No_6_June_1969 — Page 40

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A system similar to that proposed for the Hong Kong cross-harbour tunnel is being used for part of a rail tunnel under the Tamagawa River in Japan. These double-cylinders, each 80 metres long, 7.95 metres high and 13 metres wide, will be weighted with reinforced concrete and water and sunk to the river bed

tunnel. The tenderers were required to submit proposals for financing con- struction and, in some cases, depending on the source of funds, alternative designs were put forward.

The engineers' reinforced concrete design that was put out to tender is the most economic but, of course, govern- ment sponsored funds are nearly always linked with export content quotas and, as this favours high value commodities, the tendency has been for alternative designs to be of steel even though this increases the engineering costs. These costs are now estimated to be in the region of HK$220,000,000.

The Report of 1961 and after

The report discussed both bridge and tunnel on substantially the same line across the harbour. The chosen line was a compromise between traffic requirements and economic require- ments which, fortunately, do not differ greatly. It is close both to the shortest crossing and to the traffic 'desire line' between the island and the mainland. Also the location is one of the few places where there is space to accom- modate approaches and toll plaza and where it is possible to provide suitable connections to the existing road net- works on either side of the harbour.

Comparison between bridge and tunnel indicated that the initial cost of a bridge (a stayed-girder bridge with 625m. central span and 250m. side spans and shipping clearance of 52m.) would be about 10 per cent cheaper than that of a submerged-tube tunnel and also that the bridge would be sub- stantially cheaper to operate. On the other hand adverse effects on existing interests, particularly marine and avia-

2 J.O. TRESIDDER. A survey of cross- harbour traffic in Hong Kong'. Proc. Instn. Civ. Engrs, Vol. 26, pp. 549-86, London, 1963.

tion, are far greater with a bridge than with a tunnel.

The object of the traffic survey was to estimate the amount of traffic that would use the crossing. This was needed both to determine the necessary capacity of the crossing and the eco- nomic viability of the project as a toll crossing.

The survey is described in detail elsewhere2 but in essence it consisted of surveying the cross-harbour traffic in 1959 on an origin-destination basis, assessing the proportion that would be diverted to the tunnel according to the saving in time and in cost and adding an allowance for induced traffic (i.e. journeys that would only be occasioned by the existence of the tunnel). The estimate thus obtained was then increased by a factor to allow for growth.

Since the report was published in 1961 the Government of Hong Kong has sponsored major reports on other important aspects of transportation within the colony. These reports have assumed that a cross-harbour tunnel is built by the early seventies and that, in fact, additional crossings will be required within the next few years if traffic needs are to be met adequately.

It is now ten years since the tunnel traffic study was carried out but later workers in this field have confirmed that the basic findings are still correct although detailed figures have in all cases been exceeded in practice. This has been expected as the original report was prepared deliberately on a conser- vative basis.

Design

The design provides for a tunnel having two carriageways each 6.75m. wide with headroom 4.85m. The length between portals is nearly 2km and of this about 1.6km is of submerged tube

construction. Ventilation is by a semi- transverse method, fresh air being dis- tributed through grilles throughout the length of the tunnel and then passing back through the running tunnels to escape at the tunnel portals. Ventila- ting plant buildings are sited adjacent to the portals on either side of the harbour.

The vertical alignment of the tunnel is so arranged that the navigation chan- nel through the harbour has a mini- mum depth of water of 12m. This is possible with approach gradients vary- from 5 per cent to 3-1/3 per cent. Under the navigation channel the gra- dient is 1/3 per cent towards a central sump.

The steepest gradients are adjacent to either portal and inshore of the ventilating plant buildings where venti- lation ducts can be greatly reduced and the resulting saving in space utilised as an extra climbing lane for traffic leav- ing the tunnel.

Consideration has been given to the construction of the tunnel units within a cofferdam at a site 11km from the line of the tunnel although it is now likely that the units will be fabricated close by the line of the tunnel.

The tunnel could take up to about 70,000 vehicles per day and ancillary equipment will be provided to facilitate the free passage of this very heavy volume of traffic. This equipment will include full coverage by closed-circuit television as well as all the other aids normally provided in a modern tunnel.

Author

The author of this feature, Mr. K.A. Phillips, DIC, FICE, FASCE, is an Associate of Scott Wilson Kirkpatrick and Partners, consulting engineers. He recently delivered a paper on sub- merged-tube tunnelling to the First Philippine Convention on Civil En- gineering Progress.

Since this feature was prepared negotiations have advanced to a stage where agreement in principle has been reached between the Cross-Harbour Tunnel Co. and a London bank for a loan to finance the construction of the tunnel, guaranteed by the British Ex- port Credit Guarantee Department. The tunnel company is proceeding to detailed negotiation with a consortium headed by Richard Costain (Civil En- gineering) Ltd.

In this event a typical cross-section of the tunnel will be as Fig. 5(a) and units will be part fabricated on Hung Hom reclamation and side launched rather than constructed within a cof- ferdam away from the tunnel site.

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Far East BUILDER, June 1969

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