Core recovery and labelling
When removing the core from the split inner tube of a triple-tube core barrel the following procedure should be followed. First, the two parts of the split tube should be placed on a corrugated iron sheet or an angled iron rail. The upper split should then be removed, and the core photographed and logged before it is placed in the core tray by the person responsible for logging the core.
When transferring the core to the core tray, the best results are obtained by replacing the upper split with a PVC pipe that has been cut in half, rolling the combination over to transfer the core from the split tube into the cut PVC pipe, then placing the cut PVC pipe and core directly into the core tray.
To remove the core from a single-core or double-core barrel, the single-core barrel or the inner tube from the double-core barrel should be elevated to allow the core to slide out of the core barrel. The core should not be allowed to drop into the core tray; it should be captured by hand and carefully placed. Although it may be necessary to tap the core barrel with a hammer to loosen wedged core, this technique should generally be avoided to prevent artificial breaks.
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All the core, including the fines, must be transferred to the core tray in its correct order. The core should be pieced together as tightly as possible. Although an extruding piece of core may have to be artificially broken in order to fit it into the tray, this practice should be minimised. Where practicable, it is better to leave a small gap at the end of the core length than to break the core.
Damaged core can result in underestimates of rock mass strength and erroneous predictions of rock mass behaviour. Because of this, it cannot be emphasized strongly enough that the core should always be handled as carefully as possible, with the main objective being to minimise artificial breaks. It is very important to ensure that all artificial breaks are clearly marked. Proper core handling is the responsibility of the person logging the core (engineering geologist or geologist).
When the core has been retrieved it is the responsibility of the drillers to:
■ carefully wash the core to remove any drilling mud and place a depth block at the end of each run. Washing should be done very carefully, to preserve the integrity of the core. High-pressure nozzles should not be used as these cause core misplacement and further core deterioration. Great care must be taken not to wash away the fines from any weak or broken zones.
■ label the core tray with the drill hole ID, the tray serial number and an arrow pointing in the downhole direction. The proper depth should be marked on the small block after each run is recovered. Proper depth registering of the core is vital and the core logger should always check that it is being properly done. In this process two measurements must always be checked:
→ stick-up, which is the measurement that most often results in significant inaccuracies in core depth registration. This colloquial term arises from the fact that depth measurement of the drill hole is derived from (length of rod string plus core barrel outer tube) minus (length of rod string ‘sticking up’ above ground level). Since ground level is hidden under the drill rig, a convenient datum is used on the rig, usually the top of the chuck head. Measurements are made against this ‘constant stick up’, which has to be accurately assessed before drilling commences – a process that is simple in concept but frequently difficult in practice.
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→ stub length, which is the length of the stub of core left in the bottom of the drill hole after the core has been retrieved (Figure 2.12). The stub length can vary considerably depending on the driller’s skill in breaking the core at the end of each run as well as the properties of the formation being drilled. To estimate this correctly, it is necessary to inspect all breaks in the core for any grinding or similar damage. If there are no sections of grinding in a competent rock run of core, then stub length is advance minus recovery. If there is grinding, the estimate is more subjective.
Core photography
The entire core must be photographed as soon as possible after drilling, in colour and preferably using a digital camera with a minimum resolution of 3.0 megapixels. More sophisticated 2D and 3D core scanning techniques are being developed, but they are not yet widely used because of their cost and the need to rehandle the core.
It is important to ensure that there is a minimum of lateral distortion in the photographs. To achieve this, the plane of the core boxes must be parallel to the plane of the camera lens and the camera must be aimed at the midpoint of the core boxes. Each frame should include core from a single hole.
Core photo documentation includes three basic steps.
1 Photographing the core and verifying the required image quality. In all cases the core should be photographed in the core box prior to logging in order to minimise any bias caused by core damage. If split inner tubes have been used, the core should also be photographed in the split tube prior to handling. A single picture should cover one or two core trays at a time.
2 Labelling of the electronic file.
3 Photograph database management.
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The following criteria should be observed while photographing the core.
■ Lighting conditions and exposure times should be consistent throughout the project. If artificial light is being used better results can be obtained with diffused light rather than bright light.
■ The core should be photographed consistently wet or dry. Experience has shown that for geotechnical purposes photographs of dry core are more informative, although in arctic conditions this may be difficult.
■ The camera should be kept at a constant distance from the core. Wide-angle lenses should be avoided if possible as they will result in distortion.
■ The photograph should include a label, colour bar and scale. The label should provide details of drill hole ID, date, depth, starting point and direction of drilling. After the core is photographed the digital file of the photograph should be renamed so that it can easily be identified in the database. The following information should be included in the file name:
■ the drill hole ID.
■ the depth of the interval photographed.
■ an indication of whether the core is wet or dry.
The following is an example of a filename: ..\Drillhole GTS97_01\GTS97_01_204m_dry.jpg. After the core picture is properly labelled it should be stored in an appropriate database. A simple and effective solution is to store the pictures in basic folders and use a viewing program to access and review the files. This program should have the capability to view several thumbnails of photographs at the same time. An example is given in Figure 2.13.
