## Undrained shear strength

## Undrained shear strength

(OP)

Hello there, i was wondering if someone could help me interpreting this triaxial undrained test.

I need to obtain the undrained shear strength (Su) in terms of the effective vertical pressure: Su/Sigma'v.

Also cohesion and friction angle.

Im attaching the spreadsheet with the results. Thanks a lot.

I need to obtain the undrained shear strength (Su) in terms of the effective vertical pressure: Su/Sigma'v.

Also cohesion and friction angle.

Im attaching the spreadsheet with the results. Thanks a lot.

## RE: Undrained shear strength

Not sure why the lab is not giving the reporting requirements for CU-txc?

f-d

p.s., also if there is back-pressure saturation, you are assuming that the Su/P is informed only by completely saturated soils. That may or may not be the case?

ípapß gordo ainÆt no madre flaca!

## RE: Undrained shear strength

You get c' and φ' by fitting a line to the stress paths from those tests. Usually the peak (q

_{max}) or the ultimate values (at the end of the test) are used to fit the line. However, once again, the line fitting may depend on the specific application. Also, for what it's worth, c' and φ' is not a great idealisation, despite its wide application in geotechnical engineering.## RE: Undrained shear strength

You do not get Su from the P' v. q' plot. Those are effective stresses and you are looking for undrained strength.

f-d

ípapß gordo ainÆt no madre flaca!

## RE: Undrained shear strength

s

_{u}is half of q at failure. So the three tests would each have an s_{u}. But the OP wants a stress ratio, so they'll have to derive the s_{u}for each test and normalise it by the vertical effective stress. That will give three results, but the s_{u}to use would depend on the stress expected in the ground.## RE: Undrained shear strength

## RE: Undrained shear strength

Im reattaching the spreadsheet with some results. Please let me know what you think.

Thanks.

## RE: Undrained shear strength

_{0}value in your calculations: you have σ'_{v0}already (the value at the start of the test). Also, you appear to have defined s_{u}as the last value, whereas it is more typically to take the peak deviator stress and divide by 2. Are you certain that your definition of 'failure' is appropriate for your design purposes?If you want s

_{u}/σ'_{v0}then you're going to have three different results: one for each test. Determine s_{u}(typically q_{max}/2) and then normalise by your initial effective vertical stress for each test; this will give you one result per test. You can fit a line through the three results if needed, but the angle of this line won't be your internal friction angle; you do not get the friction angle from a plot of s_{u}versus σ'_{v0}(see the fifth post of this thread).## RE: Undrained shear strength

Im reataching the spreadsheet, first with phi' and cohesion results (effective). Please let me know if you think they are ok.

Thanks.

## RE: Undrained shear strength

the graphs do look good though!

f-d

ípapß gordo ainÆt no madre flaca!

## RE: Undrained shear strength