Hello,

The solution is in the Terzaghi's effective stress principle where effective stress equals total stress minus pore water pressure.

According to this equation, an increase of pore water pressure diminishes the effective stress while the total stress remains the same.

Thanks Prof. Romain Chesnaux,

I reduced the Cohesion of the joint. Is there any reason that causes normal stress increase when the Cohesion is reduced?

Thanks,

Dear Kien

If your case is one of anisotropic rock mass failure (i.e. failure along a joint surface in rock) I am not sure that the reduction of c in the joint can account for the increase of hydrostatic pressure in the joint. To begin with does the joint actually have cohesion (i.e. clay gauge, some other infilling or cementing material)? If not as is the case of most pre-existing joints in the rock mass your model should take c=0. Moreover cohesion is constant along the joint resulting in a force equal to the cohesion times the joint area. Hydrostatic pressure on the other hand is not. Its magnitude varies along the inclined joint depending on the percentage of the joint filled with water. When we deal with anisotropic failures in rock water acts as an uplift force and the weight of water is added in the normal stress acting on the joint.

Thank you very much Dr. Thomas Makedon

I had a mistake. I reduced friction angle of joint (phi) to represent the presence of water in joint. The cohesion, c, was constant.

A similar example of reducing friction angle of joint is shown in the Verification Problems and Example Applications of UDEC manual.

As we know that the increase of pore pressures will cause the reduction of effection stresses, then slip is occured. However, is there any case that reduction of friction angle of joint, then slip on joint occurs resulting the increase of normal stress? And what is the mechanism of stress behind this increase?

I also checked the example from UDEC manual (the attached file of this comment). Normal stresses are decreased during the presence of water in joint, but some points also shown the similar increase of normal stress.

Thank you very much for your consideration.

Kien

Well ok with the cohesion part. However my point was that in such analyses you should consider the hydrostatic pressure (it's not pore water pressure since this is not a porous medium i.e. soil) separately and not by reducing phi to account for the presence of water. In other words you should allow for the calculation of the added weight of water due to the rise of water level and the calculation of the respective hydrostatic pressure.

Dear Dr. Thomas Makedon ,

During the calculation, I increased the water level in the right-hand side of the model. When the water level increased to the discontinuity level in the right-hand side (point a), the fluid pressure in the joints is automatically added. Theoretically, due to the increase of the fluid pressure, it results in decreasing the effective normal stress.

However, point 2 shows the increase in both normal stress and shear stress. This is my consideration.

Thank you very much for your consideration.