Problems and countermeasures in construction of transmission line projects in permafrost regions

GuoShang Wang ,QiHao Yu ,YanHui You ,Ze Zhang ,Lei Guo ,ShiJun Wang ,Yong Yu

1.Gansu Electric Power Design Institute,Lanzhou,Gansu 730050,China

2.State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

3.Gansu Power Transmission Transformation Company,Lanzhou,Gansu 730050,China

1 Introduction

As one of the key projects of the China Western Development Program,the Qinghai-Tibet DC Interconnection Project includes the Xining-Golmud 750 kV transmission and transformation subproject,the Golmud-Lhasa ±400 kV DC transmission subproject,and the Central Tibet 220 kV grid project.The establishment and operation of the Qinghai-Tibet DC line project marks another form of important frozen soil engineering following the railway and highway projects in the permafrost regions on the Qinghai-Tibet Plateau.Unlike the highway and railway projects,the Qinghai-Tibet DC line is a type of point-line project,which is made up of a series of tower foundations.Accordingly,there are some differences in the interaction between foundations and frozen soil,and the type and the severity of secondary engineering diseases or disasters (Wenet al.,2012;Yuet al.,2012).Although there are many frozen-soil engineering activities like highway and railway constructions and corresponding researches,as well as fruitful achievements both at home and abroad,precedents in solving many key problems in transmission line project construction in permafrost regions and the application of related project management approaches are still absent in China (Zhanget al.,2013).Therefore,a review of the status and challenges of transmission line constructions in permafrost regions in the world,and relevant studies on the problems and engineering countermeasures,as well as a timely summary of the construction experiences and practices of the Qinghai-Tibet DC line project,are of great significance for economic,reasonable,and successful completion of transmission lines in such kinds of permafrost regions in the future.

2 Status of transmission line construction in permafrost regions

In the Northern Hemisphere,permafrost covers 22,790,000 km2(Zhanget al.,2000).In Russia,68%of its territory is permafrost;in Canada,50%;in China,22.4%;and 80% in Alaska in the United States (Zhouet al.,2000).Because the giant permafrost regions are in the remote north in Canada and the United States,where human activities are scarce,there are few power project constructions in those areas (Fortier and Allard,2004).Only a few have been constructed in the fringes of the permafrost regions or in deep seasonally frozen regions.In contrast,the former Soviet Union began to build transmission lines in its permafrost regions in the 1960s due to economic development and amenable permafrost distribution features.

Fairbanks,located in the mid-Alaska permafrost region,is an important city in the state with a U.S.Air Force base and rich mineral resources,where engineering activities are frequent.The transmission lines that service Fairbanks generally span southern seasonally frozen ground to the middle permafrost region(Wyman,2009).One transmission line built in 1966 from Healy (a port in southern Alaska) to Fairbanks operates at a voltage of 138 kV and runs 103.2 km.Recently,as a part of the Northern Intertie Project,the Alaska Healy-Fairbanks 230 kV transmission line commissioned by the Golden Valley Electric Association,was commenced in winter of 2001 and was completed in spring of 2003 (Wyman,2009).The other major transmission lines in Alaska’s seasonal frozen regions are the 115 kV line connecting Anchorage and Kenai and the 170-mile 345 kV Healy-Willow line,which was built and put into operation in 1985.

In Canada,the well-known Hydro-Québec power transmission system stretches more than 32,000 km from Canada’s Québec Province to the northeastern United States.The line,735 kV AC,was completed and put into operation in 1965,and some sections are located in the fringe of permafrost regions and deep seasonally frozen regions (U.S.Arctic Research Commission Permafrost Task Force,2003).

Most of Russia’s territory is located in permafrost regions,and there are rich mineral and oil-gas resources.Driven by the needs of the Siberia Development Campaign in the 1960s,the former Soviet Union started the construction of transmission lines.The transmission lines are characterized by their large scale and variable foundation types.Russia’s state power grid is mainly composed of its east,west,south,north,and central grids (Moscow United Electric Grid Company,2014).Since permafrost is mainly distributed in east-central Russia,the east,north,and central power grids are located in permafrost regions,and the east grid covers the largest permafrost area.The east grid ranges from Irkutsk,one of the largest cities in Siberia,to the right bank of northern Russia’s Angara River,the west bank of Lake Baikal,and the Lena River Basin.It is more than 400 km away from the city,and almost the entire grid is within permafrost regions.The north grid,constructed since the 1960s when the Siberia Development Campaign was waged,now spans more than 86,000 km (Irkutsk Electric Grid Company,2014).After transformation and upgrading since 1975,the north grid now has up to 644 km of 500 kV lines and 220 km of 220 kV lines in total,and all of them are within permafrost regions.The central grid had reached a certain scale in 1964,and now spans 7,000 km in various transmission lines;most of the 220 kV and 500 kV transmission lines completed in 1998 are located in permafrost regions (Irkutsk Electric Grid Company,2014).

