May_1967 — Page 34

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Fig. 3. 80-ton pneumatic tyred proving roller on fill material for runway

(iii) voids in total mix: 5% to 7% (iv) voids filled with binder:

65% to 75%

(c) Bitumen-bound base (Marshall

method) Aggregate: type (a) Gravel sand crushed or uncrushed, crusher run stone or a mixture of both or (b) Sand gravel crushed or uncrushed, crusher run stone or a mixture of both Maximum size: to be less than two-thirds of the compacted thickness of layer

Binder Type: straight run bitu- men 80/100 penetration, or straight run bitumen 60 70 penetration Compacted

thickness of base course: 12 in. (to be made up of layers not exceeding 4 in. in thickness) Design requirements

(i) stability: as determined by test but not less than 650 lb. (ii) flow: not more than 0.18 in. (iii) voids in total mix:

5% to 10%

Construction

Earthworks

Control of the quality of the works was wherever possible by means of physical tests or measurements. Some

Fig. 4. Tamping roller on fill material in paved area

13,500 soil tests, approximately 3,000 asphalt tests and 600 concrete tests of various types were carried out. A soils laboratory and a separate asphalt laboratory were set up at an early date.

It was soon found that the com- paction characteristics of the soil in the cut areas varied considerably with the depth of the cut. Type I soil was generally in the top 15 ft. of cut, Type 2 soil from 5 ft. to 30 ft. deep and Type 3 soil from 17 ft. and greater.

It was noted that the sand fraction was fairly constant at between 64% and 30%, but the silt fraction increas- ed and the clay fraction decreased with depth. The sand fraction is quartz and it is thought that the fels- pars in the original granite have wea- thered more on the surface than at depth. Undisturbed Type 3 soil has the appearance of granite. Trial com- paction tests were carried out in addi- tion to laboratory tests.

In Type soil the tamping roller was the most efficient. This was in fact the only roller capable of attain- ing the specified density and ten passes on a 6 in. thick layer gave 95% B.S. heavy compaction dry density at 3% above laboratory optimum moisture content. The above is also true for Type 2 soils with the higher plasticity indices, but Type 2 soils with the

lower plasticity indices have charac- teristics similar to Type 3 soils.

Type 3 soils have a much lower optimum moisture content (in the laboratory about 14%). This low moisture content could not be reached in the field and the only roller to achieve the specified density was a locally made 18 ton smooth wheel towed steel drum with four passes at 20% moisture content when the layer was 41⁄2 in. thick and with about 14 passes at 21% moisture content when the layer was 9 in. thick.

Having established the procedure, filling and compaction of Type 1 soil proceeded. The 100% B.S. heavy compaction was evaluated from soil from the cut a day or two before it was excavated. During filling opera- tions it was spread in layers of 9 in. loose thickness and if too moist it was disk harrowed and allowed to dry out to a moisture content of about 24%, after which it was compacted with the tamping roller. The speed of travel of the tamping roller was found to have a considerable effect on the effi- ciency, the ideal speed being approx- imately 15 mile h. In practice with Type 1 soil the roller travelled at about 11 mile, h and was still reason- ably efficient.

With the frequency of rain in Ma- laya great care had to be taken to drain the cut and fill areas by cutting and

Fig. 5. Disc harrowing fill material in paved area

Fig. 6. Inflatable formwork used in construction of slotted drains to runway, taxiways and parking aprons

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Far East Architect & Builder May, 1967