Embedded Retaining Wall Design In Accordance With Australian Design Standard AS5100.3-2017

ABSTRACT

Australian Standard AS5100 is often specified as the technical standard for embedded retaining walls in Australian infrastructure projects. The authors have identified compatibility issues between strength and stability-based criteria in AS5100 and Strength Factor analysis methods adopted by commonly used software packages. AS5100 adopts a Limit State Design approach requiring a reduction factor to be applied to passive restorative forces; whereas the Strength Factor method used in commonly adopted software such as WALLAP (Strength Factor Method option) and PLAXIS, applies a single reduction factor to all values of soil strength in the active and passive zones. The fundamental difference creates challenges for designers attempting to demonstrate compliance with AS5100 when using the widely adopted software packages.

Using a typical cantilever wall example and a range of commonly adopted soil parameters, the authors demonstrate equivalency between these methods by ascertaining the minimum Strength Factor required to achieve equal (or longer) embedment depths compared with the stipulated reduction factor per AS5100.

1 BACKGROUND

Embedded retaining walls are a common type of retaining structure to support excavations in soil and rock. Adherence to Australian Standard AS5100 – Bridge Design is commonly stipulated in the technical specifications for the design of retaining walls in Australian infrastructure projects.

The authors have identified challenges faced by designers to prove the attainment of strength and stability-based criteria in AS5100, due to the inherent incompatibility with analysis methods adopted by commonly used software packages such as WALLAP (Strength Factor Method) and PLAXIS. In this context, strength and stability-based assessments pertain to the embedded length of the pile, i.e. the length of the pile below the excavated ground surface.

For geotechnical strength (failure) and / or stability-based assessments AS5100 adopts a Limit State Design approach, requiring reduction in the restoring forces and increase of applied forces with load factors. Commonly used design software such as WALLAP (Strength Factor Method), PLAXIS2D and PLAXIS3D employ the Strength Factor method which progressively apply a single reduction factor to all values of soil strength in both the active and passive zones until a state of failure or instability is reached. The calculation methods of AS5100 and common design software are seemingly incompatible, i.e. FOS (factors of safety) calculated from WALLAP (Strength Factor Method) and / or PLAXIS are not equivalent to the FOS (inverse of load factors) stipulated in AS5100.

The incompatibility between AS5100 and design software has created inefficiencies in practice. Many designers have to perform duplicate calculations to prove that the embedded wall designs are satisfactory using their adopted design software and then verifying compliance with AS5100 Limit State Design approach. This has resulted in design inefficiencies and re-work in tight working schedules.

This paper aims to review the requirements of AS5100 and ascertain design “equivalent FOS” that are compatible with current modelling software / approaches. Note that this paper focuses on geotechnical strength design only. Structural design and serviceability should also be considered but are not included in this paper.