Shesternev D.M.，Zhang R.V.，Kuzmin G.P.，Shepelev V.V.，Pavlova N.A.，Popenko F.E.

(1.Melnikov Permafrost Institute SB RAS，Yakutsk 677010，Russia.2.Geotechnologia LLC，Yakutsk 677010，Russia)

0 Introduction

The Russian territory in the north of the Eurasian continent is more than 17000000 km2in area.Massive reserves of natural resources are concentrated here，the systematic development of which started at the beginning of the 20th century.As then，now Russia's gross domestic product is being formed to a considerable degree due to their development.Undoubtedly，its significant growth in the future is impossible without stimulating the development of production forces in the north of Russia.Taking into account this fact，after economic stagnation in the late 20th-early 21st centuries，the government of the Russian Federation，paying great attention to the development of transport infrastructure in permafrost regions，intends to increase sharply urbanization of these areas.Together with the solution of environmental and technical safety problems of utility lines，which are being designed or already functioning in permafrost zones，it is decided to increase the standards of living in such regions.

By now within the Russian permafrost zone，a network of the urbanized territories has been located in river valleys and near coastlines.River and northern sea flood risk has sharply increased due to the global climate change.Economic losses from catastrophic floods in the town of Lensk and in Yakutsk totaled several millions of rubles.Northern seacoast retreat is already posing a serious threat to villages built in the Arctic coastal areas.To prevent this in the future，levees along the Lena River have been built.At the same time，their construction made it possible not only to remove a flood threat，but also to solve the annual problem of flooding and water-logging of the floodplain area.The building experts in Yakutia Dynin V.M.，Krotov V.M.，Poleshchuk V.L.，Pyankov I.I.，Sukhanov N.V.and others have considered the floodplain area as one of general development variants of Yakutsk.The analysis of this suggestion has shown that the city development by means of building on riparian areas of the Lena River on hydraulically filled soils is possible from the technical point of view and cost-effective[1].

1 Experience of construction on hydraulic fill in permafrost regions in Russia and abroad

Experience of construction on hydraulic fill in permafrost-free regions abroad and in the USSR has a rich history and a rather considerable archive of publications on this issue.A similar experience of construction in permafrost zones is relatively slight and mostly refers to the construction of linear or hydraulic structures[1].

Hydraulically filled soils were used during the construction of the Hess Creek dam(Alaska)and Fairbanks International Airport in 1962.In the first case the foundation of the dam was frozen using a piping system with forced brine circulation.Its freezing point is－40℃.The dam and the airport are functioning successfully now.

In Russia since the 1960s，the construction of roads，airports and dams in permafrost regions using hydraulically filled soils has been carried out during the development of the West Siberian complex of oil and gas production and transportation.The developed and applied hydromechanization technology in this region was awarded the USSR State Prize in 1986.

The archive of publications concerning hydraulic fill for construction of industrial and residential objects in permafrost regions contains only 10-15 titles，the most complete of which from the fundamental and application-oriented point of view is Roman et al.[1].According to these authors，use of hydraulically filled soils for construction began in Yakutsk in the 1960s.In 1965～1967，several buildings were constructed in the river port area on hydraulically filled soils with a thickness of 2.0～4.5 m.The frozen foundation soils were preserved(Principle I).The foundations were piled，the length of piles was 6～8 m.A ventilated crawl space ensured the implementation of this principle.Further monitoring of the ground thermal regime showed that by 1980 ground temperature had been－3℃.The bearing capacity of soil allowed to increase number of storeys of constructed buildings.The second example of the Principle I for construction on hydraulically filled soils is the construction and operation of one of buildings of automobile repair plant.The size of this building according to the plan was 100×130 m.The layer of hydraulically filled soils reached at that moment 4～6 m.A ventilated crawl space together with ventilated air ducts were used to preserve the frozen state of soils.The state of this building is satisfactory.District 202，according to Roman et al.(4)，is the most perfect example.The construction on this territory was carried out on hydraulically filled soils during 1980～1988 in Yakutsk.

