In preparing the log the principle is adopted that it should be a factual record free from interpretation. Therefore, in addition to being used within the Authority these logs can be made available to tenderers and contractors.

The following comments refer to headings on the form “Geological Log of Drill Hole” (Fig. 1)

Location of the Hole.

— At the top of the log sheet the location of the hole is given in relation to the specific engineering feature it explores, and also survey co-ordinates, level and slope to enable it to be relocated if necessary on the ground and to be plotted on plans and sections.

Drilling.

—At the bottom of the log sheet information is given about the type of drill, the driller, and the date of drilling.

Description of Core.—

In this column the rock type is described by its geological name, degree of weathering, colour, grainsize, texture and mineral composition. A set of terms has been standardised to describe the degrees of weathering which can be distinguished consistently in cores of granite (Ref l). These definitions are based on simple tests or observations which can be made quickly on the core at the drill site. However, they are not directly applicable to other weathered rock types and other sets of terms have to be defined for these. The intervals from which core was not recovered are labeled " No Core ". It is to be noted that the descriptions refer only to the material recovered as core or occasionally, where so notified, to material recovered as drill cuttings.

Percentage Core loss per lift.—

The position of each drilling lift is shown by a horizontal line across this column and the core loss per lift, expressed as a percentage, is shown graphically.

Log.—

The rock type and some geological structures are shown by hatching in this column. Intervals where core was lost are left blank. When the position of the core loss within a lift cannot be determined from an examination of the core or from the observations of the driller, the core loss is shown, by convention, at the top of the lift. It is considered bad practice to fill in these blanks with the geologist’s interpretation or guess. It is best to draw attention to the losses and leave open the question of the nature of the material which was lost.

Fracture Log.—

This is a fairly recent addition to the Authority’s logs. It shows, on a logarithmic scale, the intensity of fracturing of the core. Only the fractures of natural origin (joints, faults) along which separation has occurred are counted so that cemented joints which remain tightly closed after drilling are ignored; also artificial fractures caused by drilling operations are not counted— these are usually easily recognised. The entire core from a drill hole can usually be subdivided by inspection into sections within which the intensity of fracturing is reasonably uniform. The number of pieces per ft. in each of these sections is counted and plotted as the fracture-log. The method of plotting and hatching gives prominence to the more closely fractured zones. Sections from which core was not recovered are heavily hatched across the full width of the fracture-log columns. The overall result is that the varying degree of fracture in the core, which is the main indicator of the mechanical quality of the rock, can be appreciated at a glance. But the information is also quantitative and is given in such a way that it can be transformed into a standard grading curve. Fig. 2 shows the grading curve derived from the log of drill hole 6022, 220-ft. deep, of which the first 100 ft. are shown on Fig. l.

In using such information consideration must be given to the effect of the orientation of the drill hole to the joint system in the rock mass, and also to the significance of cemented joints some of which may have parted during drilling and others not. The fracture- log provides a means of predicting the grading of rock to be excavated from quarries as well as the quality of rock in foundations. As a broad generalisation, cores tend to give a pessimistic impression about closely jointed but unfaulted rock. in foundations because even with the greatest care joints which are tightly interlocked and more or less cemented in situ become open and disturbed in the cores. However, the cores may give a more realistic indication of the sizes into which quarried rock will break along joints.

Structures.—

The properties which influence the mass behaviour of the rock are recorded. For joints these are spacing, attitude, smoothness of the joint surface, aperture (width of opening across the joints), the nature of the coatings on joint surfaces, and the nature of the material occurring as seams filling formerly open joints. Full information about the attitude of joints in the rock mass cannot usually be obtained from inspection of drill cores. The angle between the joint plane and the plane normal to the axis of the core is recorded on the log. This is the true dip angle only when the hole is vertical.