STRAIN FIELD AROUND A TUNNEL IN STIFF SOIL

An experimental tunnel excavated through a stiff silty clay (overconsolidated glacial till) has been extensively instrumented. The instruments provided data on a complete three-dimensional displacement field around the tunnel. From in-situ measured displacements, displacement gradients and thus strains have been calculated. The derived maximum shear strain field has been compared with stress-strain testing data for the till and used as a failure criterion to investigate the process of mobilization of shear strength and development of failure zones around the tunnel. The deduced mechanism of mobilization of shear strength provides an insight into the arching process around the tunnel and explains the low pressures observed on lining of tunnels in till.