EAST AND SOUTH BREAKWATERS

General

The East breakwater was required to: (1) facilitate entrance for ships, (2) protect the LPG Basin which cannot be fully protected by the South Breakwater, (3) stop the siltation moving along the beach and the inner waters. The South breakwater was to protect the LPG basin from the point where the influence of the East Breakwater stops during the N.E. monsoon, and against S.E. winds during the S.W. monsoon, and generally from various storms and swells, and stopping the general siltation caused by wave action and littoral drift. See photo of East Breakwater and cross-sectional plan.

Design Considerations

The Engineer's opinion is that the best and most economical type of breakwater for Tg. Berhala is the one with armour stone of igneous rock, weighing 5-10.5 tonnes. The problem is whether this size can be obtained in sufficient quantities. Otherwise concrete cubes or other shapes of armour, are to be used. The contract for the breakwaters was made with Kajima Corporation of Japan. It covered 1.25 million m3 of stone in placed volume. The East breakwater is positioned on a rocky reef, and the South part of the South breakwater is on deep sand layers. At the northern part of the South breakwater, there is soft clay to depths of 9.0m.

It was then examined whether to : (A) Construct this part of the breakwater as a steel piled wall in form of an A-frame, (B) Place a stone breakwater on a structural geotextile on top of the soft clay, (C) Remove the soft silty clay and backfill with sand, and place the breakwater on this.

Alternatives

Re (A) The steel piled wall.

A model test was carried out by DHI to find maximum allowable spacing between piles (actually very little, so wall was closed) and the forces on the wall from the design wave. This scheme cost slightly more than the stone breakwater with concrete cubes, and (i) although the piles would be well protected, there would be more maintenance, (ii) the stresses under design wave would be at yield. The wall would thus not stand up as well as a stone breakwater during big storms beyond the design wave, (iii) the movements of the steel piles might cause fatigue in the concrete cap.

Conclusion :

This design is more suited to more protected waters.

Re (B) Stone breakwater on a structural geotextile on top of the soft clay

It was found to be a feasible scheme, but in order to avoid squeezing out of the soft clay below the stone mound, loading would need to be slow and settlement would go on for a very long time and the breakwater might therefore require maintenance.

(C) The Adopted Design

It was found that to dredge the trench and backfill with sand was the cheapest method because a suitable dredger (Trailer Suction Hopper Dredge) was available in Singapore for this fairly small dredging work and could do it in the required short period. It was also the most durable solution. Kajima Corporation carried out a model test in early 1984, at the Kajima Institute of Construction Technology and found that the design was satisfactory. The construction period was short : from February 1984 to December 1985. During 1984 the foundation trench for South breakwater was dredged and backfilled with sand, and the abutment of the South breakwater was built using imported stone. The main work was to open up the quarry, and produce stone for stockpiles.

NEW: check out the preliminary design on this interactive calculation page.

Construction

About 1.25 million m3 of granite stone for the construction of east and south breakwaters were obtained from "Kajima" and "Kemaman" quarries at Tg. Gemok which is about 6 km from Tg. Berhala.

By drilling and blasting, stones were shattered into various sizes from the quarry face and fell onto the floor. These broken stones were then dug into by wheel loaders and hydraulic excavators and loaded onto 32-tonne dump trucks on the quarry floor. Core material (1-250 Kg. stones) in the dump trucks was transported to the rock chute at the temporary harbour at Tg. Gemok and unloaded directly onto the spilt hopper barge moored below the rock chute.

Armour stones (3-7 tonnes and 6-10.5 tonnes) in the dump trucks were transported to the storage yards or unloaded onto the flat top barges moored at the temporary harbour. There were three ramps (adjustable gangways) for the dump trucks to access to the flat top barges from the land. Filter stones (250-1,500 kg) mixed together with core materials in the dump trucks were sent to the top of stockpile and discharged along the brink to allow the stones falling along the slopes of stockpile so as to segregate them. Filter stones at the toe of this stockpile were picked up by wheel loaders and loaded onto dump trucks for transferring to the flat top barges, or, to other storage yards near the temporary harbour.

Stones in the split hopper barges and flat top barges were towed/ pushed to Tg. Berhala harbour for construction of breakwaters. Core materials in the split hopper barge were discharged directly to form the cross-sections of the breakwaters. Trimming of the core section was done by crane-barges fitted with orange-peel buckets. Filter and armour stones on the flat top barges were grabbed by the orange-peel buckets of the crane-barges and placed onto the breakwaters. Trimming of armour and filter layers to conform to design cross-sections was also done by these crane-barges.

The quarry made available and accepted by the contractor after seismic survey was developed. There was big stone in the old Kemaman quarry. The drilling data just north of the site had been interpreted as big stone formation. Nevertheless, the developed site extended up to the investigated area and big stone did not appear. The crisis continued for months, when the contractor only obtained 2% of big stone when 20% was required.

In March 1985, it became necessary to clear the old Kemaman quarry of its tremendous overburden and get the big stone which was there. In spite of this the contractor managed to finish his work in time in 1985, and sufficient for the first big LPG tanker to arrive in June 1985.

 

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