Tunnelling in sands and the response of buildings

Ruaidhri Farrell, Cambridge University
Geotechnical Engineering Group


An understanding of the response of surface and subsurface structures to tunnelling is crucial to avoid undesirable safety, legal or financial issues. Current assessment procedures, which are based on greenfield distortions, can be very conservative as in reality the presence of buildings can modify and reduce these greenfield distortions. This research investigates mechanisms associated with soil structure interaction when tunnelling beneath buildings, using both case study data and centrifuge modelling. Field observations of the soil structure interaction have been obtained from the response of two buildings to the construction of a 12m diameter tunnel with extensive jet grouting in fluvial deposits. Centrifuge modelling has been carried out on the 8m diameter beam centrifuge at the University of Cambridge. Buildings are modelled as aluminium, masonry and micro concrete beams on fine dry sand. By varying the building materials and geometry, the response of buildings with a range of stiffnesses is investigated. Observed mechanisms showed good agreement with those identified from the case study, demonstrating the benefits of centrifuge modelling. These results demonstrate that buildings can significantly modify and reduce the greenfield ground distortions, in both the vertical and horizontal planes. Based on these observations a new empirical approach for quantifying the soil structure interaction is proposed. The centrifuge modelling and field data have, however, shown that settlements beneath rigidly responding buildings can be larger than greenfield values. This arises from a redistribution of the building weight and may have implications for adjacent infrastructure.

Very large volume losses were observed ( 5%) in the case study. An investigation of the settlement and pore pressure response to the tunnelling works, indicates that a significant proportion of the volume losses were caused by the installation of horizontal jet grout columns from the tunnel face. It is postulated that low grout fluid pressure along the jet grout columns resulted in relatively unstable cavities and consequently, large volume losses for each column. While ground movements associated with individual jet grout columns were relatively small, the cumulative effect of multiple columns resulted in significant settlements. Significant excess pore pressures were generated by the jet grouting.

Further investigation into greenfield ground movements in sands has also been carried out, through comparison of three separate centrifuge tests which encompass a range of tunnel size to depth ratios. Trends in the vertical and horizontal displacement profiles are identified and a new method for predicting the settlement profile in sands, which accounts for tunnel size and depth effects, is subsequently proposed. .