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Geotechnical investigations of insulated permafrost slopes along the Norman wells pipeline using ground penetrating radar
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, Fifth International Conferention on Ground Penetrating Radar, Jun 1994, cp-300-00033
Abstract
During construction of the Norman Wells pipeline, 56 permafrost slopes were covered with a 0.5 m to 1.8m insulating blanket of wood-chips in an attempt to retard the rate ofpermafrost thaw. Warmer mean annual pipe operating temperatures (OC to 3.5C) than anticipated have led to larger thaw bulbs than predicted beneath many insulated slopes. Initially, manual probing with an active layer probe was used to monitor the development of the thaw bulbs. However, as the depth-of-thaw increased to over 2 m in some areas, manual frost probe measurements became time consuming. A series of experiments were conducted to test the ability of ground penetrating radar to determine the thermal, lithologic, and structural characteristics ofwood-chip slopes. On many ofthe slopes, gridded surveys were conducted enabling three dimensional subsurface analyses and the construction of isopleth maps of the depth-of-thaw. In this paper we show examples of lithologic mapping, including qualitative indications of the ice-content of soils, mapping the boundaries between soil units and imaging internal bedding structures within units. The thermal structure of the subsurface is examinedby interpolating the depth of frost layers on radar profiles, and by mapping the depth ofmultiple frost layers with isopleth maps. An example of mapping the lateral of extent of subsurface cavities is also discussed. In general, the depth of signal penetration ranged from as great as 10min frozen sand to as little as 3 m in unfrozen clayey soil. Surveys at 50 MHz contained the most information and the depth-of-thaw was best mapped when it was deeper than 1.5 m below the surface.