The geology is a challenging mixture of metamorphic, igneous and sedimentary rocks which have been heavily fractured and which include the Hollywood Fault. From the upstream end of the line, the tunnels encounter shales, conglomerates, basalt breccia, sandstone and granodiorite. Due to the great diversity of the geology, the station design and the tunnel linings took account of seismic shock loading. The fissured nature of the rock allowed water into the tunnel and the maximum water inflow was recorded at 8000 litres/min.

Restrictions imposed on the contractor on the working shaft sites resulted in the main tunnels being driven downhill, which exacerbated the problem of water inflow. The ‘Q’ system of rock classification was used to assess the rock properties. The tenderers were provided with this information, which demonstrated that the rock was at the poor to very poor end of the classification.

Examples of Q values: 2 to 6 = poor to fair; 1 to 0.4 = very poor; 0.4 to 0.01 = extremely poor.

The four systems for the initial support were directly related to the Q classification. The actual Q values were generally better than the predicted values but this did not prevent the contractor from claiming for unexpected ground conditions.

Related Files
Geological Profile