ISSN 01458752, Moscow University Geology Bulletin, 2014, Vol. 69, No. 3, pp. 183–188. © Allerton Press, Inc., 2014. Original Russian Text © O.G. Kistanov, 2014, published in Vestnik Moskovskogo Universiteta. Geologiya, 2014, No. 3, pp. 66–70.

The Effect of Sand Embankments on the Temperature Regime of Permafrost Base Grounds O. G. Kistanov Department of Geology, Moscow State University, Moscow, Russia email: [email protected] Received October 8, 2013

Abstract—The effect of embankments on the temperature regime of ground with continuous and discontin uous permafrost was simulated. Ground conditions are considered based on the example of the Zapolyarnoe field. The temperature dependences of the groundwater level and the capacity of the watersaturated embankment layer in the winter and summer were found. The most favorable conditions for irrigation of embankments for raising and lowering the ground temperature were ascertained. Keywords: ground water level, embankments, temperature regime, forecast model DOI: 10.3103/S0145875214030053

INTRODUCTION Experience shows that engineering and geocryo logical conditions can change during maintenance of facilities and differ from those that existed at the period of a survey. Ground conditions can change so that the state of the ground base does not satisfy a project after several years of service (Vypolnenie…, 2007). External boundary conditions mainly affect the ground temper ature, i.e., air temperature, snow depth, vegetation, etc. (Inzhenernaya geokriologiya, 1991; Metodika…, 1979). Not the least important are the effects of the subperma frost water depth and the capacity of watersaturated embankment layer on the temperature regime of per mafrost grounds (The monitoring results…, 2010). During the simulation, the effect of water satura tion in embankments on the temperature regime of base grounds at a depth of zero annual amplitudes (10–11 m) and seasonal and longterm thawing was studied. Sites with utility bridges, masts, and vacancies beyond the impact area of facilities with vented base ments are considered. A thermal field of the base grounds was calculated using the HEAT software developed at the Department of Geocryology under the guidance of L.N. Khrustalev. The main function of the software is the solution of a differential heat transfer equation by the finitedif ference method. The result of the solution is a temper

ature field of rocks at a fixed time at a specified space point. A onedimensional task was solved, since the computational domain has a horizontal surface and constant thermophysical properties of the ground. Lateral effects of facilities on the temperature field of the computational domain were neglected. PROBLEM STATEMENT Boundary Conditions When solving the problem, boundary conditions of the third kind were specified at the upper boundary, which are the monthly average air temperatures according to data from Table 1 while accounting for the radiative correction for the insolation of the embankment surface in the summer months (Inzhener naya geokriologiya, 1991). Boundary conditions of the second kind were specified at the bottom and side boundaries (the heat flow is zero at a depth of 30 m). The temperature regime is considered to be periodi cally steady throughout the forecast period and cli mate changes were neglected. The effect of thermalinsulated covers (a magni tude reciprocal of the thermal resistivity, m2 °C/W) on the heat exchange was found in the problem via the thermal resistivity of snow from the corresponding snow depth and density.

Table 1. The daytime temperature of an embankment surface during a year, accounting for radiative correction, °C Month I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

–23.9

–24.6

–16.1

–9.7

–0.8

15.4

19.5

15.5

8.3

–4.0

–17.0

–23.9

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Table 2. Parameters of snow cover on a builtup territory Month Type 1 2

Parameter Snow depth, m Snow density, kg/m3 Snow depth, m Snow density, kg/m3

XI

XII

I

II

III

IV

V

0.15 250 0.20 300

0.30 350 0.50 400

0.35 400 0.60 450

0.50 420 0.70 480

0.45 450 0.65 510

0.50 500 0.75 550

0.20 550 0.35 600

Table 3. Physical and thermophysical properties of sand in embankments according to [Engineering geocryology, 1991] Soil type Fine sand Fine sand

Total humidity Wtot, %

Dry density ρd, g/cm3

10 20

1.54 1.54

Thermal conductivity, W/m h °C

Volumetric heat capacity, W/m3 °C

thawed

permafrost

thawed

1.56 1.99

1.70 2.34

523.2 707.9

Volumetric Tempera heat of phase ture of phase 3 permafrost change, W/m change, °C 410.4 523.4

15610 30875

0.0 0.0

The distribution and character of snow accumula tion on a gastreatment plant (GTP) territory have been studied during an areal snow survey and monthly observations since 2006. Two typical ground types have been chosen, with different snow depths, where no snow compaction or removal was carried out. The first type is areas where the snow depth is not higher than 0.5 m during the winter (blow regions between roads and under utility bridges). The second type is areas where the snow depth does not exceed 0.75 m; this is typical for territories where snow reten tion occurs due to construction features of facilities and their location. The snow density is higher at the GTP territory than in natural conditions because of the redistribution of snow within the buildup area due to the winter wind pattern. The monthly snow depth and density are given in Table 2.

