While the above three techniques are for measuring stress at depth, Sigra also undertakes surface and near surface stress measurements. These involve strain-gauging the wall of a tunnel or mine opening and overcoring using a concrete coring system, so as to relieve the stress around the strain gauge. This technique has been used in excavations and within tunnels. The most notable of these is the Bogong Power project in the Snowy Mountains, where four surface stresses were measured around the periphery of a circular tunnel created by a tunnel boring machine in granodiorite. These surface stress values permitted the calculation of the overall far field stress. We believe that this tunnel was the world’s largest overcore pilot hole!
The surface stress is frequently a more important measurement than the virgin stress at depth as, combined with rock strength, it provides an immediate estimate of the opening stability.