Diamond drill core samples recommendations

Dimensions

For tests to determine the strength of the rock (UCS, TCS, BTS), the ISRM recommends core-drilled samples of minimum 50 mm in diameter, and a length between two and three times the core diameter. Long drill core samples are recommended to fulfill the size requirement after sawing and grinding.

Sampling size

The diameter of the sample should be preferably ten times (10x) the average grain size so individual crystals do not distort the test result.  This is seldomly the case for crystalline rock, therefore needs to be compensated with a larger number of samples.

A minimum of 5 tested samples per rock type is advised for a proper statistical value in homogeneous rocks, in case of heterogeneous, the minimum is 10. When the minimum of 5 samples is not possible, 3 samples might suffice under understanding of higher scatter in the values.  The more heterogeneous the rock type, the larger the sample size necessary to reduce the scatter.  The mean and its scatter are usually settled by 5 to 10 samples per rock type.

When preparing compression tests specimens in the laboratory, the selected samples are sawed to the ISRM specifications length and the ends are grinded to parallel micro-metric accuracy to ensure a proper compression test.  These processes are conducted under the most careful conditions; nevertheless, the sample might break along weaknesses present on the surface.  Taking this in consideration, it is a good practice to add a 5% spare samples per rock type to be tested

Sample selection – general recommendations

When choosing the samples to be shipped, is a good practice to bear in mind the foliation of the samples.

The compressive strength of the rock is usually the lowest when loaded at an angle of 30 – 60 degrees regarding the foliation orientation.  In the same sense, the highest compressive strength is found when loaded against or parallel to this orientation. The compressibility of the foliated rocks, the coefficient of elasticity, is the lowest perpendicular to the foliation, and the highest parallel to it.

On the other hand, the tensile strength of a rock sample is the lowest perpendicular to foliation, usually.

Shipping

When shipping the samples to the laboratory, the samples must be properly packed.  It is recommended the drill cores to be wrapped in bubble wrap and placed in a drill core box of appropriate size.  These precautions will help to avoid breakage during transportation.

When testing porous and weak stones, special care must be taken to retain the original rock moisture.  The rock samples to be tested must be packed in a moisture-proof manner as soon as obtained, since dried weak porous rocks will break very easily during transportation and preparation.

Identification

The correct identification of rock samples to for testing is essential for an accurate reporting. We encourage our clients to pre-fill the following sample list and send it to us via email when shipping the samples.
The list contains,

• Client’s sample names,
• Borehole ID,
• Geological unit investigated,
• Client’s naming convention,
• Desired tests,
• We encourage additional comments and request by the client

This minimizes errors when reporting similar rock types and provides a better control of the results for both parts.