Analysis On Collapse Characteristics And Treatment Measures Of Shallow Collapsible Loess Tunnel

Loess is widely distributed in China,from Taihang Mountain in the east,Wusheiling in the west,Qinling in the south and Great Wall in the north.The loess soil under dry condition is uniform and the compressibility is relatively small,but when the loess soil layer encounters the erosion of continuous rainfall and the seepage of groundwater,the loess strength decreases sharply,resulting in a strong collapsibility phenomenon,which poses a great threat to the engineering construction in this area.With the rapid development of traffic construction in China,a large number of engineering facilities have been built in the loess area,especially the underground works represented by tunnels are most affected by loess collapsibility.According to the characteristics of loess collapsibility,this paper summarizes the problems encountered in the excavation of collapsible loess tunnel at home and abroad.Based on the collapse accident in Zhaojiashan tunnel construction in Haiyuan County,Ningxia,the causes of the collapse and the concrete diagnosis and treatment measures are summarized according to the monitoring data.By midas/gts-nx the finite element software,under the premise of reducing the collapse deformation process of Zhaojiashan tunnel,the deformation law of tunnel surrounding rock under two different reinforcement measures is calculated numerically,and the control deformation effect of different reinforcement measures is verified,and compared with the field measured data.(1)Collapsible loess will not only reduce the bearing capacity of surrounding rock,but also cause the lining structure to bear greater soil and water pressure.There are two types of water flowing into loess strata: static infiltration and migration dominant pathway infiltration.Compared with the relaxation and stability of static infiltration process,the infiltration process fluctuation of migration dominant pathway is obvious,which can significantly change the structure of loess soil in a very short time,so it is easy to change the stable loess structure and even the uneven settlement of loess soil layer.(2)Simulation of Zhaojiashan Tunnel ZK54+526～556 with midas/gts-nx Deformation and collapse of section 526～556 during actual excavation.based on the comparison of the field monitoring data and the numerical results,the conclusion that the basic deformation trend of the two is consistent and the value is close(the maximum dome settlement of the monitoring is 34.2 cm,the numerical calculation is 38.8 cm,the difference is 11.86%),and the reliability of the numerical simulation method is proved.(3)Combined with the actual monitoring data,two measures are put forward to reinforce the collapse of Zhaojiashan tunnel: long pipe shed reinforcement and double layer small pipe reinforcement.On the premise of the reliability of the numerical method,two kinds of reinforcement measures are compared with the deformation under the condition of no reinforcement.The effect of long pipe shed reinforcement is better.Compared with double-layer small pipe reinforcement,it has been greatly improved in three aspects:maximum surface(reduced 7.4 cm),vault settlement(reduced 19%)and horizontal convergence(reduced 15 mm).Therefore,this paper considers that the reinforcement effect of long pipe shed is better than that of double layer small pipe,so it is suggested that the reinforcement measures of long pipe shed should be adopted in the subsequent excavation of tunnel.(4)The reason for the large deformation and collapse of the surrounding rock of Zhaojiashan tunnel is that the collapsibility of loess surrounding rock is obvious,the joints and fissures are developed,the strength of surrounding rock is obviously reduced under the erosion and immersion of Rain Water,and the initial support strength is obviously low.After the collapse of the surrounding rock of Zhaojiashan tunnel,the measures of backfilling the collapse cavity and sealing the face of the face are actually taken,and the temporary support of the collapsed soil is carried out and the arch of the steel arch frame is changed according to the actual situation of the tunnel invasion.