Abstract

Thin support liners (TSLs) have been used since the 1980s mainly to limit the weathering of rock. Numerous attempts have been made to create viable structural TSLs that can effectively replace 30 to 60mm of un-reinforced shotcrete (sprayed concrete) with only limited success. The structural use of TSLs is now gaining acceptance as the historical performance issues associated with them are resolved. These performance issues have been mainly associated with the long term stability of the polymer particularly under high humidity conditions and application issues related to equipment and pot life. Tunnel Guard is the most widely applied TSL internationally with sustained sales of over 1,000 tons/month in Southern Africa alone. This is enough material to cover almost 100,000 m² of exposed excavations per month. The applications in Southern Africa are mainly on deeper level tabular gold mines but the product is also used in platinum, diamond and copper mines for weathering protection and for structural support. This paper investigates the performance and cost effectiveness of the product mainly compared with shotcrete.

INTRODUCTION TO THIN SUPPORT
LINERS

Urylon Plastics Inc. from Canada and the CSIR MiningTek from South Africa basically pioneered thin support liners in the early 1980’s. Earlier membranes (from Celtite for example) had been primarily designed as rock sealants to limit weathering and were technically effective but seldom cost effective. Urylon developed a reactive (catalyzed) system based on polyurethane and MiningTek developed a non-reactive (and hence slower setting) system based on latex polymer emulsions.

TSLs are applied in a layer of typically 4 to 8 mm or less (a target of 3 to 4 mm is usually adequate and cost effective for most applications).

TSL REQUIREMENTS

  • Bond strength (adhesion) to a dry or damp substrate during the same application

  • Tensile and shear strengths

  • Elongation before failure

  • Tear strength

  • Durability (useful life)

  • Flammability and smoke toxicity

  • Resistance to chemicals, humidity (and humidity changes) and temperatures in mines (generally site specific)

  • The ability to penetrate cracks and adhere to the crack sides can also be beneficial

The performance of any membrane type of support, including shotcrete, is governed by the following main physical parameters depending on the specific application [1]:The rate of strength gain and not only the ultimate value of the tensile strength, adhesion and elongation can be important if immediate support is needed.

  • Health and safety of the material; before, during and after application

  • Substrate preparation (even more critical when using a TSL)

  • Ease of application including the cleaning of the equipment after use

  • The cost effectiveness of the placed liner

  • Pot life

  • Shelf (storage) life

The other most critical parameters are:The physical properties of membranes are constrained by the chemistry of the raw materials available. High tensile strength and high elongation are both desirable, but generally this becomes a trade-off, as a higher elongation generally results in a lower tensile strength and vice versa.

Most TSLs do not perform well under high humidity conditions whereas shotcrete is unaffected or its performance improves under such conditions.

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