In China,the early construction of transmission lines started in the northeastern permafrost regions,and the most recent ones include the Qinghai-Tibet Railway 110 kV Transmission Line built in 2005(Wei,2009) and the Qinghai-Tibet DC Interconnection Project completed and put into operation in September,2011.The latter project is considered especially symbolic of construction in permafrost regions (Yanet al.,2012).In addition,construction on the new Yushu-Qinghai Main Grid 330 kV transmission line commenced in 2012 (Chenget al.,2013).

Clearly,China is second only to Russia in terms of large-scale transmission line construction in permafrost regions.However,compared with Russia,the engineering construction is more challenging in China due to the permafrost features of high temperature,poor stability,and susceptibility to environmental changes.

3 Interaction between permafrost and foundations,and their problems in permafrost regions

The mechanical properties,stress-strain relationships,and dynamic response characteristics are distinctively different between frozen soils and thawed soils (Qi and Ma,2010).Marked creep occurs in permafrost with high temperature and high ice content due to the existence of unfrozen water and ice plasticity (Andersland and Ladanyi,1994).Therefore,the developed permafrost zones are mostly undesirable for engineering geology.Secondary engineering problems,such as frost heave and thaw settlement,often challenge project construction and operation(Wuet al.,2002b;Maet al.,2009).The frost-heave deformation of tower foundations is closely related to surface temperature in cold seasons and the depth of the active layer.The lower the surface temperature in the cold season and the deeper the active layer,the greater is the frost-heave deformation of tower foundations (Cheverevet al.,2006).Numerical simulations have shown that,due to the thermal inertia of permafrost with high ice content,the thaw depth of ice-poor permafrost near tower foundations is greater than that of ice-rich and ice-saturated permafrost (Liet al.,2013).Also,foundation types,stress conditions,slope topography,and groundwater level have significant influences on the stability of tower foundations in permafrost regions (Suet al.,2013).

The major problems in transmission line engineering are the impacts of frost heave and thaw settlement,frost lifting,harmful frozen ground phenomena,global warming,and permafrost degradation (Yuet al.,2009).The engineering practices in the permafrost regions in North America and Russia show that frost heave is the primary problem threatening the safety of transmission lines in permafrost regions.Alaska’s 138 kV transmission line in deep seasonally frozen regions employed 10-m-long timber piles as foundations.Where soils were susceptible to frost heave,in surface and ground water development zones the piles were lifted 1–2 m by frost-heaving force,and this caused enormous maintenance expenses in the operation period (Lyazginet al.,2004b).In-situ monitoring results have shown that frost lifting of the pile foundations of transmission lines is the major threat to transmission line operation in northern Tyumen,Russia.The uneven frost heave of transmission line foundations in freezing periods can reach 5 cm,and the maximum frost heave can be up to 20 cm.After 20 years of operation,the monitored maximum deviation of the towers is 2.5–2.7 m in northern Tyumen (Lyazginet al.,2003).

In northwestern China,transmission lines including the 110 kV Longren Line,the 20 kV Qirang Line,and the Erhuo Line in Daqing have experienced frost-heave-induced accidents like tower and pile collapse due to foundation instability.Another example is the Halar-Yakeshi 220 kV transmission line,which was completed and put into operation late in 1997.Until 2003 the tops of the cast-in-place piles of the N29 tower near Dongdapaozi inclined and broke the joints of the binding beam and pile body due to frost heave,which severely affected the normal operation of the transmission line (Jiang and Liu,2006).

Therefore,project constructions must address changes in the permafrost environment that can induce and intensify frost heave,which would threaten the stability of transmission line foundations.In the Qinghai-Tibetan DC Interconnection Projection,the cylindrical cone foundation design has been applied to reduce the effects of frost heave (Figure 1).

