July_1970 — Page 31

Part 1 - On Hong Kong Island

by W.A. Maslowicz

and C.H. Leung

WHEN, in August 1965, the Hong Kong Government granted a franchise to the Cross-Harbour Tunnel Co. for constructing and operating a four-lane vehicular tunnel to link Kowloon and Hong Kong island, it undertook at the same time two multi-million dollar projects to provide new road networks at both ends of the tunnel.

Scott Wilson Kirkpatrick & Part- ners, Hong Kong, in association with Freeman Fox & Partners, London, the consulting engineers for the tunnel, were appointed by Government to plan, design and supervise the con- struction of these complexes of road connections and flyovers which will facilitate dispersal of the tunnel- generated traffic into the existing road networks.

This article describes the road con- nections on the island side of the tun- nel. These are situated on the newly formed Wan Chai reclamation, just west of Kellet Island (Fig. 1).

Planning

A committee of representatives from the consultants and various public works departments was set up in late 1965 with a brief to agree the layout of connections and to co-ordinate the work of all the parties concerned. This early collaboration proved invaluable as it enabled problems to be discussed and action initiated as work progressed. Considerable time and effort was saved.

The Tunnel Company had carried out a traffic survey a few years earlier which estimated the probable traffic pattern after opening the tunnel and the road connections had to be designed to provide at least for this capacity.

Government, in keeping with its general policy to improve the road sys- tem in the colony, approved the con- struction of Waterfront Road which, when completed, would provide an urban motorway between Central Dis- trict and North Point. A section of the motorway, between Tonnochy Road and Percival Street was planned to- gether with the tunnel requirements to provide for an uninterrupted flow of vehicles entering and leaving the tunnel.

A system was eventually evolved which provided complete grade separa-

Model of tunnel access layout, Hong Kong island

tion in the area (Fig. 1), pedestrian needs being met by a number of foot- bridges.

Site investigation

Early in 1966 a contract was let for the site investigation in the area. The results showed that the existing land in Canal Road and along Gloucester Road consisted of fill material, soft marine clay and decomposed granite resting on parent rock. Standard pene- tration tests gave "n" values of between 100 to 150 at about 70 ft. below exist- ing ground level.

Marine site investigation was carried out in an area north of Gloucester Road. The depth of water varied from a few feet at the existing seawall to about 20ft, at the new seawall. The sea bed consisted of very soft marine mud, clay, gravel with shells and decomposed granite. Results showed that firm stra- tum capable of supporting foundation loads was located some 90ft. below proposed reclamation level.

Design

Structures are designed in reinforced and/or prestressed concrete to carry B.S. 153 HA loading and the stresses worked out to comply with the recom- mendations of British Ministry of

Transport (MOT) Memorandum 785- "Permissible Stresses for Highway Bridges".

In general, bridge decks are of a composite beam and slab type employ- ing prestressed concrete beams and in situ R.C. deck slab. With few excep- tions, spans are simply supported on R.C. frames having 4ft. 6in. diameter columns resting on piled foundations (Fig. 2).

Foundations on the existing seawall rubble mounds are of a spread footing type and these are pre-loaded to elimi nate excessive settlement. In each case pre-load is equivalent to 1.5 times the dead load and 2 times the live load.

Footings are constructed when set- tlement under 100 per cent pre-load does not exceed 1⁄2 inch in 48 hours. Prestressed concrete piles 18in. x 18in. are employed in piled foundations each pile being capable of carrying 100 tons.

All prestressed beams rest on rubber bearings and rubber pads complying with MOT Memorandum 802.

Railings on bridge decks are of steel rectangular hollow sections and are de- signed in accordance with MOT require- ments for "Parapets & Safety Fences".

Surface water from the bridges is discharged through downpipes cast in

·

Far East BUILDER, July 1970

27