Soil Model according to Mohr-Coulomb
Ground and slope failure
In order to evaluate the load-bearing capacity of soils and failure scenarios such as the formation of slip surfaces, the Mohr-Coulomb plasticity model is provided in InfoCAD. The model can be used for base failure analyses as well as for estimating the stability of slopes (Phi-C Reduction). The analysis can be carried out either with a associative or non-associative flow rule, whereby the respective model approach is activated by specifying a corresponding dilatancy angle.
Ground failure
In the first instance, the numerical results are verified based on the problem illustrated below (Zienkiewicz et al., 1975). Two limit cases are investigated, a rigid and a flexible foundation. Therefore, a displacement- (rigid) and a load-controlled (flexible) analysis are used, where the load is applied directly onto the weightless soil.
The settlement underneath the foundation and the lateral upward displacement of the soil (bulging) are shown in the following figure and agree very well with the results given in the Zienkiewicz paper.
The mechanism of the bearing capacity failure of foundation (active wedge, radial shear zone, passive wedge) becomes clear when considering the equivalent plastic strains. The shape of the slip surface becomes obvious.
In addition to failure scenarios, the ultimate load of the soil (max. soil pressure) can also be predicted. The analyses performed with the arc length method are in very good agreement with the reference solutions.
Slope failure
To calculate the factor of safety (FoS) of an embankment, the method according to Fellenius (1927) [Phi-C Reduction] is used in InfoCAD. The soil parameters (friction angle and cohesion) are successively reduced until the failure of the soil occurs, characterized by the sliding of the sliding body on a sliding joint (slope failure).
The numerical results are verified based on the problem illustrated below, taken from the publication by Griffiths & Lane (1999).
In order to show the influence of the calculation method and discretization (tetrahedral elements) on the factor of safety (FoS), different analysis were performed. The computed factors are included in the following Table.
| Elements | Newton | Arc Length Method |
| 1822 | 1,35 | 1,325 |
| 18922 | 1,35 | 1,325 |
| 55085 | 1,35 | 1,325 |
Influence of discretization and calculation method
For the Newton and arc length method, the factor of safety converges to the value of 1.35 and 1.325, respectively, which is independent of the discretization. The factors agree very well with the numerical reference solution of Griffiths & Lane (1999) [FoS = 1.35 (last converged state)] and the solution of Bishop & Morgenstern (1960) [FoS = 1.38].
Again the formation of the sliding surface within the soil becomes obvious by means of the equivalent plastic strains.
