I read with interest the article by Tony Barley in your April 1998 issue entitled 'Can corrosion rule slides?' I am grateful to Mr Barley for subsequent correspondence in which he identifies the location of the soil nails where he observed corrosion in September 1996.
Based on our records, these soil nails were not galvanised and had exposed heads. They were installed in 1979 as temporary remedial works to prevent further subsidence of a garden lawn platform behind a retaining wall.
The design details of these soil nails do not meet the current design standards, especially with regard to galvanisation of the steel and encapsulation of the soil nail head. In 1984, 16 of the nails were exhumed and it was found that the exposed portions of the nails were significantly corroded whereas the portions encased in cement grout were clean and without any rust.
In 1996, the area was redeveloped and the garden lawn platform was lowered to about half the original height. The heads of all soil nails concerned at the site are now covered by sprayed concrete.
Government geotechnical engineer
Geotechnical Engineering Office
101 Princess Margaret Road