Temperature study of railway embankments on ice-rich permafrost in Yakutia

Stepan Varlamov

(Melnikov Permafrost Institute SB RAS，Yakutsk，677010，Russia.)

0 Introduction

The Tommot to Niznhy Bestyakh railway traverses ice-rich permafrost(ice contents up to 0.7～0.8 expressed in fractions)between kilometer posts 692 and 734[1，2].This ice-rich permafrost is referred to in the Russian permafrost literature as“ice complex”.Solovyev[3]defines the ice complex as a distinct horizon saturated with ice wedges，which occurs as an extensive，more or less single veneer and is non-uniform in age，composition，origin and thickness.

The ice complex presents major difficulties for the construction and maintenance of the railway.These problems were discussed at several conferences held in Yakutsk[4-5]，leading to the decision to establish a permafrost and geotechnical monitoring program in the railway section over ice-rich permafrost as a testing area for railway engineering in extreme environments.Additional mitigation techniques to protect the underlying permafrost to ensure the roadway stability will be implemented after the first few years of operation.In 2007，the Melnikov Permafrost Institute began monitoring investigations of the thermal regime of the ground along the roadway and in the adjoining terrain to study the effects of railway construction and operation.

1 Roadway construction and the monitoring program

In 2007-2010，eight cross-sections were established for roadway-environment monitoring at kilometer posts(KP)692.6，693.2，693.4，708.7，708.8，708.9，717.5，and 717.9.Boreholes for temperature observations beneath the future embankments were drilled and instrumented after right-of-way clearing，except at KP 717.9 where drilling and instrumentation was made after construction of a zero embankment.Sites in the adjacent areas(clearing and forest)were established at the time of embankment construction.The monitoring program focuses on embankments and underlying soils within the depth of 5～10 m.Ground temperatures are measured with MMT-4 semiconductor thermistors mounted on cables，with the accuracy of ±0.1℃.The thermistor cables in the boreholes and their extensions were cased in polypropylene tubing(Fig.1).

Fig.1 Placing polypropylene tubes to lead thermistor cables out of the roadbed

Table 1 gives the start and end dates for right-ofway clearing，embankment placement and track laying.It should be noted that embankment construction was performed year round.

During the warm season of 2010，rockfill embankments of different designs were constructed at kilometer posts:

KP 692.4 embankment in the cut;

KP 692.6—7 m high embankment with thermosyphons on the berms to a depth of 4 m;

KP 693.2—2.5 ～3.0 m embankment，Penoplex insulation boards 5 cm thick and 8 m wide beneath the embankment and berm;

KP 693.4—7 m-high embankment;

KP 708.7—2.5 m-high embankment，sub-vertical cooling slopes;

KP 708.8—2.5～3.0 m-high embankment with overhanging snow sheds/solar screens on the side slopes;

KP 708.9—2.5 m-high embankment，thermosyphons on the berms，Penoplex board insulation below the embankment and berm;

KP 717.5—2～2.5 m-high embankment with partial removal of the active layer soil;5-m berm on the left side and up to 3 m berm on the right side of the embankment;corrugated pipes beneath the berms and embankment for drainage and soil refrigeration;

KP 717.9—2.2 m-thick zero rockfill with the active layer soils replaced.

Table 1 Start and end dates of construction operations

Results of monitoring investigations are used to evaluate changes to the thermal regime of the ground caused by construction and operation of the railway.This paper analyses the effects on the ground temperature regime beneath high，low and zero embankments.

2 Results

At KP 692.4，the ground temperature regime sharply changed toward warming after embankment construction in the cut section.Ground temperatures show a warming trend on the left side of the cutslope and a decreasing trend on the bank.Comparison of temperature data taken during geotechnical investigations in 2005(B-281/05)and after construction of the embankment in 2013 shows that ground temperatures at depths of 2.5，5 and 10 m have increased by 4.3，1.7 and 1.1℃ in the left side of the cutslope and by 1.1，1.0 and 0.5℃ below the bank to the left(Fig.2).In 2013，seasonal thaw depths determined from temperature measurements were 0.8 m in the forest，2.3 m in the bank and 3.3 m in the left side of the cutslope，indicating permafrost degradation in the adjacent cut area.

