Article by Paul Uno

Many engineers may not realise, but these two engineering topics go hand in hand. When an engineer needs to retain soil due to a cut for a roadway or an embankment or whatever scenario requires soil to be retained, the design engineer usually addresses the following Structural Strength and Stability points:

  • Soil Properties both drained and undrained
  • Bearing Resistance under the base
  • Resistance to Overturning
  • Adequate strength to resist failure via applied bending moment or applied shear
  • Slip Resistance of the complete structure (which often may entail the need for a shear key at the base).

If all these 5 aspects are correctly addressed, it does not mean that the structure is completely safe. Even though these points were ticked off one by one, there is one final design check that needs to be carried out, and that is overall stability. This form of failure can occur as a slip circle, or non-circular failure or even a slip plane (rather than a curved plane). This process involves choosing a range of points in front of the retaining wall and seeing which point produces the failure plane with the lowest factor of safety. To do this process by hand can take considerable time, and even then, you may still not have picked the weakest plane.

There is no doubt that software is available in the marketplace that can do these analyses in a matter of minutes (the majority of time is spent inputting the correct information into the program in the first place).  Software such as that produced by FINE Geo5 does these analyses very quickly and accurately. The programs they offer can be based on either standard engineering principles and proven methods of analysis (eg Terzaghi, Meyerhof, Morgenstern-Price, Bishop etc) OR they can be carried out using FEA.

I always encourage design engineers to understand the basic principles first before they even go near any software. On 24th and 25th of October 2023, ETIA will be conducting two training courses. The first will be on Retaining Wall Design and Construction (24-10-2023), where we cover various basic principles of retaining wall design and soil properties. We then solve those scenarios using the Geo5 Retaining Wall program. We will also address the Australian Standard on this topic namely AS4678 Earth Retaining Structures. The following day (25-10-2023), we will be conducting a course on Slope Stability which addresses all the standard methods (mentioned earlier) and then incorporating the use of the Geo5 Slope Stability program to see how to solve the problems quickly.

For further information, please visit the website: and refer to courses 34 and 37, respectively. Both courses are full-day workshops with worked examples and tutorials (both courses conducted via Zoom).