Figure 1 Cylindrical cone foundation of an electric transmission tower under construction

Permafrost features such as poor heat stability,strong hydrothermal activity,and high ice content make cold-region engineering projects extremely susceptible to environmental changes (Haoet al.,2010;Yang and Cheng,2011).Thaw settlement by permafrost degradation and ground ice melting can severely affect the stability of foundations of highways,railways,and so forth in permafrost regions(Wuet al.,2002a;Cheng and Ma,2006).The freezing force of transmission line foundations and the bearing capacity of the pile tips are the most important bases for maintaining the stability of tower foundations in permafrost regions (Chenget al.,2004).Permafrost degradation and melting inevitably affect the stability of transmission line projects.The construction practices in northern Tyumen in Russia have shown that the melting of permafrost under the seasonally thawed layer has a great impact on a foundation’s stability,and it may reduce the bearing capacity of and cause significant damage to tower foundations (Yanet al.,2012).Permafrost is degrading with climate warming,increasing desertification of natural environments,and engineering activities (Zhaoet al.,2010),and this leads to harmful frozen ground phenomena such as thaw settlement,subsidence,and icing.All of these give rise to decreases in the mechanical and thermal stability of permafrost (Wuet al.,2005),and thus affect the stability of tower foundations.

In a point-line project,careful adjustment and selection of the tower locations can reduce the disastrous impacts of freeze-thaw cycles on the stability of the tower foundations to some extent (Qianet al.,2009).Thus,investigating the laws of permafrost development and distribution under local factors can,to a great extent,avert freeze-thaw disasters at the stage of route selection.The major factors influencing the route selection and tower foundation location include geological environment,harmful frozen ground phenomena,differing geomorphic units,thermal stability features,and construction convenience (Liuet al.,2008;Chenget al.,2009;Qianet al.,2009).

4 Analysis of transmission lines tower foundation types

The engineered stability of transmission lines mainly relies on the interaction between foundations and permafrost,and the foundation type plays an important role in this interaction.Therefore,selecting appropriate foundation types can reduce the harm of frost heave in permafrost regions (Guan and Wu,2010).Single-pile foundations with shallow embedded depth are applied in low-voltage transmission lines in the United States and Canada.For example,on the Ontario Victor Mine Transmission Line Project in Canada,a 115 kV single-circuit line employs separate upright wooden poles with steel cantilevers,and the average span of poles is 150 m.The wooden poles were installed in the rotating holes with a depth of 2.4–2.7 m,and the excavated material and particulate packing were backfilled.In the unstable areas,stays and logs were used for reinforcing the foundations,while steel nails were used in areas with rocky ground.High-voltage transmission lines mainly employ inserted piles and cast-in-place piles.The depth of such piles is about 10 m to 30 m,depending on the voltage class,permafrost type,line location,and frost lifting strength (Guryanov,1998).

Figure 2 Foundation types selected by Qinghai-Tibet DC Interconnection Project:(a) dug foundation;(b) precast foundation;(c) cylindrical cone foundation;and (d) bored piles

In Russia,steel-pipe piles or stainless-steel screw piles with a depth of 5-9 m are used for the foundations of 6–10 kV transmission lines,and the same foundation structure but with a larger diameter (630 mm or 720 mm) are used for the 35–110 kV and 220 kV transmission lines.When the pile foundation is 10–12 m deep,impact or vibration piling or steam-hammer piling is used to install piles in permafrost (PKB Company,2014).The high-voltage transmission line in the north Tyumen Oblast in western Siberia,more than 4,000 km long,mainly uses steel pipes with a diameter of 325 mm and a foundation depth of 6–8 m,and in some areas it also employs reinforced concrete piles with a depth of 6–10 m(Lyazginet al.,2004a).Due to strong frost lifting,many of its foundations have deformed substantially.Based on in-depth studies of this problem,researchers have proposed many new types of foundation structures against frost heave,and many are in use already(Cheverevet al.,2006).

In China,transmission lines in deep seasonally frozen ground regions such as Hulunbuir apply combined open-caisson foundations,trapezoid-slope foundations,and cast-in-place pile foundations (Sun and Wang,2009).In the Qinghai-Tibet DC Interconnection Project,the predominant types of tower foundations are shown in figure 2.The selection of tower foundations in permafrost regions mainly follows the following principles:In sections with good geological and permafrost conditions,the foundation can be formed by digging and so a dug foundation is used(Figure 2a).In sections with convenient construction conditions,the precast foundation is used (Figure 2b).In sections with strong frost-heave forces and rich ice,the cylindrical cone foundation is used (Figure 2c).In sections with thick ice and vulnerable permafrost,and rivers,overflowing water,and shallow groundwater,bored piles are employed (Figure 2d).

