Laboratory Model Test On Disease And Control Technology Of Gas-pipeline In Segregated Permafrost Region

Because of the continuous cooling of the pipeline to the surrounding frozen soil,in patchy permafrost region,and the periodic variation of the atmospheric temperature,there is periodic frost heave and thaw settlement around frozen soil.In order to ensure the safe operation of pipelines,the mechanism of pipeline deformation in permafrost regions must be studied.In this paper,based on the investigation of a gas pipeline in patchy permafrost region(LA Ridge Mountain Area)in northwestern Qinghai,analysis results of typical location monitoring data and Numerical simulation analysis results,and the actual geological and climatic conditions,the laboratory model test is carried out.In the indoor low temperature environment model box where the environment temperature and its change mode are set up,the pipe-soil scale ratio model is made.Under the influence of the change of the pipeline heat source and the environment temperature,temperature,displacement and stress testing elements embedded monitor the deformation of the surrounding soil,the deformation of the pipe and the stress change,the change of the soil water content of the foundation soil.And this paper puts forward the prevention and control measures of gas pipeline in patchy permafrost area.The following conclusions are obtained in this paper:(1)The physical and mechanical parameters of the soil samples near the pipeline in the gas pipeline are measured by the conventional geotechnical test.(2)The permafrost in this area is a high temperature Unstable Permafrost.The temperature monitoring is carried out on the half section of the vertical pipe of the gas pipeline(LA Ridge Mountain Area),and a freeze-thaw cycle data is collected to analyze the variation of the temperature field under the influence of the atmospheric temperature and the thermal disturbance of the positive temperature pipeline: In the early September,the melting circle around the pipeline was the largest and the maximum melting depth could reach 6-7m;because the soil surrounding the pipeline began to be frozen,the melting circle,the transverse melting range and the maximum melting depth decreased continuously in the early September-mid March;In the middle of March of next year,the lowest annual surface temperature was,the melting zone disappeared and the surrounding soil was completely frozen.in the next year April-the next year September,because the temperature was warm,the soil around the pipeline began to melt,the melting circle,the transverse melting range and the maximum melting depth were all increasing,but the pipeline around was still in the freezing state in April and May.(3)The model test is successful for controlling the ambient temperature and the heat source condition of the model piping in the low temperature model box.The heat of the model pipe is dissipating,and the heat is transmitted in the soil around the pipe.The temperature of the soil increases and the thawing ring appears.(4)The regular frost heave and thawing phenomenon in the consolidation settlement of the model soil under the pipeline,which leads to the regular uplift and downfall of the pipeline position during the sinking process.The sinking rate of the pipe with the heat insulation model pipe is less than that of the non thermal insulation model pipe,the final settlement of the central position is reduced by 26.9%,and the displacement changes in each cycle are reduced by 28.8%-51.0%.(5)During the test,the stress value of the lower surface is irregular wave peak and trough change.The stress change at all measuring points of pipeline is lagged behind the change of environmental temperature.The maximum tensile stress of the research point of heat insulation model pipe is 61.4% compared with the model without heat preservation,and the maximum compressive stress is reduced by 59.1%.(6)Due to the large initial moisture content of the foundation soil,the moisture migration of the soil around the pipeline is large,and the phenomenon of water aggregation is obvious.Under the influence of temperature gradient,the water in the freezing zone migrated to the frozen front and cooled to ice in freezing period.In ablation season,the volume of the soil decreased and the soil subsidence was compressed,which resulted in the sinking of the model pipeline.(7)The effect of thermal resistance of heat preservation measures greatly reduces the thaw rate of surrounding soils,and acts well to protect the surrounding soil around the pipeline.It reduces the uneven settlement of pipeline and the magnitude and variation of pipeline stress,which indicates that the insulation measures are feasible,and it is worth learning from pipeline design and construction.