Deformations of a highway embankment on degraded permafrost

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uildings and pipelines, airport runways, rail beds, and<strong>highway</strong> sub-bases, cuts and fills.DiscontinuousPermafrostWinnipegPR391ThompsonChurchillContinuouspermafrost-1˚CFigure 1. Location <strong>of</strong> test site and permafrost in Manitoba,CanadaBecause the ice lenses are typically irregular inthickness and extent, out-migration <strong>of</strong> water <strong>of</strong>tenproduces differential settlements, lateral spreading, andreduced stability that lead to serviceability issues. .Typically, this thaw settlement is irregular (Hinkel et al.2003).Although the permafrost regions in northern Canadaare not heavily populated, their economic importance hasincreased substantially in recent decades owing to thepresence <strong>of</strong> mineral, petrocarbon and hydroelectricresources. Thawing <strong>of</strong> summer ice in the Arctic Oceancan be expected to lead to additional shipping into the port<strong>of</strong> Churchill. New roads and railways will have to beconstructed into northern Manitoba and Nunavut oversoils with engineering properties that may degrade withclimate change and land-use. Design, construction, andmaintenance <strong>of</strong> this new infrastructure will have to takeaccount <strong>of</strong> future warming (Hinkel et al. 2003).Figure 1 shows that Manitoba has discontinuouspermafrost north <strong>of</strong> the isotherm for a current meanannual air temperature <strong>of</strong> about 0ºC (about 2500ºC-days<strong>of</strong> frost). The permafrost becomes continuous furthernorth near the Hudson Bay coast.Construction in permafrost regions requires specialtechniques where soil pr<strong>of</strong>iles contain ice lenses.Disturbance <strong>of</strong> existing ground cover can change theenergy regime (Nelson and Outcalt, 1982). This <strong>of</strong>tenleads to thawing <strong>of</strong> underlying ice-rich permafrost andsubsidence <strong>of</strong> the road bed. <strong>Deformations</strong> can besufficiently large that the road becomes inoperable.Several methods are currently used to prevent orminimize thawing <strong>of</strong> permafrost and development <strong>of</strong>thermokarst. These include above-ground construction,use <strong>of</strong> thermosyphons, and buildings on piles or gravelpads above original ground level.Highway construction in northern Canada followsbroadly similar practices to those in warmer regions. Fillsgenerally have high thermal conductivity, leading to heattransfer into underlying layers and thawing <strong>of</strong> previouslyfrozen foundation soil (Batenipour et al. 2009b). Asphaltsurfacing absorbs heat from the sun and transfers it to the<strong>embankment</strong>. Generally, degradation <strong>of</strong> permafrostbegins at the toe <strong>of</strong> <strong>embankment</strong>s and extends laterbeneath the centre. Melting reduces strength andincreases pore water pressures when the hydraulicconductivity <strong>of</strong> the foundation soil is low. This leads todifferential settlements, lateral spreading, and instability.In order to improve local design and maintenanceprocedures, Manitoba Infrastructure and Transportation(MIT) and the University <strong>of</strong> Manitoba (UM) arecollaborating on several projects that involve fieldinstrumentation, laboratory testing and numericalmodeling. This paper reports work on a project onProvincial Road PR391, about 18 km northwest <strong>of</strong>Thompson, Manitoba (Figure 1). This is the only roadconnecting Thompson to northern mining towns,hydroelectric generating stations, and first Nationscommunities in north-western Manitoba. The site is in aregion <strong>of</strong> discontinuous permafrost.2 GEOTECHNICAL SITE INVESTIGATIONS ANDFIELD INSTRUMENTATIONS2.1 Site InvestigationThe PR391 was initially constructed as a compactedearthen road on discontinuous permafrost in the mid-1960s and then converted to a gravel road in the early1970s. In the early 1980s, it was upgraded with abituminous pavement surface. Since construction,changes in heat transfer have melted permafrost that hadbeen detected earlier, particularly under <strong>embankment</strong>s.Thawing led to large ongoing irregular deformations anddangerous traffic issues.Responses by MIT included construction <strong>of</strong> stabilizingberms and insulating peat berms beside the project<strong>embankment</strong> in the early 1990s. Over some years, theberms settled into the foundation soil and essentiallydisappeared. They currently provide no additional supportto the original <strong>embankment</strong>. Regular maintenance hasadded several metres <strong>of</strong> gravel fill since initialconstruction. Where extra maintenance is required, thewearing surface has been returned to gravel from asphalt:the asphalt pavement from the early 1980s has not beenreplaced.In 1991, drilling encountered frozen soil at depths from1.9m to 10.5m below the toe <strong>of</strong> the <strong>embankment</strong>. Laterdrilling in 2005, detected frozen soil at depths from 4.6mto 10.7m. Perhaps surprisingly, no frozen soil wasidentified in a recent drilling program in late 2008 usingcontinuous flight, solid stem augers. A later section willshow no sub-zero temperatures below the active surfacelayer. Considerable maintenance was required at the site.Records outlining maintenance procedures and annualapplication <strong>of</strong> gravel are unavailable.Figure 2 is the authors’ interpretation <strong>of</strong> the results <strong>of</strong>the site investigation at PR391 in 2008. The investigation
