Ground movements during tunnelling in sand
Thorpe, Jason Peter
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During soft ground tunnel construction, if the face pressure of a tunnel boring machine is not strictly controlled, excessive ground movements will propagate vertically upwards causing significant damage to adjacent buried infrastructure and surface structures. In order to investigate the face pressure - ground deformation relationship for tunnels in sands, the construction process was modelled using the technique of geotechnical centrifuge modelling and the resulting ground deformations were recorded using digital image correlation. In these tests a unique tunnel face boundary condition was developed which allowed the boundary condition to be initially set as a zero strain condition before it was transformed into a load-controlled boundary to investigate the instability of the face. Tests were preformed at four different burial depths in dry sand, corresponding to cover depths of 0.5, 1, 1.5, and 2 times the tunnel diameter. These results indicate that the face pressure at failure is largely independent of burial depth over the values tested. The ground deformation at the onset of tunnel face instability was found to be very small, and once initiated, the zone of ground deformations was observed to propagate upwards in a narrow chimney in front of the tunnel until it reached the ground surface causing subsidence. Further tests investigated the variation in ground deformations to be expected if a tunnel were to be passing through more complex ground conditions, including unsaturated sand, saturated sand, and the unique case of sand / clay mixed ground conditions. Ground deformations at tunnel face instability were much lower for the case of unsaturated sand, than for either the saturated or dry cases which showed broadly similar responses. In the mixed ground condition of a clay layer over topping a sand layer, the clay layer was found to only influence the tunnel face pressure – deformation response if the bottom of the clay layer was closer than 0.5 diameters above the tunnel crown.