Dear Zaryab,

It is normal. The results from the direct shear test ars bigger in general because the differences in  stress state, shearing surface, 

Hi Zaryab,

As Dr. Kai said it is normal that you have two different parameters for these tests. because loading conditions,  stress condition and stress distribution during the tests are completely different. in fact in there isn't any confining pressure.

regards

Dear Zaryab,

In the direct shear device, the failure plane is horizontal. This forced failure plane can sometimes coincide with the natural layering or planes of weakness. In the triaxial device, the failure plane is not predetermined by the configuration of the device and horizontal planes of weakness would not be expected to control the measured shear strength.

http://www.geoengineer.org/online-library/soil-mechanics/items/view/21562-a-comparison-between-the-shear-strength-measured-with-direct-shear-and-triaxial-devices-on-undisturbed-and-remolded-soils

Best regards

Thank my dear colleagues for valuable comments. Mr. Kai. The results based Direct shear and triaxial are as below:

1.      Direct shear test: Cohesion: 10 kPa and Friction angle: 39 (degree)

2.      Triaxial test: Cohesion 16 MPa and friction angle: 48 (degree)

Mr.  Kai, but here the friction angle of direct shear is bigger than triaxial. What do you think in this regard?

Cornforth (1964)  found that the drained friction angle was about 4-5 degrees (10%) higher in plane strain, compared with triaxial compression, for dense sands, and that the difference tended to diminish as sands became looser.

INTRODUCTORY SHEAR STRENGTH

Prepared by Dr. Roy E. Olson on Spring 1989

Modified by Jiunnren Lai on Spring 2004

I am agree with Dr. Ghavami. So I can not understand the results from Dr. Zaryab.

normal. the distribution of the stresses in two tests are different; in the direct test ...the failure plane is parallel to direction of applied load. while, by the triaxial , the sample placing between normal stress (vertical stress) and fluid pressure 

Hello Mr. Zaryab

You can view this link. You can find interesting stuff.

https://www.google.com.tr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwi9np2onu7RAhXMVRoKHaQBBxAQFgghMAE&url=http%3A%2F%2Fwww.lehigh.edu%2F~mepa%2Ff%2FENGR-627-Lect4.pdf&usg=AFQjCNG1yBMh5n0qpO8uSiqq3pgrwMICLQ&sig2=mS5aauLOBOXidM2xxDY93w&bvm=bv.146073913,bs.2,d.bGs

 In Direct shear tests,  the plane of failure is not governed by the properties of specimen, but by  the test itself. and the distribution of shear stress across the specimen is not uniform.

Thank you my dear colleagues for your valuable comments.