failure and only UU test result.

Thank you

### RE: Can i use undrained value for slope stability?(application of UU test)

You can use cu (phi = 0) for short term behaviour. Once the pore water pressures equalise you will need to use long term drained parameters (c' and phi).

### RE: Can i use undrained value for slope stability?(application of UU test)

(OP)

So if the road was built for almost 8 years, then i guess the right way is to use the c' and phi' value for the stability analysis. Even if it is clayey material?

The road seems to have settlement issues since open to public 8 years ago, can we assume that the PWP already equalise?

Thank you

### RE: Can i use undrained value for slope stability?(application of UU test)

After 8 years i expect you are dealing with drained strengths. As a result you cannot use your UU test result to model the slide.

Based on your post, you need to be working with the more experienced engineer to determine the best way to handle this situation.

Beginning of rant: By the way, the money spent on running UU tests was wasted. I see this sort of thing on a regular basis, and frankly there is no reason for it. Engineers need to think about what kind of data they need before going to the field, leave alone before assigning laboratory testing. End of rant.

Mike Lambert

### RE: Can i use undrained value for slope stability?(application of UU test)

(OP)
Hi,

Thank you GeoPaveTraffic.. yes agreed on the rant. I'm a junior n just assigned with this case with only UU test data.

i assumed now 1D-conso and CIU test results are important for road failure/settlement for road that had been constructed long time ago.

### RE: Can i use undrained value for slope stability?(application of UU test)

I think that if the embankment already failed, you may need to use residual strengths which can be obtained from empirical correlations (see Professor Tim Stark's website. You can download his EXCEL spreadsheet for residual strength correlations. You will need the clay content and the LL)

### RE: Can i use undrained value for slope stability?(application of UU test)

Okiryu, I couldn't find the excel spreadsheet you mentioned, can you give a link?

### RE: Can i use undrained value for slope stability?(application of UU test)

I suspect the actual strength condition (considering clay soils) is fully-softened. During the life of a clay slope, you will have undrained strength, then you will have drained strength, then you will (often) degrade to fully-softened strength. The near-surface wetting and drying, freezing and thawing and other temporal factors will influence the strength in the top 10 to 20 ft and reduce it to such softened value.

Best way to evaluate fully-softened shear strength is to push the soil through the No. 40 sieve (without air drying), hydrate the soil to its liquid limit, mold it into an odometer and then incrementally consolidate the soil to the target confinement pressure (incremental to minimizing squeezing effects). Run the direct shear and you will have a series of peak strengths on the normally-consolidated sample. Use these peak strengths to get the failure envelope for fully-softened strength. If you capture any cohesion, ignore it in design. If the stress-strain curve also shows a residual strength, that'd be the same residual strength you'd obtain from an intact sample.

There is a citation at CGPR (Center for Geotechnical Practice and Research - Virginia Tech) on these matters. Tim Stark likes the rotational shear for similar conclusions.

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Can i use undrained value for slope stability?(application of UU test)

It appears that the road was built on NC or lightly OC clay. Undrained or I drained is dependent on the drainage path length and whether wick drains were installed; after 8 years the soft clay may still be in primary consolidation if thick soft clay without wick drains is involved.

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