Study On The Impact Of Loess Collapsibility On Existing Metro Tunnel Structure

At present, there have been few metro tunnels constructed in collapsible loess stratum home and abroad. The stratum along lintong-line of Xian metro is found to be collapsible. It is necessary to do research in terms of the impact collapsible loess may have on tunnel lining. Based on lintong line of Xian metro, a systematic research on the characteristics of the collapsible loess may have on tunnel lining is done by means of centrifuge method in this paper. The following aspects are included as main contents : Firstly, exploration based on the tunnels with the loess ground collapsing evenly; Secondly, exploration based on the tunnels with the loess below collapsing unevenly. In the model test, conveying points are planned on vault,spandrel,arch foot,sidewall,inverted arch.At last, the actual model is analyzed using the method of close mold modification for collapsed loess. Main conclusions are drawn as following:The aspect of even collapsibility:Firstly, before and after the loess ground collapsed, circularstress and longitudinal stress of the lining are at the state of being compressed. The compressive stress at the arch foot is largger than other sites,valuing 2MPa.Compared with the stress before the ground collapsed, little change has been found when the ground collapsed. Obviously, the concrete strength meets the needs of the change. But, a large settlement comes up after the ground collapsed, approximately 30 cm, it’s not fit for the rules of metro operation.So the loess ground needs treatment measures.Secondly,According to the ground settlement curve when collapsed, the central area of the tunnel is found to settle the least. It’s because the inner soil has been dug out, this lead to a reduce in soil pressure of the central area. Thus, when desiging the treatment measures of the ground,it’s necessary to stengthen the bearing capability of the ground on both sides of the central area, while weakening at the central area. So as to creat a economic engeering.The aspect of uneven collapsibility:Firstly, the collapsible area with the length1.5 times the tunnel diameter and the width same to the tunnel diameter was set in the experiment. Stress less than about 2.5D away from the collapsible area center have changed greatly. Before and after uneven collapsibility,the vault, spandrel,arch foot,sidewall and inverted arch are all at the state of being compressed. The compressing stress ranges from 0.48 MPa to 1.7MPa, which meets the needs of the lining strength.Of all the conveying points, inverted arch has a bigger change than other points, which resulted from an unequal deformation between the loess ground and the inverted arch.Secondly, The aspect of longitudinal stress: whether collapsed or not, the longitudinal stress of the vault, spandrel, arch foot, sidewall are all compressive stress. While the inverted arch transformed from compressive stress to tensile stress.The larggest tensile stress values 0.5MPa. At the same time, the experiment result shows that the area which comes up tensile stress is in accrodance with the collapsible area. When designing the lining structure, attention should be paid to the tensile strength and anti cracking ability of the inverted arch.