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Comparing traditional and televiewer methods of structural data collection

SRK News | Issue 54: Rock Engineering and Slope Stability

A4   |   Letter

All mining excavations should have an appropriate geotechnical design that takes into account the rock mass structure. The level of geotechnical investigation and design required will be governed by the geological and geotechnical complexity of the deposit, in addition to the sensitivity of the slope to geotechnical risk.

Acoustic (ATV) and Optical (OTV) Televiewer Tools:

Downhole televiewers can be considered a reliable, visual, fileable method for capturing structural data. Both systems collect images of discontinuities in a borehole using downhole geophysical survey equipment that can be orientated for structural logging. The acoustic televiewer transmits ultrasound pulses from a rotating sensor, recording the amplitude and the travel time of the signals reflected from the borehole wall to provide an image of the borehole wall.

While downhole televiewers have previously been considered expensive, the costs are not adversely comparable to using traditional core orientation. Such methods require the daily hire of the orientation tool with geotechnicians and geologists supervising and logging (often twenty-four hours a day) at each working rig. The quality of data captured and the influence that high confidence structural data can have on a slope design can far outweigh the upfront costs of using the televiewer system.

The use of downhole televiewer methods can result in much improved confidence in rock mass conditions and structural data sets, which can produce greater confidence in slope designs. Such systems can be used in all rock masses. Very heavily fractured rock masses can be lined with PVC to ensure borehole stability and orientation by ATV. The use of televiewer systems maximises data from boreholes and is a minimal cost in relation to a drill programme -- typically 10% to 20% of the total drill cost -- but can cost much less if using existing open holes or very deep boreholes. The advantages of using the system are:

  • Increased Rock Mass Rating (only logging natural open structures)
  • More accurate calculations of fracture frequency/joint spacing
  • Reduced variability in orientating major joint sets
  • Higher confidence in small scale structures within the rock mass
  • Increased confidence in the orientation, locations and thickness/ intensity of major structures
  • Provision of fileable data that can be reassessed if necessary
  • Suitable for stable old boreholes and doesn’t always require new drilling programmes

These, benefits can manifest themselves in greater optimisation of pit slopes, which in turn can lead to financial benefits when designing economically sensitive slopes, and confidence in design when developing slopes near critical pit infrastructure.

Max Brown:
Karl Llewelyn:

SRK Africa