Permafrost and thermal interfaces from Normal Wells pipeline ditchwall logs

Abstract

During the construction of the Normal Wells pipeline by Interprovincial Pipeline Ltd. in the early 1980s, a continuous ditchwall log was created during ditching for pipeline burial. The ditch was typically 1.2 m deep and stretched continuously from Normal Wells in the Canadian Arctic to Zama Lake in northern Alberta, a total distance of 869 km through the discontinuous permafrost zone. The ditch was logged by experienced geotechnical field personnel every 50–100 m, depending on changing conditions. Every transition from unfrozen to frozen soil was logged based on visual criteria such as color change, visual presence of ice or moisture in the ditchwall, etc. Recently, the authors have carefully studied the ditchwall records and compiled a data file containing all of the relevant data pertaining to permafrost distribution. This provided a unique opportunity to study the amount and distribution of permafrost along a continuous transect through discontinuous and sporadic permafrost regions in Arctic Canada. The number of thermal interfaces per kilometre is an extremely important input parameter for studies relating to pipeline frost heave and thaw settlement in the Arctic. In addition, a knowledge of the percentage of frozen ground is important when deciding whether to operate a gas or pipeline above or below freezing. The number of frozen–unfrozen interfaces have been summarized in the paper by pipeline spread and geological terrain unit. The overall percentage of frozen ground decreases from up to 95% in the north to a low of around 16% at the south end of the study area, as might be expected. The number of interfaces typically varies between about one and three per kilometre, with the highest number occurring in some of the organic terrain units in the southern discontinuous zone. Finally, comparisons are made with the amount of permafrost and number of interfaces as logged by electrical geophysical surveys carried out and published in advance of pipeline construction. There is reasonable agreement in terms of the overall amount of frozen ground; however, the geophysical surveys may have overestimated the number of thermal interfaces in some areas. Key words: ditch logs, permafrost, Normal Wells, pipeline, thermal interfaces, frozen ground.