6. Method for evaluating the safety of mountain tunnel linings

Earth pressure in mountain tunnels causes deformation of the tunnel and compressive bending failure in the tunnel lining, which in turn leads to spalling of the tunnel lining which is a threat to train running safety. Nevertheless, to date no method has been developed to quantitatively evaluate the impact of compressive force induced spalling on safety. Consequently, a method has been proposed to evaluate the safety of tunnel linings after having carried out numerical analyses and performed model tests.

Model tests were performed taking into account the variety of materials used for tunnel linings (plain concrete, short-fibre reinforced concrete, RC concrete, bricks, etc.). At the same time, numerical analyses were carried out to reproduce these tests considering compressive bending of the tunnel lining. The results of the above tests showed that regardless of the material used in the lining, it became clear that was possible to evaluate the occurrence of compressive bending(Fig.2 )by indexing the horizontal deformation ratio(Fig.1). Furthermore, real tunnel conditions were reproduced using numerical analysis, which displayed the quantitative horizontal deformation ratio when compressive bending occurred, for all the different types of tunnel lining. (Fig.3).

In order to render this method practical enough to be used in maintenance management, further work was carried out to clarify the relationship between horizontal deformation ratios and safety levels in terms of spalling caused by compressive bending, for each type of tunnel structure and rock mass type(Table 1), which was then compiled into a manual on tunnel lining safety. Apart from being applied in maintenance management as a screening method to detect mountain tunnels susceptible to lining damage from compressive bending caused by earth pressure, this method can also be useful in reviewing the limit values in mountain tunnel design specifications for future mountain tunnels, as a performance control method.