Evaluation of tunnel face stability by transparent soil models

Accurate estimation of tunnel face support pressure is necessary for economical and safe shield tunneling in cohesionless soils. This paper presents measurements of tunnel face support pressure and associated soil movements obtained using a transparent soil model that simulates shield tunneling in medium dense saturated sand. The use of a transparent soil surrogate permits measuring the internal soil deformations within the model soil. Soil deformations associated with various face support pressures are presented for 4 cover-to-diameter (C/D) ratios. Failure is found to be sudden with sand flowing into the tunnel leading to a prismatic wedge in front of the tunnel face and a vertical chimney of soil above. A minimum support pressure was achieved with support pressures as low as 10 ± 1% of the effective vertical stress at the tunnel axis. The stability of the tunnel face was related to the coefficient of active earth pressure with C/D ratio having a small effect on the magnitude of required pressure at collapse.