Civil Engineering Association

This lecture deals with tunnelling under squeezing rock conditions. Following an outline of the main factors influencing squeezing, the definition of this type of behaviour, as proposed by ISRM (International Society for Rock Mechanics) in 1995, is given. An overview of the methods used for identification and quantification of squeezing is presented, along with the empirical and semi-empirical approaches presently available in order to anticipate the potential of squeezing tunnel problems. A brief historical retrospective is reported on the excavation and support methods used in Italy in order to cope with squeezing conditions at the end of 1800, when the first railway tunnels were excavated.
Based on the experiences made and lessons learned in recent years through important tunnelling works in Europe, an attempt is made to trace the state of the art in modern construction methods, when dealing with squeezing conditions by either conventional or mechanised excavation. The closed-form solutions available for the analysis of the rock mass response during tunnel excavation are described in terms of the ground characteristic line and with reference to some elasto-plastic or elasto-visco-plastic stress-strain models for the rock mass. Also described are the equations for the support characteristic lines.
Then, the use of numerical methods for the simulation of different models of behaviour and for design analysis of complex excavation and support systems is considered, also including three-dimensional conditions near the advancing tunnel face.
Finally, a brief discussion on monitoring methods is given, in conjunction with a short description of a case study.