CEEN 341 - Lecture 19 - Intro to Shear Strength and the Direct Shear Test

This lecture introduces the basic concepts of shear strength of soil. Soil friction is described, and the friction angle is derived. The Mohr-Coulomb failure criteria are introduced, which state that soil shear strength is a combination of cohesion and friction. The direct shear test is introduced and explained. An an example calculation with the direct shear test is performed to develop a Mohr-Coulomb failure envelope.

Learning Objectives:
1) Derive and/or explain what the the friction angle is in the soil.
2) Derive the Mohr-Coulomb equation for shear stress in the soil, and explain when failure occurs in the soil.
3) Explain the operation of the direct shear test and list some of its advantages and/or disadvantages.
4) Using direct shear test results, compute the friction angle of a sand.

Critical Questions to Consider:

1) What are the two sources to soil shear strength?

2) Why can't a Mohr circle ever cross the Mohr-Coulomb failure envelope? What happens once a point on the circle touches the envelope? How does the point of tangency where the Mohr circle touches the failure envelope relate to the orientation of the failure plane in the soil?

3) What are some advantages and disadvantages of the direct shear test?

4) What does it mean for a shear test to be a "drained" shear test? How do you think this would differ from an "undrained" shear test, which we will talk about in the next lecture or two?

[ngonidzashekimbini]

Hands down best Geotech lecturer on YOU

[marshmelows]

Great presentation indeed, love your diction and rhythm while presenting!

[lbdco]

Wow! Thank you so much!! I have been struggling so much until I found your lectures. I really appreciate your efforts to make these complex concepts accessible to students from different backgrounds, learning pace and and learning style. Thank you!!

[hassammajid]

excellent sir, plz complete the whole course your lectures are very amazing

[ahmedtaha]

Thank you so much for posting your excellent lectures on YouTube. Regards from Saudi Arabai.

[srinivasamurthy]

The way you explain the concepts is outstanding, sir. thanks a lot regards form INDIA

[kingshukmukherjee]

Thank you so much sir!! I couldn't grasp the concepts and had literally lost complete interest in soil mechanics course of my curriculum. But your videos have been extremely helpful . Thanks a lot sir!!!

[rossanaherrera]

Thank you so much Dr. Franke. The lecture is very clear. I am writing from Peru

[josecohen]

You're the man, thanks from Canada :)

[alideep]

Thank you so much, that's a lot of valuable knowledge

[lisro]

Do you by any chance have a foundation design or earth structures lecture videos. I am studying for the FE and was looking for material. Your lecture videos are great and detailed.

[mahbubn]

Dear Professor, Thanks for your great effort.
I have mailed to you extending my greetings.

I think you are so busy to answer.
Anyway, I wish you will continue your great work to enrich our knowledge with your lectures.

[bamispeciaal]

Great presentation. Thank you professor Franke!

[ronaldlake]

Fantastic lecture! Thank you Dr Franke!!

[kyawmyohtet]

Cant pity myself enough. I just found out this lecture in a few days to graduate.

[matkontakos]

Hi prof, im really into geotechnics but it's quite hard to really understand the concentps.Could you be more specific about the symmetrie of mohr-coulomb criteria, the symmetrie means that the element fails at to planes? one above and one below? or at one only and the direction is govern by the nature of the element?

[thenineminus]

Does anyone understand how the angle Professor Franke found on 20:11? I don't understand why.

[mehulkatakiya]

at 43:28, while calculating tau, a contact area between soil decrease. shouldn't we consider?