Long term performance of thin spray-on liners for permanent support in underground mines

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

III 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.