Thaw Settlement Research Articles

Long-term thermal effect of asphalt pavement on permafrost under an embankment

Abstract Permafrost temperatures have been monitored through six boreholes drilled up to 15 m in depth under asphalt pavement along the centerline of an embankment along the Qinghai–Tibetan Highway. Temperatures have been measured twice monthly from 1996 through 2007. Soil temperatures at 1.0 m depth under asphalt pavement in the northern areas of the Tanggula Mountains have shown a significant increasing trend, but a significantly decreasing trend in the southern Tanggula Mountains during the period of record. Mean annual permafrost temperatures at 10 m depth have increased from 0.26 °C to 0.64 °C, with an average increase of about 0.39 °C from six monitoring sites during the period of record. The artificial permafrost table depth has increased significantly over the period of record for all six monitoring sites, varying from 0.16 to 2.6 m, with an average of 1.67 m. The heat accumulation produced by heat absorption of the asphalt pavement dominates the long-term thermal effect of asphalt pavement on permafrost. With permafrost thawing and permafrost temperature rise, thaw settlement increased significantly. The amount of thaw settlement is positively correlated with the permafrost thawing rate. A noteworthy feature is that lower temperature permafrost's response to climate change is more sensitive than to engineering; however, the response of high temperature permafrost to engineering is more sensitive than to climate change.

Estimates on thermal effects of the China–Russia crude oil pipeline in permafrost regions

Abstract Using the finite element method, thermal effects of the Chinese–Russian crude oil pipeline in permafrost regions were estimated under two construction modes (conventional burial and aboveground embankment), two thermal control techniques (bare and insulated pipe), and three climate conditions along the route (marked under the natural mean ground surface temperatures of − 0.5, − 1.0, − 1.5 °C, respectively). The results show that with the silt embankment on gravel berm and the insulated pipe, the thaw of permafrost under the pipeline could be prevented during its service life. However, thawing of the permafrost under the conventionally buried pipeline cannot be prevented regardless of climate conditions or thermal insulation around the pipe. So it is strongly recommended that over-excavation and/or thermosyphons be applied for the conventional burial construction mode, especially in the warm and ice-rich permafrost sections to mitigate frost heave and thaw settlement in the subsoil.

Thermal Characteristics of the Embankment with Crushed Rock Side Slope to Mitigate Thaw Settlement Hazards of the Qinghai‐Tibet Railway

Abstract:Permafrost (perennially frozen ground) appears widely in the Golmud‐Lhasa section of the Qinghai‐Tibet railway and is characterized by high ground temperature (≥–1°C) and massive ground ice. Under the scenarios of global warming and human activity, the permafrost under the railway will gradually thaw and the massive ground ice will slowly melt, resulting in some thaw settlement hazards, which mainly include longitudinal and lateral cracks, and slope failure. The crushed rock layer has a thermal semiconductor effect under the periodic fluctuation of natural air. It can be used to lower the temperature of the underlying permafrost along the Qinghai‐Tibet railway, and mitigate the thaw settlement hazards of the subgrade. In the present paper, the daily and annual changes in the thermal characteristics of the embankment with crushed rock side slope (ECRSS) were quantitatively simulated using the numerical method to study the cooling effect of the crushed rock layer and its mitigative ability. The results showed that the ECRSS absorbed some heat in the daytime in summer, but part of it was released at night, which accounted for approximately 20% of that absorbed. Within a year, it removed more heat from the railway subgrade in winter than that absorbed in summer. It can store approximately 20% of the “cold” energy in subgrade. Therefore, ECRSS is a better measure to mitigate thaw settlement hazards to the railway.

