MECHANICAL STUDY ON STEP-BY-STEP TIMELY SUPPORTING FOR DEEP-BURIED TUNNELS

Due to the lack of theoretical studies of step-by-step supporting time for deep-buried tunnels, a mechanical model was established using the composite lining structure of ‘initial lining + bolt + steel arch frame + second lining’. Based on the Mohr-Coulomb criterion, the ‘space effect’ of excavation, the time-effect characteristics of lining and the time of step-by-step application of supporting structure were considered. The analytical expressions of the stress and deformation in the plastic zone, and the supporting pressure in the process of excavation and supporting were derived. Results from the derivation and numerical modelling were compared and the credibility of the proposed model was verified. The variation of stress in the plastic zone, displacement of the tunnel wall, supporting pressure and the supporting time were studied. Moreover, the optimal time of step-by-step supporting for tunnels were suggested. Results show that the radial stress in the plastic zone in the surrounding rocks gradually strengthened. The tangential stress of the peak area transferred to the tunnel wall and the thickness of the plastic zone decreased greatly during the installation time. Bolts and steel arches played important roles. After optimizing the time of application of the initial support, the maximum displacement of the tunnel wall and the load sharing of the secondary lining were reduced by 27.4% and 12.2% respectively. Furthermore, the optimal time for the application of supporting structures was recommended. This study is expected to provide a theoretical guidance for step-by-step supporting in tunnel engineering.