The strength reduction method affecting the strength characteristics of the soil materials is a progressive procedure that applies a reduction factor to both cohesion and friction coefficient until the loss of stability or failure of the slope occurs or until the calculation is operative in the numerical method. The reciprocal of this reduction factor is identified as the factor of safety associated with the modeled soil slope.

Consider that this approach is starting with classical Mohr Coulomb criterion, and that in reality the same reduction progression could not be the only possible.

It is possible to run and to compare this procedure in many codes now available.

Regards

The reduction Ph/c in Plaxis is easy to understand. What kind of model did you use to define soil (Mohr-Columb, Critical state, or hardening-softening)? Was it clay or sand?

We know that soil mechanics, soils show a rapid increase in shear stress reaching a peak value at low shear strains and then show a decrease in shear stress with increasing shear strain (strain-softens), ultimately attaining a critical state shear stress. The strain-softening response generally results from localized failure zones called shear bands. These shear bands are soil pockets that have loosened and reached the critical state of shear stress. Between the shear bands are denser soils that gradually loosen as shearing continues.

The soil mass within a shear band undergoes an intense shearing, while the soil masses above and below it behave as rigid bodies. The development of shear bands depends on the boundary conditions imposed on the soil, the homogeneity of the soil, the grain size, uniformity of loads, and initial density.

When a shear band develops in some types of overconsolidated clays, the particles become oriented parallel to the direction of the shear band, causing the final shear stress of these clays to decrease below the critical state shear stress.

It is the so-called critical state of soils. The difference between phi (peak) and ph(decrease) is the so-called dilatancy angle (represent an expansion of soils).

The shear strength decreases and the factor of safety hence start to decrease.

it is Mohr Columb and two different layers. I mean the slope part is sand without cohesion and the foundation soil is clayey sand that has both friction and cohesion.. i expect that it will reduce phi/c and the factor of safety increases till the failure and peak will be accepted as factor of safety but the behavior is not regular it has partly desreases and increases randomly.

Now I understand exactly your problem. If the factor of safety change randomly, this may be from numerical error and re-considering modeling is needed

Thank you very much for help..

I hope i can solve the problem and let you know the reason and solution.

Best regards