Centrifugal modelling of tunnel construction in soft clay

Robert Mair, Cambridge University, Geotechnical Engineering Group


This dissertation is concerned with the stability of shallow tunnels under construction in soft clay. Prior to erection of the permanent linings, temporary support is often required in the unlined tunnel heading to maintain its stability and prevent unacceptable ground movements. Experimental and analytical studies were undertaken to investigate the relationships between support pressures, deformations and overall stability of unlined tunnels in soft clay.

The experimental method throughout the investigation was centrifugal modelling using the Cambridge Geotechnical Centrifuge. Model tunnels were constructed in soft clay at 1/75 and 1/125 scale and tested at 75 g and 125 g respectively to establish the internal consistency of the method. Tunnel behaviour was observed as compressed air support within the tunnels was steadily reduced until failure occurred. A first series of two-dimensional tests on plane-section tunnels in clay of constant undrained shear strength with depth investigated overall stability. In a second plane-section test series the clay was brought into equilibrium in an overconsolidated state on the centrifuge before tunnel cutting; deformations and pore-pressure responses around the tunnel could then be compared with finite element predictions for clay with the given stress history. A further test series modelled three-dimensional tunnel headings and investigated" the influence of heading geometry on deformation behaviour and stability.

Tunnel stability in the first test series was well predicted by plasticity theory. Observed collapse mechanisms in the centrifuge models led to the development of new and improved upper bound solutions. The geometry of the collapse mechanisms predicted by plasticity theory was consistent with field data of surface settlements during tunnel construction. The development of techniques for pore-pressure measurement within the body of clay models in the second test series enabled full effective stress histories to be monitored throughout model preparation and during subsequent centrifuge testing. Good agreement was obtained in deformation behaviour between model tunnels tested at different scales.

Finite element predictions of pore-pressure responses around the deforming model tunnels, made with the Modified Cam-clay program 'CRISTINA', generally agreed well with the observed responses. Ground loss into the tunnels was also reasonably predicted. However the predicted strain patterns and surface deformation behaviour were in poor agreement with the experimental observations. Zones of contained failure close to the tunnel were satisfactorily predicted by the analysis.

The test series conducted on model tunnel headings showed the stability to be strongly influenced by the heading geometry. The reduction in stability as the length of the unsupported heading increased was accompanied by the deformation behaviour becoming increasingly two-dimensional.