June_1966 — Page 64

Flooded excavations for pier 5, where a 7 ft. diameter culvert was found

As designed the piles were to be manufactured from a 1:6 mix con- crete but test driving indicated that a stronger pile would be required and it was decided to experiment with different mixes.

In the event an economical and generally satisfactory pile was produc- ed using the 1:6 mix in the body of the pile and a 1:4.5 mix in the first 5 ft. of the head and around the shoe of the pile. The 1:6 mix in the body of the pile was difficult to compact satisfactorily without constant super- vision and cannot normally be recom- mended for hard driving conditions.

Most of the piles were fitted with conventional chilled C. I. shoes, but those driven into the bottom of the old dry dock (pier 2), were fitted with rock shoes.

Due to various factors such as difficulties in obtaining steel to meet the contract specification, and the slowing down or cessation of work because the contractor had more

Pile casting yard adjoining the site

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urgent commitments for the PWD. during typhoons "Sally" "Ruby" and "Dot", the first piles were not cast until the beginning of November 1964 and were not driven until December 14, 1964.

When once the routine was established, pile casting, curing, lift- ing, and transporting proceeded until all the contract requirements of 160 piles were met in March 1965.

After 28 days curing period, the piles were transported to the piling frame by being underslung from two four-wheeled steerable bogies fitted with penumatic tyres and chain blocks. Each chain block was fitted with a ring and two stirrup shaped steel straps to locate in each of the lifting holes provided in the pile by means of a pin locking into the open end of the stirrups. The piles were therefore supported at four points throughout their journey.

Pile driving was done with an 80 ft. high lattice steel piling frame fitted

with steadying guy ropes and the usual diesel driven twin drum winch, catheads, ropes and blocks to enable it to be manoeuvred on tubular steel rollers turning on timbers previously laid on the ground.

The piles were lifted and pitched on the frame with two hoist blocks. Each hoist block was attached to two of the slinging points on each pile by a single rope sling passing through a pulley beneath the hoist block. The pile was therefore again supported at four points throughout the pitching operation. The second pulley beneath hoist blocks allowed the slings to run through smoothly and the pile to change from a horizontal to a vertical position without undue strain on the pile.

Once in position on the frame, the pile was plumbed by jacking and in- serting packing pieces beneath the bed of the pile frame and driven with a 5 ton drop hammer.

Obstructions

Pile driving conditions on the site were generally difficult and progress was hampered by obstructions. The site is on fill, in what used to be the old Naval Dockyard installations. Full plans of these installations were not available and the obstructions to piling which were caused by numerous brickwalls. machinery bases, culverts etc. were not discovered until the work progressed, but had to be re- moved before pile driving could continue any further.

Fortunately most of these obstruc tions were sufficiently minor for them to be penetrated by the piles after a certain amount of hard driving or else cleared by comparatively shallow excavations.

However, on two of the piers, piling was seriously obstructed by old cul- verts. The first to be encountered. was immediately beneath and running parallel to the centre line of pier 8. This horseshoe shaped concrete cul- vert took six weeks to cut out and in the end the contractor had to do tidal work for a further two weeks to cut the 12 in. diameter timber matress poles used as a foundation.

As is usual with timber that is constantly submerged the wood was in remarkably good condition and due to the restricted space was ex- tremely difficult to remove. After clearing all the obstruction the excaca- tion was backfilled with selected material and piles were then driven in the usual way.

Later a 48 in. diameter culvert was discovered crossing the pile group for pier 5. together with another well below high tide level and to one side of this. Investigation of this latter culvert by a diver showed the dia- meter to be about 7ft. with apparently open access to sea. In view of the difficulties and time that would have been taken in attempting removal, the original single pile group layout was abandoned in favour of two separate groups linked by a stiff beam over the obstruction.

Far East Architect & Builder June, 1966