Over the past ten years a thin support liner (TSL) has been developed with appropriate application equipment for use as part of an underground support system. The material has very unique properties (and is safe to handle and apply) that enable it to be used under adverse conditions such as in the presence of running water. The material can have an adjustable set time from seconds to minutes and has a very rapid strength development with an ultimate tensile strength of over 14 MPa (2000 psi) and an unusually high bond strength. Two case studies are presented; a shallow civil tunnel in Hong Kong that had weak ground and water ingress, and a shaft system on a mine in Namibia where water ingress threatened the stability of the entire infrastructure. The TSL was successfully used in both applications saving time and money on both applications
1.1. Support underground
Support underground is critical to ensure the safety of the workforce and the uninterrupted use of the excavation during its useful (planned) life. The following points influence the need for some form of support in any underground excavation:
? The stress state around the excavation periphery.
? The change in the stress state during the useful life of the excavation (usually as a result of mining in the vicinity).
? The shape and cross sectional area of the excavation.
? Local and regional geological features (such as faults, bedding planes and joints/cleats).
? The in situ rock properties (the uniaxial compressive strength etc.).
? The final use for the excavation.
? The planned life of the excavation.
The main functions of support over the useful life of the underground excavation are therefore to:
? Limit unplanned over-break (during the excavation process).
? Prevent local rockfalls.
? Reduce deterioration with time.
? Maintain the designed cross sectional area and shape.
? Absorb sufficient energy in dynamic conditions if applicable (i.e. during seismicity).
This is generally achieved using a combination of support types to create an interactive support system. Commonly some form of reinforcing member (e.g.: a grouted rebar or cable bolt) is used with a liner (for areal cover) support (such as shotcrete or screen/mesh). Generally, the role of the reinforcing member is to keep the larger rockmass (key blocks) in place, whilst the liner?s role is to keep the smaller rocks between the reinforcing units in place (stop unraveling). Frequently the liner support design is not given the attention that it deserves and this can create potentially hazardous conditions with time. .