Detection of Cavities in Karstic Terrain. Road Subsidence — Snowy Mountains Highway Near Yarrangobilly, State of New South Wales — Australia

Following subsidence of the Snowy Mountains Highway, due to collapse of underground cavities in limestone, electromagnetic (EM) methods were employed to try and define the extent of the cavities. Drilling investigations and some ground-radar probing indicated that these cavities were shallow (3-6 m below road surface) and occurring essentially within the soil profile. The subsequent EM conductivity survey clearly indicated the very changeable sub-surface materials profile, being an intricate array of limestone pinnacles alternating with deep troughs of Terra Rosa soil. It is concluded that such surveys are useful for the above stated purpose, but only to a limited degree, and must be supported by other methods of investigation. That is, the use of this geophysical method will not provide definitive answers to the size and shape of cavities, but it will indicate their probable presence.