20.9 Rock

When planning an intrusive investigation likely to encounter rock, the following should be taken into account.

  • a) The use of geophysical profiling methods to determine the depth to rockhead should be evaluated for sites where access or ground conditions are problematic for boreholes, or where the ground conditions are likely to vary on a scale smaller than the practical distance between boreholes.

NOTE 1 A combination of cable percussion boring (or dynamic sampling) and rotary core drilling with an overlap around the rockhead might be an option but selecting the depth at which to change methods can itself affect the understanding of the rockhead profile.

  • b) Where weaker rock occurs at shallow depth, excavations (pits or trenches) might be used, as block samples can be obtained from these for laboratory testing. Excavations can facilitate in-situ inspection of the soil/rock boundary and of the mass characteristics of the rock, the latter can also be investigated in situ by inspection in shafts or headings (see 24.2, 24.3 and 24.4). Boreholes can be progressed in some weaker rocks by the cable percussion method, using the clay cutter, shell or chisel (as appropriate to the character of the rock), or by dynamic sampling methods.
  • c) Disturbed samples from the drill tools associated with these exploration techniques are generally Class 5 but might be Class 4 or 5 if recovered by the cable percussive clay cutter.

NOTE 2 in many extremely weak to weak rocks, samples might be recovered using the driven 100 mm diameter tube sampler. Driving the sampler can cause severe disturbance and the sample might shatter, or break up, making it very difficult, if not impossible, to identify the natural structure of the rock. The samples are often in the quality class range 3 to 5.

NOTE 3 Cable percussion boring in rock gives results of limited value, at best it provides a general lithological profile. In many cases, as for example when rock is overlain by boulders, the method generally cannot identify the interface between soil and rock with any certainty.

NOTE 4 In weaker rock and in stronger rocks that are closely bedded, jointed, or affected by faulting, there might be difficulty in recovering cores of satisfactory quality and larger-sized equipment producing cores of about 100 mm diameter or greater and the proper selection of barrel and bit type helps to improve the core recovery. The ground investigation contractor can assist in selecting the best combination of equipment.

  • d) Boreholes formed by rotary core drilling should be used where it is necessary to obtain better quality information about the identity and character of the rock. There are many combinations of drilling methods, tools, and core barrel and bit designs in rotary core drilling (see 24.10); the selection of flushing medium should also be considered carefully. It might be possible to use resonance drilling as an alternative in some circumstances. Rotary core drilling usually produces samples representative of the character and engineering properties of the intact rock material and which might give some indication of the frequency and dip of discontinuities but not their orientation. Soil-like or weak discontinuity infill might be lost in rotary core samples.

The limited duration of most ground investigations does not usually allow for much experiment to achieve the best results, so the investigation should be designed to employ the most effective method of sampling both weathered and Unweathered rock. This should be done in consultation with the ground investigation contractor.

The use of in-situ testing, either within a borehole or an excavation, should also be evaluated when planning the intrusive investigation.

Investigation of rock masses requires a range of tests to characterize the in-situ condition; these should include a selection of the following:

  • 1) The standard penetration test (see Clause 39) can be used to give a rough indication of the strength and compressibility of extremely weak and weak rock. This test does not provide direct measurement of these parameters; they are derived from the test results via published correlations.
  • 2) The permeability test or the Packer or Lugeon test can be used to give a measure of the mass permeability, which in turn can give an indication of the presence of open joints and discontinuities.
  • 3) Plate tests (see Clause 45) and dilatometers such as the pressuremeter (see Clause 41) can be used to investigate deformation properties and possibly also the strength. Both of these tests can provide a number of useful geotechnical parameters for design, that cannot be obtained from other forms of investigation.

In interpreting the in-situ tests, the effects of drilling disturbance of the ground should be taken into account.

20.10 Discontinuities


In most rocks and some soils the behaviour is controlled by the material and mass properties. The mass properties depend largely on the geometry and nature of the discontinuities present. This can require the engineering properties to be measured in the plane of the discontinuities along specific orientations determined by the anticipated directions of the stresses to be applied. Discontinuities also occur in some soils and can control the mass strength and deformation characteristics.

In soils, the discontinuities are often destroyed by the investigation techniques that are used and so the influence of discontinuities is not always considered. It is, however, possible to obtain high-quality large diameter (>100 mm) cores. Such cores can provide good information on such discontinuities and following careful sub sampling can provide samples suitable for laboratory testing that allow the effect of discontinuities on shear strength and compressibility to be measured.

Where discontinuities are important to the engineering project, this should be taken into account when planning the investigation.

Investigation of discontinuities in rock requires a range of tests to characterize the in-situ condition; these should include a selection of the following:

  • a) Rotary core drilling usually gives some indication of the frequency and dip of discontinuities but not their orientation. The use of impression packers, downhole viewers, core orientation devices, formation micro-imaging (FMI) and crosshole geophysical techniques might be useful where greater information on the discontinuity characteristics are required.
  • b) Natural or man-made exposures allow data to be obtained on the frequency, orientation and nature of discontinuities. This is best carried out on three orthogonal surfaces.
  • c) Block samples from pits can provide class 1 samples. The advantage of these samples is that they can be oriented and the discontinuities in the pit described.

After initial investigations using interpretation of aerial photographs, outcrop logging and the drilling of vertical and inclined oriented holes, further measures should be undertaken if warranted. It might be necessary to undertake mapping of full surface exposure, large diameter boreholes, trenches, pits or adits to allow visual inspection around and within the undisturbed ground mass, and measurement of the relevant discontinuity data.

NOTE In some projects, suitable exposures might be provided in excavations necessitated by the permanent works. The extraction or in-situ preparation of orientated test samples can be carried out in these excavations together with oriented field tests. The orientation of the excavations controls their intersection with the discontinuities and, consequently, the discontinuity data that can be obtained. Normally, three orthogonal exposures are required to define fully the spatial distribution of the discontinuities. The extent of the excavations is governed by the spacing between discontinuities and the size of the works.