April_1966 — Page 71

PLAN

BREASTING

(which will be realistic if the correct balance has been struck between risk and cost).

Esso being experienced in the berth- ing of tankers were asked to state what design criteria they wished to apply at Nga Ying Chau. Based on their requirements, the design criteria actually used are given in Appendix 1.

Design

Ground conditions are such that it was apparent that it would not be pos- sible to obtain satisfactory penetration of piles for them to either take large uplift forces or be assumed to be completely encastre. Furthermore in places it was doubtful whether the raking piles would support their own weight immediately after being driven.

To improve these conditions partial dredging of soft mud and its replace- ment with granular fill was carried out where piles were to be driven for the central pier and the four mooring dolphins. All piles were driven to re- fusal on rock and were assumed to be pin-jointed at their bases.

In view of the time available, the lengths required (of the order of 70 ft.) and the driving conditions which would either be penetration under vir- tually their own weight or refusal at rock, it was decided that steel H piles would be the most satisfactory to

use.

Consideration was given to the question of protection for these piles. Fortunately we were able to examine steel piles which had been withdrawn after about 40 years submergence in Hong Kong waters. It was found be- low the wind and water line that cor- rosion had not been excessive except for isolated spots below sea bed level where pitting had occured, presum- ably due to particularly corrosive par- ticles in the sub-soil.

In view of this and the fact that for painting or similar coatings to be at all effective preparation of the metal must be carried out, which is both expensive and requires more time than was available, it was decid- ed that the piles should be driven un- treated but should afterwards be ne- cased with concrete down to -3 ft. P. D. and that they should be bonded

SECTION A-A

DOLPHIN

Fig. 2

that cathodic protection may be ap- plied later if necessary.

Breasting dolphins (Fig. 2)

These dolphins were originally de- signed as circular caissons with heavy bases and capping slabs. However. the successful contractor put forward an alternative proposal which was ac- ceptable and is the basis of the pre- sent design. This consists of a 71⁄2 ft. thick reinforced concrete cap car- ried on twenty 4 ft. diameter in-situ cased piles certain of which are an- chored to bed-rock.

The original intention was that the stability of the structure should rely solely on gravity restraining forces but when analysed in detail it was found that the "mechanism" is such that large uplift forces are required to be withstood in certain of the piles. This is being done by anchoring Macalloy

rods well into bed-rock and then post- tensioning them.

Mooring dolphins (Fig. 3)

Four mooring dolphins are provid- ed for the berth. Each consists of a 4 ft. thick cast in-situ reinforced concrete slab carried on groups of raking and vertical steel H section bearing piles.

These dolphins are provided with fender systems inshore for the berth- ing of oil barges up to 1,500 d.w.t. The pile groups were arranged so as to resist the pull on the mooring bol- lard and the berthing forces from a barge respectively.

Pier head unloading platform (Fig. 3)

This platform is a beam-and-slab composite structure of precast and in- situ concrete supported on groups of raking and vertical steel H piles. The pile capping beams were cast in-situ and the precast slabs were laid to form soffit shuttering to support the concert topping. Loads on the plat- form include those imposed by a hose- handling frame and wave and wind forces.

Intermediate platform (Fig. 4)

The purpose of this platform is to provide space to accommodate facili- ties for loading transhipping oil bar- ges. The structure consists of a cast in-situ 3 ft. thick slab on steel H piles and includes fender systems on either side.

Driveway and pipeway (Fig. 4)

The 8 ft. driveway and the product pipes are supported on pile bents formed of in-situ concrete beams on steel H piles raked at 1:5. The longi-

Site from N. W. View taken in February 1966

together into a continuous circuit so The "William M. Allen" (69,000 d.w.t. with 55,000 tons on board) coming alongside

Far East Architect & Builder April, 1966

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