Safe re-entry time with In-Cycle Shotcrete for support of underground excavations

In-Cycle shotcrete is an important component of support for many underground excavations in rock. In particular, the safe re-entry time after shotcrete has been sprayed needs to be considered as it influences the productivity associated with development. It is proposed that the shear strength of the shotcrete matrix is a better parameter to consider than the Uniaxial Compressive Strength of the overall shotcrete material including coarse aggregate. A number of shotcrete paste mixes were prepared and evaluated for strength gain. It was found generally that the shear strengths of shotcrete pastes increase exponentially during the first 4 to 8 hours of curing in all shotcrete mixes except when the pastes included a hydration stabiliser. It was found for shotcrete mixes with accelerator, synthetic fibres and aggregates that an average layer of about 50mm thickness develops sufficient shear strength of about 20kPa within about one hour to support a tetrahedral block with 1m edge lengths and the layer mass. For the same shotcrete layer, it was found that a cubic metre block and the shotcrete layer can be supported within about 3 hours and 50 minutes when the shotcrete shear strength is estimated to be approximately 97kPa.