Evolution of loosened zone of surrounding rock for tunnels constructed by multi-partition excavation method

Based on Wumengshan No. 2 railway station tunnel with four tracks, numerical simulation is performed to comparatively study the differences of loosened zone between multi-partition excavation and full face excavation. Based on the field monitoring data, the evolution of loosened zone by multi-partition excavation is analyzed. The results indicate that: from the viewpoint of thickness of loosened zone, the full face excavation makes it greater than the multi-partition excavation does, with the maximum value being 12.6% greater and the minimum value being 41.2% greater. From the viewpoint of distribution of loosened zone, the full face excavation makes it generally symmetrical, but the multi-partition excavation does not. Besides, for the multi-partition excavation, the thickness of loosened zone decreases from the crown to the side wall, but for the full face excavation, the thickness has a distribiotion of “crown > arch toe > arch shoulder > side wall”. For the multi-partition excavation case, the loosened zone near the earlier excavated part will increase owing to the later excavation of other parts. This influence of “up and down position” is greater than that of “left and right position”. Furthermore, for the left and right position, the loosened zone near the later excavated part is little greater than that near the earlier excavated part. This makes it clear that the earlier excavation has deteriorating effect on the later excavated part. The final loosened zone is the result of excavation interaction of all parts, so it is highly process-related.