Ground vibration during the bentonite tunnelling process

Abstract

The research was carried out during the bentonite shield tunnel drive for the Acton Grange trunk outfall sewer at Warrington, Cheshire. This tunnel is driven through cohesionless Drift deposits beneath a built-up urban environment, with a cover of less than 6m. The environmental effects of the ground vibration caused by the excavation process are investigated with particular regard to ground settlement by compaction. The geology of the area and the technical and commercial factors which led to the choice of the bentonite tunnelling system are described. Previous work on compaction by vibration is critically reviewed and methods to assess a soil's potential for compaction are given. The vibration instrumentation is described and relevant wave propagation theory is developed with emphasis on body waves from underground sources. Vibration data were recorded from transducers located in boreholes, on the pavement 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 3.90 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 and structural damage to property above the tunnel was observed. 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. The vibration caused by the excavation process caused ground compaction which contributed to ground settlement and the ensuing damage to the overlying structures. The vibration was not likely to have damaged these properties directly but did cause considerable nuisance to the residents.