Tunnel or Bridge Across Waitemata?

Sun (Auckland), Volume III, Issue 761, 6 September 1929, Page 11

Tunnel or Bridge Across Waitemata?

RELATIVE MERITS ENGINEER’S COMMENTS That the need for the duplicating of the trans-harbour traffic facilities, either by a bridge or a tunnel, will shortly become a thing of the past, owing to the development of aittransport was the opinion expressed by the chairman, Mr. T. Walsh, speaking at a meeting of the Waitemata Chamber of Commerce at Devonport last evening. The meeting, which was addressed by Mr. D. F. Harkness, C.E., of the Auckland University Engineering School, examined the idea of a harbour tunnel and its probable advantages as compared with a bridge scheme. The lecturer, after a world-wide review of the recent developments in both bridge and tunnel building, supported the chairman’s remarks, by declaring that any above-water structure erected over the harbour would have to be regarded as being a possible obstruction to air transit facilities. There would be flying boats in two or three years, working from the harbour, and in a very few' years intercolonial flying boats connecting New Zealand and Australia would have a base on Auckland Harbour, too. He was aware that a company was now arranging. to bring out flying craft. BRIDGES—FOR AND AGAINST

Traffic could be taken across navigable waters by three methods: ferry, and over or under-water methods. Ferries had the disadvantage that traffic congestion was likely; but they had the advantage, where the navigable waters were not otherwise crowded, that the ferry services could be built up as required or supplied in localities needing them: without undue capital outlay. Bridges allowed continual traffic and were often pleasant routes for scenic travel. Their disadvantages were several. Over harbours that were of commer cial importance engineering practice, supplemented by naval requirements, insisted on above-water clearances of up to 180 ft and at least one long span to permit of handling vessels under adverse conditions of wind and tide. This meanUlengthy approaches to permit of maximum grades for traffic. This entailed buying land, often at high price, for as far back as half a mile from the shore ends of the bridge. Land about the approaches usually decreased in value, due to complaints about noise, and interferj ence otherwise. ! There was frequently great congestion about the bridge terminals and traffic delays; any bridge had to be j built to carry what would be considered a maximum load in the future, for it wag not feasible to add to a bridge as the load developed. This meant heavy capital expenditure which could be never used to full capacity. Brooklyn Bridge had more than £2,000,000, spent on it in the last 50 years in an endeavour to keep it strong enough to carry the increasing loads. Big spans meant big costs, and needed strong foundations to carry them. Maintenance was heavy and in wartime bridges were the most vulnerable points in transit syster# •>. Mr. Harkness illustrated his remarks by reference to bridges in New York. San Francisco, London, Detroit and Sydney. MODERN AND ADAPTABLE

Drawing on his personal experiences of tunnels, such as the famous Holland twin tunnels in New York, the Mersey Tunnel, which was built after a bridge scheme had been examined and found less suitable, and of instances in which engineers were now examining both tunnel and bridge schemes, Mr. Harkness pointed out that it had been found in some places that three tunnels could be built for the cost of one bridge. The tunnels could be spaced to meet the traffic needs in separated localities or built as needed. Modern practice had provided mechanical ventilation and tunnels did not get obsolete because traffic loads increased, since they rested on solid ground or suppqrts. Maintenance was a minimum and by spacing the inwaru traffic opening and the outward traffic exit congestion of traffic and interference with right-angle traffic at the terminal points was reduced to very small proportions. PRELIMINARY INVESTIGATION

Stressing that in America sums of £12,000 to £200,000 had been spent in thoroughly examining the relative merits of bridges and tunnels in cases where it was required to carry traffic over water, the lecturer insisted that no selection of either method or site should be attempted in Auckland until all available data, as much information as possible about the harbour bottom, and a proper traffic survey had been tabulated and sifted by experts.

In reply to Mr. H. Greville. Mr. Harkness said he could not give anydefinite figures as to the * increase in traffic where a bridge had eliminated a ferry. It had been found in New York that where a bridge had been built in competition with , a ferry 55 per cent, of the bridge traffic had come from previous ferry traffic. Mr. Greville said that in Canada, in several places, bridges had. killed the ferries and the increase in traffic had been 100 per cent. Answering another question, Mr. Harkness slid ferro-concrete construction was not feasible for high level bridges. He did not think bridges with lifting spans were proper—they caused traffic delays. The regular tolls collected on bridges and tunnels in America was 15 cents on cars, and five cents a person (passenger and pedestrian).

HARBOUR BRIDGE HISTORY RECOUNTED Introducing Mr. D. K. Harkness, who lectured on bridges and tunnels before tlio Waitemata Chamber of Commerce last evening at Devonport, the chairman, Mr. T. Walsh, remarked that the idea of bridging the harbour was not new. It had been quite fully examined before a Royal Commission j in 1921, but so far back as ISG3 there i had been schemes for bridging the j harbour. One of them put forward by the ! late Air. T. T. Masefield, a well-known ironfounder, was for a bridge of boats |to Stanley Point. Mr. Harkness menj tioned that recently one of the pro- ; posals for bridging from San Francisco ! to Oakland was for a bridge of con- | crete pontoons with a span in the I centre to permit shipping to pass.