At KP 700.5，a borehole was drilled in 2011 at the bottom of the cut.Here the rockfill is 2 m in thickness.Ground temperature at 10 m depth was －1.2℃.Seasonal thaw depths in 2012 and 2013，determined from temperature measurements，were 2.2 and 3.3 m，respectively.Temperatures at depths of 5 and 10 m were－1.0 and－1.1℃in 2012 and－0.5 and －1.0℃ in 2013.A geotechnical investigation in 2006 indicated that natural ground temperatures at these depths in the forest stand(KP 700.2，B-4/06)were －2.4 and －2.1℃，respectively.Thus，permafrost temperatures at 5 and 10 m depths beneath the cut increased by 1.8 and 1.3℃over the 7-year period(see Fig.2).

Fig.2 Temperature profiles for the undisturbed area(B-281/05;B-4/06)，cutslope(B-4/12)，soil bank(B-5/12)and cut bottom at KP 692.4，700.2 and 700.5

At KP 692.6 in larch wetland，the ROW was cleared in the spring prior to snowmelt.In September 2007，the seasonal thaw depth was 1.02～1.12 m，while ground temperatures at depths of 1.5，3.0 and 5.0 m varied from －0.3 to －0.6，from －1.3 to －1.6 and from －1.9 to －2.7℃，respectively.In winter 2007/08，ground temperatures at these depths were －2.6 to －3.4，－2.0 to －2.5，and －1.5 to－2.4℃，respectively.By the end of the thaw season，thaw depth was 0.1～0.2 m greater than year before，while ground temperatures at 3 and 5 m depths were warmer by 0.1 and 0.2～0.3℃，respectively.In April 2009，builders began to place embankment to a height of 1.5～2.0 m.Ground temperatures at 3 and 5 m depths cooled by 3.1～3.6 and 1.0～1.9℃compared to the previous winter.By the autumn，the permafrost table had risen by 1.0 m beneath the embankment centerline and by 0.5 m beneath the berms.Ground temperatures at depths of 1.5，3.0 and 5.0 m beneath the embankment were －1.4 to －1.5，－2.4，and －2.4 to －3.2℃，respectively.After completion of a 7-m-high embankment in 2010，a thaw layer 0.3 to 0.5 m in thickness was still present，although ground temperatures tended to lower.The strong cooling effect in the first year was likely due to placing embankment in winter on frozen ground，while the loss of this effect in the second year could be due to the warming effect of the high embankment during the previous summer.Temperature readings indicate that the permafrost beneath the centerline of the 7-m-high bermed embankment has risen and aggraded into the fill during the period of 2010～2013(Fig.3).The underlying ground exhibits a cooling trend.In the end of the warm season of 2013，ground temperatures at 0.2 and 1.5 m beneath the centerline were －1.1 and－1.3℃，respectively.It should be noted that the underlying ground at 1.5 m depth in the end of the warm season of 2010 had a minimum temperature value equal to －1.7℃.Beneath the right berm with a height of 1 m，the ground temperature shows a stronger cooling trend at the expense of winter cold accumulation.In the end of the warm season of 2013，ground temperatures at depths of 1.5 and 5.0 m were－1.6 and －3.0℃.Thus，the high embankment has，on an annual basis，a minor cooling effect on the underlying ground，while the low berm has a stronger effect.In 2013，the upper permafrost was at 0.9 m depth beneath the right berm.Fluctuations in permafrost table(0.5 to 0.9 m)have been observed to occur in response to interannual variations of meteorological elements(air temperature，snow accumulation regime，etc.).

Fig.3 Changes in temperature isotherms and permafrost table beneath the embankment at KP 692.6

