Abstract:
Soil-nails and anchors as means of lateral support in surface excavations require stability
analyses as part of design. Generally, the acceptance criterion is some arbitrary ‘factor of
safety’ which can be routinely computed using well-established principles of limit equilibrium
analyses. These methods have been tested over many years and have been shown to be
adequate for design. There is an increasing trend towards the use of the finite element strength
reduction methods to determine factors of safety in lateral support design. Differences are
often reported between factors of safety calculated using these methods. There is also a danger
that engineers might use finite element modelling without full appreciation of the impact of
choices and assumptions made in using the software.
This paper compares the results of limit equilibrium and finite element calculations to assess
factors of safety. Under certain conditions the factor of safety from the finite element strength
reduction technique is comparable with limit equilibrium methods, provided that the same
failure mechanism is evaluated. In addition, in the case of anchors, the same capacity must be
specified in both analyses. In defining in-situ stress states, the friction angle and Poisson’s ratio
should be specified so as to not violate the yield criterion. Modelling parameters (mesh grading
and boundary distances) were found to have relatively minor influences on the factor of safety
for the strength reduction technique.