Development of freezing–thawing processes of foundation soils surrounding the China–Russia Crude Oil Pipeline in the permafrost areas under a warming climate

The proposed China–Russia Crude Oil Pipeline (CRCOP) will be subjected to strong frost heave and thaw settlement of the surrounding soil as it traverses permafrost and seasonally frozen ground areas in Northeastern China. The freezing–thawing processes, the development of the maximum frozen cylinder in taliks and thawed cylinder in permafrost areas, and the variations in the maximum freezing depths under the pipeline in taliks and thawing depths in different permafrost regions near Mo'he station, the first pumping station in China, were studied in detail using numerical methods in this paper. The inlet oil temperature at Mo'he station was assumed to vary from 10 to − 6 °C in a sine wave form during the preliminary design phase. Research results showed that the freezing–thawing processes of soils surrounding the buried pipeline had distinct differences from those in the undisturbed ground profile in permafrost areas. In summer, there was downward thawing from the ground surface and upward and downward thawing from the pipeline's surface once the temperature of the oil rose above 0 °C. In winter, downward freezing began from the ground surface but upward and downward cooling of the cylinder around the pipeline didn't begin until the temperature of the oil dropped below 0 °C. However, in the undisturbed ground profile, downward thawing from the ground surface occurred in summer and downward freezing from the ground surface and upward freezing from the permafrost table occurred in winter. The maximum thawing depths and thawed cylinder around the pipeline in warm permafrost enlarged with elapsing time and decreasing water content of the soils. In taliks, the maximum freezing depths and frozen cylinder around the pipeline kept shrinking with elapsing time and increasing water content of the soils. The freezing–thawing processes and development of the thawed and frozen cylinders around the pipeline were muted by any insulation layer surrounding the pipeline. Insulation had better thermal moderating on the heat exchange between the pipeline and the surrounding soils during the early operating period. But its role slowly weakened after a long-term operating. Research results will provide the basis for assessment and forecasting of engineering geological conditions, analysis of mechanical stability of the pipeline, foundation design, and pipeline construction and maintenance.

Environmental Conditions and Vegetation Recovery at Abandoned Drilling Mud Sumps in the Mackenzie Delta Region, Northwest Territories, Canada

Historical data from oil and gas exploration in the delta of the Mackenzie River, Northwest Territories, in the 1970s provided an opportunity to estimate decadal-scale impacts of exploratory oil and gas drilling on native plant communities in low Arctic tundra. We assessed changes in vegetation composition and associated environmental gradients across seven drilling mud sumps in the Kendall Island Bird Sanctuary, Mackenzie Delta. Three decades after disturbance, drilling sumps had developed vegetation coverage equivalent to that in undisturbed areas, although bare soil persisted in ponded areas and where a salt crust was present. Vegetation on sumps was composed of communities dominated by forbs, grasses, and tall shrubs that were distinct from adjacent, undisturbed sedge and low shrub communities. The area of altered vegetation around a sump was generally larger in upland or saline environments than in lowland areas. Pooled water observed around many sumps was likely associated with thaw subsidence that occurred following construction, which was subsequently compounded by snow drifting and increased soil temperatures along the margins of the sump mound. Changes in drainage, active-layer depth, and surface salt concentrations appear to be key environmental factors that have helped shape plant communities established on drilling sumps in the three decades after disturbance.

Forecasting the oil temperatures along the proposed China–Russia Crude Oil Pipeline using quasi 3-D transient heat conduction model