ness is 1.8 m. This section has been chosen as the most typical for the territory under study;

Characterization of the Territory under Study

Characterization of FillUp Ground

On the basis of an analysis of the geocryological permafrost structure in terms of depth, annual average ground temperatures, and continuous or discontinu ous types of permafrost under the GTP territory, four types of ground section with an embankment height of 1.0 and 2.0 m were chosen for each soil. Ten engineer ing and geological elements were selected depending on the composition, ice content, and moisture of soils (Fig. 1): —Section Ia: loams with stratified sand and clay bands; the permafrost top depth is 7.0 m; the season allyfrozen layer (SFL) thickness is 1.0 m. This section has been chosen for the calculations as the most dan gerous and, probably, most variable under external effects, which is adverse for bases in these conditions; —Section Ib: loams with stratified sand and clay bands; continuous permafrost, the active layer thick

Embankment works on a territory under the GTP3C (Ia and Ib typical sections) started in April 2002 and finished in April 2003; under the GTP2C (IIa and IIb typical sections), started in April 1999 and finished in November 2001. Filling of sand was carried out from hydraulically filled clamps on natural vegetative ground cover and frozen ground. The calculation was carried out for an embankment that was 1.0 and 2.0 in height, i.e., for the minimal and typical ones for this territory. The calculations for Ia and Ib soils started from May 15, 2002, and for soils of typical sections IIa and IIb, from May 15, 1999, since these dates were the closest dates of thermometrical surveys in boreholes during engineering and geological investigations. The physical and thermophysical properties of the sand in the embankment are given in Table 3.

—Typical section IIa: loams with stratified sand and clay bands, which are underlain by fine sands from a depth of 6.0 m; the permafrost top depth is 4.0 m; the active layer thickness is 2.0 m; —Typical section IIb: loams with stratified sand and clay bands, which are underlain by fine sands from a depth of 6.0 m; continuous permafrost; the active layer thickness is 1.5 m. The thermophysical properties of the soil sections are given according to Russian Construction Norms and Rules 2.02.04.–88 while accounting for labora tory studies of physical and mechanical properties (Vypolnenie…, 2007).

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Embankment height, m

1

2 Filled soils (fine and dust sands)

Depth, m 0 2

1

1

6

3

7

6

2

8

4

4

8

10

9

9

9

9

6 8

3 5

4 10 12 5 14 16 26

5

5

28 30 Ia

IIa

Ib

IIb

Types of engineeringgeocryological sections Permafrost during the surveying period

Active layer during the surveying period Boundary of engineering geological elements

5 Engineeringgeological elements

sand (fine, dusty) loam with sandy clay and mud bands Fig. 1. Engineeringgeocryological sections for the computational domain.

Ground (Subpermafrost) Water Depth in the Embankment Fieldobservation data received during GTP terri tory monitoring in the falls of 2006–2011 and the MOSCOW UNIVERSITY GEOLOGY BULLETIN

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study of drainage and groundwater accumulation con ditions allowed us to ascertain that there is a stagnant (subpermafrost) groundwater level in areal dust fine sand embankments with an almost horizontal surface No. 3

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Table 4. The subpermafrost water level in an embankment that is 1.0 m in height Number of variant Groundwater level, m Summer Winter

11

12

13

14

15

16

1.0 1.0

0.5 0.5

0.5 1.0

0.3 0.3

0.3 0.5

0.3 1.0

Table 5. The subpermafrost water level in an embankment that is 2.0 m in height Groundwater level, m Summer Winter

Number of variant 21

22

23

24

25

26

27

28

29

210

2.0 2.0

1.5 1.5

1.5 2.0

1.0 1.0

1.0 1.5

1.0 2.0

0.5 0.5

0.5 1.0

0.5 1.5

0.5 2.0

in the summer–autumn seasons. The only exception is the embankment periphery, where the level drainage (decrease) occurs by the beginning of winter. It is mainly supplied totally due to atmospheric precipita tions. The level depth is often 0.3–0.5 m (rarer, 0.5– 1.0 m) in the summer and before the winter beginning in embankments of 1.0 in height. The groundwater depth is often 1.0–1.5 m (more rarely, –5.–1.0 m) in embankments that are 2.0 in height. During the prob lem solution, the groundwater depth was specified by the sandmoisture limit; it was taken as equal to 10% above the limit (from field tests) and 20% below the limit; the humidity was taken close to the total water saturation of sand. Six values of groundwater depth are considered for a 1.0m embankment; they are presented in Table 4. Ten values are considered for a 2.0m embankment (Table 5). Temperature, °C 0 –0.1 –0.2 –0.3 –0.4 –0.5 0

5

10

15

Section Ia, Ib

20

25

30

35 Year

Section IIa, IIb

Fig. 2. Variations in the temperature at a depth of 10 m for areas with discontinuous permafrost.

