Backfilling a giant karst cave with grouted engineering spoil as a new countermeasure for tunnels through giant karsts cave is studied in this paper. The numerical models of sections with different distribution characteristics of karst cave and tunnels are established for studying the deformation of surrounding rock and mechanical response of tunnel lining with and without grouting, respectively. The results illustrate that the countermeasure scheme is feasible. In order to ensure that the countermeasure can perform as expected effect, the inadequate grouting sections are recognized and verified using field grouting records and single-hole grouting quantity analysis. Finally, the application effect of the countermeasure scheme is evaluated by field monitoring of horizontal convergence. The result shows that the grouting can reduce the deformation of surrounding rock at the side wall and bottom of tunnel by 70–80% and reduce the stress redistribution range of surrounding rock. However, due to the great differences between the limestone and engineering spoil, the grouting cannot change the share of distribution of load between corresponding region surrounding rock, the max principle stress of tunnel lining is almost identical with and without grouting. The grouting reinforced engineering spoil backfill the giant karst cave can meet the requirement of excavation stability. The inadequate grouting sections caused by groundwater and through crack are identified effectively, and are verified by coring observation method. The horizontal convergence of the tunnel is less than 30 mm, and the stable state can be reached within 20 days, which demonstrate that the remarkable engineering results is achieved. The countermeasure of giant karst cave can provide a useful reference for similar project.

The excavation of large-span triple tunnels using drilling and blasting method inevitably causes complicated load transfer effects and induces potentially damaging ground vibrations. In this study, the structural responses (including the surrounding rock pressure, normal-contact pressure between the primary and secondary linings, internal forces in the secondary lining) and the seismic responses (including peak particle velocity and corner frequency), are systematically recorded. It is found that the first-excavated left tunnel is influenced heavily by the excavation of the last-excavated middle tunnel, whereas it is hardly affected by the excavation of the second-excavated right tunnel. The load carried by the primary lining is approximately three times as that carried by the secondary lining. The middle tunnel was in the least desirable state due to the formation of the large Protodyakonov’s equilibrium arch (PEA). Based on timely feedback of the comprehensive monitoring system, a series of vibration-reducing techniques were applied and effectively guaranteed safety during blasting construction. By referring to Chinese codes, the minimum safety factor of the secondary lining is 1.3; the maximum PPV (0.15 cm/s) is lower than the allowable value; and the corner frequency (40-140 Hz) will not cause resonant vibration of the Great Wall.

To exploit the influence of the tunnel face and the distance between the diameter and the orifice of a blast pipe on the ventilation effect in symmetric tunnel construction, this paper uses Fluent to establish a three-dimensional model and numerical simulation. Firstly, the accuracy of the numerical simulation is tested and then the distance between the orifice and tunnel face and the influence of the air duct diameter on the ventilation effect are studied, respectively. The results show that the ventilation effect is best when the wind pipe is arranged on one side of the tunnel wall (an asymmetrical layout), although the space in the tunnel is axisymmetric, and that the error of the numerical simulation is less than 5% of the measured value. When the distance between the orifice and tunnel face is 5 m, the uniformity of the air flow field near the tunnel face is poor; when the distance is 10 m and 12 m, an obvious vertex area appears in the tunnel. Furthermore, the uniformity of the wind velocity flow field is optimal when the distance is 8 m. When the air duct diameter is less than 1.4 m, there is a uniformity of the flow field near the tunnel face of the upper and lower benches; when the air duct diameter is more than 1.4 m, the tunnel face of the upper bench near the ground shows more obvious backflow. Therefore, it was determined that taking the air duct diameter as 1.4 m and the distance between the orifice and tunnel face as 8 m was the best combination for the design of ventilation in this project. It was also found that a better ventilation effect can be achieved when the distance between the nozzle of the ventilator and the tunnel face is 6 m–9 m and the wind speed of the nozzle is 6 m/s–8 m/s. In practical engineering, the wind speed and the required air volume should be taken into consideration to determine the diameter of the ventilator.