The research was carried out during the bentonite shield tunnel drive for the Acton grange trunk outfall sewer at Warrington, Cheshire. This tunnel was driven through cohesionless drift deposits beneath a built-up urban environment, with a cover of less than 6 m. The environmental effects of the ground vibration caused by the excavation process were investigated with particular regard to ground settlement by compaction. Vibration data were recorded from transducers located in boreholes, on the surface, on the tunnelling machine and on the concrete tunnel lining. These records were processed to characterise the vibrations in terms of peak particle velocities, frequency spectra and spatial attenuation. The maximum measured ground vibration (expressed in terms of resultant peak particle velocity) was 3.9 mm/s. The vibration was characterised by random high velocity particle motions resulting from impacts between the machine's disc cutters and glacial boulders in the tunnel face. Surface and subsurface settlement measurements were made along the tunnel line. Laboratory tests and other field data showed that the ground in this area was likely to settle at levels of vibration lower than those measured from the tunnelling machine. Vibration from the excavation process probably caused ground compaction which contributed significantly to ground settlement and its effects. The vibration was not likely to have caused any direct damage by dynamic stressing but the associated low level noise did cause concern among local residents. (Author/publisher)
Abstract