VISITSM
MELBOURNE TELEPHONE TUNNELS.
On Tuesday, September 29th, members inspected the tunnels which are being driven beneath Melbourne for the accommoda- tion of the telephone and telegraphic services. The invitations from the Commonwealth Public Works Department and from the con- tractor (Mr. A. G. Shaw) were conveyed through Mr. T. Hill (C.P.W.D. designing and supervising engineer), and Major H.
V. Champion (consulting and supervising engineer' to the con- tractor), respectively. By the courtesy of Messrs. D. A. and J.
W. Swanson, members had also the privilege of accompanying
—under the guidance of the Architect, Mr. J. S. Brittingham- the Royal Victorian Institute of Architects through the new tele- phone exchange buildings.
Briefly the salient points of the tunnel system are as follows :- OBJECT :
The ultimate complete undergrounding of the telephone and telegraph lines of the central—"city"—area of the metropolitan district and connection with suburban arterial systems and with minor conduit reticulations within the city proper. The tunnels are for these purposes exclusively.
VISIT-MELBOURNE TELEPHONE TUNNELS. 153 for railway purposes, some years ago, three 70 feet piles spliced end to end, had failed to bottom at some parts.
Could Mr. Hall enlighten them as to whence the great flow of lava—"bluestone"—which filled the old drainage valleys in the Melbourne district had proceeded?
Further, whether the Yarra, at its present rate of flow, was sufficient to account for the subsequent erosion?
Mr. T. S. HALL said there was considerable evidence that the rivers were formerly less than they were now. But, at the same time, the elevation of the land had been 170 or 200 feet greater, and with the steeper grade the cutting power of the streams must have been greater also. It was the velocity of the water, and the chance it had to discharge its load, rather than the volume, which determined the erosion. He did not think there was any need to call for a larger discharge if they assumed the steeper gradient.
As regarded the lava : It was surprising to note how many old volcanoes there were near Melbourne. Although nearly all very much denuded, there were at least one hundred of them in the county of Bourke, but he could not refer to any particular volcano as the source of any particular lava flow.
154 VICTORIAN INSTITUTE OF ENGINEERS:
EXTENT :
About 13,000 feet lineal. The main reticulation sub-traverses the following streets :-
Lonsdale, from Queen to Elizabeth streets.
Little Bourke, from Spencer to Russell streets.
Little Collins, from William to Swanston streets.
Little Flinders, from William to Queen streets.
Queen, from Little Lonsdale to Lonsdale and from Little Bourke to Flinders streets.
Swanston, from Little Collins to Flinders streets.
GENERAL CONSTRUCTION :
Invert and sides in 5 : 1 concrete, cement rendered ; arch yin.
brick with -in. collar of cement, except through about 1,000 feet of wet country, where the construction is wholly con- crete.
DETAILS :
The cables are to be each of abo pairs—metallic return—paper insulated and lead coated. These will rest in sets, at intervals, on successive groups of W. I. side brackets-2o brackets pee group.
Depth and Grade: To avoid sewerage and other reticulations, the tunnels are dropped to levels carrying them beneath all ex- isting works. The maximum depth, to the invert, is 38, the mini- mum 21, and the average 26 feet below street level. The grade, determined by surface contour, varies from a maximum of 1 :1r at the entrance to a minimum of I : 260.
Access: By manholes opening from main street footpaths, and by circular brick shafts of heavy construction sinking directly to the tunnels at some twenty other points. The latter shafts are for feeding in heavy cables, etc.
GEOLOGICAL FORMATION :
Older and later basalts, both solid and decomposed ; silurian ; schists, with ironstone bands ; alluvial ; and in some parts quartz drifts and fire clay.
DRAINAGE :
The grading is to sumphs, to be provided, if it is found necessary, with electrically-driven centrifugal pumps. It is, how- ever, anticipated that the completed work will be dry.
COST :
The contract price is £48,000.
At the conclusion of the inspection the PRESIDENT, supported by Mr. R. O. THOMPSON, thanked Mr. A. G. Shaw and Messrs.
Swanson Bros. for the opportunity which had been afforded the members of the Institute to examine an important development in Melbourne's telephone service.
Mr. A. G. SHAW and Mr. D. A. SWANSON briefly responded.
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