STABILISATION OF THE KAI TAK RUNWAYS
BY BARBER-GREENE MACADAM OPERATIONS
Aircraft using the newly completed section of the runway.
In November of 1949 a contract was negotiated by the Hong Kong Government with Messrs. Gammon (Malaya) Ltd. of Singapore for the work of applying a bituminous macadam pavement to the concrete runways at Kai Tak airfield, using Barber-Greene mixing and spreading machines.
This treatment of the runways was necessitated by the rapid disintegration of the "Japanese" concrete under the enormous loads of four-engined aircraft and was therefore a matter of great urgency. It is only by the employment of mechanical means such as the Barber-Greene apparatus that a really high quality macadam pavement could be obtained, with the highest possible density and perfect finish, and at the speed which the urgency of the job demanded.
The work comprised two distinct operations namely, a base coat of 14" bituminous macadam of varying compacted thickness 5" to 2" followed by a wearing carpet of 3/8′′ bituminous macadam, 1′′ compacted thickness. The areas so treated were approximately 200,000 super yards and 245,000 super yards, respectively.
Preliminary works leading to the actual mixing and spreading operations were, of course, very considerable, not the least of which was the opening up of a new quarry at Ngau-chi-wan, one mile from the airfield, and the installation there of quarrying plant which had to be removed from one of Messrs. Gammon's quarries at Singapore and shipped to Hong Kong.
It is here worthy of mention that though the first ship- ment of quarrying plant at Singapore was not possible before the 7th of November, 1949, the whole installation was in operation by the 15th of December, 1949.
The plant comprised one 20 ton hour primary crusher and two balanced secondary crushers, Screens were of the electrically-vibrated type and the whole installation was driven by one 300 H.P. Crossley diesel engine which, until a few years ago was in marine service.
Rock drilling was performed using "Holman" pneumatic drilling machines with the patent tungsten carbide-tipped bits also manufactured and supplied by Messrs. Holmans.
At the peak of production 275 tons per day of crushed and screened granite three grades down from 1"-were pro- duced and delivered to the mixing site on the airfield.
Preliminary works in connection with the mixing plant were also somewhat extensive, involving the shipping from Singapore of bitumen boilers, air compressors, power rollers, pressure spraying machine (for tack coat) 10 "Dodge" tipping lorries, power driven conveyors, D4 tractor, welding set, steam boiler, bulk storage tanks and numerous ancilliaries.
At the outset of negotiations Messrs. Gammon's Barber- Greene machines were operating at the new Changi airfield in Singapore and could not be spared for the Kai Tak project within the required time. This difficulty was overcome by the purchase by Messrs. Gaminon of a new combined Dryer and Mixer direct from the makers in the U.S.A. This set reached Hong Kong on 26th December, 1949, and was put into operation on 20th January, 1950, the date on which the actual mixing and spreading work began. Thus was opened a new page in the history of Public Works in the Colony.
The mechanical Spreader which was manufactured by Messrs. Barber-Greene Olding of England was supplied by Messrs. William Jacks & Co., Ltd.
Plate 1 shows, diagramatically the complete production operation from the cold, damp aggregates in the stockpiles to the "piping-hot" macadam being discharged into the tipping lorry.
The Feeder 'B' has two adjustable gates 'E' through which is ejected, by a reciprocating action the desired quantities of aggregates. These are then discharged into the drum of the Dryer by the bucket elevator 'C', and subjected to intense heat from the flaws of a pressure oil burner which heats
and drys the aggregates to the specified temperature, when they gravitate onto the "hot" bucket elevator 'J' and so into the Mixer hopper 'Q. The Apron Feeder P then conveys the hot aggregates through the adjustable "hot" gate 'R' into the Pug-mill 'U' where the bitumen, at the specified tempera- ture is added through a conical jet under pressure from the pump '0'.
The "Fines Feeder" 'B' was not employed in the operations here described as prior sieve analysis of the aggre- gates to be used showed there was sufficient "fines" present to provide the necessary filler i.e. material passing B.S. Sieve 170.
Incidentally, the Dust Collector, 'H' is provided to recover the "fines" which would otherwise be discharged into the open air by the forced draught of the Blower. This clever device is based on a cyclonic principle whereby the velocity of the "fines" is reduced to a minimum in the dusts and these are returned to the "hot" end of the Dryer which is not affected by the draught. The Dryer and Pug-mill mixer were each originally provided with a Le Roi gasoline engine but these have since been replaced by Caterpillar diesel engines.
The granite aggregate used in the Base-coat macadam was graded material which had passed through a 1" vibrating screen and been retained on a 1′′ screen. The aggregates for the Wearing Carpet were of two grades, the "coarse" having passed a screen and been retained on a 1/8" screen and the "fine" being all the material having passed the 1.8" These were batched in the proportions of 60% and 40% respectively.
screen.
Straight-run bitumen of 60:70 grade was used as the binder and was added to the aggregate at the rates of 3.5% te 4 and 6.5% and 7% by weight in the case of the Base- coat and Wearing Coat materials, respectively.
The temperature of the "dried" stone as it entered the Pug-mill was usually in the region of 325°F and the bitumen was added at like temperature, thus giving perfect adhesion of bitumen to stone and resulting in a high-quality macadam. The mechanical Spreader and Finisher used has a standard width of 10′0′′ but can be modified to given widths of 8, 9, 10, 11 or 12 feet as required. The variable speeds at which the Spreader may be operated range from 7 feet per minute to 40 feet per minute, thus enabling the operator to adjust his spreading rate to the delivery of materials and so keep his machine in continuous operation which is preferred to inter- rupted and spasmodic progress.
"Dodge" tipping lorries were used to convey the macadam from the Mixing Station to the Spreader, the average length of the haul being 1 mile.
The areas which were required to be paved consisted of two runways each approx. 1500 yards long of which a strip 190 feet wide throughout their whole length was treated plus the turning circles and turnouts.
The existing surface was first tack-coated with a cut- back bitumen applied hot by a mobile pressure sprayer which operated at the rate of 0.05 to 0.15 gallon per super yard, as determined by the Engineer.
It was stipulated that the work must be executed in four distinct phases, each phase being one half the width of a runway over its full length. Flying took place on the half of the runway not being worked over and needless to say, those operating and attending the Spreader had their full measure of thrills, particularly when spreading at or near the centre of the runway.
It was originally intended that the Base-coat should be of 24" compacted thickness but it was later decided in the case of the main runway to increase the thickness to 5" at the centre, diminishing to 21" at a point 35 feet on each side of the centre-line. This was done to improve the run-off of stormwater to the side-drains and undoubtedly achieved its purpose as was observed during the wet season.
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