Ver4.0.06 of Retaining Wall, released on 2021-07-26, implemented the single source principle.

Background

Most design codes distinguish between favourable (stabilising) and unfavourable (destabilising) actions on walls. This is reflected in the partial factors that are applied to each. Typically, stabilising actions are decreased, while destabilising actions are increased. Generally, there are three stability limits that need to be checked, that is: bearing, sliding, and overturning. It often happens that one action is unfavourable for bearing, but favourable for sliding and overturning. A good example would be the soil on the heel of a cantilever retaining wall.

This process can become complex at times. Consider the case of the following retaining wall:

In terms of overturning, the forces, Pw, Pwb1, Pwb2 are destabilising, while Pwt1 and Pwt2 are stabilising. One might think that the components of hydrostatic pressure ought to be factored differently. However, it is illogical to treat the components of hydrostatic pressure differently since they come from the same source. Dividing actions into orthogonal components is a convenient way for engineers to understand the influence of forces, but that should not mean that the components are treated differently.

Eurocode 7 addresses the issue with the ‘Single-Source Principle’:

Unfavourable (or destabilising) and favourable (or stabilising) permanent actions may in some situations be considered as coming from a single source. If they are considered so, a single partial factor may be applied to the sum of these actions or to the sum of their effects.

[EN 1997-1 §2.4.2(9)P NOTE]

Our approach

The program first determines the total influence of an action, thereafter it can be classified as stabilising or destabilising. The resultant force is then factored appropriately.

Users might see different results when reviewing old design files where hydrostatic pressures are present. In most cases, safety factors will increase when designed after Ver4.0.06, because the previous approach tended to be too conservative.