Study On Criteria For Stability Of Loess Tunnel With Big Cross Section

The special features of loess determine that patterns of stress for the loess tunnel vary. During excavation for the loess tunnel with big cross section, the cross section would always be divided into several small sections, which makes it difficult to lay out surveying points and to carry out displacement monitoring by the current method of points layout. As the criteria for judgment of tunnel stability specified in all standards and technical guidelines are based on general data of single-lined or double-lined railways, it would be considered improper to adopt such criteria to the loess tunnel with big cross section.The thesis makes analysis on rock deformation characteristics and rock stressing mechanism of the loess tunnel with big cross section, and makes presentation by finite element method on structures of tunnels in different depths, which shows that there are two patterns of rock stress imposed on support: one pattern for depth of approximately 15m and the other pattern for depth of more than 35m; it also shows that the two patterns gradually combines into one unique pattern as depth grows deeper.Take primary support as normal shaped-steel concrete, and calculation is made on support under different patterns of rock stress by the mode of load—structure, where it shows that support on the waist part tends to be fragile for 15m-depth pattern and support for the cross section at the foot tends to be fragile for 70m-depth pattern (this cross section can be used as the controlling cross section for supporting structures). The displacement of so-called controlling cross section depends not only on the rock stress on the section, but also on the thickness of the section and the elastic modulus of materials used as the support. The thesis contains a diagram showing the relationship between displacement of so-called controlling cross section and thickness of the section and the elastic modulus of materials, which shows the displacement value is small and support is stable when cross section is thick and material elastic modulus is big and which also shows that cross section thickness has an bigger impact on displacement.