Abstract The China–Russia Crude Oil Pipeline (CRCOP) faces significant challenges due to differential frost heaving and thaw settlement resulting from significant variations of oil temperatures along the pipeline. Oil temperature distribution along the pipeline during the long-term operation period is a very important factor in pipeline foundation design under future climate warming and various frozen soil conditions. It is important for the assessment and prediction of differential frost heave and thaw settlement of the pipeline foundations soils, forecasting the development of the seasonal and inter-annual frozen and thawed cylinders around the operating pipeline, stress–strain analysis of the pipeline, and mitigation of subsequent frost hazards. A quasi three-dimensional computational model was developed to predict the oil temperature along the pipeline. It was verified by analytic solutions of the minimum oil temperatures along the route provided by the Daqing Oilfield Engineering (DOE) Co. The oil temperatures were predicted and analyzed for two proposed annual oil flow rates of 15 million tons (0.3 mbpd) and 30 million tons (0.6 mbpd) with and without mitigative measures (only pipe insulation was considered here) during the operation period. Also, the inter-annual variations of oil temperature at key typical locations were investigated to understand the impact of climate warming. The results indicated that the maximum oil temperature cools southwards, but the minimum oil temperature warms southwards (with the inlet oil temperatures from − 6 to +10 °C). However, the average annual oil temperature decreases southwards in the northern part of the pipeline, then it starts to slowly increase. The amplitudes of oil temperature change will decrease southwards. Oil temperatures will slightly increase with elapsing time due to the imposed boundary conditions of climate warming. The oil temperatures with a lower flow rate vary more significantly than that with a higher flow rate because the oil temperature with a low flow rate is more affected by the thermal regime of the surrounding soils and the external environments. Insulation around the pipeline tends to reduce the oil temperature variations along the pipeline during pipeline operation period. Therefore, pipe insulation can effectively reduce the development of frozen and thawed cylinders in the permafrost zone. The phase change of water in soils around the pipeline has a distinct influence on the oil temperature during the freeze–thaw transition periods. The oil temperature tends to be equal to the ambient ground temperature around the pipeline with southward distance and with elapsing operation time. The pipeline oil temperature is controlled by the incoming oil temperature and the surrounding ground temperature before the equalization. It would be mainly controlled by the ground temperature around the pipeline afterwards.

Measurement of ice movement in water using electrical capacitance tomography

The permafrost with the highest altitude and largest area in the mid and low latitude is located in the Qinghai-Tibet Plateau. As most frozen soils contain ice particles which are very sensitive to temperature and other external parameters, thus influencing the stability of the embankment in permafrost regions, it is very important to develop techniques to prevent damages to railway embankments due to thaw settlement. In this paper, the electrical capacitance sensors are designed to study the freezing front movement in a vessel and ice movement in water, which is the first step to apply the ECT system to the study of frozen soil. Two sensor arrangements are put into use. First, the traditional closed electrode sensors are put into use. In this arrangement, the electrodes are attached to the outside of the pipe or vessel, and the cross-sectional distribution of ice and water could be reconstructed from the capacitances measured. Also, the ice moving track at the cross section could be reflected thoroughly. Since the traditional closed electrode sensors can not meet the needs of measuring the ice freezing front movement, a new electrode sensors structure, that is, the unclosed electrode sensors are designed to satisfy the specific test of frozen soil. In this arrangement, several pairs of electrodes are arranged along the height of the vessel. A sudden decrease in the measured capacitance is observed when the freezing front advances past the electrodes. Therefore, according to the capacitance variation, the ice movement can be reflected. In summary, electrical capacitance tomography has the advantages of being non-intrusive. With different electrode sensor arrangement, ice movement and ice freezing front can be obtained. The electrical capacitance sensor system can be applied to investigate the complicated phenomena in frozen soil.

Thaw lake expansion in a two‐dimensional coupled model of heat transfer, thaw subsidence, and mass movement

