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Can corrosion rule slides?

The editor welcomes letters at 151 Rosebery Avenue, London EC1R 4QX; UK fax: +44(0)171 505 6642

e-mail: paulw@construct.emap.co.uk

Can corrosion rule slides?

Your brief article ('Slide rules' GE February) summarising slope stabilisation difficulties and remedial measures taken in Hong Kong raises two particular aspects with regard to durability of the tensile members installed in the ground:-

1)As illustrated in the accompanying photograph of a soil nail retaining a wall on Victoria Park, Hong Kong Island, steel soil nails corrode badly, (Photo 1). Equally, sev- ere corrosion below the plate could also be observed. In the UK, the concept of the designing of permanent steel soils nails based on sacrificial rates of loss of steel is recommended in BS8006. However, even in the environment of a cracked body of cementitious grout, the loss of steel section will not be uniform and will be concentrated at the crack locations. Whether the tabulated rates of a uniform loss of steel, apparently derived from corrosion rates in fill, are realistic in soils nails is questionable and in consequence so is the lifespan of the nail and hence the long term stability of the slope.

The other argument for limiting the corrosion protection requirements of steel nails is that steel nails are of low carbon steel only and nominally loaded and thus less likely to corrode than highly prestressed steels. Although the illustration shows no contribution from cement alkalinity which may be present around the nail in the ground, the exposed corroded steel is low carbon material and is unstressed. Further-more, where a nail corrodes within the cracked grout body, the presence of the expanding corroded material is likely to be deleterious to the bond, a critical component of the working nail.

This begs the question:

Are all steel soil nails installed in Hong Kong required to be totally isolated from the environment by plastic protection system, or are they designed for sacrificial loss of steel? Or, is a different approach to durability of nails applied to steel soil nails in slopes with low consequential damage in event of failure, as compared to slopes where extensive damage or loss of life would occur?

2)The second point relates to use of modern fully tested ground anchors in which high quality double corrosion protection systems are factory applied prior to delivery to site and installation. The British Standard for Ground Anchorage's, (BS8081) the European Standard, (EN 1537) and the Hong Kong Geotechnical Engineering office specification all demand use of systems that fully isolate the stressed steel from the environment and testing of every individual anchor. Associated with the use of such refined proven systems may be a greater cost than that associated with installation of nominally larger unstressed steel bars in the ground (soil nails) but is it worth economy for a potentially reduced lifespan solution.

The illustrated use of fully protected individually tested ground anchors retaining large reinforced concrete slabs offers a safe, yet flexible, solution (photo 2, below). In a more recent slope stabilisation contract in Southampton the lightly reinforced bearing slabs were buried in the slope to ensure an 'environmentally friendly' finish.

AD Barley, director of engineering

Keller Ground Engineering

Thorp Arch Trading Estate

Wetherby,West Yorkshire LS23 7BJ.

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