The report by Ted Brown, one time professor of rock mechanics at Imperial College on the tunnel collapse at Lane Cove, Sydney is curiously inconsistent and incomplete (NCE 26 January).
First, the inconsistencies.
The standards of procedures and methodology of design and its control are described as complying with best practice. Yet, at the site of the collapse, geological sections had not been prepared along the tunnels, from which the likelihood of striking the dolerite dyke in the critical area could have been predicted.
There was also no modelling of the junction, which, with a clear span of around 20m, was a unique feature of the project and beyond the limits of experience in the Ashfi eld shales.
Nor was there forward probing to explore rock conditions at the junction which could have provided time to design variation of profile or support.
The report is incomplete as, for one, it lacks any discussion on the tunnel profi le. This appears to have a remarkably flat roof and vertical side walls which in poor quality jointed rock would be likely to produce high rock stresses locally to the shoulder.
There is also no description of the excavation scheme and no mention of instrumentation to measure convergence or other critical features.
My main object in writing is that this report implies that the collapse was an unfortunate feature of a well-organised project. If this were the case, which I question, the future prospects for sprayed concrete lined tunnels in built-up areas must be in doubt.
Alan Muir Wood, Franklands, Bere Court Road, Pangbourne, Berks RG8 8JY