Reason Analysis And Disposal Measures Study On Concentrated Collapse In Shallow-buried Soft Rock Section Of Large-span Highway

With the continuous development of social economy and logistics industry in China,the scale of the road network construction is expanding year by year.Along with the great development of the design theory and construction technology of the highway facilities,the technical threshold of large-span highway tunnel is decreasing and the volume of construction is developing rapidly.In the shallow-buried soft rock conditions for large-span tunnel,the stress characteristic of large-span and flat section shape is completely different from the traditional circular cross-section,so it is more prone to geologic hazards that take collapse as the main thing.Reasonable tunnel excavation method is the primary means of disposal of landslides disaster,and the double side drift method is proved to the most ideal method which is applied in the excavation and supporting of large-span shallow tunnel.In construction of the cross section of a large span tunnel that the span is in 15~18m by using the double side drift method,the one-time completion of excavation heading in large area is difficult,so the upper and lower step excavation is adopted instead and the temporary horizontal support is not provided from the construction of heading excavation in the engineering experience,this deficiency of component will affect the stability of the retaining structure seriously and cause collapse occurrence.Therefore,it has great significance to study the necessity of temporary support and the support effect of temporary support in different setting conditions.The project of Yingerling tunnel in the highway of Rongwu as the background,through on-the-spot investigation and follow up the tunnel construction,the engineering geological characteristics,engineering geological evaluation,tunnel construction method and flow can be understood.By basing on the stratum structure method,combining with finite element analysis theory and using Midas GTS NX geotechnical finite element analysis software,the continuous concentrated collapse of Yingerling tunnel in shallow buried soft rock section was simulated,and the side heading excavation with different forms of temporary invert of supporting effect was analysed comparatively,and drawing the following conclusions :(1)In the side heading on the excavation,primary designed support structure is constructed sectionally and is not into a closed loop by using temporary support,which is the main reason for the collapse of the accident.Adding temporary inverted arch can restrain the deformation of the supporting structure into the tunnel,control the settlement,improve the stress condition of the surrounding rock,and control the expansion of the plastic zone,which can prevent the collapse accident from occurring.(2)On the left and right line of the tunnel,the selection of the heading upper step in the reasonable height and the choice of structure form in temporary invert arch are flexible.The right heading of right hole in 4m high upper step and the left heading of left hole in 4.5m high upper step can satisfy the security requirements by the concrete temporary inverted arch,The left heading of left hole in 5m high upper step and the right heading of right hole upper step in the same height need to choose 20 b I-beam or steel reinforced concrete temporary inverted arch to satisfy the security requirements because of their larger rigidity.