A Research On The Impact Of The Voids Behind Lining And Lining Thickness Deficiencies On The Safety Of A Double-arch Tunnel Structure

The structure of double-arch tunnels has more complicated stress state than common tunnel;its special structure form makes it easier to be involved in many kinds of lining diseases during service life,such as voids,cracking,water leakage.Therefore,in order to find out the impact of voids behind tunnel lining and insufficient thickness of the lining for the lining safety,a symmetrical double-arch tunnel was selected as study subject.The theoretical analysis,experimental investigation and numerical simulation were conducted during the research,the earth pressure between the surrounding rocks and the tunnel linings(under the situation of voids behind tunnel lining and lining thickness deficiencies),deformation,failure form of the double-arch tunnel linings were analysed.A fuzzy comprehensive evaluation model was developed for the prediction of the structural safety of a double-arch tunnel.The main contents and results are as follows:(1)The stress,deformation characteristic and failure form of the symmetrical double-arch tunnel under normal condition(properly contact between lining and surrounding rock,no lining deficiencies)were revealed.Results showed that the earth pressure between surrounding rocks and linings,deformation and failure form of lining structure was symmetrically distributed along the symmetry axis of the double-arch tunnel.The largest earth pressure appeared at bottom of the side wall of the double-arch tunnel,and followed by the bottom of the middle wall and the inverting arch.The largest bending moment of lining structure appeared at middle wall cormer,which experience the most serious damage.(2)The distribution of contact pressure(between the surrounding rock and tunnel lining)under the impact of voids and insufficient lining thickness was studied.Compared with the normal condition,biasing effect induced by the asymmetric distribution of the contact pressure made the pressure of surrounding rocks moving towards the opposite tunnel.The contact pressure at the position of insufficient thickness of tunnel lining which is significant lowered,while the pressure is relative increased at adjacent area.(3)The rules of stress change of symmetrical double-arch tunnel under the impact of voids and insufficient lining thickness were analyzed,and the distribution of internal forces of lining were clarified.Compared with the normal condition,the internal force distribution of the double-arch tunnel structure was significantly changed,and the structural stress state of the double-arch tunnel was deteriorated.Impacted by insufficient lining thickness,the sectional moment of inertia and stiffness was changed and the axial force and bending moment of the lining structure were significant reduced.As a result,the lining structure was unevenly loaded and larger asymmetric deformation was formed.(4)The distribution rules and propagation of cracks in the symmetrica]double-arch tunnel under the impact of voids behind lining and insufficient lining thickness were uncovered.Compared with the normal condition,the structural failures of the double-arch tunnel were asymmetrically distributed resulting in much more complicated cracks when there was a void behind the lining.The arch crack in the opposite tunnel of voids appeared much earlier,and the contact part between middle wall top and the crown was prone to crack;when the thickness of the lining was insufficient,the structural damage of the insufficient thickness part became more serious with the thickness of the lining continuous decreased.(5)Based on the fuzzy comprehensive evaluation method and the consideration of the structural stress state,the deformation law and failure characteristics of the double-arch tunnel,the safety evaluation model of the symmetrical double-arch tunnel lining structure under the effect of voids behind linings and insufficient lining thickness was established.The safety-grading standard of the symmetric double-arch tunnel structure was proposed,which could be used to evaluate the safety of the symmetric double-arch tunnel structure under the situation of impact of voids behind lining and insufficient lining thickness.