Thaw lakes, widespread in permafrost lowlands, expand their basins by conduction of heat from warm lake water into adjacent permafrost, subsidence of icy permafrost on thawing, and movement of thawed sediment from lake margins into basins by diffusive and advective mass wasting. We describe a cross‐sectional numerical model with thermal processes and mass wasting. To test the model and provide an initial investigation of its utility, the model is driven using historical daily temperatures and permafrost conditions for the northern Seward Peninsula, Alaska (NSP; thick syngenetic ice, mean annual air temperature (MAAT) −6°C) and Yukon coastal plain (YCP; thin epigenetic ice, MAAT −10°C). In the model, lakes develop dynamic equilibrium profiles that are independent of initial morphology. These profiles migrate outward episodically and match the morphology of profiles from lakes that were measured at each site. Modeled NSP lakes expand more rapidly than YCP lakes (0.26 versus 0.10 m a−1) under respective modern climates. When identical climates are imposed, NSP lakes still grow more rapidly because their deeper basins and steeper bathymetric slopes move thawed insulating sediment away from the lake margin. In sensitivity tests, an increase of 3°C in MAAT causes 2.5× (NSP) and 1.6× (YCP) faster expansion of lakes. An 8°C decrease essentially halts expansion for both sites, consistent with paleostudies which attribute basins to postglacial warming. In the model, basins expand monotonically but lakes do not. The 1σ interannual variability of lake expansion is 0.51 (NSP) and 0.44 m a−1 (YCP), with single year rates of up to ±8 m occurring because of instabilities from thermal/mass movement coupling even under a stationary climate. This variability is likely a minimum estimate, compared to natural variability, and suggests that long measurement time series, of basins not lake surfaces, would best detect thermokarst acceleration resulting from a climate change.

Time‐dependent morphology of thaw lakes and taliks in deep and shallow ground ice

The shape and depth of thaw lake basins depends on lake age, on whether the talik (thaw bulb) is at steady state, and on the distribution of ice in the ground. To investigate implications of this broad hypothesis, we use a numerical model of conductive heat transfer, phase change, and thaw subsidence of ice‐rich sediment beneath a lake in cross section. For ground thermal properties with lake temperatures and dimensions consistent with measurements, modeled talik depth approximately increases with except in lakes in deep ground ice which deepen more rapidly because of consolidation on thawing. In deep ground ice environments, basins achieve depths of ≈20 m in ≈5 ka. Expanding lakes with disequilibrium taliks have deep basins with broad, 100+ m wide, inclined margins. In shallow ground ice settings (either original epigenetic ice or in permafrost that has reformed in drained basins), lakes are <3 m deep and flat bottomed. Unaffected by preexisting topography and ground ice variations, first‐generation lakes in deep ground ice are rounder and grow larger in area than later‐generation lakes. These predictions are consistent with GPS, sonar, and remote sensing measurements of bathymetry and plan view shape of first‐ and later‐generation lakes in substrates with deep syngenetic ground ice (Pleistocene loess, northern Seward Peninsula, Alaska) and shallow epigenetic ice (Yukon Arctic coastal plain).

InSAR detects possible thaw settlement in the Alaskan Arctic Coastal Plain

Satellite interferometric synthetic aperture radar (InSAR) has proven to be an effective tool for monitoring surface deformation from volcanoes, earthquakes, landslides, and groundwater withdrawal. This paper seeks to expand the list of applications of InSAR data to include monitoring subsidence possibly associated with thaw settlement over the Alaskan Arctic Coastal Plain. To test our hypothesis that InSAR data are sufficiently sensitive to detect subsidence associated with thaw settlement, we acquired all Japanese Earth Resources Satellite-1 (JERS-1) L-band data available for the summers of 1996, 1997, and 1998 over two sites on the Alaska North Slope. The least amount of subsidence for both study sites was detected in the interferograms covering the summer of 1996 (2–3 cm), interferograms from 1997 and 1998 revealed that about 3 cm of subsidence occurred at the northern Cache One Lake site, and about 5 cm of subsidence was detected at the southern Kaparuk River site. These preliminary results illustrate the capacity of the L-band (24 cm) wavelength JERS-1 radar data to penetrate the short Arctic vegetation to monitor subsidence possibly associated with thaw settlement of the active layer and (or) other hydrologic changes over relatively large areas.

