No_6_June_1969 — Page 35

TOLL PLAZA

N. VENT. BDG.

How the Hong Kong cross-harbour tunnel will be constructed

Tunnelling by the submerged-tube method

S. VENT. BDG.

by K. A. Phillips

WHEREAS the When and the Who have long been the subject of negotia- tions, the How has never been in doubt. Hong Kong's cross-harbour tunnel pro- ject, for which a franchise was granted in 1965, will be constructed by the submerged-tube method.

This method, developed over the past 40 years, has helped to remove or overcome many of the problems as- sociated with subaqueous tunnelling through poor ground. In its simplest form it is the fabrication and in- stallation of a very large precast pipe.

So far as is known, the first attempt at constructing a submerged-tube tun- nel was made in London about 150 years ago when two units of tunnel were built on the lower reaches of the Thames. The units were formed of bonded brickwork bound with wrought iron straps and sealed with hemispheri- cal brick bulkheads. Construction was carried out on a barge which was then sunk to launch the unit.

The units were let down into posi- tion by winches set up on scaffolding in such a way that at low tide one unit

could be entered through a manhole and the puddled clay and gravel seal examined. The experiment was success- ful but the projected tunnel was aban- doned owing to lack of funds.

Since this attempt, the development of submerged-tube tunnelling has taken two basic forms which will be referred to as the European and the American types. The differences between the two types reflect the differing relationship of cost of material to cost of labour between Europe and North America. The former is of reinforced concrete and has its origins in civil engineering whereas the latter is a steel shell, pro- tected and ballasted with concrete and in many ways is closely related to ship building.

Cross-section (Fig. 1)

The differences between the two types show up most clearly in the methods of fabrication and launching. The European type is usually built in- side a specially constructed cofferdam, while the American type is often built in a shipyard, launched and then fitted

out when afloat. American type tun- nels have either a single circular tube or two interconnected circular tubes; they are relatively simple forms to fabricate in steel and make very strong sections.

European type tunnels are rectangu- lar. This is the most convenient shape for placing concrete. Another big ad- vantage with the rectangular section is that overall construction depth is less than for a circular section and there- fore, for a given clearance over the tunnel, dredging is less in quantity and approach ramps can be shorter and/or flatter.

From considerations of buoyancy alone, it is necessary for about 45 per cent of the cross-section to be solid (assuming that concrete is used for bal- last). Thus stresses are relatively small except, possibly, during placing.

There have been proposals that de- signs should incorporate composite action between steel and concrete in order to economise in the use of steel. Because of the relatively low stresses, prestressed concrete is seldom worth-

Far East BUILDER, June 1969

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