Factor of safety in AS4678: Earth retaining structures

Factor of safety is used to provide safety margin over the theoretical design capacity to allow for uncertainties in loading, material strength and design process. Design of earth retaining structures has traditionally been based on the overall factor of safety method. However, the current Australian Standard for Earth Retaining Structures, AS4678-2002, is based on partial factors of safety method. In this paper, cantilever retaining walls and embedded sheet pile walls have been designed based on the recommendations of AS4678-2002 to examine the overall factor of safety inherent in the standard. Various wall heights and soil parameters are used in the designs. The overall factor of safety is then back-calculated for each wall based on its designed dimensions. The results of analysis are presented in the form of the overall factor of safety associated with the dimension of the walls and soil properties. The overall factor of safety of walls in cohesionless soils varies between 1.7 and 2.3; shorter walls have higher factor of safety. However, when the backfill soil has some cohesion, the overall factor of safety is generally higher than 2 and becomes more than 5 for soil cohesion greater than 30 kPa. For embedded sheet pile walls in cohesionless soils, the factor of safety remains constant for one particular type of soil, regardless of the height of the wall. The results of analyses of these walls in cohesionless soils also show that the factor of safety increases slightly as the friction angle of the soil increases. For the walls embedded in cohesive soils, the overall factor of safety is higher compared to those in cohesionless soils and this behavior is consistent with the one observed in cantilever retaining walls.