5 Role and influence of the construction process

The construction process itself may destroy the vulnerable ecological environment in permafrost regions,and warm-season-related problems,such as water development in frozen layers,permafrost thawing,and wetland water accumulation,can influence the construction process.Considering all of these factors,construction in winter is widely undertaken all over the world in permafrost regions,particularly in vulnerable permafrost areas.For example,the Alaska Healy-Fairbanks transmission line was built in winter,even though it was polar night in the Northern Hemisphere.A similar approach was taken in the Qinghai-Tibet DC Interconnection Project,where the construction schedule was adjusted or delayed until mid to late October in areas where serious water gushing due to water development on frozen layers and foundation ditch excavation could occur.In the swampy wetland areas,the construction was scheduled in December.

The construction experience of the Qinghai-Tibet DC Interconnection Project shows that "rapid construction" is also one of the important principles to ensure the success of construction during the winter.In this approach,the construction time of the permafrost foundation is strictly controlled by means of adequate preparation,quick excavation,fast pouring,and timely backfill,which reduces the disturbance of some environmental factors like sunlight reaching the permafrost in the excavated foundation ditches.Advanced techniques can also help rapid construction.For instance,the construction of the Alaska Healy-Fairbanks transmission line resorted to helicopters for lifting in some sections.Application of machines and tools like large-scale rotary drills and heavy cranes in the Qinghai-Tibet DC Interconnection Project ensured the success of the construction.

Because the construction quality has a great impact on the stability of tower foundations,it is necessary to strictly control the pouring quality of concrete foundations (Zhu and Wang,2011).For pouring cast-in-place concrete,the higher the molding temperature,the better the pouring quality.However,this can significantly impact the permafrost temperature field around the foundation.Therefore,the molding temperature of concrete should be controlled as follows:no less than 6–8 °C when the air temperature is above-15 °C,and increasing to about 10 °C when the air temperature is below-15 °C (He,2008;Maet al.,2013).The backfilling quality of foundation ditches also influences the foundation stability.The compactness of the unfrozen coarse-grained soils for layer-by-layer tamped backfilling of large excavated foundations should be not less than 80% (Ganet al.,2011).

6 Research on engineering measures

A study by Duan and Naterer (2004) on the freeze-thaw process of active layers near transmission line tower foundations based on a metal-bar mold buried in soil showed a remarkable heating effect of metal tower foundations on surrounding areas.The increasing freeze-thaw depth and cycle of the active layers caused adverse impacts on the stability of the tower foundations.To eliminate or relieve the adverse impacts of frost heave and thaw settlement on transmission line construction,many measures are usually applied to ensure the stability.These measures mainly fall into three categories:

1) To eliminate the impacts of tangential frost-heave force on the foundation,measures include lubricating grease on foundation faces,glass-steel formwork,backfill with soil insensitive to frost heave,sloped foundations,increase of foundation embedded depth,burial of non-frost-heave materials,set-up of frost-heave separators,and burial of permeable non-woven geotechnical fabric to separate foundation from surrounding soil (Yu,2002;Wang and Zhang,2004).

2) To protect the permafrost environment,measures like slope drains and vegetation restoration are used.

3) To protect permafrost and to control the frost process,measures like thermosyphons,thermal insulation materials,sun louvers,and some active cooling and heat-stabilizing equipment are used (Wan,2008).

To slow increases in ambient temperature or reduce the impacts of the construction process on permafrost degradation and heat stability,active cooling of permafrost foundations is an important method.For instance,thermosyphons are used in Russia to increase the freezing force of pole-tower foundations and surrounding permafrost,and effectively prevent frost heave in foundations.Numerical simulations have shown that thermosyphons can improve the thermal stability of tower foundations (Kondrat’ev,2004;Wang and Xu,2010).In China,a large amount of thermosyphons are in use in the Qinghai-Tibet DC Interconnection Project.A maximum of 16 thermosyphons were used for one electric tower,where four thermosyphons per foot of tower were installed(Figure 3).Altogether,there are about 7,000 thermosyphons used in this project.

