Modified criterion for prediction of tunnel deformation in non-squeezing ground conditions

Abstract

The strength factor-based criterion for tunnel deformation prediction proposed by Hoek in 2000 is reckoned to be the most practical criterion among the tunnel deformation criteria since 1980s. In view of the fact that Hoek's criterion was obtained based on case studies under squeezing ground conditions, it has a limited range of applications, applicable mainly in squeezing tunnels. In this work, this criterion was modified by taking into consideration the conditions of eight unstable stations in three non-squeezing tunnels in Iran to amplify its application range. Relative displacements and strength factor (defined as a strength-to-stress ratio) were obtained in unstable stations first. Then, the parametric values were plotted on a Cartesian coordinate's plane. By using the optimal regression line passing through all points, a criterion for prediction of tunnel deformation was proposed finally. A comparison made between the measured values and the ones predicted by the new criterion shows that the variance accounted for was 77% and the root mean square error was 2%, an acceptable accuracy and a small error implying the effectiveness of the new criterion in tunnel deformation prediction. In addition, the results show that the strength factor of 0.38 can be used to determine the boundary between squeezing and non-squeezing conditions; the behavior and type of failure of the tunnel would be predictable by use of the strength factor.

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