BRIDGE FOUNDATIONS

Sun (Auckland), Volume III, Issue 839, 6 December 1929, Page 1

BRIDGE FOUNDATIONS

Depth of Mud is Big Factor

DISCUSSED BY ENGINEERS

ENGINEERING difficulties in connection with the proposed harbour bridge, particularly in connection with foundations, were discussed before the Bridge Commission this morning. Mr. F. W. Furkert, engineer-in-chief to the Public Works Department, said that recent borings in the harbour had penetrated 80 feet of mud 40 feet under water without coming to the bottom. Mr. F. E. Powell, civil engineer, who has examined the proposals for the Bridge Association, said it was important that the engineers should see the borings. The foundations would present a big job, but he did not think that they would raise a really serious difficulty.

The members of the Commission are Xr. J. Marchbacks, engineer to the Wellington Harbour Board (chairman), Mr. F. W. Furkert, engineer-in-chief to the Public Works Department, and Captain C. McDonald, of Dunedin, retired shipmaster. Evidence as to the probable cost of bridges was given yesterday afternoon by Mr. Stanley Jones, constructional engineer. Working on a basis of an soft vertical and 370 ft horizontal clearance, with a 40ft roadway, he estimated the cost of the Beaumont Street bridge at £718,050. Objections to the site were that it would cut the St. Mary's Bay boat harbour in two, the run of the tide through the navigation span was not square to the centre line of the bridge, and the sufficiency of vertical clearance possible with satisfactory grades was doubtful. These disadvantages were overcome, but at a greater cost, on the Point Erin site. For this bridge he allowed clearances of 135 ft and 635 ft, and estimated the cost at £815,850. The cost of the under-water concrete work was estimated at £6 a cubic yard. The Chairman: It occurs to me that your estimate is too low. Answering further questions, Mr. Jones said that no allowance had been made for the cost of causeways to link Northcote with Bayswater and Stanley Point. Dr. J. W. Craven, of Birkenhead, gave evidence as to the inconvenience and danger of transferring patients at night to the general hospital. He gave instances of cases where the tossing experienced during rough weather was detrimental to the patient. An estimate for a bridge on the Shoal Bay site to cost £941,000 was submitted by Mr. R. F. Moore, constructional engineer. The length was given at 4,000 feet, width 66ft, and vertical clearance 86ft at high water. In deep water there would be several spans of 200 feet. He did not think that aDy importance could be attached to the upper waters of the harbour for large or heavy navigation, and discounted the suggestion of danger to steamers manoeuvring at the wharves if the bridge was erected. This morning Mr. E. Aldridge. Mayor of Devonport, was cross-exam-ined on his previous evidence. He said that before the resolution in regard to the council’s attitude was passed it was discussed at length by the council. To Mr. R. 11. Greviile: He would admit that Devonport, in late years, had not progressed as quickly as other districts. He had not seen the population figures of other boroughs from 1926, the date of the last census, but in comparison with the 1926 figures of other boroughs he would consider that Devonport was progressing satisfactorily. In regard to the distance given from Devonport Post Office to Queen Street Post Office, he would admit that a mistake had been on the decimal point, but which ever way one took the distance it was two miles. Mr. F. E. Powell, civil engineer, and formerly chief assistant-engineer to the Auckland Harbour Board, gave evidence regarding the bridge proposals. He said he had extensive Practical experience in boring and bridge construction work in Auckland and other harbours. He was a member of the association and as such considered it desirable not to design any proposed bridges but examine and report on those submitted. He had examined several sites proposed by the association’s engineers and had favoured the Shoal Bay site for convenience and general development. He and the committee considered the danger to navigation and the possible spoiling of Shoal Bay for future harbour development. It was considered j that those views were unwarranted, j THE MUD PROBLEM

The question of a tunnel was considered but thought to be impracticable at the present time. It was also agreed that a low level bridge w as undesirable. It was not easy to determine what the minimum level should be, but it was thought desirable to keep it as low as possible consistent with the needs of navigafion. A span of 400 was thought unnecessary. Bad weather conditions were not frequent in the harbour and the coincidence of a large ship navigating at that particular time was considered to be too remote an eventuality to be a really serious factor. The construction of the bridge, either in steel or concrete, offered little difficulty. Mr. Furkert: What depth of mud “id you count on? Mr. Powell: We are a little in the dark about that and for that reason the engineers on our committee would like to view the boring work in operation. had taken into account 'tie possibility of 100 ft at the deepest Point. This, they were aware, would Present fairly considerable difficulty. To the Chairman: I appreciate that 'here would be fairly considerable difficulty in sinking the casings. In s ay, 60ft of mud, 1 would seriously consider another method, possibly a ***** pile method.

Mr. Furkert: You said the bridge would not present much difficulty. In one boring done recently 80ft of mud had been penetrated 40ft under water and that was not the bottom. That depth had been bored in a day and a-half. Mr. Powel: I think we could s(iy the foundations will be a big job, but I don't consider that there would be any serious difficulties. The cylinders would be of steel, and he anticipated no difficulty in getting them down. Mr. Furkert: With a span ol 1.400 ft. as suggested you would need big cylinders. Then there might be 60£t. or 80ft. of water and a similar depth of mud to get through. To Mr. R. McVeagh: The plan of the proposed bridge gave a horizontal clearance of 400 ft. at the water-line, but not at the top, where the arcltes came in. Mr. Powell again stressed the necesisty of the engineers seeing tile result of the borings. The chairman said there was no reason why this should not be done. He remembered that when the American Fleet came to Auckland a portion of it was anchored in the water up above the proposed Shoal Bay bridge site. DESIGN OF BRIDGE Regarding the construction of the bridge, Mr. Powell said two designs for a bridge in concrete both offered a simple and practicable method of construction, either by built-out steel cantilevers cairying their own formwork for subsequent concreting, or by preformed trusses lifted into place and again carrying their own boxing tor concrete. Designs based on well-known steel truss systems offered no obvious difficulty beyond lifting the trusses into place, and plant capable of doing that work already existed iij the harbour. Unless some totally unexpected conditions were disclosed by borings, no extraordinary difficulty should be experienced in providing sound foundations for even very large spans, either by piling or by caissons. Immediate investlga'mn of the character of the softer material overlying the papa rock was essential so that a definite idea could be formed as to the size of the caissons and methods of sinking. Mr. G. T. Murray, civil engineer, said he was district engineer at Auckland from 1915 to 1920. He favoured the Northcote Point site on account of the elevation possible and the fact that it would not obstruct the Beaumont Street reclamation or Shoal Bay as the Bridge Association’s proposal would. The elevation above water is 80ft and there was a straight r un off there by an existing street, while the southern bridge head and approaches at a point in St. Mary’s Bay would not obstruct access to the Beaumont Street reclamations, nor to the western boat harbour, as; the association scheme appeared to do. The approach would start from Fanshawe Street, skirt St. Mary’s Bay, but keeping out sufficiently to allow boating activities inside an easy curve of 20 chains or even 40 chains radius, then a 10-chain straight on to the same alignment of the bridge. The bridge alignment crosses the mole immediately to the westward of the westward opening to the boat harbour, thus not interfering with boating on that side. He considered that a bridge at the point indicated might be more costly than a tube tunnel, but would be preferable to the travelling public from safety, comfort and hygienic points of view, though with an opening span it would have certain disadvantages. (Continued on page 11.)