Experimental validation of passive permafrost cooling systems

In areas of warm permafrost, the thawing of interstitial ice in the permafrost layer can cause man-made infrastructure to suffer from thaw settlement damage and failure. For highway and railway embankments, thermosyphons and air convection embankment (ACE) layers are two methods that can be used to reduce embankment and foundation soil temperatures and prevent thaw settlement. These techniques utilize the low air temperature during winter to enhance freezing of the foundation soils and maintain the thermal integrity of the permafrost foundation. In the present paper we report on the application of these techniques in a full-scale field application, carried out as part of a new road building project near Fairbanks, Alaska. Three separate cooling techniques were utilized in the project, including hairpin thermosyphons, horizontal ACE layers, and ventilated shoulders in three different configurations. Each of the three configurations was instrumented with a thermal monitoring system in an effort to evaluate the cooling effectiveness and predict the long-term impact on underlying permafrost temperatures. This paper describes the cooling systems that were used in the project including the thermal monitoring system, and presents performance results for the first year of operation.

Thaw Settlement in Soils of the Arctic Coastal Plain, Alaska

ABSTRACTOn the Beaufort Coastal Plain of northern Alaska, thaw settlement in permafrost soils occurs whenever natural or human disturbances result in an increase in the depth of seasonally thawed soil (the active layer). Knowledge of the potential magnitude of thaw settlement is important for assessing the long-term recovery of disturbed land and for developing rehabilitation plans and performance standards. To address this need, we analyzed thaw strain and thaw depth data from soil cores distributed across the central Beaufort Coastal Plain to evaluate potential thaw settlement at landscape (e.g. terrain units) and regional (e.g. ecodistricts) scales in connection with various oilfield studies during the period 1998–2003.Mean thaw strain values ranged from 1% for meander active channel deposits to 55% for delta inactive overbank deposits, and tended to be highest at 1–2 m below the ground surface. The potential thaw settlement of specific terrain units was evaluated based on thaw strain of soils and the range of changes in active layer depths typically found after disturbance. Mean estimates for potential thaw settlement for an active layer adjustment to 110 cm after disturbance varied from ∼0 cm in sandy soils with thick active layers associated with meander active channel and overbank deposits to 86 cm in very ice-rich silty soils with thin active layers associated with delta inactive overbank deposits. Potential thaw settlement also varied by region, with values for terrain units tending to be higher in the central Beaufort Coastal Plain (Prudhoe Bay and Kuparuk Oilfield areas) than in the western Beaufort Coastal Plain. The highest estimate for potential thaw settlement was 103 cm for delta inactive overbank deposits on the Colville Delta. The thawing of ice wedges will further contribute to thaw settlement and effect local hydrology and topography following thermokarst.

Settlement of embankments in permafrost regions in the Qinghai-Tibet Plateau

Settlement of embankments in permafrost regions results from several simultaneous processes, each with a different mechanism. The first is the well-known process of thaw settlement due to the degradation of permafrost underneath. This has been considered to be the most problematic and has attracted great attention in previous studies. The second process is that of creep. Field observations show that due to the construction of embankments, the temperature in permafrost layers may rise remarkably, forming a large ‘warmer’ frozen layer with a higher tendency to creep. At the same time, creep of unfrozen soils can take place in the active layer in warm seasons under new loading conditions. Third, because freeze-thaw cycling in the active layer can change the soil properties, additional settlement would also take place in the first years. These three processes are interactive, which makes any attempts at calculation very difficult. The article first reviews the basic methodologies and their limitations for settlement estimation. Then, based on the practice in the construction of a highway and a railway on the Qinghai-Tibet Plateau, the possible settlement sources are analysed. In order to investigate the whole settlement comprehensively with clear mechanisms, the concept of an equivalent permafrost table is proposed.

