Factor of Safety Considerations in Reinforced Soil Structures

Abstract

A comparison is given of the conventional method of design of reinforced soil structures, involving the use of overall or ‘lump’ factors of safety, with an alternative method which allows a more realistic assessment of the contribution made by the soil. The results of the comparisons for both internal and external stability are presented with respect to the specified factor of safety determined in the usual way. The study has shown that relative to the specified factors of safety, the values calculated by the proposed method are generally smaller, apart from at the limiting condition when they are in agreement, provided that the assumed movement of the wall is outward rotation about the base. For the case of outward rotation about the top, even at the limiting condition the results generally disagree. The study has also shown that, regardless of the fact that only one safety factor may be specified, there are at least two different safety factors influencing behaviour. This result thus provides support for the use of partial factors in design as has been recently proposed.

The importance of considering strain behaviour in design is discussed in the paper. Although the method referred to above does not truly consider strain, it provides much greater consistency and compatibility of soil-reinforcement design while at the same time preserving the simplicity of existing methods. The method seems best suited to design involving “stiff” reinforcements.

The increasing use of polymeric reinforcements has highlighted the need for an alternative approach which considers strain behaviour directly. One such method has been described in the paper and the equations for internal local stability are presented. The analysis has been limited to the simple case of linear variation of strain of the soil and reinforcement but could be readily extended to more complex situations.