Geotechnical logging of diamond drill core is used to characterize the rock mass and the structural setting. The collected data is applied in geotechnical studies to design mine excavations (open pit slope angles and underground mine excavation sizes and required ground support schemes). Any feature that will influence excavation stability needs to be captured by logging. Geotechnical logging can occur on standard resource definition holes or specifically designed geotechnical investigation holes. The purpose of the hole often influences who will be doing the logging of the core. To ensure that the best possible outcome is achieved, diamond core drilled for geotechnical logging must be prepared to a much higher standard than regular exploration core.
A significant amount of geotechnical logging is done by contractors/consultants. Low-quality core processing does result in the visiting geotechnical consultant or logger spending time bringing the core up to the required standard prior to logging. This could result in a lost opportunity to acquire accurate structural and rock mass data and in the worst cases, may require redrilling of the holes.
This article is intended to provide a basic guideline for the level of quality needed for reliable geotechnical data collection and will consider:
- Core orientation;
- Core loss;
- Meter markup;
- Geology markup;
- Feature allocation;
- Industry standards.
Core orientation
For geotechnical structural data collection, the geotechnical logger needs to take measurements on every single open natural break i.e. driller-induced breaks are excluded from the analysis. Hence drillers must be marking their breaks on the core that result from fitting the core into the tray or removal from the rods. Regular checks of the core recovery process at the rig by the supervising geologist/geotech is useful in this respect.
It is imperative that as much core as possible is connected to the driller’s orientation marks (a mark placed to indicate the bottom of the hole in cross section view), even if it is only one or two marks as the geotechnical logger will determine the reliability of the driller’s markup. Orientation lines should be drawn no-lock (section of core that cannot be joined) to no-lock, taking care to extend through orifails (driller was unable to place an orimark), poorly placed core loss blocks (see next page) and cave-in (debris that has fallen into the hole). Core loss and cave-in should be tested by attempting to lock solid ground across the interval.
Typical errors identified when orientating core are:
Not racking up enough runs to accurately assess the reliability of driller’s orientation marks. Ideally, at least two runs should be laid out drawing the oriline on the core surface. This requires longer core jigs to be used particularly for surface drilling.
Relying on the orimark drawn on the surface (along hole length) of the core for accuracy. The orimark drawn on the core cross-section is often more accurate. Acceptable deviation for a driller’s mark from the drawn line is about 10-15 degrees. The best place to anchor the line and determine reliability is the point where the driller’s mark on the core face meets the mark on the core surface. Drawing the oriline over the top of the driller’s surface mark aids in the QA/QC (quality assurance and quality control) of the driller’s mark and eliminates the risk of error when the driller’s mark is crooked.
Out of order core where the core has been dropped or put back into the core tray incorrectly. I have reoriented entire runs of core placed in the trays backwards!
Running orilines through fragmented core. Drillers will often place loose fragments at the ends of the tray rows or the bottom of the run. If the oriline hits a no-lock do not give up straight away, cross-check against other no-locks and search for similar looking surfaces to the no-lock. Sometimes one of those little pieces at the end of the rows can make the lock happen. You do not need to match up every single piece of broken core to make a lock happen. For further information on drill core orientation best practice, see Brett Davis (2012).
Core loss
Always ensure that core loss has been located at the point where it occurred. The drilling teams have been typically trained to assign core loss to the bottom of the just completed run. This location is not necessarily where the core loss occurred. Validate the core loss by attempting to match the core across the gap. If not here, reassign to the most likely location in the run. Some geologists like to disperse the core loss through several locations in the same run. When doing this, do not assign core loss to locations where clearly no core loss occurred. Sometimes core loss is caused by core being dropped in the rod, this core loss may be picked up by driller in the next run (core gain); take care to allow for core gain when calculating core loss location and meter marks. Always record core loss locations on the tray in the format start depth-length lost-end depth. Geotechnical rockmass logging is tolerant of multiple core loss reconciliation techniques; however, please only use one technique per hole and only one set of core loss markup at a time.
Meter marking
For geotechnical purposes, it is important to establish the actual location of major structures and the geotechnical domain boundaries. Meter marking is typically undertaken by the site technicians (particularly in fresh rock) prior to the commencement of geotechnical logging. The tape measures used are regularly observed to be in poor condition or have a non-zero start.
There are several places where meter marking can be cross-checked against an absolute borehole depth point. These are Start of Hole (SOH), End of Hole (EOH) and anywhere the driller did a rod count. These locations are calculated by the number of rods used x rod length, less height of first rod above ground (stick up) and are a good guide for determining whether the other driller’s blocks have been labeled correctly. Meter marking that has been done correctly nearly always comes back to one of these points. Always do meter marking after full core orientation has been completed.
