Geotechnical Insights Into Circular Shaft Performance In Clay

Shafts are a key aspect of most underground tunnel schemes. Circular shafts are generally preferred over other plan geometries because they are inherently stiffer and more efficient at resisting in-situ horizontal earth pressures. However, there is limited understanding of the performance of these circular shafts and the associated ground movements due to their construction. This paper describes field observations of circular shaft construction assembled from three major tunnelling projects in the United Kingdom and novel centrifuge model tests of shaft excavations in clays that were conducted using the geotechnical centrifuge at the University of Cambridge. Most of the field observations were of settlements adjacent to the shaft; one case study provided novel measurements of longitudinal bending and circumferential hoop strains in a circular shaft lining using innovative fibre optic instrumentation. The wished-in-placed circular shaft in the centrifuge tests was instrumented with strain gauges to measure longitudinal bending and hoop strains. Both the field measurements and the centrifuge tests demonstrated that hoop strains are far more significant than longitudinal bending strains. Miniature displacement transducers installed at varying distances from the shaft measured the adjacent ground surface movements during shaft excavation. Key findings from the field and centrifuge model tests are described.