FIELD MONITORING FOR FROST-HEAVING AND THAWING-SETTLEMENT OF SOIL AROUND GAS PIPELINES IN PERMAFROST REGION

Pipeline is one of the most commonly used methods in petroleum transportation. There are different engineering problems when pipelines go across different geological conditions. In order to solve the diseases like thawing collapse of pipelines and frost heaving of engineering structures, an investigation of gas pipeline was undertaken in a certain permafrost region in Qinghai Province. We selected typical sections and buried temperature sensors and magnetic settlement rings to monitor the temperature and displacement of the soil around pipelines during cold and warm seasons. We studied the law of ground temperature variation around the pipelines and the law of frost-heaving and thawing-settlement, which provides reference for the design, construction, operation and disease treatment of gas pipelines in permafrost region. The studies show that ground temperature changes greatly with the temperature during cold and warm seasons in this area, and the closer to the pipelines, the greater the annual amplitude of ground temperature. The ground temperature of frozen soil in this area is -2℃~1℃, which means that the temperate zone belongs to basically stable zone. Heat disturbance of the gas transmission in positive temperature causes the thawing settlement of soil. Due to the heat release of pipelines, ground temperature of the soil above the pipeline turns positive, and the impact range is about 1.5m. Ground temperature from 1m to 4m under the ground surface is influenced by multiple factors. The annual ground temperature range is relatively small under the depth of 4m, all of which are negative temperature. Frost-heaving occurs from the depths upwards. There is a hysteresis in time. The freezing-thawing cycle is very severe in October and November and should be monitored in time.