Dear Sinnappoo,

There are many field & laboratory tests to measure the shear strength of the soil.

Field tests may measure shear strength directly such as van test and dutch cone test, or indirectly like SPT and CPT tests, while lab tests measure shear strength directly such as direct shear test and tri-axial test.     

Reliability of lab tests are lower than of field tests because of sample disturbance and effect of field conditions. on other hand, lab. tests are more controlled and can simulate many loading conditions

So, if you study a certain case study, use  field test results, on other hand, if you are working on general research not related to certain site use lab tests ( or general empirical values)

Generally, variations ( S.D. / Mean)  of soil cohesion and internal friction angle are about 40% and 15% respectively.

good luck

apply UU and CIU triaxial test for slope stability analysis integrating the parameter of suction value in low pressure during fully saturated and partially saturated soil.

Dear Ahmed

Thanks for explaining applicability of both lab and field tests and the variation. I liked it.

Best regards

Ravi

Good afternoon Noorul

Thanks for the advice on shear strength test. So, triaxial tests are required for the slope stability analysis including suction parameter. I will work on it.

Best regards

Ravi

Hello Sinnappoo (Ravi) Ravindran

please check attachments

For in-situ test, SPT test is economic and quite reliable, co-relations can be used to determine shear strength. 

For laboratory assessment, for cohesionless soil, friction angle can be determined from direct shear test. On the other hand, for cohesive soil, only unconfined compressive strength must be determined.  

Dear Sinnappoo;

First of all, you should define your slope stability problem clearly and in detail. If you are interested in determining shear strength of a slope that has already failed before; Direct  or simple shear in laqb. are the simplest and most practical method.But there is a point must be mentioned. It can be more helpful to determine residual values of cohession, friction angle and even secant modulus by applying sufficient passes. moreover, ıf you have also groundwater and the slope fails relatively high strain rate, ı recommend you to perform UU test. In the field, you can measure and follow the deformation of slope by inclometers.

Good afternoon Nirmala

Thanks very much for sharing 2 research papers. I will definitely use it for studies. Test methods and sample disturbance affect the shear strength according to these findings.

Best regards

Ravi

Good afternoon Debojit

Thanks for the feedback. I totally agree with you that SPT co-relation (in the field), direct shear and UCS to work out shear strength.

Thank you

Best regards

Ravi

Dear Sinan

Thanks for the advice. I am still trying to define slope failure. I agree with you that UU test, direct shear test and inclinometer readings will be useful the stability analysis. I have commenced direct shear tests on soil samples.

Best regards

Ravi

Dear Sinnappo;

At least two parameters can affect the results of shear resistance of the soil: scale effect and sample disturbance/undisturbance. So, as much as you, in your test, could achieve these important factor you could attain the accurate value of shear resistance of the soil. In gravelly soil you have to have a big shear box (both in lab or field) to take the scale effect and in fine soils the soil would be avoided against the disturbance. In any case my experiences show the field tests are better.

good luck   

Dear Zaman

Thanks for your advice. I totally agree with you that both scale effect and sample disturbance are key factors. In addition I understand the importance of field tests.

Best regards

Ravi

If you want to observe the "shear zone" it self, consider the improved triaxial (also referred to as "the Danish" triaxial, by L.B.Ibsen).

The improved triaxial can sustain isotropic stress field within a triaxial specimen. Thus every point inside the specimen yields at the same time. Consequently, Isotropic yielding takes place - the "shear zone" expands across the whole specimen. The sample does not "crack" into two peaces. Instead - it retains near perfect cylindrical shape during yielding.

A sample has been compressed up to 50% strain without cracking (bifurcating) by I.Vardoulakis using this type of apparatus. And numerous sand specimens were pulled back to initial length after plastic yielding as well (during our own tests).

In some tests the "shear zone" is just a thin surface in a big picture. They are concerned with behavior of the system - not the sher zone it self. The improved (Danish...) triaxial apparatus takes a different approach, it can take measurements directly inside the "shear zone" it self.

I hope that I am summing up the essence of what those above have written when I say that because of the divergence of scale, disturbance, geometry, confinement and many other factors between the tests and what is occurring in the soil of an actual slope and in the model, one must always be using correction factors on the "measured" shear strengths for use in  slope stability modeling software.  The best way to arrive at these factors is to back calculate the shear strength in failed slopes in the same materials and compare them to measured values.  Additional insight is gained by inferring required minimum shear strength in un-failed slopes.  With experience, one can extrapolate these correction factors to similar materials and settings.

The key in all this is to recognize that the values from any test of shear strength cannot be assumed to be a value that can be uncritically used in slope stability modeling software.  This uncertainty is why one should always include a sizable factor of safety when assessing or designing a slope.

Good Morning Matthew

Thanks very much for the feedback. I really appreciate it. I like the idea of correction factor on measured strength. In addition, I understand the importance of the shear strength of unfailed slope (which I never thought of). The industry practice here is to use Factor of Safety from 1.3 to 1.5. I learnt that shear strength is affected by many factors such as sample disturbance, shear strain rate, degree of saturation, consolidation etc. Thanks again for providing advice and more ideas.

Best regards

Ravi

Good evening Tomas

Thanks for the feedback. I would like to learn on "Danish Triaxial Testing" and isotropic yielding. I will see if I can try this test out. Once again, thanks for sharing.

Best regards

Ravi

For slope stability, I suggest Direct shear Test.

Also for a long creep landslide, Ring Shear Test is the best

Hi Mohsin

Please excuse me for late reply. Thanks very much for your suggestion. I have already started direct shear tests. About ring shear test, we may have to purchase it.

Thank you

Best regards

Ravi