Ground Temperature The temperature of a ground section at a corre sponding initial time of the forecast was specified from data that were received during engineering and geolog ical surveys on the territory that was planned to be builtup. Four values of ground temperature at a depth of 10 m were considered. It was taken equal –0.3°C for typical Ia and IIa sections and –0.5, –1.0, and –1.5°C for Ib and IIb sections. Fill sands were initially thawed; the temperature in the embankment body was 2.0°C at the initial time point of the computation. The above temperature values characterize the geotemperature regime in natural conditions for most soils; this was ascertained from thermometric studies during the surveys. STUDY RESULTS AND DISCUSSION A (30 year) forecast was made without accounting for the migration of moisture and convective heat exchange in the ground; a longterm simulation was carried out. After filling an embankment with a known groundwater level regime, forecasted values of the soil temperature and SFL/active layer depth were fixed for October 15 at 5, 10, 20, and 30 years after the filling. The soil temperature gradually decreased to ⎯0.2°C at a depth of 10 m for all areas with continuous permafrost at a snow depth of 0.75 m, independently of the embankment height and groundwater level (Fig. 2). For a snow depth of 0.5 m, an embankment height of 1.0 m, and a groundwater depth of 0.3–0.5 m (sum mer and the period before the winter begins), the annual average temperature insignificantly decreases (by 0.1°C) and the seasonal frost depth increases by 0.2–1.0 m and more with formation of a new perma frost layer at depths of 2.2–4.0 m. The tendency toward a decrease in tav remains in an embankment that is 2.0 in height at the same areas, while an increase in the seasonal frost depth depends on the groundwa

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Embankment height, m

Embankment height, m

0 Summer Groundwater level 0.5 m –0.5 Temperature, °C –1.0 0 –1.5 –2.0 –0.5 –1.0 –1.5 –2.0 –2.5 0

5

10

15

Snow 0.50 m

20

–0 Winter Groundwater level 0.5 m –0.5 Depth, m –1.0 –1.5 –1.5 –2.5 –2.0 –3.5 –4.5 –5.5 –6.5 –7.5 0

35 Year Snow 0.75 m 25

187

30

5

10

15

20

Snow 0.50 m

35 Year Snow 0.75 m 25

30

Fig. 3. Forecasted soil temperatures under the effect of an embankment with a constant groundwater level at a depth of 0.5 m versus the snow depth.

Fig. 4. The dynamics of the activelayer thickness and per mafrost top depth at a constant groundwater level versus the snow depth.

ter level by the start of freezing, i.e., the deeper the level is, the larger the frost depth.

Areas with section IIb, an embankment height of 1.0 m, and snow depths of 0.5 and 0.75 m are charac terized by a temperature increase throughout the sec tion for all values of the groundwater level. The temper ature attains –0.2°C at the 30th year of the embank ment’s existence. The permafrost top decreases and is located at depths from 3.0 to 7.9 m depending on the groundwater level: it is at a minimum at a groundwater depth of 1.5–2.0 m and a maximum at a depth of 0.3– 0.5 m. This can be explained by an increase in the pos itive part of the annual heat budget due to a higher thermal conductivity of saturated soils as compared to airdry soils and a higher volumetric heat of the phase change during thawing of icerich sands (Table 3). The maximum depth of the permafrost top decrease in the IIb section (Fig. 1) might well be connected with sand deposits at a depth of 6.0 m. Soil temperature maxima and minima at a depth of 10 m for continuous permafrost during the survey at all

On areas with an engineeringgeological section of type IIb, initial values of the annual average perma frost temperature from –0.5 to –1.5°C, and embank ment heights of 1.0 and 2.0 m, the temperature decreases throughout the section as compared to the initial conditions at any groundwater level (Fig. 3). The minimum temperature is –2.3°C and the maxi mum one is –1.2°C at a depth of 10 m at the 30th year of the embankment. The active layer depth is from 1.9 to 2.1 m for an embankment of 1.0 m in height and from 1.8 to 2.3 m for a 2.0 m embankment. Permafrost thaws at a snow depth of 0.75 m, which is accompanied by a change of the continuous to dis continuous type of permafrost. The permafrost top depth is at a maximum (7.5 m) at a groundwater depth of 0.5 m in the summer and winter.

