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I was very surprised at the tone of your article on stone curtain wall systems ('Breaking cover', NCE 22 January).
It is important to stress that the number of cases of cladding failures where stone is in danger of falling off a building have fortunately been very few. Those that have occurred have largely been the result of poor workmanship rather than failure of the stone itself.
Most designers will be aware that sedimentary rocks such as sandstone and limestone have natural partings or potential planes of weakness such as bedding and joint planes. Due consideration has traditionally been given to these features when using stone in buildings and these guidelines apply similarly to modern construction. The presence of such fractures in rocks such as granites is, I agree, less well known and this is where the technique of dye testing as described in the article may be of value. In my experience most of the cracking observed in granites on building facades was already naturally present in the stone before installation.
With regard to fatigue, there is now much evidence that rocks lose strength throughout the natural process of ageing on buildings. This is well documented with much work having been carried out in the US before the problem was appreciated in the UK. In many cases this strength loss will not be critical, as stone is traditionally designed with appropriate factors of safety. Should the fatigue lead to cracking then this may be a cause for failure, but not all fatigue causes cracking.
Surprisingly, while the article discussed the case of granites where very few failures are known, it entirely ignored the much more significant issue of the failure of marbles when used externally. There are several well documented cases of Carrara marble 'bowing' and cracking, the most famous being the 82 storey Amoco Tower in Chicago where this behaviour lead to all 43,000 marble panels being replaced on the building. Here, and in other cass such as the Lincoln First Bank Tower in Rochester, New York, the failures were not 'sudden and without warning' as the article suggests such failures might be, but were observed and monitored for some time before the decision to replace the marble claddings was taken.
I believe that the potential problems with stone are best tackled at initial selection stage with the geomaterials engineer. The process then moves to the quarry where the design team can view the materials insitu and formulate the inspection and testing program.
There is the need for better guidance in the specification and selection of stone for cladding systems and the British Standard is soon to be reviewed. In addition the Bath Centre for Window & Cladding Technology has just published its guide to the selection of thin stone for cladding which should provide a reasoned discussion of the issues.
Bruno Miglio, chief engineering geologist, WS Atkins Consultants, Woodcote Grove, Ashley Road, Epsom, Surrey KT18 5BW.
Car free Oxford
Your recent correspondents who criticised the present conditions in the centre of Oxford can be reassured that subject to a successful outcome to the public inquiry that started on 27 January, the city centre will be transformed. The successful traffic restraint policies of the past, which have established traffic at 1960s levels and resulted in Oxford being one of the few places where bus patronage has increased (50% in the last 10 years) are to be extended with further restrictions on cars and the creation, at last, of a properly pedestrianised area - all without any new roads or tunnels under the city centre.
The criticism of Mr Hairsine that little progress has been made since the demise of trams and that Oxford would be an ideal place for electric- driven vehicles comes a little hard when you realise that Oxford's horse- drawn trams (the university and others opposed conversion to electric power because of the unsightly overhead gantries and wires) were discontinued prior to the First World War!
We have, in fact, made significant progress, including operating electric buses in Oxford for the last four years. Sadly the unique electric bus service is due to be withdrawn at the end of March as a result of the county council's financial difficulties.
Roger Williams, chief transport planner (M), Oxfordshire County Council, Speedwell House, Speedwell Street, Oxford OX1 1NE.
We are third year civil engineers studying at the University of Bristol. For our major research project we are looking into 'Public Commemoration of British Engineers' in the UK.
We are trying to discover how well engineers are commemorated publicly, by statues, plaques, street names, postage stamps, bank notes; in fact anything that recognises an engineer's work publicly.
It would be of great help to us if any of your readers had any such information and would be kind enough to let us have the details.
Alastair Shanks & Stephen Walters, 27 Roslyn Road, Redland, Bristol BS6 6NJ.
. . . and finding it
I enclose an example of the national pride Denmark takes in its civil engineering achievements - a label from a bottle of beer! The beer, named 'Bridge Beer', was specially brewed to commemorate the completion of the Western Storeblt Bridge. Such a celebration of that achievement has an obvious wide spread appeal 'reaching the parts of the population other ideas cannot reach', which benefits the status of its engineers who are highly regarded in that country.
Maybe some similarly popular, less highbrow ways of promoting what is 'probably the best profession in the world' could work here?
B Wadge,(M) 6 St Michaels Green, Beaconsfield, Bucks HP9 2BN.
I read with interest the article 'Battling against Channel leaks' (NCE 9 October 1997). To those of us who worked on the construction of the Channel Tunnel in the UK 'wet zone' it may not be surprising to read that some sealing of leaks at segment joints is still necessary. Although I hope that the metaphor 'limited skirmish' would be a more apt analogy than the 'never ending battle against leakage' referred to by Mr Cargo.
I urge Eurotunnel to publish a technical report on its systematic approach and findings for maintaining the Channel Tunnel lining. Such a report would not only be of considerable benefit to those involved in the maintenance of other transportation tunnels, but would also be a valuable reference to spur more effort during the design and construction phase of future tunnels, for applying mitigation measures to minimise costly maintenance.
Roger Daniel (AM), 20 The Cloisters St Johns Road, Hastings, TN37 6JT.
In NCE 22 January incorrect reference has been made to the existing partnership that is being employed on the Docklands Light Railway, Lewisham extension. The text refers to Mitsui as carrying out the tunnelling work.
Work is being carried out by LRG Contractors, a JV between Mowlem and Nishimatsu Construction Company. It is NCC that is responsible for the construction of two PCC segmentally lined tunnels, beneath the Thames, between Island Gardens and Greenwich.
In turn, NCC has a JV arrangement with Mitsui, however NCC has brought its experience to bear on the construction of the tunnels and the hand- excavated cross passage between the two running tunnels.
project manager, nishimatsu_millwall@