A major city centre road collapsed after the construction of a new sewer twenty metres below the ground surface. The client swiftly acted to make the road safe but there were then concerns that several other areas could potentially have developed voids associated with the sewer construction. One option was to carry out a series of boreholes but drilling down could potentially have caused any geological weaknesses to fail and the local road authority also wanted an approach that did not cause further disruption to the city centre traffic. A geophysical survey was able to determine if there were any voids present with minimum disruption to members of the public and no risk of causing any further collapses.
The first stage of work was a ground penetrating radar (GPR) survey within the sewer itself before it was put into operation. The sewer was 4 m in diameter but using scaffolding it was possible to carry out a GPR survey of the tunnel roof and sides directly below the collapse to check that there were no additional voids present immediately above the sewer. No voids were detected.
The second stage of work involved a microgravity survey of the road surrounding the collapse and a survey of three additional roads. All of these surveys were undertaken over 5 nights shifts. Microgravity is sensitive to surface vibrations but by carrying out the work at night, when traffic was at a minimum, and using traffic management to close off adjacent lanes it was possible to obtain good quality data. The data was processed and no areas of significant voiding or loose material were detected. As a result of the geophysical survey it is believed that the road collapse was due to an isolated geological failure.
The client could now hand the sewer over to the water board to be put into operation and the local authority were confident that there would be no more holes appearing under their roads! All this was achieved without the need for any intrusive work and with minimum disruption to the public.