At KP 693.2，the ROW was cleared in the spring before snowmelt.On 16 September 2007，the seasonal thaw depth was 0.72～0.92 m，and ground temperatures at depths of 1.5，3.0，and 5.0 m were within－0.5 to －0.8，－1.2 to －1.4，and －1.8 to －2.0℃，respectively.In April 2008，ground temperatures were －1.1 to －3.4，－1.1 to －2.2，and －1.4 to－1.9℃.By the end of the thaw season，thaw depth was 0.21～0.42 m deeper than year before，while ground temperatures at 3 and 5 m depths were warmer by 0.1～0.2 and 0.2～0.3℃，respectively.The lower intensity of cold accumulation compared to the larch wetland site(KP 692.6)is explained by the later closure of phase boundaries due to the increase in thaw after clearing.In the winter of 2008/2009 Penoplex extruded polystyrene foam boards were installed in the embankment and berm during construction.This had a notable cooling effect in the upper 4～5 m of the underlying ground，with temperatures at depths of 0.2，1.5，3.0 and 5.0 m lowered to within －7.8 to－8.9，－4.9 to －6.8，－2.5 to －3.4，and －1.5 to－1.6℃，respectively.By the end of summer 2009，temperature readings indicated a rise of the permafrost table by 1 m at the center of the embankment and by 0.5 m beneath the berm slopes.In the winter of 2009/2010，ground cooling varied across the embankment foundation.Temperatures at depths of 0.2，1.5，3.0 and 5.0 m were －6.0，－3.4，－1.9 and－1.6℃ beneath the centerline，－ 8.6，－ 6.4，－4.2 and －2.7℃ beneath the right berm slope，and －4.6，－2.8，－1.3 and －1.4 ℃ beneath the left berm slope.The ground temperature at 5 m depth beneath the right berm was 1.2℃colder compared to the previous winter，while beneath the centerline and left berm the temperatures were warmer by 0.1 and 0.2℃，respectively.

This was likely due to ponding of suprapermafrost water by the left berm and the insulating effect of Penoplex boards at the centerline.By the end of the summer，the permafrost table beneath the embankment centerline and berm slopes was at the same level as year before.Ground temperature in the 5-m layer was 0.1～0.3℃colder than in the previous season beneath the right berm，while beneath the left berm and the centerline the temperatures increased by 0.1～0.8℃ and 0.1℃，respectively.In winter 2010/2011，passive cooling devices were installed on the berms.From 2010，ground temperature beneath the centerline shows a clear cooling trend.More notable cooling is observed beneath the shaded berm on the right.Ground cooling is weaker beneath the more exposed berm on the left(Fig.4).In winter 2011，seasonal frost beneath the centerline penetrated to the permafrost table only by the end of the season，resulting in lesser cold accumulation compared to the left and right berms.Ground temperatures within the 5-m layer were－1.0 to －1.5 beneath the centerline，－2.0 to－4.3 beneath the left berm，and －3.4 to －10.1℃beneath the right berm.In summer 2011，virtually no changes in the ground thermal regime were observed.It should be noted that ground temperature at 0.2 m depth beneath the left berm slope shows an increasing trend due to the effect of suprapermafrost water of the active layer.

Fig.4 Changes in temperature isotherms and permafrost table beneath the embankment at KP 693.2

Penoplex polystyrene insulation has been extensively used in roadway construction over the ice-rich permafrost.Temperature observations in boreholes beneath the berms at KP 693.2 and 693.5 have shown that the insulating effect depends on depth of placement.With insulation placed at a depth of 0.7 m at KP 693.5 the depth of seasonal thaw beneath the berm was 1.70 m，while placement at 1.1 m(KP 693.2)resulted in thaw depth of 1.95 m.Three-year observations in boreholes located at varying distances from thermosyphons indicate that their cooling effect is strong within a 0.5 m radius(B-18)and is sharply reduced at a distance of 1.3 m(B-9)(Fig.5).At KP 693.5 the permafrost table beneath the berm has risen to Penoplex placement level(0.7 m)，whereas at KP 693.2 it has remained at the same level(1.95 m).

Fig.5 Temperature profiles in and beneath the berm at KP 693.2(B-9)and KP 693.5(B-18)

At KP 693.4，effects on the ground temperature regime beneath the 7-m-high embankment are different from those at KP 693.2.Here the soils of the active layer are wetter because of the lower elevations with greater slopes.The range of temperatures in the winter and summer were greatest due to these factors.Seasonal thaw depth was 0.90～1.03 m in the first season after right-of-way clearing and was 0.04～0.24 m deeper in the second season.By 23 September 2007，ground temperatures at depths of 1.5，3.0 and 5.0 m were within －0.4 to －0.6，－1.2 to －1.6 and－1.9 to －2.2℃，respectively.Temperatures increased by 0.1～0.2℃ by the autumn of 2008.After placement of a 3 ～4 m embankment，a notable，albeit non-uniform，cooling occurred in the underlying soils in winter 2008/09.On 2 April，ground temperatures at depths of 0.2，1.5，3.0 and 5.0 m were －9.8，－6.2，－3.0 and －2.0 ℃ beneath the centerline，－6.5，－4.6，－2.8 and －2.1℃ beneath the right sideslope，and －13.4，－10.3，－10.8 and －3.7℃ beneath the left sideslope.This difference is explained by the embankment construction procedure.Temperature data for 11 September 2009 indicate that the permafrost table has risen by more than 1 m beneath the centerline，about 1 m beneath the right lower sideslope and less than 0.5 m beneath the left sideslope.Ground temperatures at depths of 1.5，3.0 and 5.0 m vary within －0.9 to －1.4，－1，5 to －2.2，and－2.0 to－2.5℃.

