Assessment On The Safety Of Lattice-Girder-Reinforced Shotcrete Of Tunnels

To a tunnel under construction, a prompt assessment to the safety of the lining through in-situ measured data is of practical importance both for the dynamic design and to guarantee the stability of the tunnel. Among all the parameters measured during tunnel construction, displacement is widely used and thus accepted as a very essential quantity, due to its advantages in convenience, low cost and high credibility.In terms of the displacement-based method proposed to evaluate the safety of tunnel support structures, the influence of shrinkage and creep of shotcrete on the stability of tunnels is approached in this paper. The properties of shotcrete on shrinkage and creep are discussed. The reinforcement's influencing factors on concrete creep and shrinkage are discribed, and the formulas to calculate the internal forces of the shotcrete lining with shrinkage and creep considered are derived out. the cohension between them are considered, When solving the displacement vector caused by shrinkage between reinforcemnet and concrete, which remedies defects that the concrete shrinkage is the only choice., then build the strain formulas of shrinkage. Consider the effect of the reinforcement ratio on the deformation, the potective layer curing condition, etc. when solving the creep. Finally the performance function on large bias and low bias component are gived. During the computing the Monte Carlo method are applied to calculate failure probability P_f and reliability indexβof the function. Then do some example analyse.The results show that the method gives larger reliability index when shrinkage and creep of the shotcrete are taken into account. That is to say, the reliability index was underestimated in the past. In comparison with creep, shrinkage doesn't have much contribution to the increase of the reliability index, which means that shrinkage could be neglected in the analysis. The influence of creep depends on the dimension of the internal forces of the lining. When the forces are small, the influence of creep could be ignored. However, when the forces are big, the influence of creep should be taken into consideration.This work improves the accuracy of the displacement-based method.