The Yakutsk branch of Krasnoyarsk Institute of Siberia and the Far East in the field of industrial design(PromstroyNIIproject)，Poleshchuk V.L.(CEO)，Roman L.T.，Tseeva A.N.etc.(researchers)made a significant contribution to the development of construction technologies.The regularities of formation of physical and mechanical properties of hydraulically filled and underlying soils thawing after hydraulic filling were studied by them in a testing area.The technology of two-stage hydraulic filling was developed.The building foundations of three types were tested:spread footings，piled foundations and mat ones.Precast spread footings adopted as main foundations during the construction of District 202 showed the highest technical efficiency.The thickness of the hydraulically filled sand layer was 6.11 m and the foundation depth was 4.5 m.Undoubtedly，the experience in construction of foundations in the river port and building of the foundation repair plant was taken into account.At the same time，analytical studies were conducted.According to Roman et al.，these studies were a perfect confirmation of Principle I for construction of buildings in District 202.In addition，in the end of the 20th-the beginning of the 21st centuries，a complex of multistory buildings on hydraulically filled soils using Principle II(with thaw bulb formation)was constructed in the riparian zone of the Lena River in Yakutsk.This made it possible to use basements as parking places(Fig.1 a，b).

Fig.1 Exterior of the building constructed according to Principle II(a)，basement used as a parking place(b)

The further operation history of the residential complex in District 202 showed that the buildings were based on thawed foundation soils.Hot water heating system leaks are considered to be the primary cause of permafrost degradation and its warming.We thick that hydraulically filled sand，the thermal conductivity of which is only 0.5 W/m* ℃，as well as additional warming of soils during spring and summer flood periods contribute significantly to this process.In this case，ventilated crawl spaces do not provide the expected rise of the permafrost table into the hydraulic fill.

The transition of the foundation soils from frozen to thawed state should have caused significant deformations of buildings.Apparently，the precast spread footings(spread footings are based on a concrete slab)provided technical stability of buildings and structures due to redistribution of differential deformations caused by the slab.Thus，the buildings constructed according to Principle I now are being operated according to Principle II.In this case，the experience in construction and operation of buildings in District 202 demonstrates the necessity to conduct an additional analysis of the building site conditions where permafrost is degrading.The effect of thickness of hydraulically filled soils on temperature of the underlying permafrost should be estimated once again.

The limitation of thaw penetration during construction according to Principle II calls for various thermal insulation measures as well as stabilization of the permafrost table.

In this regard，taking into account the experience in construction of District 202 and other engineering objects in Yakutsk，the development of several new districts of enhanced comfort on hydraulically filled soils within the complex of Lena River terraces is planned.District 203 is one of them(Fig.2).

Fig.2 Model of District 203，Yakutsk，to be constructed on hydraulic fill allowing permafrost degradation

A part of the district 202 constructed in the 1980s could be seen in the north-western part of the Fig.2.Its fundamental difference is that it was built according to the Principle I，while the district 203 is being built using the Principle II(formation of a thaw bulb during construction and operation of buildings).

2 Geocryologicalconditionsand their role in selection of construction principles

It is known that the average air temperature，the structure and amount of precipitations due to global climate change have changed significantly.The territory of Yakutsk is not an exception.A fundamental statement that the formation and stable development of permafrost can be possible only under certain necessary conditions is known[1-4].The first necessary condition is satisfied when the long-tern average annual air temperature is negative，the second one requires the longterm mean annual ground temperature in the base of active layer to be negative.It follows that even if a necessary condition is satisfied，permafrost may not be formed.An example is an open talik beneath the Lena River.There is no seasonally frozen layer in this area.The second example，the heating influence of snow cover on the territory of negative annual average air temperature is so considerable，that the mean ground temperature of the active layer base is positive.In this case，there is no permafrost in such regions.

During recent years we have seen different interpretations of air temperature changes dynamics in the literature.These interpretations differ not only in values，but also in directions.For example it is showed in the monograph by Roman et al.[1]that the average air temperatures during the period from 2000 to 2010 were changing from positive to negative.It follows that since that time a new phase of climate cooling has begun.The data of Skachkov[5]and Khrustalyov[6]are similar.According to them，the long-term average air temperature will be increasing until 2060.The average annual air temperature in Yakutsk by this time will have increased from －10.7℃in 1971 to－3～－4℃in 2050 ～2060[6].Such increase of the long-term average air temperature will affect the structure and properties of high permafrost layers.It will also affect negatively all the buildings and structures constructed within the permafrost regions.

