PROJECT DESCRIPTION

YTL Cement operate a 4Mt per year limestone quarry supplying 2 cement kiln lines. Situated on a 400m mogote hill, the working area is accessed from the base of the hill via an inclined tunnel, which also hosts the main conveyor and glory hole infrastructure. Tunnel integrity could not be compromised whilst developing an alternative access roadway. The client required clear guidance on determining the minimum acceptable crown thickness between the tunnel roof (and lateral offset) from the reducing working quarry floor areas.

CHALLENGE

Prior to GWP involvement, the accepted crown pillar thickness was a minimum distance of 50m. It was recognized by the Client that, for the footprint area of the working quarry, the required 50m standoff would necessitate a reduction in production rate and make the installation of an alternative access impossible to install.

SOLUTION

GWP undertook an extensive geotechnical field mapping exercise to determine the rock mass characteristic of the ground making up the tunnel. Numerical modelling techniques were used to determine the critical crown conditions for tunnel security. We concluded the tunnel crown thickness could be reduced to 35m, enabling a further 2 years of uncompromised production, before installation of an alternative access ramp and tunnel decommissioning.

IMPACT

YTL Cement engaged GWP for quarry phasing and design and geotechnical monitoring of the tunnel to assess the need for any reinforcement design. GWP provided a detailed phasing design for quarry development, output, tunnel decommissioning, and an alternative access road for continuing quarry operation.