Analysing The Hazards

Does your geotechnical assessment really provide the right information?

By Stephen Beauchamp, geotechnical specialist, The Mineral Planning Group (MPG)

The short answer to the above question is: ‘Well, that depends on what materials you are quarrying and how steep your excavations and tips are’. Under the current Quarry Regulations 1999 geotechnical assessments of excavations and tips are required if any excavations or tips are classified as constituting a ‘significant hazard’. Regulation 33 of the Regulations defines hazards as significant if such a failure would directly or indirectly be:

• liable to endanger premises, roadways or other places where people are likely to be found off site; or
• likely to kill or seriously injure anyone.

In addition to the general assessment of significant hazards, the Regulations specify criteria in relation to the height of excavations. For example, in moderately weak or stronger rocks hazards should be treated as significant if:

• the vertical height of any individual face is more 15m; or
• the overall vertical height of any adequately benched face or slope, measured from toe to crest, is between 15m and 30m, and the overall face angle is steeper than 1 horizontal to 1 vertical (45° to the horizontal).

Other specific criteria are set out in Regulation 33 for weak or very weak rocks and engineering soils, tips and lagoons, defining what constitutes a significant hazard and, hence, which section of the quarry workings require a mandatory geotechnical assessment. Irrespective of the various specific situations outlined above, an excavation, tip or lagoon may still be a significant hazard, perhaps due to some local geological feature or the proximity of an excavation to a tip. All excavations, tips and lagoons assessed as representing a significant hazard should be assessed by a suitably qualified geotechnical specialist, at a frequency of every two years or less.

So, what can an operator expect from the geotechnical specialist? What sort of report will be produced and will it answer the fundamental question: are the excavations, tips and lagoons within the site safe? The type of analysis that is required for the geotechnical assessment will depend on the nature of the materials that have been assessed as constituting a significant hazard. 

In the case of quarries where moderately weak or stronger rocks are extracted, the most likely cause of failure is movement along planes of weakness, rather than a failure of the rock mass.  Within the rock mass there are planar features (discontinuities) along which blocks of rock may slide and, ultimately, fall from the face. Discontinuities include such structures as bedding planes, joints, faults, cross-bedding surfaces, mineral veins etc. Generally, three basic modes of failure are possible:

• Planar failures – sliding of blocks along one discontinuity set which dips out of the faces.
• Wedge failure – sliding of blocks along the intersection of two discontinuity sets.
• Toppling failure – blocks falling from the face due to steeply inclined or vertical discontinuities which run parallel or nearly parallel to the direction of the face.

In order to assess the potential for any of the three modes of failure to occur, it is first necessary to geotechnically map the exposed faces, where it is safe to do so, and record the orientation, attitude and condition of the main discontinuity sets. This basic information can then be plotted as a stereographic projection, which is a method whereby three-dimensional data is represented on a two-dimensional diagram. Additional data are added to the stereographic projection, such as the orientation and attitude of the working faces and the basic friction angle for the rocks being quarried. By combining this information the geotechnical specialist can produce an assessment of whether or not planar, wedge or toppling failures are likely to occur within each face in the quarry. Without an appropriate geotechnical assessment potentially unstable faces will not be detected until after the failure has occurred, ie when it is too late!

Unfortunately, stereographic projections do not appear to be widely used in Quarry Regulations assessments – geotechnical specialists preferring instead to make a visual assessment of face stability, based on photographs or reference to previous mapping carried out by others, or to carry out apparently ‘sophisticated’ computer analysis using finite element analysis. The reluctance to employ stereographic projection analysis by some geotechnical specialists is perhaps due to the dwindling numbers of universities offering undergraduate engineering geology courses, where such techniques are taught. Even rudimentary geotechnical mapping is not taught on many current geology degree courses and hence the fundamental data for the stability analysis may not be recorded.

In quarries where weak rocks and engineering soils are worked, the mode of failure is often a rotational or circular failure through the rock or soil mass, rather than along pre-existing planes of weakness. Generally, this type of failure would be analysed using one of the standard slope stability computer programs, such as SLOPE, STABLE etc. For such an analysis to provide reliable results site-specific geotechnical data is required. Information regarding the bulk density and shear strength properties of each of the material types that are intersected by the quarry faces is needed. Typically, this information is obtained by drilling boreholes within the quarry and extracting samples for laboratory testing. The presence of groundwater within the faces will have an adverse impact on the shear strength of weak rocks and soils, increasing the risk of slope failure. Therefore, groundwater level data derived from piezometers installed within the quarry perimeter will be required as one of the inputs into the slope stability analysis.

So, how much slope stability analysis is necessary? Once a computer model has been set up it is relatively easy to modify the input parameters and produce a large number of different scenarios.  However, if the input values are assumptions based on the experience of the geotechnical specialist rather than site-specific laboratory results, there may be a tendency to generate an excessive number of computer printouts that do not necessarily improve the understanding of the potential for slope failure. Equally, it may not be appropriate to use data from other quarries, perhaps owned by the same operator, in place of geotechnical parameters specific to the quarry which is being assessed. In short, the cost of laboratory testing of samples from the quarry may save the costs associated with extensive and potentially unnecessary computer runs.

Geotechnical assessments are a vital part of the safety of operational quarries and are a statutory requirement for any parts of a quarry designated as a significant hazard. However, an operator should make sure that the geotechnical assessment is appropriate for the conditions prevailing in their quarry. Can your geotechnical specialist produce a stereographic projection analysis?

For further information, contact MPG on tel: (01274) 884599 or 884699; fax: (01274) 884664.