CHARACTERIZING RAIL EMBANKMENT STABILIZATION NEEDS ON THE HUDSON BAY RAILWAY

The Hudson Bay Railway (HBR) is a 510 mile railway completed in 1929 in northern Manitoba, Canada. It connects domestic locations in North America with international destinations through the Port of Churchill. Permafrost was encountered during construction at milepost 136 in isolated peat bogs which continued in a gradual northward transition from discontinuous to continuous permafrost. Over the past 80 years, warming climate combined with poor engineering properties of the railway embankment material has resulted in further thawing of the discontinuous permafrost leading to differential settlement along the rail embankment and high annual maintenance costs. In a bid to understand the geothermal regime of the embankment, underlying subsurface condition, and to seek for solutions to stabilize the embankment, extensive work has been done from 1977 to the present time. This paper seeks to review reports of the past projects and compare the results against current conditions at selected test locations. INTRODUCTION The Hudson Bay Railway (HBR) is a 510-mile railway built within the permafrost region of Northern Manitoba, Canada. Its construction began in the early 1900s and was completed in 1929. The line was operated by Canadian National Railway (CN) from 1929-1997 before being sold to the current owner, OmniTRAX. It runs north from The Pas to Churchill, along the route shown in figure 1. During construction, isolated permafrost bodies were first encountered in Wabowden (milepost 136). As the line pushed forward to the east and north, an increase in the size and frequency of permafrost bodies was encountered. From the Pas to Churchill there is gradual northward transition of permafrost condition from non-existent to sporadic discontinuous to extensive discontinuous and finally to continuous permafrost in Churchill (figure 1). Construction was temporarily suspended between 1917 and 1926 and during this time little or no maintenance was done on the line. When work resumed, it was observed that the line had settled in many places which necessitated the placement of large volumes of additional granular fill to restore the embankment to usable condition once more. This subsidence experienced was a precursor to long term behavior of this rail corridor which has since necessitated significant and continuous maintenance. Track settlements were termed ‘sinkholes’ by Canadian National Railway (CN) maintenance personnel and these sinkholes consist of short sections of track that may experience as much as 100 to 150 mm of settlement during a single summer thawing season. These seasonal sags reflect thaw-settlement of ice-rich permafrost soils within the subgrade (EBA, 1977). To obtain a better understanding of the processes responsible for sinkhole development, EBA Engineering Consultants Ltd. (EBA) was engaged in 1976 by CN to investigate the technical feasibility of mitigating the railway grade settlements. This paper presents a comprehensive review of the studies done by EBA from 1976 to 1991 and compares their results to the current condition of the line at spot locations, as determined through recent field investigations.