Long term performance of thin spray-on liners for permanent support in underground mines
Date
2020
Authors
Kanda, Mpoyi Jacques
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Abstract
Thin spray-on liners (TSLs) have been implemented for over 25 years. The
feedback from their strength assessments has demonstrated high tensile strength
and elongation capability. However, these supporting advantages are not always
evidenced in the underground mining environment where it is often applied. This
lack of practical constancy in TSL performance has given way to scepticism over
its utilization, despite good results propagated by some laboratory tests. Thus this
thesis’s research study aims to assess the practical underground performances of
TSL, taking into consideration its long-term performance aspect, so that its
reliability might be clearly defined and placed at the reach of the users. Included in
this thesis’s research, is a critical review of existing literature provided by suppliers
and other researchers, which assesses TSL performance capabilities. The TSL
assessment techniques opted for in this research, are the field assessments,
laboratory trials and numerical modelling analyses.
In summary, existing suppliers and research literature were found to present the
following: TSL’s high areal-support capabilities are its high tensile and adhesive
strengths, as well as its high elongation capacity. The available reviewed laboratory
testing was undertaken in “traditional” laboratory conditions, and only a few recent
research-initiatives started examining the long-term performance of TSL. The latest
long-term results show preliminary signs of TSL limitations when such are selected
for permanent support. Thus due to the importance of the long-term aspect of TSL
assessment, time-dependency information was reviewed in detail in this thesis. Yet
throughout the whole review, no information was encountered on assessments of
TSL that combined underground mine-like conditions such as humidity and
temperature with relatively longer timing of exposure.
During the fieldwork activities, two South African underground platinum mines
were visited: a deep mine and a shallow mine. The mine operators were still not
sure where these liners were performant, but the suppliers highly advocated their
utilization in most of the mines’ openings. The following laboratory tests were
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conducted on three TSLs in this thesis: uniaxial tensile strength (UTS), Brazilian
indirect tensile (BIT) strength test and membrane displacement test (MBT), which
is a punch-through test. In these tests, the focus was directed on the influence of the
parameters that are characteristic of some deep underground mines, as well as on
the performance behaviours of TSLs, particularly after long periods of exposure or
long-term testing. To achieve this, three different environmental conditions were
considered for testing, namely: the traditional laboratory condition (called room
condition), the humid (saturated) condition, and a combined humid and high
temperature (50°C) condition. The long-term performance of TSL under loading
was assessed through creep testing.
The thesis’s BIT tests were performed on TSL discs and on coated and uncoated
shotcrete discs. Through these assessments, the impact of TSLs on coated shotcrete
discs was assessed through short-term and long-term testing after prior longer
exposures to various environmental conditions. The MBT testing method followed
the testing conditions established for the UTS and BIT tests. Two assessing
parameters were suggested for standard assessment of MBT performance.
Numerical modelling analyses were also carried out. A coated BIT model was set
using a Rocscience program, namely RS3.
From the short-term testing results, it was found that TSL generally shows a tensile
strengthening trend in room conditions, and a weakening trend in humid conditions
and in combined humid and high temperature conditions. An exception was
however observed for water-based TSLs, which could still display a strengthening
trend in humid condition, but which was overcome when the temperature parameter
was added. Concerning the long-term performance of TSL, it was found that TSL
could accommodate larger deformations for static and lighter loads. BIT shotcrete
specimens coated with TSL could permanently succeed in resisting disturbing
forces. Among the recommendations, TSL1-like materials should preferably not be
used as permanent support in humid underground mines, unless combined with
other mining supports. Also, it is inappropriate to design TSL underground support
systems based on liner strength and deformation in room conditions.
Description
A thesis submitted in fulfilment of the requirements
for the degree of Doctor of Philosophy in Engineering to the Faculty of Engineering and the Built Environment, School of Mining Engineering, University of the Witwatersrand, Johannesburg, 2020