involved two cross-sections, one <strong>of</strong> which was designatedas ‘stable’ and the other as ‘unstable’. The stable sectionis only about 2m high (Figure 2a) and has not deformedsignificantly. The unstable section is also about 2m highabove the surrounding natural ground. It has settledconsiderably and now contains about 5m - 6m <strong>of</strong> gravel(Figure 2b). The gravel is partly from the originalconstruction and partly from ongoing re-grading. The‘zero’ depth in Figure 2 and in subsequent figures isreferenced to the level <strong>of</strong> the original ground surface and<strong>of</strong> the surrounding undisturbed land.near the toe. At the mid-slope, the soil is primarily silt toclayey silt. The toe <strong>of</strong> the unstable section consists <strong>of</strong>1.0m <strong>of</strong> clayey peat-silt, 1.0m <strong>of</strong> fine gravel, followed by alayer <strong>of</strong> highly plastic clay. This clay is firm, brown, siltyclay at upper levels and becomes very s<strong>of</strong>t and grey to adepth approaching 18m. The mid-slope <strong>of</strong> the unstablesection consists <strong>of</strong> almost 2.0m <strong>of</strong> clayey peat-silt, over2.0m <strong>of</strong> loose fine gravel, followed by firm brown silty claythat transitions to grey and s<strong>of</strong>t clay. Both sections areunderlain by gneissic bedrock. The surrounding area ispoorly drained - there is free-standing water withinapproximately 20m <strong>of</strong> the <strong>embankment</strong> toe when snow isnot present.2.2 InstrumentationClusters <strong>of</strong> instruments were installed at the shoulder,mid-slope and toe <strong>of</strong> both the stable and unstable sections(Figure 3). They include thermistor strings at 1m intervals,vibrating wire piezometers and standpipes, surfacesettlement plates, slope inclinometers, and lateraldisplacement extensometers at the toe <strong>of</strong> the<strong>embankment</strong>. The piezometers and standpipes wereused to identify possible upwards or downwards hydraulicgradients.Temperatures have been collected during two wintercycles. Readings were taken monthly on a dataacquisition system and downloaded manually. Telephoneaccess was not available at reasonable cost at therelatively remote site.3 DATA FROM FIELD INSTRUMENTATIONFollowing sections analyse the results <strong>of</strong> the fieldobservations. The data include surveyed results from thesurface settlement plates, displacements measured byinclinometers and horizontal extensometers, pore waterpressures, and monthly temperatures from the thermistorstrings. Inserts in following figures show the positions <strong>of</strong>the instruments for which results are being reported.3.1 Movements measured by surface settlement platesFigure 2. a) stable section; b) unstable sectionThe terms stable and unstable are here used in thesense <strong>of</strong> a serviceability limit state and not an ultimatelimit state. There are no indications <strong>of</strong> deep-seatedrotational movements.Boreholes were drilled at the mid-slope and toe <strong>of</strong>each section to examine the stratigraphy, collect samplesfor laboratory testing, and install instruments to record thebehaviour <strong>of</strong> the foundation soils over several years.Batenipour et al. (2009a, b and 2010) provided additionalinformation about site characterization, instrumentationand material properties. This paper provides informationon two full years <strong>of</strong> field measurements.Figure 2 shows that soil conditions below the originalground level (at depth ‘0’ in the figure) at the two sectionsare considerably different. The stable section consists <strong>of</strong>approximately 4.0m <strong>of</strong> s<strong>of</strong>t-to-firm clayey silt/silty clay withpeat intrusions that vary from thin stratifications to pocketsFigures 4 and 5 show displacements <strong>of</strong> surface settlementplates in metres in both the stable and unstable sectionsat the shoulder <strong>of</strong> the road, at mid-slope, and at the toe <strong>of</strong>the <strong>embankment</strong>s. Vertical movements are shown aselevation changes in Figure 4, and horizontal (lateral)movements perpendicular to the centreline <strong>of</strong> the road inFigure 5.Figures 4a, 4b, and 4c show the vertical movements atthe shoulder, mid-slope and toe <strong>of</strong> the two sections. Bothshow seasonal movements that are largely, but notcompletely recoverable. Because <strong>of</strong> the gravel in the<strong>embankment</strong>, the seasonal movements at the shoulderare small. There appears to be little cumulativemovement at the shoulder <strong>of</strong> the stable section. Incontrast, there has been about 0.15m <strong>of</strong> settlement at theshoulder <strong>of</strong> the unstable section. Seasonal heaves arelarger at mid-slope, and larger again at the toe. Again thestable section shows little cumulative movement, while
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Deformations of a highway embankment on degraded permafrost

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