GGU-STABILITY: General information on General wedge method and Vertical slice method

Calculations according to General wedge method or the Vertical slice method are explained in the DIN 4084:1996. With these procedures force polygons are constructed. The force polygon is only closed in special cases ( = 1.0 or 1/f = 1,0). Otherwise, an additional force dT is required to close the polygon. If this force dT acts excitingly, the safety is larger than 1.0. If the force dT acts resistingly, the safety is less than 1.0.

For calculations with partial factors, it is sufficient for verification that the safety factor is > 1.0 or the utilisation factor < 1.0. However, one is generally also interested in the safe distance to the failure condition. In order to calculate this safe distance and/or to carry out a quantitative comparison of various failure bodies, the force dT could, in principle, be brought in. The size of this force is, however, mainly dependent upon the size of the investigated slip body. A comparison of two very differently sized slip bodies using the force dT can easily lead to a false impression. The program therefore not only calculates the force dT, but reduces or increases the friction angle and cohesion as long as is necessary to close the force polygon. The safety factor or the utilisation factor 1/f is then calculated from

For calculations after General wedge method or the Vertical slice method, as opposed to the procedure after Janbu, the shear forces in the intermediate slip surfaces are considered.

This means that, in comparison to calculations after Janbu, higher safety factors are achieved. The program also offers the possibility, however, of setting the shear forces in the intermediate slip surfaces to "0".

When calculating according to General wedge method, if several soils are defined within a principle or intermediate slip surface, the program calculates with averaged values for the friction angle and cohesion. If an intermediate slip surface is vertical, the shear forces will not be considered in any case.

When calculating with the Vertical slice method, the intermediate slip surfaces are vertical and earth pressure forces in the slice flanks are considered, the inclination of which results from the pressure line. According to DIN 4084:1996, the pressure line may run between the centre and the lower third of the slice flank. The program therefore calculates the pressure line at a height h to:

h = 0,416 · H = H · (0.5 + 0,333)/2

H = height of slice flank

If the inclination of the pressure line, and thus the inclination of the earth pressure force, is larger than the average friction angle in the corresponding slice flank, the average friction angle will be used for calculations. In the examples in the DIN 4084:1996 the friction angle is averaged using the earth pressure coefficients. The program does this simply via soil thickness.