the usual method of well drilling.
Features of the machine are follows:
as
a. The depth of the protection casing is 3 to 5 metres, making the level of water in the casing 2 metres higher than the underground water line so that a minimum pressure of 0.2 kg. per sq. cm. is exerted on the wall of the drilled hole to prevent it from collapsing.
b.
Circulation water can be river water, sea water or running water.
A
slurry is formed during drilling by mixing clay from the ground with water. This protects the whole wall with a thin coating of clay ac- cumulated through pressure of slurry on the wall.
C. No vibration or ground slipping occurs and holes may be drilled to a depth of 200 metres and to a diame- ter of 1.2 metres.
d. Various types of bit are pro- vided for different terrains from sedimentary clay to hard shale.
Operation of the reverse circulation method is in three stages. First the protection pipe is sunk. This is ac- complished by a 1 ton driving ham- mer which is raised by a winch powered by a petrol engine.
Next the hole is drilled. The bit is attached at the end of a pipe which is rotated by a gear train coupled to a motor. As the bit works down into the earth a suction pump takes slurry from the bottom of the hole to а storage tank where the drilled debris
Cantilever girders in place above solid reinforced concrete piers
is settled out.
For sandy soil or clay the sinking rate is 4 to 5 metres an hour and for shale, 0.5 to 1.0 metres an hour. A pile hole of 1.02 or 1.27 metres diameter and 30 metres depth takes on average about 15 hours.
Steel wire is fabricated into cages of 10 metres length and these are placed into the pile hole with a boom. The ends of the cages are arc welded. Premixed concrete is then poured into the hole through a Tremie tube to be set underwater.
Cost
By this method the eight founda- tion piles needed for each pier of the Hwa Kiang Bridge take about 16 days. Piling and making a footing take about one month compared with the three months normally needed in Taiwan to complete an open-top reinforced con-
crete caisson, 30 metres deep.
Total expenditure on the Hwa Kiang Bridge will be about NT$82.8 million, which is being made available as a loan at an interest rate of 6 per cent per annum from the Sino- American Development Fund (SADF). The loan will be repaid by collecting tolls from the traffic over 15 years from the beginning of 1969.
The work, which was started in July 1966, should be completed this summer. Total amounts of construc- tion materials are estimated at 1,550 metric tons of reinforcing bars, 252 metric tons of high strength wire and 8,030 metric tons of cement.
Main contractor for the substruc- ture and superstructure of the bridge is the Retired Servicemen Engineering Agency (RSEA). The reverse cir- culation piling is being carried out by the New Taiwan Foundation Engineering Co.
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stributors:
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'dine House, 12th Floor, H.K. Tels. 229477 & 225563.
Far East Architect & Builder March, 1968
59