Sampling

Author: Warren Newcomen
BGC Engineering Inc.

Introduction

Various types of lab testing can be carried out on core samples collected at site.  The standard types of tests include:

• point load tests (PLT), and
• unconfined compressive strength (UCS) testing on intact rock samples (to be correlated to PLT results);
• direct shear (DS) testing on discontinuities; and
• grain size (GS) and Atterberg limits (AL) tests of fault infill material.

Selecting samples to send to the lab

Samples sent to the laboratory should be labelled with

• the hole ID,
• depth to and from,
• test type and
• sample number.

Natural moisture content should be preserved where possible, so rock samples should be wrapped in plastic and soil samples should be double Ziploc™ bagged. For UCS samples, several PLT tests must be done on the same interval of core.

When in doubt take a sample.  Before samples are sent to the lab, sort through them to remove any that have broken in transport, and confirm that the most representative samples are going to be tested. A small percentage of samples will break in transport, and at the end of the program it is better to have shipped additional samples back to the office and not have tested them all than to be short on samples for testing.

Point load test samples

Point load test (PLT) samples can be marked at the drill rig. This type of testing can also be carried out at the drill rig but it is usually conducted at the core shack after the core has been logged geologically but before the samples are removed for laboratory testing. Point load test samples specified for testing can be identified as per the symbol outlined in the Core symbols table, and should be taken approximately every 4 to 5 m. However, the exact number and frequency of sampling/testing will largely depend on the lithologies and alteration types encountered. If numerous lithologies and alterations are encountered then the frequency of PLTs should increase to ensure that a sufficient number of tests are completed in each type. The frequency of PLT tests should be increased in close proximity to the UCS samples selected, with 2 tests conducted directly above and 2 tests directly below the sample. Each PLT sample should be free of major flaws that will influence the test results and its length should measure at least twice the diameter of the core (e.g. 9 cm for NQ3 core) along the core axis.

Unconfined compressive strength samples

Unconfined compressive strength (UCS) samples should be taken approximately every 50 m in each hole; however, the exact number will largely depend on the lithologies and alteration sequences encountered. The samples should be spread out over the lithology and alteration types with a focus of the major types encountered. Each UCS sample should be free of any major flaws that will influence the test results (i.e. micro fractures) and should measure a minimum of 12 cm along the core axis (i.e. greater than twice the diameter of NQ3 core); longer samples are preferred.

Direct shear samples

Direct shear (DS) samples should be taken approximately every 50 m in each hole; however, the exact number will largely depend on the lithologies and the discontinuity types encountered. The samples should be spread out over the lithology and discontinuities types with a focus of the major types encountered. Each DS sample should have at least 5 cm of intact rock perpendicular to it on either side of the discontinuity.

Atterberg limits and grain size

Significant fault zones should be sampled for Atterberg limits, grain size testing, and/or XRD analysis. Remember that even non-plastic gouge provides valuable information for design. Often it can be difficult to find proper sample sizes so if the opportunity arises, take it.

Mapping

Photogrammetry

Photogrammetry is a process in which measurements of discontinuity length and orientation are made through the use of high resolution photographs. A structural feature is photographed from at least two locations, producing two lines of sight. The intersection of these lines of site produce three-dimensional coordinates for the feature, so the size, shape, and orientation of the feature can be measured. It is advantageous to choose lines of site from different elevations to reduce occlusions. One of the strengths of photogrammetry is that it allows data to be gathered from inaccessible areas. Also, a more distant vantage point makes discontinuity persistence and interaction easier to observe.