ABSTRACT:

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 4m diameter sewer tunnel being driven through mixed soil conditions under compressed air at a depth to the crown of about 18m. Emergency grouting was conducted from tunnel level and the road surface after the road surface had subsided over 400mm. Over 130m3 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 4m wide and up to 10m 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.

INTRODUCTION

During tunnelling for the North West Sewer Project through the north western suburbs of Melbourne a road collapse occurred on a major arterial road above the tunnel alignment. Initially the collapse manifested itself as blistering pavement and escaping air from the compressed air operations associated with the construction of the 4m diameter sewer using a tunnel boring machine (TBM). The collapse area continued to expand over the ensuing days until it covered an area of 8m by 4m to a depth of 400mm. Emergency grouting from the tunnel and road surface was carried out to stabilise the collapse zone. To assist in identifying the extent of the collapse zone latest geophysical techniques based on innovative use of seismic waves to scan the earth similar to CAT (CT) scans used in medical diagnoses were employed.

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