Research On Safety Degree Of Existing Tunnel Based On Strength Reduction Method

Strength reduction method is adopted to study the safety of operational tunnel, and safety factor is used as safety standard, ground-structure model is built in homogenous ground, mechanical behavior of tunnel construction is analyzed. Destruction mechanism of different buried depths, geometrics is studied. Safety analysis is proposed considering surrounding rock degration and lining degration respectively. Moreover, safety of permenant lining considering its partial thickness sufficiency is calculated. The main conclusions of this paper are as follows:(1) By using numerical simulation software FLAC3D, based on strength reduction method, supporting timing under homogenouse ground condition has been simulated by controlling the release extent of loads and mechanical behavior has been analyzed. Results show that supporting timing has little impact on the safety factor of the tunnel in the end. Such numerical modeling could be a reference for safety analysis of operational tunnel.(2) Failure mechanism of different buried depth and tunnel shapes is analyazed. It could be a reference for analytical solution of operational tunnel.The results show that when buried depth increases, the safety factor will first increase, then decrease. For different tunnel shape, damage form of rectangular cavity is not affected by embedded depth, it damages in the arch crown of tunnel; The vault of straight wall arch tunnel is firstly destroyed in shallow buried section, and the tunnel is firstly destroyed in the arch foot in deep buried section; The destruction mechanism of circular tunnel and hoof tunnel are similar, damage is in vaults area when tunnel is shallow buried, while damage is in the wall when tunnel is buried deep. Circular transeverse tunnel is the safest, hoof tunnel is the second, and rectangular tunnel is the most unsafe.(3) Both the full degradation of surrounding rock and lining are fullly degradaded, tunnel safety factor decreases with the increase of the degradation degree. Degradation of surrounding rock in different depths, when the depth of deterioration is in 1 m to 2 m,3 m to 4 m, safety factor begins to reduce considerablly;When partial degradation is made in different parts of the surrounding rock, inverted arch is the most detrimental area. When primary lining turns into full degradation, safety factor decreases rapidly at preliminary stage; When different part of rock mass is degraded, inverted arch is the most detrimental area. The partial degradation of primary lining, the arch foot part degradation is the most adverse part, sidewall and inverted arch are the second detrimental parts; When the secondary lining becomes all degradation, with the increase of degradation degree, safety factor decreases rapidly; While permenant lining becomes partial degradation, vault and haunch areas are the most detrimental ones.When thickness of permenant lining becomes thiner, with the decrease of the thickness, safety factor decreases consistively. And the vault and sidewall are the most detrimental parts.