Page 561 - Rock Mechanics For Underground Mining

18 Monitoring rock mass

performance

18.1 The purposes and nature of monitoring rock mass performance

Monitoring is the surveillance of engineering structures, either visually or with the
help of instruments. In a general geomechanics context, monitoring may be carried
out for four main reasons:

(a) to record the natural values of, and variations in, geotechnical parameters such
as water table level, ground levels and seismic events before the initiation of an
engineering project;
(b) to ensure safety during construction and operation by giving warning of the
development of excess ground deformations, groundwater pressures and loads
in support and reinforcement elements, for example;
(c) to check the validity of the assumptions, conceptual models and values of soil
or rock mass properties used in design calculations;
(d) to control the implementation of ground treatment and remedial works such as
ground freezing during shaft sinking or tunnelling throughwater-bearing ground,
grouting, drainage or the provision of support and reinforcement.
In mining rock mechanics, most monitoring is carried out for the second and third of
thesereasons.Monitoringthesafetyoftheminestructureisaclearresponsibilityofthe
mining engineer. Monitoring to check the rock mass response and, as a consequence,
adjust the overall mine design or take remedial measures, is equally important. It
will be appreciated from the discussions presented throughout this book, that rock
masses are extremely complex media whose engineering properties are difficult, if not
impossible, to predetermine accurately ahead of excavation. It will also be clear that
the models used to predict the various aspects of rock mass response to different types
of mining procedure, are based on idealisations, assumptions and simplifications. It
is vitally necessary, therefore, to obtain checks on the accuracy of the predictions
made in design calculations. As illustrated in the design flow diagram of Figure 1.7,
monitoring the behaviour of the rock mass surrounding the mining excavation is
an integral part of a mine rock mechanics programme, and provides the feedback
necessary to close the design loop. In some cases, the design may be based largely
on the results obtained by monitoring trial excavations or the initial behaviour of
the prototype excavation, with little or no reliance being placed on pre-excavation
design calculations. This use of field observations of the performance of structures
is central to the general practice of geotechnical engineering in which it is known as
the observational method (Peck, 1969).
Monitoring systems used in conjunction with modern large-scale underground
mining operations can be very sophisticated and expensive. However, it should be
remembered that valuable conclusions about rock mass response can often be reached
from visual observations and from observations made using very simple monitoring
devices. Items that may be monitored in an underground mining operation include
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