Applying Innovative Geophysical Methods To Assist Identification And Repair Of Road Collapses

Seismic tomographic imaging (STI) is applied to assess the extent of a road collapse that occurred in a major arterial road through the north-western suburbs of Melbourne. The road collapse was associated with a 4 m diameter sewer tunnel being driven through mixed soil conditions under compressed air at a depth to the crown of about 18 m. Emergency grouting was conducted from tunnel level and the road surface after the road surface had subsided over 400 mm. Over 130 m³ of grout was introduced to stabilise the area.

Downhole seismic testing and STI were used to assess the disturbed ground which extended over a zone approximately 4 m wide and up to 10 m deep. Further imaging after the initial grouting works indicated that significant voiding still remained and a further 20m³ of grout was introduced through surface boreholes. Subsequent imaging clearly showed a significant increase in seismic velocity within the disturbed zone. No further movement has occurred in the collapse area over the following years.