Long-distance Pipeline In Loess Dark Hole Impact Analysis

The potential danger posed by the gas pipeline in a bad geological environment have caught the attention of people,the flammable and explosive medium in the pipeline can be extremely dangerous after the pipe is broken.Therefore,there will be of important theoretical significance and practical value for the study of the influence of the loess dark holes on the pipe in the loess area.Based on the second pipeline of west-east gas DL069 Tongguan section,this paper discusses the geological conditions,environmental conditions,and the characteristics,development rules,causes of the dark holes around the pipe in the research area.By adopting on-site investigation,exploration,indoor and outdoor experiment and theoretical calculation and numerical simulation analysis,some valuable results and conclusions are obtained through the analysis of the development rules,genetic mechanism of the dark holes around the pipe and the stress relation of the pipe.(1)Based on the development characteristics of loess dark holes around the pipe in the research area,the development reason of loess dark holes around the pipe is put forward,including(a)internal cause(soil property conditions,joints and fissures,groundwater);(b)external causes(backfill soil,rainfall,vegetation coverage and drainage structures).The formation mechanism of the subsidence of backfill soil and the loss of the pipe subsoil is proposed.This can be roughly summed up as " the subsidence of backfill soil,negative terrain in subsidence,the formation of the subsidence fissures,runoff water seeped along the subsidence,pipe top sinkhole forming,soil contact gap increases,pipe soil erosion,the formation of loess dark holes around the pipe".(2)The flow solid coupled model of pipe and loess dark holes around the pipe is built,the stress and strain characteristics of the pipeline and the critical parts of the loess are investigated when the surface runoff seeps under rainfall mode.The stress of the pipe wall is greater than the bottom of the tube.Diminishing strain by tube center to both sides,but on the border of backfill soil and undisturbed soil is maximum,it shows that in the undisturbed soil with a border of backfill soil,contact gap is the main channel leads to the formation of a water infiltration sinkhole.This paper expounds the formation mechanism of the dark holes around the pipe.(3)The overburden load,internal pressure and temperature difference stress of the pipe are analyzed.Based on the mechanism of pipe soil,the model of non-fixed end single-span pipeline was established.Assuming pipe grade X80(type),tube 2 m depth,get the limit span of security for 37 m will be gotten through theoretical calculation,this can provide reference for the later maintenance of the west-east gas pipeline.