Introduction

Diamond core drilling, in spite of its very high cost, is very widely used for subsurface exploration for sites of dams, tunnels and other major engineering structures, when these are in rock. There is no adequate substitute for diamond drilling, although it can, and usually should be supplemented by other kinds of exploration. Although diamond drilling is widely used it is far from being a standardised procedure and many details of the planning, execution and evaluation of the diamond drilling programmes need careful and systematic attention to obtain the best results.

The procedures used for geological investigations in the Snowy Mountains Scheme have been described in outline previously (Ref. 1). The purpose of this paper is to give details of current procedures in diamond drilling, including modifications introduced as a result of experience.

Planning the Drilling Programme

Details of planning vary greatly with the type of project and whether the project is in the preliminary investigation or the detailed design stage, but the same general principles apply.

First the engineering objectives of the exploration programme are defined.

Then an exploration programme is drawn up taking into account all geological information available at the time. Drilling is only one form of exploration which must be co-ordinated with geological mapping, sub-surface exploration by trenching, test pits, adits, etc., and by geophysical methods, especially seismic refraction. In particular it is always highly desirable that topographical and geological mapping should precede diamond drilling.

The objectives of the programme seldom remain constant in all details, being modified as the engineering studies develop and also as the results of exploration become progressively available. Care is taken to inform the field staff promptly of any changes in objectives which may require changes in exploration targets. Conversely, results from the field are relayed back to the investigation or design engineers as soon as they become available.

For the efficient deployment of diamond drilling crews and equipment it is necessary to estimate the total quantity of drilling, the number and depth of holes, their approximate locations, and special requirements. However, within these limits, the greatest degree of freedom is maintained as the drilling progresses to modify the programme to take into account any changes in engineering and geological objectives. In terms of useful information obtained it can be very wasteful and inefficient to draw up, in advance, a large detailed drilling programme and to adhere to it rigidly, although this I certainly would facilitate the drilling operations.

The site of each hole is marked out on the ground by the geologist who is expected to modify the position and slope from those given on the programme if local topography or geological conditions should make this desirable. The geologist then draws up a specification for the hole. This consists of a large scale plan and section through the hole, and a written specification which includes a statement of the engineering objectives, the geological objectives, the anticipated drilling conditions (rock types, faults, etc.) and any special requirements. The specification is reviewed by the engineers concerned and then given to the drillers.