The Structure and Dynamics of Earth Hummocks in the Subarctic Forest Near Inuvik, Northwest Territories, Canada

ABSTRACTSurface microrelief, permafrost table configuration, ground-ice, and soil organic-matter contents are described for three sites near Inuvik, N.W.T., that are characterized by collapsed, poorly developed, and well-developed earth hummocks, respectively. The diameters of collapsed earth hummocks were significantly greater than those of well-developed vegetated hummocks. Hummock relief and interhummock distance increased along the continuum of forms, with the widest spacing and greatest relief measured at the site with well-developed vegetated hummocks. A bowl-shaped permafrost table mirrored the surface relief of most hummocks, but the collapsed hummocks were underlain by a planar or domed permafrost table. Segregated ice lenses parallel to the permafrost table, and small bodies of intrusive ice, were observed beneath the developing and well-developed hummocks. The configuration of the permafrost table and hummock relief, long-term observations of active-layer and hummock change, and hummock response to surface manipulation indicate that formation of a bowl-shaped permafrost table in association with organic accumulation and development of near-surface ground ice thrusts soils inward and upward causing hummock growth, whereas thaw subsidence may cause outward spreading and hummock collapse. Reaction-wood rings in black spruce trees growing on hummocky terrain indicate that tree tilting was associated with active-layer thinning and hummock growth. Cessation of reaction wood coincided with a period of active-layer deepening, degradation of ground ice, and outward spreading of the hummocks. In subarctic forests, hummock dynamics may be driven by ecological change associated with the fire cycle or climate change.

Field instrumentation for real‐time monitoring of periglacial solifluction

AbstractHere we describe new field instrumentation recently installed on a non‐permafrost solifluction slope in Dovrefjell, Norway, and a continuous permafrost solifluction slope in Svalbard. The equipment is designed to provide continuous data on soil thermal status, hydraulic condition, phase change, soil volume strain and soil shear strain. Supporting frames were constructed from steel scaffolding tubes and provide a stable mounting for two LVDT displacement transducers. These are arranged in an inverted triangular configuration with the triangle apex linked to a small steel footplate embedded in the ground surface, allowing continuous monitoring of frost heave, thaw settlement and downslope surface displacements. In addition, thermistors and Druck PDCR 81 miniature pore pressure transducers were installed to a depth of 0.8 m in Dovrefjell and 1.2 m (approximately 20 cm below the permafrost table) in Svalbard. Thermistors were also mounted above the ground surface to 2 m to measure air temperature. All instrumentation was logged at 1 h intervals using a Campbell CR23X logger and multiplexer. In Svalbard an automatic digital camera recording one image per day was installed adjacent to the site in order to monitor snow depth. Simple Rudberg columns will be excavated after several years to observe the profiles of soil movement. Copyright © 2007 John Wiley & Sons, Ltd.

Permafrost Destabilization and Thermokarst Following Snow Fence Installation, Barrow, Alaska, U.S.A

ABSTRACTIn autumn 1997, a 2.2 km-long, 4 m-high snow fence was constructed east of the coastal village of Barrow, Alaska. A large drift develops each winter on the downwind side of the fence, and a smaller drift forms upwind. To monitor the thermal impact on ice-rich permafrost, nine monitoring sites were installed near the fence in 1999 to measure soil temperature at 5, 30, and 50 cm; an additional three sites were located in the undisturbed tundra as a control. Maximum thaw and snow depth were measured annually. The results of the 6-yr study indicates that soil temperatures beneath the drift are 2 to 14°C warmer than the control in winter due to the insulting effects of the snow. Since the drift persists 4 to 8 wk after snow has disappeared from the undisturbed tundra, soil thaw is delayed and soil temperatures in summer are 2 to 3°C cooler than the control. The mean soil temperature over the 6-yr period of record has warmed 2 to 5°C, and the upper permafrost has thawed. The ground surface has experienced 10 to 20 cm of thaw subsidence in many places, and widespread thermokarst is apparent where snow meltwater ponds. Both direct soil warming and the indirect effects of ponding contribute to local permafrost destabilization.

