Guidelines for the design and testing of Thin Sprayed Linings

Thin Sprayed Liners (TSLs), as well as shotcrete and mesh, are mainly used to provide a better support coverage in mining excavations. While it seems evident that increased support coverage leads to improved stability around mining excavations, it is not obvious how the actual support performance of surface-support liners should be quantified.  The behaviour and performance of TSLs are addressed in this project. On the basis of underground and laboratory work, in combination with numerical modelling results, it was possible to formulate potential instability mechanisms that may be controlled by a TSL. The emphasis in this study is on the mechanical interaction between a TSL and a rock mass and the relevant and associated material properties. While it is acknowledged that a TSL may have other functions, such as the prevention of weathering through sealing, such other functions have not been directly addressed here.
 
The main objective of this project is to formulate a standard testing methodology and to develop testing equipment in order to quantify relevant TSL parameters. During the course of this project, a series of international seminars was organised between the Australian Centre for Geomechanics, the University of the Witwatersrand, and the Université Laval in Canada. The objective of these seminars was also to formulate and reach consensus on internationally acceptable standard testing procedures for TSLs. By actively participating in these seminars, insights were shared between local and international participants. The testing methodology that is presented in this report is therefore not only relevant to South African applications, but is also broadly supported by the international mining community.
 
The support performance of a TSL is largely determined by the substrate onto which it is attached. In addition, a TSL is typically part of a support system that contains other support components such as tendons. The requirements for a TSL are therefore strongly associated with the detailed morphology of the substrate as well as with the characteristics of the other support components. A simple mechanical model that quantifies the interaction between a TSL, the substrate, and the retaining support is described in this report. With the aid of this model, the required TSL parameters can, in principle, be quantified.
 
One of the most revealing findings from this project was the strongly time-dependent behaviour of some of the TSL products. In essence, the strength of such products is directly associated with the duration of loading, and strength therefore needs to be carefully defined in those products. High strength and high stiffness are associated with rapid loading, while relatively low strength and stiffness are associated with slow loading. Loading rate is thus extremely relevant when the strength and stiffness of a particular material are being determined. The testing equipment that was developed for this project is capable of subjecting test specimens to variable loading rates. The proposed standard testing procedure requires the specification of loading rate in a tension test.
 
Two standard tests were formulated, namely a tension test and an adhesion test. 
 
The test procedures have been optimised based on laboratory and underground testing and guidelines for the standard testing procedures are presented in the report. The adhesion test was designed for in situ evaluations as well and can be used to assess the underground performance in relation to potential performance under laboratory conditions. While it is not the intention here to prescribe detailed requirements for TSL products, it is apparent from this study that only tough and strong bonding materials are capable of providing effective support resistance when large deformations need to be accommodated. However, certain brittle and stiff materials may provide very effective support when they are not subjected to large deformations. In fact, the perception that large deformations need to be accommodated seems to be misplaced in many of the observed cases. It is therefore important that any potential application is carefully assessed. 
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