Retractable triple tube core barrel.—This modification was developed by the Authority in conjunction with Triefus Pty. Ltd. to obtain undisturbed cores of completely to highly weathered granite containing residual boulders of fresher granite. The cutting head of a driven tube sampler is attached to the end of the stationary inner tube and projects ahead of the cutting face of the bit. The other end of the inner tube is fitted with a coiled spring against which the inner tube tends to retract according to the resistance to penetration met by the cutting face. When material is encountered which cannot be penetrated by the cutting face, it retracts fully into the outer tube and the diamond or tungsten carbide bit then cuts through the obstruction. At other times the bit only cuts clearance behind the cutting head of the sampler. The core passes into a third tube of plastic and at the end of a run this tube containing the sample is pushed gently out by hydraulic pressure. The samples are undisturbed except in an outer skin and are at field moisture content. The plastic packaging enables them to be kept in this condition during transport to the laboratory for testing. Quite fragile residual granite soils can be sampled in this way. A major advantage of this technique is that the work is done with conventional diamond drilling equipment which can be used to continue the hole into hard rock.

Core Orientation Device.—A fairly simple modification of the double or triple tube core barrel is now available for fixing the orientation of the cores. This consists essentially of a diamond, set inside the entrance to the stationary inner tube which scratches a line longitudinally on the core as it enters, used in conjunction with oriented drill rods. This technique has not yet been tested on the Snowy Mountains Scheme.

Skill and Motivation of Drillers.

Obviously it is desirable to have well trained competent drillers, as well as suitable equipment. However it is also necessary to give these men an incentive to obtain the maximum possible core recovery, since drilling for maximum core recovery generally requires more frequent lifts and hence more work and slower drilling than would be the case if maximum footage per shift was the objective. It is necessary to inform the drillers that the emphasis is on complete recovery and maximum information rather than on maximum drilling rate.

This creates difficulties where the drilling is being done by contract and paid for per ft. of hole drilled. The problem cannot be overcome by paying on core recovery or by penalties for core losses, the basic difficulty being that it is not possible to know what should be maximum core recovery obtainable. In many areas this could be 100 per cent and it would be unwise to be satisfied with less. ln other difficult areas it may be impossible to avoid some losses and thus unfair to penalise the drillers for them. The Authority has largely overcome the problem by employing its own drillers and operating a bonus scheme based on core recovery.

Care of Core

Core is placed in boxes lift by lift as it is drilled. It is disurbed as little as possible during transfer from the core barrel. However pieces commonly become separated during the process and these must be fitted back compactly to avoid distributing the core over a greater length in the box than in the ground. The bottom piece in each lift is immediately marked with paint or non- fading ink preferably with its depth, and a corresponding depth mark is placed on the core box. A small allowance may have to be made for the fact that sometimes the core does not break off flush with the bottom of the hole but an inch or two above with the result that the core is shorter by this amount than the depth indicated by the drilling rods plus core barrel. However the shortage is overcome in succeeding runs and there is no cumulative error.

The length of core boxes used is always made to correspond with the length of hole so that, when the core is properly packed, all core losses appear as gaps also of the correct length. These are filled with square-section timber painted red for emphasis.

If the position of the core loss within the lift is not known then, by convention, it is shown at the top of the lift. The length of each box is five feet so that the depths which are marked at the bottom of each compartment read 0, 5, 10,...etc. In practice this method of boxing requires core to be broken at intervals of five feet and since this,cannot be done precisely the boxes are made about live feet one inch long to allow for irregularities.