Prediction of Ground Conditions Ahead of an Advancing Tunnel Face by Quantification of Vector Orientation

Abstract

Observation and Measurement of displacements at the crown of the tunnel is an integral part of NATM (New Austrian Tunneling Method) in complex rock masses to verify the construction parameters, support systems and safety requirements. However, the prediction of ground conditions ahead of an advancing tunnel face is also an application, often explored less in the field by site engineers. Literature suggests displacement vector orientation as a good indicator of weak ground or fault/shear zones ahead of the tunnel face. There has been no quantification between the vector orientation and the properties of the weaker ground ahead. An attempt is made to quantify the effect of the properties of the weaker ground on the vector orientation numerically. The model results are initially compared with analytical solutions, followed by benchmarking with the results of other researchers. The factors which affect the vector orientation are the diameter of the tunnel, stiffness ratio of the rocks and insitu stress ratio. The effect of each of these parameters is studied independently. A correlation between the vector orientation and the variation in ground conditions is then established to predict the properties of the weaker ground ahead of the advancing tunnel face.