Assessment On The Safety Of Primary Support Of Tunnels

Owing to the complexity of tunneling,it is needed to monitor and measure the mechanical behavior of both the ground and the support structures in the course of construction,which leads to the alleged observational designing in building of underground tunnels.On the one hand,displacement of lining is of a very good representation of the mechanical behavior of the ground and the support structures.On the other hand,measurement of lining displacement is easy to take,asking simple instruments and conceiving little interruption with construction work,thereby resulting in low cost and being widely incorporated in practice.The current Chinese codes for tunnels have made suggestions on how to measure and how to use the displacement of the lining.However,the code method has some drawbacks in assessing the safety of a tunnel under construction:the method is deterministic without considering the variability of influence factors;the ultimate displacement given in the code is of a wide range and is inconvenient to use;because the ultimate displacement is related to the dimension and shape of a tunnel,the code method cannot be used on all tunnels.Therefore,it is of great theoretical and practical significance to better and widely use the measured displacement in the evaluation of the safety of a tunnel.A displacement-based method is established in this paper to assess the safety of primary lining in tunnels under construction.The main work includes:(1) Formulas are derived to compute the internal forces of primary lining through measured displacements.Performance functions are set and steps to calculate the reliability indexes are given.The variation of basic random variables is discussed and their statistic characteristics are determined.(2) The influence of different curves on the reliability indices is studied.Results show that parabola is the most suitable one for the displacement-based method, following with cosine curve being the second and then circle the third,while hyperbola is unfit for the method.(3) A derivation is done on the formulae to compute the internal forces of shotcrete supports with shrinkage,creep and temperature considered.The influence of these factors on the safety of tunnel supports is approached.Results show that shrinkage makes the reliability indexes only a bit higher,so its influence could be neglected in practice.The influence of creep is related not only to the magnitude of internal forces, but also to the time length the forces act on the supports.The larger the forces are and the longer the forces act,the larger the increase of the reliability indexes get.On the contrary,the reliability indexes will get less increase.Creep should be taken into account in practice.The influence of temperature depends on the temperature difference. The larger the difference,the larger the change of the reliability indices.When the difference is above 5 degrees,temperature should be considered in practice.(4) A computer program is made to compute the reliability indexes of tunnels. Case studies are done using this program.The computed indices for the tunnels are all consistent with the in-situ situation,which proves the correctness of the proposed method.Our work improves the deterministic method to become a probabilistic one in the assessment of the safety of tunnel supports.The information within the measured data of displacement is better used in this method.An objective evaluation can be made through the proposed method.This study can help enhance the technique level of tunnel construction.A highly effective utility of the measured data will reduce our reliance on the measurement of other parameters in tunnel construction,which surely help save the cost of the project.