Permafrost Foundation, Qinghai-Tibet Railway Roman Poudyal1

Abstract

Ground having temperature below freezing and remained frozen for at least 2 year to thousands

of years are permafrost ground. Permafrost can carry sustainable amount of load in its freezing

condition. It can be treated as stable foundation on freezing state but if thawing occurs different

problem arises causing failure of structure. Consolidation, deformation, heaving, jacking are

some problem that arises in permafrost ground. Proper design and technique must be used while

constructing structure on such ground. This paper describes different types of foundation that can

be constructed on permafrost ground and give brief example of Qinghai-Tibet railway

construction as example of construction in permafrost area.

1.0 Introduction

About one fifth of world is covered by permafrost. Building stable structure in such area

requires proper technique as per site condition. Constructions of any structure done with

conventional way alter the heat flow pattern of area leading to thawing of permafrost. To

construct stable foundation on permafrost, permafrost must remain frozen. When thawing of

permafrost starts all problems starts leading to failure of structure.

Warm, cold, thaw stable, thaw unstable are different type of permafrost on their annual

mean temperature and soil type or water content. Cold and thaw stable type of permafrost

1 Research Assistant, School of Civil & Environmental Engineering, Oklahoma State Univ., Stillwater. Email:

roman.poudyal@okstate.edu

require relatively large amount of heat for thawing while warm and thaw unstable permafrost

are highly heat susceptible. Problem in permafrost foundation start to occur after thawing of

underlying permafrost. Consolidations, differential settlement, frost heave, frost weakening

are some problems that are seen in permafrost foundation.

Post pad, pile, refrigerated and elevated foundations are some foundations that are

constructed in permafrost region to prevent thawing. These techniques are used separately or

in combination for better result. All these technique tries to prevent additional heat flow to

ground and not to disturb natural heat flow for maintaining thermal equilibrium of

permafrost.

Qinghai-Tibet railway is one of successful example of railway construction in permafrost

area. About half of total length of this railway passes through permafrost region. Also it is

located in average altitude of 4000 m which makes it more difficult in construction. Different

techniques have been used there to maintain freezing of permafrost and cope with increasing

temperature.

This paper presents description of different types of foundation build in permafrost region

and present example of Qinghai-Tibet railway construction and technique there.

2.0 Permafrost

Permafrost or permanently frozen ground is defined as soil or rock ground having soil or

rock or other material at temperature below freezing point for two to thousands of years and

it may occur in unconsolidated soil, in gravel, or even in bedrock (Clarke 2007). About one

fifth of earth land contains permafrost. Permafrost are formed when soil temperature remain

lower than freezing point due to different reasons like climate, latitude, solar aspect or

vegetation patterns. Permafrost possess state of equilibrium that is total heat gain in summer

is balanced by winter cold so, they remain frozen but this equilibrium can be disturbed by

different human activities (like vegetation, construction) or by natural causes (global

temperature change, erosion). Permafrost continuously goes freeze and thaw cycle on

freezing soil water matrix increase volume and on thawing it decreases volume (Clarke

2007). So, proper techniques are needed for constructing stable structure in permafrost

ground.

Permafrost is categorized in two groups based on their temperature. Permafrost with

mean annual temperature higher than -1 C is warm permafrost while permafrost having

temperature lower than -1 C is cold permafrost. Warm permafrost is highly susceptible of

thawing while cold permafrost needs lot of heat for thawing (Wu et al. 2010). Permafrost

with volumetric ice content greater than 20% is ice-rich permafrost.

Based on behavior of permafrost they are categorized into two groups a) Thaw-Stable

and b) Thaw-Unstable. If soil gets saturated with frozen water (void spaces of soil filled by

frozen water) on thawing of such permafrost volume of such permafrost will not change such

permafrost is called thaw-stable. But if soil moisture content is greater than saturation on

freezing volume will increase due to separation of sol grains and on thawing of such

permafrost volume decreases and settlement occurs. This type of permafrost is called as

thaw-unstable (McFadden 2001). Generally coarse grained and well drain soil form thaw

stable permafrost while fine grained and poorly drain soil form thaw unstable permafrost

(Wu et al. 2010).

Permafrost is different from seasonal frost which forms during winter season. Seasonal

frost only remains frozen during winter and thaws on summer. These frosts generally present

over permafrost layer and termed as active layer (Clarke 2007). Unfrozen ground underlying

permafrost or in between active layer and permafrost is called talik. General soil profile with

permafrost is shown in Fig. 1 and temperature pattern is shown in Fig. 2.

Fig. 1: Ground profile with permafrost (Clarke 2007)

Fig. 2: Temperature profile of ground (McFadden 2001)

3.0 Problems due to Permafrost

Any structure for performing better and providing intended purpose for expected lifetime

it totally depends on its foundation. Foundation should be strong enough and it should not

shift, heave or settle. But in area with permafrost it is hard to achieve such foundation

condition with general method of construction. Permafrost support substantial load if it is in

frozen condition but when it thaws, more problem arises. So, construction with some

preventive technique or design is needed for stabilizing permafrost foundation.