When selecting a construction principle for the development of District 202，it was supposed，that foundation soil temperature in the whole area of the district would have reached －1～ －2℃ in 2～3 years.However，monitoring of the territory and its parts at a distance of 2～3 m from the buildings shows that the temperatures at a depth 10 m and more are positive now.In our opinion，water leaks are not the only reason for this.A complicated interaction between ground and floodwater of the Lena River，which are an additional strong source of heat involved in the formation of the ground state，plays a significant role(Fig.3).

Fig.3 Penetration of the seepage line into hydraulic fill during seasonal flooding(spring-summer)，1981.(Data of the Yakutsk State Design and Research Institute for Construction)[4]

In addition，we have established that insufficient thicknesses of hydraulically filled soils have practically no effect on the thermal state of underlysing ground and they become thermal insulators when their thickness is more than thicknessofseasonalfrostpenetration(Fig.4).

Fig.4 Depth to the permafrost table(horizontal axis)vs hydraulic fill thickness(vertical axis)in District 203，Yakutsk(derived from the 2013 ground investigation data)

3 Construction and operation of buildings on hydraulically filled soils in Yakutsk

Residential district 202 is about 30 ha in area.Before hydraulic filling it was a floodplain of the Lena River.This surface with an anabranch of the Lena River protected by a levee was covered with numerous lakes，mostly oxbow ones，which were seasonally flooded partially or completely.The floodplain was mostly flat with extended ridges and lows.Sections between the ridges were swamped or covered with oxbow lakes with a depth of about 3 m and a length of about 1～1.3 km.Their banks were overgrown with bushes(mostly willows)and wild grasses.

The archival materials concerning permafrost soil conditions prior to development are available in a corresponding cadastre and other scientific-technical and technical reports[7-15]as well as in the published paper[1].

The analysis of archival material shows that Jurassic rocks(sands and weathered sandstone with siltstone beds)in the area are overlain by Quaternary alluvium.Its thickness is 18～21 m.Its base is presented by gravelly sands with pebble and small boulders.The middle part of this section consists mostly of small and medium sands.The layer surface is presented by sand，clayey and sandy silts with layers of fine-or rarely me-dium-grained sands.

Prior to hydraulic filling，the District 202 and adjacent areas including the area of District 203 now under construction，had complicated geocryological conditions.Engineering-geocryological sections of this territory were notable for permafrost of various types and newly formed permafrost contacting with the hydrogenic talik.Closed taliks of partially flooded floodplain had a depth of 3～5 m.The thickness of taliks beneath the annually flooded floodplain was 4.2 ～9 m or more.The bed of the City anabranch and the left bank of its floodplain are zones of hydrogenic talik formation with a thickness of up to 30 m.

The hydrological monitoring observations in District 202，conducted in 1992，showed that buried(relict)taliks with a thickness of up to 28～30 m were preserved under hydraulically filled soils.

Temperature field monitoring of the geocryological environment(1979～1980)revealed that permafrost had thawed to a depth of 3-5 m or more beneath District 202.During 1983～1985 members of the Yakutsk State Design and Research Institute for Construction measured the temperature in District 202 at the experimental building 1-B(now building 6).These measurements showed a decrease in temperature in the region of its positive values at the base level of hydraulically filled soils.The magnitude of the marked decrease in temperature was 2÷3℃.

The objects built in District 202 were operating with violation of the existing building codes.Emergency water leaks from heat-emitting utility lines occurred in this district repeatedly.Such leaks led not only to complete thawing of the newly formed permafrost in some areas，but also increased the temperature of hydraulically filled and underlying soils to high positive values(Table 1).

The Yakutsk State Design and Research Institute for Construction during 1992～1996 continued temperature measurements to a depth of 18 m in restored and newly drilled boreholes in the District 202 area，covering both hydraulically filled(floodplain and alluvial)and underlying soils.As a result of the measurements it was established that the temperature of both soils continued to decrease.Thus，soil cooling was registered in the period 1992～1996 in borehole TC-6 on the territory of the building 1-B(now building 6)beneath its central part at a depth of 16～18 m.The same situation was registered in borehole TC-11 beneath the northeastern corner of the same building.Soil temperature in this borehole at a depth of 17 m and 15 m was about －1 ℃[4].

Table 1 Maximum ground temperature(tMAX)at depth H，m beneath various objects in District 202 during emergency water releases*from utility lines according to the data in[4]

During 2009～2011，periodic soil temperature measurements in District 202 were carried out，as part of the City of Yakutsk Geocryological Monitoring Program，in boreholes drilled near the foundations of multi-story buildings.The boreholes were drilled near the buildings at a distance of 1.5(boreholes 4，5，6)to 3.0 m(borehole 47)in areas with different exposure to the sun.These year-round measurement data allow to estimate the thermal state of soils at a depth of 10 m(Table 2).

