Tunnelling in squeezing rocks: Case histories

Squeezing rock conditions have posed and continue to pose a major obstacle to the construction of tunnels through mountains, as experience dating back more than a century shows. The paper deals with the study of past experiences in the light of present geotechnical engineering knowledge. Many of the transalpine tunnels were constructed before geotechnical engineering had been developed, and the principles underlying squeezing were not yet understood. Also construction techniques have changed with time. By studying past experience in the light of our present knowledge in geotechnical engineering (rock and soil mechanics), one may gain more insight into the nature and causes of squeezing ground behaviour. Here, a number of older and newer case histories are summarised, providing substantial insight into the phenomenon of squeezing rock. Squeezing rock behaviour is influenced by rock type and structure. Usually, in squeezing zones the rock is strongly jointed and fractured and has low strength. Overburden has also a significant effect and squeezing behaviour may occur abruptly in a tunnel once a limiting overburden has been exceeded. Water pressures in strongly jointed and often crushed rock are important and so are the adopted construction procedures and sequences. A support of substantial structural strength may be necessary to prevent long-term deformations and to withstand increased loading on the tunnel liner from the rock mass surrounding the tunnel.