June_1970 — Page 33

sunk in position on a prepared founda. tion.

A steel caisson with concrete base slab as ballast has been constructed at Taikoo Dockyard. After launching, the caisson will be completed internally to provide the reinforced-concrete sub- structure to the ventilation building. This will increase the draft of the cais- son which will be completed and sunk in position on a prepared crushed- stone bed. The laying of the sub- merged-tube tunnel will start from this building once it is in position and ready.

All work on the South ventilation building and ramp will be constructed in the dry within a temporary earth and rockfill cofferdam (Fig. 4). The ventilation building will be of conven- tional reinforced-concrete construc- tion while the ramp will be a rein. forced-concrete twin box where it is covered and a reinforced concrete U- shape where it is in cutting.

The open section will have to be anchored down against flotation by means of prestressed anchorages.

Tunnel units

There are two principal systems for a submerged tube tunnel: either a

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North ramp open section during construction

structural steel unit which is launched like a ship and then lined with rein- forced concrete or a rectangular rein- forced concrete section which is com- pleted in dry dock and then floated out. The former, which is the system being used in Hong Kong, is usually constructed in a shipyard, while the latter is usually constructed within a specially-formed cofferdam.

If there is anything unique about the Hong Kong tunnel, it probably is

that a conventional shipyard has not been employed to fabricate the steel units. Instead, a temporary fabricating yard complete with slipway has been set up specially at Hung Hom adjacent to the line of the tunnel. A plan of this yard is shown in Fig. 5.

First, steel plate 36ft. x 6ft. 3/8in. is unloaded across Hung Hom seawall and stacked ready for use. Next it passes through straightening rolls and then on to a Hancoplater which con-

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oll plaza

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King's Park

Ferry piers

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HOL

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Hung Hom

4000

1 Mile

5000 feet

Far East BUILDER, June 1970

R.C.struts

light louvres

R.C, struts

Fig. 2. Typical deep (louvred) section

R.C. arches

precast concrete facing unit

4'-0" dia. contiguous bored piles

ventilation duct

R.C. struct

Base slab varies 4' 9"

5' 9" thick

4'-0" dia. contiguous

bored piles

Fig. 3. North ramp

cut and cover

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