Table 2 Soil temperatures at a depth of 10 m(t10)in District 202 during 2009～2011

These data should be supplemented with source data.This information，which allows estimating the distribution of maximum(tmax)，minimum(tmin)and average(tav)soil temperature，is presented in the tables below(Tables 3～6).

Table 3 Distribution of maximum(tmax)，minimum(tmin)and average(tav)soil temperature with depth(H，m)in the period from 5.11.2009 to 20.10.2011，cross-section of B-4

Table 4 Distribution of maximum(tmax)，minimum(tmin)and average(tav)soil temperature with depth in the period from 23.11.2009 to 20.10.2011，cross-section of B-5

Table 5 Distribution of maximum(tmax)，minimum(tmin)and average(tav)soil temperature with depth in the period from 23.11.2009 to 20.10.2011，cross-section of B-6

Table 6 Distribution of maximum(tmax)，minimum(tmin)and average(tav)soil temperature with depth in the period from 27.10.2010 to 20.10.2011，cross-section of B-47

If the primary data(before hydraulic filling)are compared with the averaged data obtained during recent years(Tables 2-6)and with the lithologic description of B-4，5，6，47 sections the following can be stated:

The hydraulically filled soils in District 202 consist mainly of sands.This layer has existed for about 30 years.Now it is characterized as a thick(6.5 ～8.5 m，sometimes up to 14 m)consolidated ground layer，which does not cause significant settlements of soils with a low(usually less than 9%)gravimetric water content.Only sometimes gravimetric water content of hydraulically filled sands reaches 14% ～22%.

The soils and its underlying deposits are changed differentially during processes of freezing and thawing.These processes affect the territory of District 202 in different ways.Thus，the temperature of the soil in B-4 at a depth of 2 m drops below －23 ℃ in winter.Sands at the same depth(2 m)warm up to 8℃in summer.As a result，a newly formed warm permafrost with a thickness up to 1.5 ～2 m can be registered.Soil temperature at the depth of 10 m is positive(0.7℃).The B-6 has a similar geocryological engineering structure.

There is seasonally frozen soil layer in B-5 and B-47.The active layer consists mostly of sands and its thickness is up to 4.5 m in these boreholes.The seasonally frozen layer is underlain by soils of technogenic taliks.The temperature(t med)of these onshore taliks at a depth of 5 m is between 0.1℃(B-47)to 2.5℃ (B-5).The technogenic talik is underlain by soils of natural hydrogenic one(B-47).

We consider due to many years'experience of successful urban development on technogenic layer(hydraulically filled soils)that soils created by such hydromechanical method and construction on them are a perspective direction in modernization and further development of Yakutsk engineering infrastructure.

It follows from the experience in construction and operation of District 202 that the foundations are in non-frozen state now due to technical effects.This contradicts the regulations，which stipulate preserving them in frozen state during construction according to the Principle I.A similar distribution of foundation soils temperatures is typical for residential buildings 1-B and 1-B，at collectors.For foundation soil of the building 1-B both negative(TC-11，TC-6)and positive values of temperatures to a depth of 16～17 m are presented[4].

Monitoring of temperature changes on the territory of District 202 also showed that the soil temperature at a distance of 1.5～3 m from the foundations to a depth of 10 m is positive.These data confirm the fact that new permafrost zones has not been formed for more than 25 ～30 year period on hydraulically filled soils.The only exception is pereletok(up to 2 m)，as a rule at northern walls.

Currently members of Yakutsk State Design and Research Institute for Construction，namely Prof.Roman L.T.and other experts，strongly recommend to use the experience of building District 202.They also provide their kinetics calculations of soil temperature within District 202，as well as actual data concerning soil temperature studying.Calculations of thawing foundation soil deformations excluding application of the Principle II are also provided.

The analysis of literary sources and materials of technical as well as scientific and technical reports conducted in Melnikov Permafrost Institute SB RAS showed that engineering-geological investigations and researches had been carried out before and after the hydraulic filling[5-8].

It follows from the content of the mentioned works that hydraulically filled soils and rhythmic changes in hydraulic grade line level of suprapermafrost water during flood periods explain the formation of permafrost of degradation type(Table 7)in the area of interaction of engineering structures.