Numerical simulation of frost heave with coupled water freezing, temperature and stress fields in tunnel excavation

Artificial ground freezing is an effective ground improvement technique to deal with diverse geotechnical construction problems as it serves to cut off the water and also improves the ground soil strength. However, ground freezing may produce frost heave and thaw settlement at the ground surface. Predicting and controlling the frost heave is a challenge to engineering construction in a heavily populated city. This paper proposes an analysis model that couples the water freezing, temperature and stress fields. This model is first applied to an underground excavation problem of a corridor where ground freezing is used. The numerical predictions are compared with field measurements. It is then applied for a model tunnel problem to study the effect of the overlying soil thickness, frozen soil wall thickness, excavation radius and brine temperature on the frost heave. It is found that the vertical component of the frost heave follows a normal distribution with a maximum at the tunnel axis, while the horizontal component reaches a maximum at a distance from the tunnel axis. This distance is directly proportional to the thickness of the overlying soil. A critical brine temperature is also found at which the frost heave at the ground surface reaches a maximum.

Site Characteristics and Plant Community Development Following Partial Gravel Removal in an Arctic Oilfield

This paper describes the results of a revegetation experiment involving partial removal of gravel fill followed by various revegetation treatments on five sites in the Prudhoe Bay Oilfield on Alaska’s Arctic Coastal Plain. Gravel fill was removed to a residual thickness of approximately 25 cm. Revegetation treatments were transplanted tundra plugs and fertilizer; seeding with indigenous graminoids and fertilizer; seeding with native-grass cultivars and fertilizer; fertilizer only; and no treatment. We monitored surface stability, soil characteristics, and vegetation response from 1990 to 2003. Thaw settlement of 17–40 cm occurred over most areas (with >1 m over areas with ice wedges) between 1990 and 1997; sites had mostly stabilized by 2003. Soil properties important for plant growth generally were poor. The establishment of vegetation dominated by indigenous species was similar when adding only fertilizer as compared to also adding plant materials. Although total live vascular cover was similar among fertilized, tundra-plug transplant, and indigenous graminoid seed treatments (26.1–38.3%), species richness was highest for the indigenous graminoid seed and tundra-plug transplant treatments. The results from this study will drive decisions about planting and fertilization schemes for future North Slope rehabilitation projects.

Influence of Density Inversion on Thawing Around a Cylinder in a Frozen Saturated Porous Medium

An analytical study is made of the convective flow field produced when a warm cylinder maintained at a fixed temperature above freezing is buried in saturated frozen porous medium. The flow field is shown to have a double cell pattern due to the density inversion of water at ~ 4°C, with downward convection of heat dominating at cylinder temperatures of below ~ 10°C and upward heat convection dominating at temperatures greater than this. The analysis uses a perturbation technique to determine the first-order convective correction to the flow and temperature fields around the cylinder for a quasi-static case. It demonstrates that the porous medium permeability and the cylinder temperature are the dominant factors in determining the point at which convection heat transfer becomes significant, with convection expected to be insignificant for Darcy permeabilies lower than 10−5 m/s. The analysis also gives an indication of the rates of thawing occurring in different directions without resorting to numerical methods. The practical implications of a thawing pattern significantly different to that predicted by conduction theory only are discussed briefly with respect to the problem of differential thaw settlement of arctic pipelines.

Movement, moisture and thermal conditions at a turf‐banked solifluction lobe, Kluane Range, Yukon Territory, Canada

AbstractA turf‐banked solifluction lobe was instrumented for the continuous recording of surface and internal movement, thaw settlement, soil volumetric moisture content and thermal conditions during the spring and summer of 2002. Strain probe measurements showed that gelifluction occurred as a series of discrete and abrupt displacements near the thaw plane, followed by retrograde movement. Most gelifluction events took place when thaw rates were high and the soil was saturated almost to the surface, but rain was also capable of inducing gelifluction at depth. The rapid and localized character of the observed displacements suggests that gelifluction results from micro‐shearing of the soil, possibly along the interface of thawing ice lenses. Copyright © 2005 John Wiley & Sons, Ltd.