In the second winter season，ground temperatures at 0.2，1.5，3.0 and 5.0 m depths beneath the centerline were －6.9，－4.3，－2.9 and －2.4℃，respectively.Warmer temperatures compared to the first season are explained by the attenuation of temperature variation with depth，i.e.，completion of the embankment to the design 7 m height.In the right lower slope area，ground temperatures at respective depths in the first winter season were －14.0，－11.3，－8.1 and－5.3℃.Ground cooling was stronger than at the centerline.In the left lower slope area，cooling was weaker(－4.3，－2.3，－1.3 and －1.5℃)due to accumulation of suprapermafrost water along the embankment and latent heat effects in refreezing of the active layer similar to KP 693.2.In September 2010，ground temperatures were 0.1～0.3℃colder beneath the centerline，0.6～1.3℃ colder in the lower part of the right sideslope and 0.1～0.2℃warmer in the lower part of the left sideslope as compared to the previous season.In winter 2011，no cold accumulation occurred beneath the centerline，and a tendency toward warmer temperatures was observed.A similar pattern was observed beneath the left sideslope.Beneath the right sideslope，on the contrary，cold accumulation was notable，albeit less than in the previous winter.Seasonal thaw depth by the end of summer 2011 was nearly the same(1.10 m)beneath the right sideslope，whereas beneath the left slope it was deeper than 1.80 m.Ground temperatures at depths of 0.2，1.5，3.0 and 5.0 m were warmer as compared to the previous season，with the values of －0.9，－1.1，－1.7 and－2.1 ℃ beneath the centerline，－0.1，－1.2，－2.0 and －3.1℃ beneath the right sideslope，and 1.1，－0.7，－1.2 and －1.5 ℃ beneath the left sideslope.

In 2013，ground temperatures beneath the embankment centerline showed no significant changes compared to the previous year.It is however evident from temperature data that the permafrost table was raised above the original surface into the embankment(Fig.6).By the end of the warm season of 2013，ground temperature at 0.2 m depth beneath the embankment was－0.9℃.Soils beneath the right sideslope showed a notable cooling trend，whereas beneath the left sideslope no changes were observed due to high moisture contents of the active layer.Ground temperature at 0.2 m in the end of the warm season of 2013 was－0.1 beneath the right sideslope and+0.3℃beneath the left sideslope.This confirms our earlier conclusion about the warming effect of surface water on the ground temperature regime in this section.

At KP 708.8，seasonal thaw depths following ROW clearing in 2008 were 1.05 to 1.29 m，or 0.25～0.49 m deeper than in the forest.The soils were 0.5～1.0℃ warmer compared to the forest site，with temperatures of－0.3 to －0.5，－0.8 to －1.0，and－1.2 to －1.3℃ at depths of 1.5，3.0 and 5.0 m，respectively.In April 2009，cold was accumulated only within the upper 4 m of the ground.Temperatures were －2.6 to －3.3，－0.9 to －1.3，and －1.1 to－1.3℃ at these depths.A 2.5 m of embankment fill was placed in late August 2009.In early September，ground temperatures were high，with values for 0.2 and 1.5 m depths of 4.0 and 0.6℃below the centerline，－0.5 and －1.2℃ beneath the lower right slope，and －0.4°and －1.2℃ beneath the left sideslope.The ground temperature regime in winter 2009/2010 after completing the embankment with wooden snow sheds，about 1 m in width，on the sideslopes was as follows.Ground temperature beneath the centerline almost did not cool(－0.1℃)at 1.5 m depth，and at 0.2 m the temperature decrease was only to－0.7℃.Hence，the freeze front did not penetrate into the permafrost.Seasonal frost reached the permafrost in the lower slope area，but cold accumulation was insignificant and ground temperature lowered only by 0.4～1.4 ℃ compared to the autumn values.