Different problem may arise due to change in state of permafrost. If structural are

constructed with conventional techniques more heat will be passed into ground and also

cooling effect is restrained resulting thaw of permafrost. Then the strain is induced, it is

consolidation and settlement of soil. Another problem called thaw weakening also occurred.

It is loss of shear strength of foundation soil may occur due to slow dissipation of pore water

which slowly dissipates due to drainage restriction by unfrozen soils. Increase of volume

occurs on freezing of soil water mixture which causes swelling of soil, frost heave. If soil in

contact with foundation structures such as pile freezes frost heave thus occurred pushes the

structure up called frost jacking. On melting uplifted portion again filled by more water and

frost jacking continues which results in serious deformation of structure on longer

time(Clarke 2007).

a. b.

Fig. 3: Different failure occurred in permafrost a) differential settlement b) railway track failure

So mainly settlement, consolidation of ground occurs which leads to uneven movement,

deformation, non-functionality or failure of structure. Fig. 3 shows some failure that occurred

in permafrost foundation structures. Time at which problem in soil with permafrost depends

on many factors like thickness of active layer, type of soil, degree of saturation (McFadden

2001). But sudden collapse doesnt occur in permafrost failure, problem progress in slow

rate.

4.0 Permafrost Foundation Design

Best way to prevent permafrost problem is to avoid permafrost during construction if

possible (McFadden 2001). But it is not possible so making stable structure in permafrost

layer different technique should be applied for keeping thermal equilibrium of frost to

prevent thawing. Following techniques and foundation are constructed to make stable

foundation:

Post and Pad Foundation:

This foundation main technique is raising structure above the ground (decoupling the

structure from the ground) to prevent heat transfer and allow cooling. This is only applicable

for low load and small structure. Building with heavy floor load or roads, railways cannot be

constructed on post pad (McFadden 2000). Structure raised above ground using post or pad

as shown in Fig. 4 which creates free space between structure and ground allowing air flow.

This approach is more feasible in area with sufficiently cold weather where winter

temperature sufficient to freeze active layer and maintain the thermal equilibrium of

permafrost (Clarke 2007).

Fig. 4: Typical post and pad foundation (Clarke 2007)

Adjustable Foundation (Post Pad):

This is modified form of post pad with mechanical jack which can be used to releveling

and strengthening of foundation. Water level is used to see the settlement or heave of

foundation and adjustment was made from jack bar (McFadden 2000). This foundation is

only applicable for small building with lower loads.

Pile Foundation:

It is more advancement or modification with same basic principle of post pad; it can bear

more load structure. Pile are inserted in permafrost deep enough to provide support for

structure and also frost heaving and jacking. Pile extended above ground (generally around 2

feet) to allow air flow (McFadden 2000). Length of pile should be adequate to transfer and

bear load and not less than twice the thickness of active layer. Different types of pile like

timber, steel, precast concrete, and cast-in place concrete can be used for foundation. This

type of foundation can also be used for roadway, railway construction by constructing big

bore pile in permafrost area like bridge.

Refrigerated Foundation:

It is not always possible to elevate structure above ground. Heavy structure, roadways

need to be constructed on ground level, elevated structure for such will be very costly so, on

such condition additional measures are taken to maintain frost freezing. Here additional

cooling measures are applied in addition to natural cooling using natural convection or

artificial cooling (McFadden 2000). Based on external energy use refrigerated foundation

system can be classified as active and passive. When system doesnt use pumps, blowers or

any other mechanical devices to aid heat transfer then it is called passive foundation system

(Clarke 2007). Periodic maintenance and operation is required in active system while passive

system is self-operating.

Active System: Additional measure to enhance natural cooling is used in active system.

Mainly there are three types of active system chilled liquid system, force air convection and

direct expansion system. Chilled air system consists of mechanical chiller and array of pipe

in ground. It is generally used for temporary freezing mainly during construction. Force air

convection system cools ground by blow of air thorough pipe. Direct expansion system is

efficient way of foundation cooling. Mechanical condensing system extract heat from

refrigerant which take heat from surface (Clarke 2007)

Passive System: Air conduction pipe, thermoshyphons are example of passive system. Air

conduction pipe are open vertical pipe free to air flow (warm air flow up and colder air flow

down). If vent is closed in summer more better performance can be obtained.

Thermosyphons are closed pipes filled with refrigerant fluid, inserted deep in ground and

extended above ground. Extended portion may contain fins to facilitate heat transfer. These

are one way heat transfer device when the air temperature is less than ground temperature it

remove heat from the ground but when the air temperature is greater than the ground

temperature heat transfer ceases. Typical shape of thermosyphon and air conduction pipe is

shown in Fig. 5. Its performance depends on wind velocity and temperature.

