Engineering geology

A Theoretical Example – core boxes

Figure 1 is a drawing of three core boxes to illustrate some of the points made above.
The drawing Fig. 1 is two dimensional and limited in what it shows by the constraints of the graphics programme used for its construction. Details are as follows:

  1. Three core boxes are illustrated containing cores obtained by rotary core drilling from 15.00 to 38.06 m depth. The upper two boxes contain 160 mm diameter core, the lowest 120 mm diameter core. The driller has marked the beginning and end of each core run, which are measured from the drill rods, and has also estimated the depth of the end or beginning of the core in each core box. If there has been any core loss then these latter depths may be inaccurate.
Fig. 1. A theoretical example of a series of core boxes
  1. The cores have been placed in the core boxes to more-or-less fill them without breaking the cores more than is necessary. However, there will be some ‘fitting’ breaks and one of the first tasks in logging is to identify them. One such break is to be found at point ‘A’; there are others. They would not be counted as natural discontinuities for RQD measurements etc.
  1. The cores in the first box show a ‘fining upwards’ succession of rocks (conglomerate⇒ sandstone⇒sandy mudstone⇒mudstone⇒coal). The stick at ‘B’ shows this succession inverted and has thus been placed upside down in the box.
  1. The rocks at ‘C’, just below the 18.75 m core run marker are rather broken and may represent an over-drilled core stub, left in the hole at the end of the previous run and disturbed at the beginning of the next run.
  1. The fragment of rock at ‘D’ would not appear to be in the correct place and may come from the bottom of the run at 22.50 m.
  1. The joint at ‘E’ may be open. If, when its two surfaces are placed in contact, the thickness of the core is less than the diameter drilled then the joint is open and the aperture can be calculated.
  1. At ‘F’, the core is very broken and at this point, the driller decided to reduce core diameter, presumably as a result of encountering drilling difficulties. Such difficulties would be recorded in the daily drilling log. The implication is thus that the breaks in the core above and below 31.05 m are a consequence of natural phenomena and not the drilling itself. The same remark could apply to the broken core just above 38.06 m but here it might also be the core cracking in the vicinity of the core spring on breaking off the core to pull it to surface.

Describing Lithologies

There are clear differences between the strata above about 22.70 m, from 22.70 to about 31.00 m and below this last depth. In the case of the strata above 22.70 m there would appear to be cyclic deposition in fining up sequences. There are various ways of describing the sequence. If the project concerns putting foundations on the rock mass it would probably be sufficient to record the number of cycles of deposition and to indicate their nature. If the rocks are likely to be exposed in a slope or tunnel wall it would be prudent to emphasise the presence of the coaly laminae which could squeeze or which could form planes on which sliding might take place. Alternatively, a more factual approach could be taken and the rocks closely described layer by layer with measurements of depth and thickness.

At about 22.75 m a conglomerate is found composed of limestone pebbles and may well mark an unconformity. A check on the relative ages of the limestone and overlying cyclic deposits would be of value for the unconformity surface could be that of a very irregularly eroded landscape. The limestones show fractures, perhaps solution opened, and healed by calcite veining. The end of the core shows solution features, there is significant core loss, and the driller may have recorded encountering cavities as the hole was drilled.

The limestones are not all the same. They are generally pale grey but two pieces, perhaps once joined by a calcite vein, are dark grey and show different bedding dips.
This could be a consequence of faulting. Below about 30.90 m the mudstones and sandstones are again encountered although lacking indications of cyclic deposition. However, while their bedding below about 32 m is regular and thin between 31 and 32 m it becomes variable with indications of folding. This can be syn-sedimentary, but is probably faulting.

Structure and Strength

Measurements of bedding dip would be taken as frequently as the dip varied. The relative orientation of joints could also be measured. Strength may be assessed by the Equotip rebound number or some other method but indicated in the log by the terms used in the scale of strength for rocks, e.g. moderately strong.

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