Fig.6 Changes in temperature isotherms and permafrost table beneath the embankment at KP 693.4

During the first two winters，snow sheds on the embankment sideslopes did not provide sufficient cooling at the embankment toes.Near-surface(0.2 m)ground temperatures at the end of the first winter were－2.8 beneath the right slope and －4.5℃beneath the left slope，which were significantly higher compared to the sitesdiscussed above.Near-surface ground temperature beneath the right slope lowered to－3.9 ℃ in the second winter.In summer 2011，ground temperature within 1.5 m beneath the centerline did not fall below －0.2℃;the depth of thaw 2.31 m at the left berm toe and 1.18 m at the right berm.In 2013，ground temperatures at 0.2 and 1.5 m depths beneath the centerline were－0.3 and －0.4℃，respectively.The permafrost table was almost at the original ground surface.Slight cooling was observed beneath the right sideslope(Fig.7).The permafrost table lowered 0.3～0.5 m below the natural ground surface.Beneath the left slope the permafrost table is at 1～1.3 m depth.Ground temperatures beneath the sideslopes show little change.In 2011，thaw depth on the 2 m rock bank to the left of the embankment was about 2.2 m.Ground temperature at 10 m depth was－1.5℃.In 2012 and 2013，thaw depths estimated from temperature measurements were 2.7 m and 2.6 m，respectively，while ground temperature increased to －1.1℃.At the toe of the left sideslope，seasonal thaw depth was 2.2～2.3 m and ground temperature was 1.2℃，indicating permafrost degradation in this area.

At KP 717.9，temperature boreholes were drilled and instrumented in late August 2009 after removal of the active layer and placement of fill material.Drilling showed that the depth of thaw beneath the 2.2 m rock-fill was 2.5～3.0 m from the fill surface.This provided favorable conditions for talik development in the underlying ground，which is confirmed by temperature data.Temperature at 1.5 m in the fill lowers to －5.0℃ in winter and rises to 4.3～7.1℃ in summer.Permafrost at 5 m depth remains persistently warm(－0.8 to－1.5℃).The 0℃ isotherm is estimated to be at 2.8～3.2 m and seasonal frost seems not to penetrate into the permafrost.By the end of the thaw season in 2013，ground temperatures at 5 m were －0.8 and －1.5℃ (Fig.8).

Fig.7 Changes in temperature isotherms and permafrost table beneath the embankment at KP 708.8

Fig.8 Changes in temperature isotherms and permafrost table beneath the embankment в at KP 717.9

3 Conclusions

1)Ground temperature observations during construction and temporary operation of the railway revealed the initiation of ice-rich permafrost degradation on the cut slopes and ditches in the cut sections.

2)During winter placement of fill material，the permafrost table was raised，the rate of rise being greater for higher embankments.The placement of a zero embankment in summer with removal of the active layer resulted in the development of a perennial thaw bulb.

3)After roadbed completion，the lower embankments have，on an annual basis，a stronger cooling effect on the underlying ground as compared to the higher embankments.

4)Thermosyphons and Penoplex insulation were found to significantly lower ground temperatures beneath the shaded right berm(downhill side)，whereas beneath the more exposed left berm(uphill side)they have had a minor effect.The deeper is Penoplex placement，the lesser is its insulating effect.The cooling effect of thermosyphons is strong within a 0.5 m radius and is sharply reduced at a distance of 1.3 m.No expected benefit has been found from the snow sheds installed on the sideslopes of the 2.5-3 m high embankment.

5)Soils beneath the shaded right sideslopes(downhill side)exhibit a cooling trend.Beneath the more exposed left sideslopes(uphill side)the permafrost table was lowered due to the warming effect of surface water.

6)Seasonal thaw penetration into the embankment depends on rockfill thickness.The thicker the fill，the deeper is seasonal thaw.

7)Placing additional thermosyphons is recommended to prevent ground warming and progressive thawing at the more exposed left side of the roadbed.Reductions in ground warming and thawing can also be achieved by periodic snow removal or compaction which promote intensive freezing of the active layer and cold accumulation in the underlying permafrost.

8)Instrumental and visual observations of the roadbed during temporary railway operation revealed sections subject to dangerous deformations.Annual roadbed inspection is therefore recommended along the entire route.

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