Performance of double-arch tunnels under internal BLEVE

Double-arch tunnels,as one of the popular forms of tunnels,might be exposed to boiling liquid expanding vapour explosions(BLEVEs)associated with transported liquified petroleum gas(LPG),which could cause damage to the tunnel and even catastrophic collapse of the tunnel in extreme cases.However,very limited study has investigated the performance of double-arch tunnels when exposed to internal BLEVEs and in most analyses of tunnel responses to accidental explosions.The TNT-equivalence method was used to approximate the explosion load,which may lead to inaccurate tunnel response predictions.This study numerically investigates the response of typical double-arch tunnels to an internal BLEVE resulting from the instantaneous rupture of a 20 m^(3) LPG tank.Effects of various factors,including in-situ stresses,BLEVE locations,and lining configurations on tunnel responses are examined.The results show that the double-arch tunnels at their early-operation ages are more vulnerable to severe damage when exposed to the BLEVE due to the low action of in-situ stress of rock mass on the response of early-age tunnels.It is also found that directing the LPG tank to different driving lanes inside tunnels can affect the BLEVE-induced tunnel response more significantly than varying the configurations of tunnel lining.Moreover,installing section-steel arches in the mid-wall can effectively improve the blast resistance of the double-arch tunnels against the internal BLEVE.In addition,the prediction models based on multi-variate nonlinear regressions and machine learning methods are developed to predict the BLEVE-induced damage levels of the double-arch tunnels without and with section-steel arches.

Investigation of cracking mechanism of the first tunnel lining during double-arch tunnel construction

To solve the engineering problem of the first tunnel lining cracking caused by the second tunnel construction of double-arch highway tunnels,a research method combining distributed optical-fibre monitoring,inversion analysis and numerical simulation that can reflect lining cracking was presented.Optical fibres were laid on opposite sides of the steel arches inside the first tunnel lining.Embedded optical-fibre monitoring was conducted continuously during the second tunnel driving.Based on the fibre-optic strain profile,the lining cracking was deduced and warned in time.The mechanical behaviour of the steel arch was investigated by the inversion analysis,which took into consideration the integrated impact of axial force and flexural moment.A two-dimensional(2D)load-structure method–based numerical model was established,considering the influence of different load distributions in each construction condition.The total strain rotating crack constitutive model was applied to reflect the cracking behaviour of concrete lining in the simulation,and the model was calibrated and verified in the laboratory.Comparative analysis between the simulated strain distribution and the distributed optical-fibre monitoring results was carried out.The deformation mode and crack distribution of the lining were analysed.The cracking mechanism was explained.Specifically,the second tunnel construction led to the loading at the top of the middle partition wall and the release of rock pressure in the first tunnel.Under these load changes,the secondary lining of the first tunnel cracked on the inner side of the top of the middle partition wall owing to tension,and compression-bending failure occurred near the right arch foot.Finally,the influence of the parameters on the lining force was analysed,and a construction optimisation scheme was proposed.

Stability analysis and optimization of excavation method of double-arch tunnel with an extra-large span based on numerical investigation

The Xiamen Haicang double-arch tunnel has a maximum span of 45.73 m and a minimum burial depth of 5.8 m.A larger deformation or collapse of the tunnel is readily encountered during tunnel excavation.It is therefore necessary to select a construction approach that is suitable for double-arch tunnel projects with an extra-large span.In this study,three construction methods for double-arch tunnels with extra-large spans were numerically simulated.Subsequently,the deformation behavior and stress characteristics of the surrounding rock were obtained and compared.The results showed that the double-side-drift method with temporary vertical support achieves better adaptability in the construction of such tunnels,which can be observed from both the numerical results and in situ monitoring data.In addition,the improved temporary support plays a critical role in controlling the surrounding rock deformation.In addition,the disturbance resulting from the excavation of adjacent drifts was obvious,particularly the disturbance of the surrounding rock caused by the excavation of the middle drift.The present findings can serve as the initial guidelines for the construction of ultra-shallowly buried double-arch tunnels with extra-large spans.