The monitoring results of the freeze-thaw process of modular precast foundations in the Wudaoliang District of the Qinghai-Tibet DC Interconnection Project show that backfill disturbances of permafrost intensified the ground temperature fluctuations and increased the thawing depth of the permafrost,but the degree and scope of influence were limited.Construction of modular precast foundations in winter and retention of appropriate porosity in the freezing-thawing active layers can not only speed up permafrost refreezing,but also improve the compaction by natural solidification and thaw settlement of soils.In warm seasons this weakens the diffusion of heat into foundation depths and keeps foundations frozen (Chenget al.,2012).

Figure 3 Thermosyphons used in the Qinghai-Tibet DC Interconnection Project

7 Environmental protection measures

The ecological environment in permafrost regions is fragile,being extremely susceptible to natural environment changes and engineering activities that cause vegetation degradation,plant community changes,ground ice melting,permafrost table drop,and permafrost thickness reduction (Wanget al.,2004).The ecological environment can also undermine permafrost engineering stability (Wuet al.,2003).Therefore,it is of great importance to provide protection for the fragile ecological environment in permafrost regions during the construction process.

Vegetation in permafrost regions in North America is extravagant and the wetlands are rich.The resulting limits on transport during the construction process and the impacts of construction processes on the environment are the major problems.For these reasons,construction of the Alaska 230 kV Healy-Fairbanks transmission line was scheduled in winter to protect the environment and ensure normal operation.Because there were no actual roads in that primitive area,road sprinklers were used to build snow-ice roads to protect the natural ecology.The roads built in winter would melt and disappear in the warm seasons,and had less impact on the environment (Wyman,2009).

In the Qinghai-Tibet DC Interconnection Project,several methods to protect the natural environment have been adopted in route selection,design,and in the construction stages.For example,at the design stage the route was set to avoid nature reserves and approaches to the Qinghai-Tibet Highway and Railway;at the construction stage,measures like shortening temporary roads and setting up separate protection zones for construction were used to minimize the potential impacts on the ecological environment (Figure 4a).To further protect the permafrost environment,part of the tower foundation construction was scheduled in December (Figure 4b).Construction in winter and vegetation restoration have been particularly effective in reducing the impacts on the ecological environment and ensuring engineering stability (Linget al.,2012).

Figure 4 (a) Setting up separate protection zones for construction (left);(b) tower foundation construction in December in the Fenhuo Mountain region (right)

8 Conclusions

1) Transmission line constructions in permafrost regions have specific characteristics in different countries.In the permafrost giants like Canada and the United States,the permafrost regions are in the remote north where human activities are scarce,so there are fewer power transmission line construction projects.Russia has the longest history of power transmission line construction in permafrost regions,characterized by large construction scale and multiple types of transmission lines.Based on improved engineering practices from the Qinghai-Tibet DC Interconnection Project,China is achieving fruitful results in such projects in permafrost regions.

2) The primary problems that challenge tower foundations in permafrost regions include frost heave,thaw settlement,frost lifting,and harmful frozen ground phenomena.Additional factors like global warming and permafrost degradation increase the uncertainty of these problems.Therefore,all countries have to pay attention to the protection and treatment of permafrost foundations in permafrost construction.

3) The frost-heave deformation of tower foundations is closely related to the surface features in cold seasons,and the temperature and the thickness of the active layers.The mechanical characteristics of frozen soils are highly sensitive to temperature.Considering the complexity of the freeze-thaw process,attention must be paid,during the route selection and investigation,to the analysis of engineering geological conditions such as permafrost distribution,permafrost types,and harmful frozen ground phenomena along transmission lines.This could help to avoid or reduce potentially disastrous impacts of freezing-thawing when transmission lines are put into operation.

4) In most countries,the choice of tower foundation types depends on the voltage class of the transmission lines.Pile foundations with options of inserted piles,inserted cast-in-place piles,and screw piles are widely used.China has rich engineering experience in these key tower foundation types and has achieved good results.

5) To ensure the stability of transmission line tower foundations and protection of the permafrost environment,engineering measures like thermosyphons are effective in reducing freezing-thawing hazards.Also,construction in winter is a common practice in permafrost regions,particularly in vulnerable permafrost areas.

This work was supported by the National Key Basic Research Program of China (973 Program) (No.2012CB026106),the Program for Innovative Research Group of the Natural Science Foundation of China (No.41121061),and the fund of State Key Laboratory of Frozen Soil Engineering (No.SKLFSE-ZT-16).The authors thank for the correction in English and valuable comments from Dr.Pan Xica and other reviewers.

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