At some stage in your engineering geological career you are going to have to log core. Without good core logging any foundation design is worthless and the onus is on the engineering geologist to produce high quality logs on which the engineer can base his design.
So how do you go about it? Well, you are in the hands of the driller to some extent, and they have a duty to supply you with good quality, intact, well organised core. This sometimes is beyond the ability of some drillers and you may have a battle on your hands to improve core recoveries and to ensure that depth blocks and core packing are up to standard.
Assuming however that you have some good quality core with all the depth markers in place, then this is how to go about logging it. First thing is to make yourself comfortable. Every geologist has stories of squatting over core boxes in the hot sun for days on end, making what is at times a fairly difficult job a completely painful one. Find yourself a table – even if it means making a temporary one out of core boxes, get some shade over it, organise yourself a seat and a supply of beverages and then get to work.
Equipment needed is as follows:
- A logging table
- Kettle, tea, coffee and sugar
- Steel tape measure
- Paint brush
- Spray bottle
- Pocket knife
- Indelible pens
- Kodak colour chart
- Photoboard and letters
Once you have all the gear sorted it is time to tidy up the core and ensure that the various lengths can be joined together down the entire length of the hole. It is good practice, and in fact imperative if the core has been orientated, to line the core up so that its orientation is the same for the entire borehole. This will assist in recognising joint sets and making them more apparent in the core photographs.
Once this job is complete the next step is to put metre marks onto the core with blackboard chalk. If the drilling is of a good standard then this shouldn’t be much of a problem, if not then you have a job on your hands. The best approach is not to start at the top of the hole and log downwards as this will produce errors. What is required is to look for bottom breaks or redrills. A bottom break comprises a ridge of rock at the end of a drill run and which extends slightly beyond the nominal diameter of the core. If the drillers have measured their drill string and stick up correctly this should be an accurate depth.
Redrills are also useful for tying down depths. When the driller finishes a run he then pulls back on the rods to break off the core from the base of the hole. This sometimes breaks right at the base of the hole where the kerf of the bit has cut into the rock to give the distinct bottom break discussed above. But only sometimes, and in most instances a piece of core is left sticking up in the hole and a length of core slightly less than the length of the run is recovered. Once the core has been recovered from the core barrel, down the hole it goes again and the next run is drilled. In many instances a ridge is ground into the piece of core still sticking up in the hole before the bit starts cutting into new rock, and this ridge marks the redrill. This should correspond exactly to the calculated depth of the previous core run. It is good practice to draw a chalk arrow from the core depth marker to the redrill mark to indicate the depth to which the bit actually drilled.
Using the driller’s blocks, redrill marks and bottom breaks it should be possible to put the metre marks onto the core. Working up hole and down hole from known depths allows for fairly accurate positioning of the metre marks. There will be discrepancies, particularly where there are weathered areas with associated core losses, and by working up and down the hole from the known depths it is possible to ascribe core losses to their correct places.
Once the metre marks are on the core and the positions of core losses have been determined, you can get on with the logging. This might all sound like hard work but it is imperative that you go through this procedure to make your life easier in the long run. The metre marks allow for measurements to be made of geological features, discontinuities and weathered zones with ease, rather than having to constantly revert back to some earlier driller’s block or other arbitrary point. It is also a fine way of checking the accuracy of the driller’s blocks and core recovery.
Now choose another chalk colour and work your way down the hole, marking up changes in lithology, hardness, weathering, discontinuity spacing and any other features which are geotechnically or geologically significant.
Only now can you pick up your pencil and beginning putting data onto the log sheet. But your life will now be a good deal easier thanks to all the hard work you have put in aligning all the cores, ensuring that they have been put into the boxes the right way, that there are no mistakes with the driller’s depths and that there are metre marks from which you can take immediate measurements.
The logging should be carried out according to the procedures outlined in a paper by Jennings, J.E., A.B.A. Brink & A.A.B. Williams 1973 and titled ‘Revised guide to soil profiling for civil engineering purpose in South Africa’. This paper has been expanded by the SAIEG, AEG and SAICE and can be accessed here. It is not my intention to expand any further on this, except to say that I would rather log rock as per the International Society of Rock Mechanics (ISRM) criteria – certainly when it comes to the discontinuity descriptions at least.
Core Orientation and Measuring Discontinuity Orientations
Often one of the requirements, particularly in tunnelling or dam jobs is for orientated core. The reasons are fairly obvious, as discontinuity orientations can then be derived from the core. Question is, how to measure these from the orientated core? What is needed is a goniometer, made from wood or aluminium so as not to affect the compass. A quick aside is due now – many boreholes are inclined so as to intersect both vertical and horizontal discontinuities and to assist in the core orientation procedures. When core has been orientated it should be put in the box with the top of the core facing upwards. A line should then be ruled down the entire length of the hole with an indelible pen indicating the orientation. With this reference line in place the next step is to set up the goniometer so that it is orientated exactly the same direction as that of the borehole.
The edge of the base plate and by default the cradle is aligned to the borehole direction and the dip of the goniometer is set against the protractor to reflect the angle at which the borehole was drilled. Once the goniometer is set up, the cores can then be placed into the apparatus with the line marking the top of the core facing upwards and aligned with the centre line of the goniometer. The discontinuity orientations are then measured with a compass and recorded on the log sheets at the necessary depths. Obviously the core pieces will be of different lengths and a few pieces of timber should be available to provide a spacer on which the base of the core can rest while the orientation of the discontinuities can be measured.
Of importance in core logging is to record the Rock Quality Designation (RQD), fracture frequency and total core recovery (TCR). RQD is a tricky subject and you are referred to a paper by Norbury, Child and Spink on the subject. It has been defined in BS 5930 as “The sum length of all core pieces that are 10 cm or longer, measured along the centre line of the core, expressed as a percentage of the core drilled.” Beware of this, as there are all sorts of ambiguities which can creep in here and the method has its critics. RQD is still used in assessing the stability of rock masses and should in all cases be recorded.
- Total core recovery is the length of core recovered for each run divided by length of the run and expressed as a percentage – say for instance 2.75 m of core is recovered for a 3.0 m run – this will equate to 92 percent recovery.
- Fracture frequency is the number of fractures per core run. This does not include driller’s breaks or drilling induced fractures. However it is at times difficult to separate out natural and drilling induced fractures, particular in shales and fissile rocks, and in this light some comment should be made about the difficulties encountered.