Study On Mechanical Properties Of Unsaturated Soil Strata And Supporting Structure Stability Of Tunnel For Oil And Gas Transmission Pipeline

When building a pipeline tunnel in unsaturated soils (loess and expansive soil), because of the special nature of loess and expansive soil, tunnel engineering would be serious insecurity. Especially the engineering properties of collapsibility, vertical joints of loess and expansion and shrinkage characteristics of expansive soil is not treated properly, such as the support structure delayed and inappropriate, it will cause excessive deformation of surrounding rock, destruction of support structures, consequently affect the construction schedule.With the methods of literature search, data collection, laboratory test and numerical simulation, this paper carried out at two aspects:vertical joints of loess and swell-shrink characteristics. The main contents of the work were shown as follows:(1)By tri-axial compression test, the shear strength of unsaturated soils (loess) at different initial states is studied. And the purpose is to grasp the relationship between shear strength of loess and water content, dry density.(2)Vertical joints existed in the loess in tunnel construction process, will have an impact on the tunnel itself and surface deformation. Based on finite element numerical simulation, the deformation of surrounding rock, the deformation of surface, mechanical features of support structures with different spaces and different depths of vertical joints in shallow tunnel is studied. It will provide guidance to the design and construction of loess tunnel with vertical joints.(3)Due to pipe tunnel’s small section, finite element method is used to study the variation of surrounding rock deformation and supporting structure stress state. The dynamic relationship between different construction method, step sequence and surrounding rock deformation, supporting structure stress state is established, thereby forming the appropriate construction method to ensure the safety and stability of the tunnel face.(4)Based on the relation between saturation and swelling rate obtained by laboratory test, considering the influence of moisture content changes on the shear strength, the influence of the swelling-shrinkage characteristic on the support structure and the influence rules of swelling effect on the surrounding rock deformation and internal forces of support structures with different laying depth is studied.