GGU-STABILITY: Safety factor definitions

In accordance with DIN 4084 (old), the program uses the safety definition after Fellenius:

For bearing capacity analysis according to DIN 4017, the safety factor is acquired from a comparison of the failure load V_b and the working strip footing load V_work.:

Other comparisons are usual for different calculation procedures. For verification of the external stability of a soil nail wall, e.g., the nail forces are used in the safety definition.

Such safety definitions can also be created indirectly with GGU-STABILITY. If, for example, you would like to calculate the bearing capacity safety of a strip footing within a slope, enter the strip footing load as a permanent load and increase the actual load value by the required safety factor (e.g. = 2.0). For the following slope failure calculation, you need only verify that the slope failure safety factor is above 1.0. If you interested in the actual safety factor value, you must vary the size of the load by hand until you get a safety factor of 1.0 according to DIN 4084. For this special case the program has a routine to do this work for you. For verification of a strip footing in a slope, the DIN 4084 (Supplement; Section 4) contains an appropriate, but very carefully formulated note. In principle, for bearing capacity analysis on the basis of the DIN 4084, you will not get the same safety factors as with DIN 4017, as the theoretical basis is not identical. In particular, the DIN 4017 assumes no shear strength above the footing base. In the DIN 4017, areas above the footing base are only considered with respect to surcharge loads.

Procedure for partial safety factor concept

As described above for the global safety factor concept, the forces V and H are modified until the utilisation factor for the slope stability analysis is "1.0" This gives the ultimate bearing capacity V_b and from this the utilisation factor for bearing capacity analysis = V/V_b. Using the partial safety factor concept loads must be increased by the partial factors for actions and the partial factor for bearing capacity. The partial factors for the friction angle, cohesion and permanent actions are set to "1.0".

In certain systems the analysis method may lead to non-converging solutions.

It is safer to deactivate the "Bearing capacity" check box in the "Editor 1/Analysis options". Then define the V and H loads as permanent actions. Using the partial safety factor concept actions must be increased by the partial factors for actions and the partial factor for bearing capacity increased. When using the global safety factor concept the loads are increased by the required bearing capacity factor. A traditional slope stability analysis can then be performed.

If utilisation factors ≤ 1.0 are achieved sufficient bearing capacity is verified.