Mechanical characteristic test of tunnel support structure in weak loess stratum

In order to analyze the mechanical characteristics of tunnel support structure in weak loess stratum and optimize the design theory of loess tunnel, large scale field tests of the sections of miscellaneous fill, deep depth and shallow depth were conducted for a loess tunnel of the highway through Xining City. In the aspects of space distribution, time distribution, calculation method and contrast verification, the contact pressure between surrounding rock and primary liner, the contact pressure between primary liner and secondary liner, and the strains of secondary liner and inverted arch were systematically studied. Analysis result shows that the vault and spandrel are the most dangerous parts of loess tunnel for their large pressures and prone to collapse. The calculated values of surrounding rock pressure are very different from the measured values, and the existing method has a big error in the calculation of surrounding rock pressure at the loess section. The time for surrounding rock pressure to stabilize is shorter at deep depth section than the value at shallow depth section, and the time at the section of miscellaneous fill is the longest. Following the guidance of New Austrian Tunnelling Method for the loess tunnel and the design concept of "strong primary liner and weak secondary liner", the loading proportions of the secondary liner have been kept in the range of 11%-36% and the primary liner has played an important role in controlling the deformation of surrounding rock. The secondary liner has played as safety reserve.