Ideally, meter marks should never deviate more than 10 to 20 cm (3.94 to 7.87 in) away (preferably less than 5 cm or 1.97 in out) from the value on the driller’s core blocks (some drillers do round to the nearest 10 cm/ 3.94 in). Drillers do make mistakes and sometimes the core blocks can be wrong. In most cases, these mistakes are detected and corrected going forward by the drillers.
When reviewing meter markings, cast an eye on the deviation keeping in mind that some technicians subconsciously add or remove 5-10 mm (0.19-0.39 in) of measurement per meter (creeping drift). When the difference between meter marks and core blocks is deviating too much, always go back and check for creeping drift first then continue to see if the blocks come back. Do not under any circumstance insert fake core loss or additional material into geotech core to correct for poor meter marking.
Geology markup
Geotechnical rockmass logging must lock into the major geological contacts. It is preferable that these are labeled directly onto the core with a simple geology code ‘+/-’ alteration. Geological logging of geotech core needs to be as simple as possible and consistent.
Elaborate geological code systems that subdivide geology based on grain size, alteration, mineralization and/or texture are subject to misuse and are difficult to reconcile for modeling purposes. For example, if you give a geologist 20 possible codes for sandstone there is a very good chance there will be at least ten codes being used for a sandstone unit that has the same geotechnical properties. Keep the geology logging to significant changes in rock type and alteration. Keep the logging relevant to the target mineralization.
Recommended spacing for geology for an open stoping operation with similar geotechnical characteristics is firstly tens of meters when in gangue, for example, barren turbidites; secondly, at the meter scale when in outer alteration halo, for example, distal chlorite and or carbonate in a gold system; thirdly at the decimeter scale when in the inner alteration halo and or mineralization, e.g. areas of stringer and massive sulfides.
Do QA/QC of contacts. It is quite common for geologists to assign incorrect depths to contacts either via incorrect measurement from meter marks or ignoring the meter marks completely and measuring from the core blocks. If meter marks are determined to be incorrect during geological logging it is the responsibility of the logging geologist to ensure the marks are corrected.
Feature allocation
It seems to be quite common for geologists to lump fault zones into large geotechnically heterogeneous domains labeled as fault zones or shears but ignoring lithology. It is preferable to subdivide fault zones at the decimeter- to meter-scale via geotechnical characteristics using the dominant apparent host lithology. For example, a fault zone containing a healed dolerite breccia, a quartz vein, then a sheared black shale with a fault gauge, should be logged as separate domains. Logging the above as one unit would conceal the position of the shale contact and conceal the potentially problematic shale-hosted active fault beneath the quartz vein from numerical analysis.
Although geotechnical logging relies on the collection of every single open feature, structures measured for geological purposes can be useful. It is very important that any structure measured for geological purposes is clearly identified. It is a too common occurrence for a discontinuous healed micro-fault with a 1 cm (0.39 in) offset to be logged with the same moniker as an open major fault with several meters of offset. It is a good idea when selecting structures to ask the question ‘if I drilled a hole 10 m (32.8 ft) away could I find the exact same structure?’. Useful structural measurements for
geotechnical purposes are open faults, shear planes, foliation, bedding and of course open joints (preferably all open joints observed should be measured).
It is important to ensure that the logger is using the correct sized orientation tool (of the many different forms available) and knows how to use it correctly.
Industry standards
Industry standards with regards processing core for geotechnical data collection have been slipping over the past five years. This slippage has occurred due to the combined effect of an influx of untrained staff, caused by the recent increased demand in the mining industry, and a legacy attitude towards the value of holding on to trained core shed staff from the employer dominated market of several years ago.
To resolve the slipping industry standards in geotechnical data collection there needs to be two parts:
- A reset towards training and retention of competent staff.
- An understanding what geotechnical data is collected for and that adopted collection standards need to comply with how they were intended to be collected and utilized.
Conclusions
This article has covered the common issues that can result in a slowing down and inaccuracies in geotechnical logging. Each section provides advice that will speed up the logging process for the geotech logger and ensure or improve quality data collection improving the reliability of the rock mass characterization and hence forecasting of rock mass response to excavation. The author does not expect potential clients to strictly adhere to the advice above, however implementation of these processes will pay back in the results achieved.
References
Brett Davis, 2012. Drill core orientation – An Inconvenient Truth (Part 1 of 3). Drill core orientation – An Inconvenient Truth (Part 1 of 3) (orefind.com)
Brett Davis, 2012. Drill core orientation – An Inconvenient Truth (Part 2 of 3). Drill core orientation – An Inconvenient Truth (Part 2 of 3) (orefind.com)
Brett Davis, 2012. Drill core orientation – An Inconvenient Truth (Part 3 of 3). Drill core orientation – An Inconvenient Truth (Part 3 of 3) (orefind.com)
