Tunnel face support pressure and associated risk

Prediction of ground movements and risk assessment of damage to adjacent buildings and utilities have become an essential part of the success of tunneling projects in urban environments. First stage assessment of risk in tunneling zones involved evaluation of face stability conditions and ground movement induced by tunneling. This paper presents experimental work on the stability of a tunnel face using transparent soil models to estimate the magnitude of tunnel face minimum supporting pressure, failure pattern and extension of failed zone which can further be applied to tunnel risk analysis. The model tunnel was installed in a plexiglas box filled with transparent soil and was supported at the face by means of water pressure. Tests were performed by reducing supporting pressure until collapse occurred. Model tests were sliced optically with a laser sheet and images were captured for every decrement in tunnel supporting pressure. Later, these images were analyzed by Digital Image Correlation (DIC) and vector fields of incremental displacements were obtained. The value of minimum support pressure was obtained and geometry of the failure zones was found to be of chimney shape with a limited extension in front of the face. Contour plots of resultant ground movement at pre-collapse and post failure indicated that a minimum distance equal to the tunnel diameter should be considered above and a way form the tunnel for underground utilities risk assessment. A minimum support pressure was achieved with support pressures as low as 10% of the effective vertical stress at the tunnel axis. The stability of the tunnel face was related to the coefficient of active earth pressure. In general, results of the study were in agreement with current knowledge of full-scale situations.