Table 6. The maximum and minimum soil temperatures (°C) at a depth of 10 m at the 30th year of an embankment for all values of the subpermafrost water level Section type Ib

Embank ment height, m

Permafrost tempera ture, °C

1.0

Minimal Maximum Minimal Maximum Minimal Maximum Minimal Maximum

2.0 IIb

1.0 2.0

Snow depth, 0.50 m

Snow depth, 0.75 m

initial temperature

initial temperature

–0.5

–1.0

–1.5

–0.5

–1.0

–1.5

–1.9 –1.2 –2.3 –1.4 –0.7 –0.2 –0.9 –0.2

–1.9 –1.2 –2.3 –1.4 –0.8 –0.2 –1.0 –0.2

–1.9 –1.3 –2.3 –1.5 –0.8 –0.2 –1.0 –0.2

–0.9 –0.2 –0.2 –0.2 –0.2 –0.2 –0.2 –0.2

–0.9 –0.2 –0.2 –0.2 –0.2 –0.2 –0.2 –0.2

–1.0 –0.3 –0.2 –0.2 –0.2 –0.2 –0.2 –0.2

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values of the groundwater level are given in Table 6, which was constructed while accounting for the type of engineeringgeocryological section in terms of the embankment height, groundwater temperature during the survey, and snow depth during service. The maximum difference in the temperatures of a soil section of the IIb type attains 0.5–0.6°C for an embankment that is 1.0 m in height and 0.7–0.8°C for an embankment that is 2.0 m in height, at any initial tem perature of the permafrost and a snow depth of 0.5 m. The maxima and minima are equal to –0.2°C for a snow depth of 0.75 m and all values of the groundwater level. CONCLUSIONS 1. A high groundwater level in the active layer increases the annual heat budget in soils, which affects the increase in their annual average temperature and seasonal thawing depth. 2. A snow depth of 0.75 m prevents cooling of soils at all values of the subpermafrost water level. The soil temperature at a depth of 10 m tends to –0.2°C at this snow depth, independently of the embankment height and type of soil considered in the model. 3. The presence of groundwater in an SFL/active layer promotes an increase in the permafrost top depth at a snow depth of 0.75 m for all types of sections and a height of 0.5 m for the IIa and IIb types. 4. A difference in the soil temperatures at a depth of 10 m at the 30th year of the existence of a 1.0m embankment attains 0.7°C for type Ib and 0.5–0.6°C for type IIb, depending on the subpermafrost water level in the summer and winter. The difference attains

0.8–0.9°C for a section of type Ib and 0.7–0.8°C for type IIb for a 2.0m embankment. 5. The most favorable conditions for a decrease in the base soil temperature are dry embankments in the summer and winter. REFERENCES Inzhenernaya geokriologiya (Engineering Geocryology), Ershov, E.D., Ed., Moscow: Nedra, 1991. Metodika merzlotnoi s”emki (Geocryological Survey Methods) Kudryavtsev, V.A., Ed., Moscow: Izd. Mosk. Univ., 1979. The monitoring results of the roadbed of the KhralovSokh onto site during the construction of a new Obskaya– Bovanenkovo railroad line, in Aktual’nye problemy mekhaniki, prochnosti i teploprovodnosti pri nizkikh tem peraturakh. Teoriya i metody zamorazhivaniya gruntov: Matly XII konf. (Proc. XII Conf. “Actual Problems of Mechanics, Strength, Heat Conductivity under Low Temperatures. Theory and Methods of Soil Freezing”), St. Petersburg: SPbGUNiPT, 2010. Vypolnenie merzlotnykh inzhenernogeologicheskikh i geofiz icheskikh izyskanii, sostavlenie prognoza vliyaniya tekh nogennykh nagruzok na sostoyanie geologicheskoi sredy po ploshchadkam UKPG1V i UKPG2V Zapolyarnogo GNKM i zona vliyaniya. V 4kh kn (EngineeringGeo logical and Geophysical Surveys on Permafrost and Compilation of Forecast of Influence of Technogenic Factors on the Geological Environment Within the UKPG1V and UKPG2V Sites at the Zapolyarny Oil and Gas Condensate Field (OGCF) and a Zone of Its Influence. Four Books), Moscow: FGUP Fundament proekt, 2007.

Translated by O. Ponomareva

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