Engineers Australia

Soil nailed retention – A practical approach to field verification

Peter McDonald and Brian Ims

Abstract

In Melbourne, soil nailed retention has been used on a number of major road projects for about 20 years and since that time the walls appear to have performed satisfactorily. During the intervening period to the present, there has been much debate about how the system of soil and embedded nails actually works and world wide a number of design approaches have been developed. These design approaches fall into three main categories:

  • bi-linear slip surfaces and a limiting equilibrium analysis method (UK and USA – NailSolver, 1990; SnailWin)
  • circular slip surfaces and limiting equilibrium analysis method (Australia – STARES)
  • numerical analysis methods (FLAC and PLAXIS)

The nails form passive resistance elements that become stressed as excavation proceeds and the nails strain as there is load redistribution between the soil and the nails. It is now generally accepted that the contribution of the shear (bending) resistance of the nails is minor and most current design methods ignore the nail shear resistance. However, there is much debate about various aspects of design and it is still unclear what distribution of nail forces exists in practice, with various researchers finding a large variation in actual nail forces compared with the forces predicted according to the design methods (FHWA, 1999). The design for head force can be a particularly complicated issue, with the FHWA design method recognizing the contribution arising from both the flexural strength of the wall facing and punching shear modes of failure.

If the nail force distribution is not well predicted by the various design methods it would seem reasonable to adopt a simplified type of analysis in which the nail head force is limited to a nominated value and the wall facing pressures are then determined according to the limiting nail head forces established, providing that near-face failures cannot occur after assessing possible shallow face failure modes. The computer program STARES developed by the University of Sydney is one design approach that allows ready analysis of soil nailed walls using this approach.

A major factor in soil nail analysis is the bond between the nails and the ground and this is one of the most critical aspects of soil nailed retention design because if the bond fails, large volume wall failures can potentially occur. It is therefore important to carefully consider soil-to-nail bond in design and this is one aspect that can be checked by a careful program of field verification and testing. Unfortunately, in many cases the testing and field verification procedures do not relate explicitly to particular soil nail designs and are often derivatives of test specifications for stressed anchor systems. For production soil nails, it is often impossible to carry out testing to the loads required to validate design bond values without nail yield so proof load testing of completed nails may be of little practical use in confirming design bond values.

A carefully designed program of field testing can however be used to verify design bond values and provide confidence in this critical aspect of soil nail performance.