Smart structural health monitoring of mining support units.

Abstract

In the South African mining industry, the design of tunnel support systems is generally based on empirical methodologies that consider rockmass characteristics as well as the type of loading (e.g. seismic) that the excavation experiences. The design methodologies are by no means infallible, and work is continually being conducted to improve the classification of excavation conditions and thereby improve the selection of a suitable support system.

This study is concerned with finding a means to monitor the installed support units

rather than with improving the classification methodologies. It is postulated that

with the extraction of accurate information describing the state of any support unit

at any given time, areas of instability in the tunnel can be readily identified and

strengthened~ Also, the information gathered as to the behaviour of the support

units in a particular region can be used to assist in understanding the

environmental characteristics of that region (rockmass, loading, etc.).

A material survey was conducted to identify suitable candidates that could feasibly

be used in either a passive (feedback when interrogated) or active (constant

feedback) structural health monitoring system. The preferred candidates identified

in this study are the group of passive smart materials referred to as TRIP steels,

which are a subset of strain memory alloys. TRIP steels exhibit microstructural

changes from paramagnetic austenite to ferromagnetic martensite as a function of

increasing deformation at a given temperature.. The strength of the magnetic field

at critical locations provides an indication as to the health state of the component.

Because of their high strengths and ductility, TRIP steels can be used as what

amounts to a self-monitoring support unit (interrogation apparatus required).

Finite element methods are a practical means of predicting the mechanical and

magnetostatic behaviour of TRIP steel structural members once material equations

have been established by experiment.