The Safety Evaluation Of Cracked Highway Tunnel Lining In Loess Area

The loess highway tunnel lining crack disease in the construction and operation of the process is very common, many tunnels with lining cracks can still work safely. The thesis taking a western tunnel as an example studies the safety of the tunnel lining with cracks by use of fracture mechanics theory and numerical simulation, and builds a security warning system.In this thesis, the following conclusions are drawn:1) In this thesis, causes and hazards of the loess tunnel lining crack disease are analyzed, the distribution, shape and mechanics of cracks are summarized. Combined with statistical analysis of lining cracks on the western Loess Tunnel, we draw a few of conclusions that the width of most cracks are more than 0.3mm and the longitudinal cracks parallel to the longitudinal axis of tunnels bright a lot of harm to the construction of loess tunnel, more than three quarters of which are distributed in the tunnel dome. The depth of the lining cracks has a large span, the deepest of some is arrive to the thickness of the tunnel lining.2) Stress at the crack tip is more concentrated, which is the main cause of the tunnel lining fracture in the low stresses. Stress intensity factor used to indicate the stress field of the crack tip, combined with the fracture toughness and fracture criterion, are summed up a formula to calculate the crack tip safety factor K1. Cracks destroyed the integrity of the lining structure, so that the lining section bearing capacity decreases. Therefore, the bearing capacity of the cracked lining needs to be recalculated by three kinds of circumstances. Then the safety factor K2 of cracked lining ultimate bearing capacity can be calculated. In conclusion, a new formula, K=min(K1, K2), to calculate the safety factor of cracked lining is derived.3) Several working conditions are settled based on the immersion degree and zone of the surrounding loess rock as a result of surface water immersed along the surface cracks. The safety factors K of cracked lining of the several working conditions are solved by use of theoretical calculation and numerical simulation. And variation of the safety factor K depends on the depth of the tunnel dome lining cracks is analyzed. There are several conclusions: the deeper the tunnel lining cracks is, the larger the rate of descend of the safety factor K is, the more sensitive the safety factor K are to changes of tunnel working conditions are. Finally, the corresponding range of security alert level table is provided, after safety warnings of security tunnel lining with cracks under several conditions.In this paper, a new evaluation method are provided for safety of cracked Lining, which can effectively take into account the lining crack tip stability and section bearing capacity. And the corresponding security warning level interval tables are provided, as well. It has significance on further study of cracked highway tunnel lining.