We would like to thank Roger Bailey (Ground Engineering December 1999) for his interest in the article 'Stainless steel corrosion in spent oil shale' (GE September 1999). At the time the article was written our investigation was, as clearly stated, ongoing.
Our investigation of the wall at Edinburgh is now complete. The main conclusions are that both the type of backfill and the type of reinforcement (Type 430 ferritic stainless steel) used were, independently, unsuitable for this application. We have also identified two other UK structures built using the same system in the early 1970s.
As a point of detail the reinforcement sections were 80mm by 1.5mm and not 60mm by 1.5mm as stated by Bailey.
It is pertinent to note that staff from the British Steel Corporation (Lee and Edwards in TRRL Supplementary Report 457) expressed in 1977 the opinion that Type 430 reinforcement 'will suffer significant pitting corrosion in clay or sandy soils during the life span of a structure'. The decision by the Reinforced Earth Company to cease the use of this type of steel in the UK in 1975, and other comments in Bailey's letter, would appear to support this assertion.
To date, no copy of a document from any of the companies involved with the construction of the wall at Edinburgh has been located that warns The City of Edinburgh Council of the risk of collapse of the structure due to corrosion of the stainless steel reinforcements.
We do not know what the situation is regarding the other two reinforced walls in the UK which were built using ferritic stainless steel reinforcing strips.
Dr Mike Winter and Dr Andrew Butler
Transport Research Laboratory, Edinburgh