June_1970 — Page 36

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control room

hoppers

screed trolley

main hoist blocks

counter weight (85 t.)

Fig. 6. Screed and lay barge

ballast tank

screed box

a.) Screeding

The barge is shown placing and screeding crush- ed stone. During screeding the pull on each counter-weight is adjusted to 30 tons and the barge levelled by moving water ballast. As the screed trolley moves forward and material in the hoppers varies correct level has to be main- tained by moving water ballast and adjusting the screed box cables.

b.) Laying

The screed box is hoisted up out of the way and the tunnel unit towed in and main hoist blocks connected. Then the unit is ballasted until each main hoist carries 100 tons with the top of unit at sea level. As the unit is lowered the main holst winches must be adjusted to en- sure that tensions on all four hoists (indicated on load cells) remain equal.

dredged trench

crushed stone

bulkhead plates

top hood plate

steel shell

tee stiffener

pneumatically placed concrete

tapered pin

DETAIL OF TAPERED PIN

bottom hood plate

Fig. 7. Detail of joint

tremie concrete

hood plate

dam plate

tapered pin

level. Dredging is carried out by means of a 2400 Lima crane mounted on a pontoon and carrying a 6 cu.yd. buc- ket. The crushed rock is bottom- dumped in the prepared trench and then screeded with the special rig shown diagrammatically in Fig. 6.

Units are placed in position and joined together, starting from the North ventilation building. When ready, each unit is provided with tem- porary access and survey towers and floated into position within the screed- and-lay barge (Fig. 6). Once this barge has been accurately positioned, the unit is ballasted with tremie con- crete until it has about 400 tons nega- tive buoyancy when it is winched. down carefully into position.

When it is almost in positions 25 ton ratchets, two to each hood plate, are attached to pull it up against the previous unit. Once the two units re- gister, tapered steel pins are inserted to connect the units (Fig. 7). When this has been done the hood plates are caulked up and dam plates slotted into position.

Finally the joint is concreted up by placing 600 cu.yd. of tremie concrete within the dam plates. Later the inter- bulkhead space is drained, the shell plates welded together between units, bulkheads removed and concrete lining completed.

After units are placed and joined, fill is placed around them. Initially the fill is placed by tremie, then by bottom dumping and finally completed with a 7ft. thick protective rock blanket.

The design minimum factors of

Far East BUILDER, June 1970