STUDY ON DEFORMATION CHARACTERISTICS OF SUPPORTING STRUC-TURE UNDER LOESS CRISS-CROSS TUNNEL EXCAVATION

The triaxial test was utilized to evaluate the stress-strain characteristics of remould loess with different water contents,and the strain variation of support structure and the excavation influence range of crossing tunnel in loess were revealed by the model test. The investigation shows that the deformation of the support structure is related to the position of the tunnel face,and the deformation of the steel arch was observed when the distance between tunnel face and monitoring section was within 1.5 D(D is the diameter of the tunnel),and when the distance exceeded 1.5 D from the monitoring section,the deformation was stabilized,which indicates that the horizontal influence range of excavation was about 1.5 D before and after the tunnel face. Besides,the deformation of the support structure is most obvious in the range of 0.5 D before and after the monitoring section,which accounts for more than 70% of the total strain release. It indicates that the deformation can be controlled by the support structure well,and also large deformation of surrounding rock occurs immediately after excavation,hence timely support should be provided to ensure the stability of the tunnel after excavation is completed. The largest deformation of the tunnel in the vault of the intersection was observed,which was the result of the different stress conditions between vault,hance and vertical wall,and the vault is subject to the vertical load,the hance and vertical wall is subjected to the lateral load. After the excavation of the main tunnel is completed and the deformation of the support structure being stabilized,the secondary deformation of the support structure in the intersection can be influenced by the excavation of the bifurcation tunnel. The deformation of the vault accounts for about 2/5 of the total deformation,and the further the monitoring section is away from the intersection,the deformation of the monitoring section was already very small. These results show that the closer the distance to the intersection,the greater the deformation is. So,the stability of the intersection is the key to the stability of the tunnel.