According to field observations and modeling in a testing area，freezing of hydraulically filled soils with a thickness of 6～7 m may take more than 20 years.We consider that upper boundary conditions were established without taking into account additional heat sources and thermal-physical characteristics of hydraulically filled soils were significantly overestimated.The integral action of unaccounted factors is evident due to soil temperature increase.Therefore，there is a stable process of permafrost degradation in hydraulically filled soils with a thickness of more than 8～10 m(see Fig.4).

Table 7 Soil temperature(℃)according to observations in the B-1 without hydraulically filled soils(by Pavlov A.V.et al[3])

The hydraulically filled area of District 203 is currently an area where the layer of seasonal frost penetration does not extend to the permafrost table at a depth of 17.2～21.9 m.The underlying permafrost began to thaw during the hydraulic filling and subsequent a rather long period.According to the drilling data(2012～2013)，there are watered grounds in the bottom of the thawed layer beginning with a depth of 8～10 m which confirm water filtration of the Lena River.At the same time，they determine the soil thermal regime.Under these conditions，during the development of District 203 only Principle II with strengthened foundations can be applied.Such foundations take and redistribute force caused by differential soil settlement.

In 2005，members of Melnikov Permafrost Institute concluded that hydrostatical pressure of river water(during flood periods)affects hydrogeological condi-tions of thawed and unfrozen soils on the hydraulically filled territory.Moving groundwater with a high heat capacity carry a significant amount of heat and have a strong thermal effect on the thermal regime of surrounding soils.The intensity of heat transfer carried out by waters depends on their speed，thickness，intensity and temperature.The higher these parameters are，the greater the amount of heat is transferred by water.

Due to absence of hydraulic and thermal parameters of occasional water flows it is impossible to conduct forecast calculations of foundation soil temperature formation.However，having measured soil temperatures of District 203 and the calculated values without taking into account the thermal effect of moving groundwater it is possible to estimate how this water influence the formation of foundation soil temperature regime.The measured temperature is positive to a depth of 15.0 m.The calculated temperature values obtained without taking into consideration filtering water flows are negative.This confirms that ground waters have a significant effect on the temperature regime of foundation soils and the permafrost table movement will be determined by thermal effect of these waters.Under these conditions，the permafrost table stabilization based on ventilated crawl spaces and cooling devices with reasonable costs is almost impossible.

4 Conclusions

1)Principle I for construction can be applied only in case of ground cooling to a solid frozen state beginning with depth of foundation bed.It is up to －1℃for sand and up to －3 ℃ for clay soils.Furthermore，this thermotechnical regime of foundation soils must be maintained for the entire period of building operation.The experience of building operation in District 202 showed that using ventilated crawl space does not provide a sufficient effect.

2)In this regard and taking into account the experience of building in District 202 we can conclude that the requirements of regulations within District 202 are practically not met.The buildings are based on thawed during their operation foundations.This corresponds to the construction norms and specifications 2.02.04-88[7]for construction according to the Principle II.

3)Mat foundation should be used during the development of District 203 in Yakutsk according to the Principle II.

4)Design consideration for construction in new districts on floodplain territories of the Lena River in Yakutsk and other permafrost areas of Siberia should be developed individually.The existing geocryological conditions and the kinetics of their changes in space and time taking into account the impact of climate change and technogenic impact should be taken into consideration.

5)The above mentioned statements confirm that it is appropriate to use the Principle I for construction on hydraulically filled soils with the thickness less than the thickness of potential seasonal soil freezing.If the layer of hydraulically filled soils is thicker than potential seasonal soil freezing it is appropriate to use the Principle II.

6)The requirement of the section 5 is being implemented now during development of District 203 in Yakutsk.Constructive and meliorative measures are taken to prevent possible deformations during thaw bulb formation.To these measures belong mat foundations and insulating materials to increase the thermal resistance of floor in basements.

7)Design temperatures obtained without taking into account thermal influence of underground waters are negative，while measured temperatures are positive.This fact confirms that these waters are a strong heat source.

8)A multilevel engineering geocryological monitoring is being developed to identify the influence of extensive factors(heat emission from buildings directly into foundation soils)and intensive ones(changes in heat exchange rate of the territory of District due to changes in components of radiation thermal balance)as well as warming impact of flood waters on the kinetics and mechanics of geocryological environment on District 203 territory.

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