Fig. 5: Typical shape of thermosyphon (Clarke 2007)

Crushed Stone Elevated Foundation:

This type of foundation is basically useful in construction of roadway, railway. Porous

crushed rock embankment allow strong air convection during winter due to unstable air

density but in summer density gradient is relatively stable so air flow ceases. So this

embankment helps in cooling of ground and keeping permafrost frozen (Zhizhong et al.

2005). Typical section of such foundation is shown in Fig. 6.

Fig. 6: Crushed rock embankment (Qingbai et al. 2008)

5.0 Qhangai-Tibet Railway

Qhangai-Tibet railway consist most of its way above 4000 meters (13,500 feet) above sea

level. It passes through very susceptible weather and environmental condition. So, the

success of this railway lies in preventing permafrost underlying rail track from thawing using

different techniques.

5.1 Project Description

Qhangai-Tibet railway located in very high altitude, average altitude of track is 4000

meters (13,500 feet) above sea level. It is words most elevated track with highest altitude of

5090 meters (16,700) feet. Total length of track from Xining (capital of Qhangai) to Lhasa

(Capital of Tibet) is 1,956 km (1215 mile) and eighty five percent of total length is located in

forbidden zone or death zone, called because of thin air, harsh and unpredictable weather,

fierce sandstorm and high uv-radiation.

Qhangai-Tibet railway was completed on June 2006. Out of total length 550 km is in

continuous permafrost region and 82 km is in sporadic permafrost region (Qingbai et al.

2008). Here 275 km is warm, 124 km is ice rich and 171 km is cold permafrost.

5.2 Problem Encountered

Tibet is the last province which remains unconnected to railway network due to lot of

obstacle present. Main problem encountered are challenging mountain barriers, unstable

permafrost and swampy wetlands and fragile ecosystem. Also low temperature, low oxygen

at these high altitudes is also other major problem for construction. So, high altitudes, steep

grades, plummeting temperature, howling winds are problem that makes construction of

railway more challenging and dangerous. Also special design for train engines which can run

on low oxygen and pressurized compartment to keep passenger safe from suffering altitude

sickness is needed.

5.3 Solution Implemented

Considering global warming effect, effect of human activities, different measures are

implemented for keeping permafrost in stable condition. Temperature increase rate of 1 C

per 50 years is adopted for design of railway foundation (Qingbai et al. 2008). Main

techniques involved are crushed stone embankment, crushed stone embankment with

thermosyphon or ventilation duct and dry bridges.

Crushed Stone Embankment:

Convective effect of air in crushed rock is utilized to cool soil. In summer air voids prevents

(air bad conductor) heat movement in adjacent rock and soil while in winter cold air infiltrate

towards bottom while warm air goes out. The coarse rock act as thermal semi-conductor and

cool down underlying soils (Cheng 2005). It is effective and relatively cheap method. In

some area these embankments are fitted with ventilation duct pipe to allow air flow. Low

thermal conductivity of air and small contact area of the stones crushed stone layer act as

thermal insulator limiting het flow in summer and allowing more cooling in winter. Fig. 7

shows typical design of these crushed stone embankments.

Fig. 7: Crushed rock embankment

Thermosyphon:

Thermosyphon are sealed tube which is pressurized and filled with liquid ammonia. It is

embedded in ground to a depth of about 5 meters depth and has fins on outer projected part to

facilitate heat transfer (Ferrell and Lautala 2010). These are installed in crushed rock

embankment to achieve additional cooling effect for ground. It is based on principal of

natural heat convection by natural convection. Base on embankment height different length

of thermosyphons are used and total in 34 km length these are installed (Wei et al. 2009).

Fig. 8 shows embankment with thermosyphon.

Fig. 8: Thermosyphon ((Qingbai et al. 2008))

Dry Bridges:

High tall pile foundation are build and erected above ground which helps in cooling ground

by allowing air flow and also shield direct sun in warm weather. It is effective and can

support heavy loads but it is too expensive so it is applied only on those regions where other

methods are not effective. It is applied in warm and ice-rich area (Cheng 2005).Generally

when settlement and thaw were considered unavoidable even with the utilization of

engineering solution this method is used. Fig. 9 shows dry bridge constructed in railway.

Fig. 9: Dry bridges ((Cheng 2005))

Problem of thin air (low oxygen) is solved by providing oxygen cylinder for worker while

working in tunnels and also high pressure oxygen cabin was also build to recover worker

from altitude sickness. Also concreting on such temperature is challenging. Mainly to cope

with heat generated during concreting all dry bridges construction was done in winter season

and also concrete is cooled before pouring to ground.

6.0 Summary and Conclusion

Permafrost ground as long as kept on its thermal equilibrium state doesnt create any problem

but construction of structure on it surely creates some disturbances so, proper measure must

be taken in to account to avoid permafrost related problem. Conventional construction

technique should not be followed while doing construction in permafrost area. Special type

of foundation and special measure and technique should be followed while construction.

From successful construction of Qinghai-Tibet railway it is confirmed that if proper method

are employed long lasting structure can be built in permafrost with maintaining its natural

stability.

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