Hydrotechnical Construction, VoL 29, No. 1, January, 1995

HYDROPOWER CONSTRUCTION AND THE ENVIRONMENT

M O D E L O F P R O T E C T I N G T H E G E O L O G I C A L ENVIRONMENT

IN THE PERMAFROST REGION

A. A. Kagan

Human engineering and economic activity inevitably cause disturbances in the environment, including in its geological

component. It is impossible, in principle, to completely eliminate such disturbances, but it is possible and necessary to minimize

them, having provided the normal vital activity of the environment and normal functioning of structures. The solution of the

"geological part" of this problem should figure in the tasks of engineering-geological surveys and investigations as their

essential part. The extent of disturbances in the geological environment is determined, as a rule, by the size of the structure, intensity

of its effect on the environment, and the degree of stability of the latter. Hydraulic structures deserve special attention in this

respect if the disastrous situations occurring as a consequence of gross errors during designing of the structure and equipment,

construction, or operation, of the Chernobyl nuclear power plant incident type, are not taken into account.

A model of protection of the geological environment can be of great help in solving the stated problem. The first step

on the path of constructing such a model is an analysis of the effect of the structure on the geological environment. Then the

size of the region of influence of the structure and changes in the geological environment are predicted, after which the

significance of the indicated changes and their extent are evaluated. In the next stage the economic and social consequences

of the changes are established and, if this proves necessary, recommendations on minimizing their negative influence are given.

In accordance with the recommendations, engineering measures on geological environmental protection are worked

out and evaluated from an economic and social viewpoint, which makes it possible to make decisions about realizing the

construction project.

A diagram of constructing the geological environmental protection model is shown in Fig. 1.

Let us examine in greater detail the works on the model directly pertaining to the competence of the engineering

geologist for the example of the region of permafrost development which most markedly and keenly reacts to human

interference. Many examples of disturbances of the geological environment when conducting construction works and operating

various structures in this region are known. With respect to hydraulic structures, it is sufficient to mention the Khantaika

hydrostation reservoir, where thermokarst, thermal abrasion, and solifluction occurred widely as a consequence of the thawing

effect of water on the severely icy swollen silty clay soils of the reservoir banks and bed, which led to dredging and, as a

consequence, to an increase of its water area. According to the data of Ya. A. Kronik and T. S. Onikienko, the increase of

the capacity of the reservoir due to thermal abrasion was about 15 % [5]. Considerable thermokarst depressions, thermal erosion

channels along the network of polygonal vein ice, and solifluction movements with pronounced disturbance of the vegetation

cover are observed in the zone adjacent to the reservoir.

On the banks of the Vilyui reservoir there are stretches up to 1 km wide composed of dolomites underlain by weak

sandstones. The sandstones and dolomites are broken by tectonic deformations, in the zones of influence of which the soils are

distinguished by a high ice content and swelling. After filling the reservoir the sandstones thawed, were invaded by water, and

after thawing of the crush zone, settlement of individual blocks began with the formation of pits and small bays.

The hydraulic structure influences the geological environment by physical, chemical, physicochemical, and thermal

effects occurring as a consequence of the joint action of the structure and water on this environment. Here the effects of the

hydraulic structures on the geological environment cause a response of this environment and vice versa. For example, a change

Translated from Gidrotekhnicheskoe Stroitel'stvo, No. 1, pp. 5-11, January, 1995.

0018-8220/95/2901-0001512.50 �9 Plenum Publishing Corporation

Prediction of Prediction of significance of External effect the size of the changes in the extent of changes region of influence changes in the geological environ- in the geological of the external geological environ- environment effect ment ment

t Prediction of the [ ~ Economic ~ Recommendations on minimizing I I Development of measures to h c~176 ~'~'r ' negative consequencesofchanges]-,,~,] minimize negativecnnsequences environmentin the geological IINMJ| Social ill the ge;,Iogical environment _[ [ environment~ changes in the geoh,gical

t IEvaluati~176 ~ Economic ~

to minimize negative Project consequences of changes decisions in the geological environment Social

Fig. 1. Diagram of constructing the geocryological environmental protection model.

_ ~ ~ [ f Effect of stnacture

J

t... ...zs Geodynamic processes and phenomena

_& -'I Ph,s,., I

I DYna~ [ [ "~StStatic

1 Fig. 2. Diagram of the effect of a hydraulic structure on the geological environment.

in the chemical composition of the subsurface waters can lead to worsening of the soil properties, worsening of stability,

increase of deformability and permeability of the soils for water with subsequent negative changes in the geological environment, and in a number of cases not only in it.

In turn, the geological environment also affects the structures. A diagram of the effect of hydraulic structures on the geological environment and the results of such an effect are

shown in Fig. 2, and a diagram of the effect of the geological environment on the structure is shown in Fig. 3. In the permafrost region hydraulic structures are usually located both on thawed (within the underflow talik) and on

frozen (a large part of the river and reservoir banks) deposits. In the first case the structure has at first physical and then physicochemical and chemical effects. In the second case the role of the indicated effects is displayed substantially only after realization of the thermal effect. As a rule, the areas occupied by permafrost considerably exceed the talik zones. Therefore,

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The width of the subirrigation zone in thawed soils L (km) can be determined by the dependence

L = 0,481 lgKp+b,

where Kp is the permeability coefficient, m/day; b is a parameter dependent on the dam height H and operating time of the

structure:

H,m .................... > 100 75 25 < 25

b for ......... 50 1,08 0,98 0,71 0,70

t, yr ........... 100 1,23 1,15 0,95 0,91

An analysis of the given dependence shows that subirrigation of dams with a height less than 25 m with a permeability

coefficient of the soils up to 0.01 m/day does not have a substantial effect on the geological environment. To localize the

negative consequences as a result of subirrigation of dams with a height of more than 15 m with large soil permeability

coefficients, it is required to use anitseepage measures and the more serious ones, the greater the dam height and permeability

of the soils.

Within the region of influence of the reservoir the moisture content of soils of the active layer increases as a

consequence of microclimatic changes, and under certain condkions increases its thickness. According to the available data,

such changes are noted at a distance of 1,5-2 km from the waterline in the reservoir. Together with an increase of soil moisture

content as a result of subirrigation, this leads to the occurrence and activation of solifluction, landslide, thermal abrasion, and

thermal erosion processes.

Mass wasting as a result of intensification of solifluction is usually observed on slopes with a steepness of 4-10 ~ and

in rare cases up to 20 ~ [4]. Since the load created by soils of the active layer is small, the pore pressure in the thawed soil,

as a rule, is virtually not dissipated and its strength in most cases is determined by cohesion of unconsolidated soil. Figure 4a

shows the values of the cohesion parameter for which movement has still not started. If the slope is covered with turf, soil

movement can occur along the contact of the thawed and frozen ground or near the contact in the zone of increased moisture

content, which is especially characteristic for clay varieties.

When analyzing the effect of hydraulic structures on stability of slopes it is necessary to take into account the history

of their development. If the slope was formed in perennially frozen soils, especially icy and swollen ones, then their thawing

inevitably leads to a decrease of slope stability. Here it is necessary to keep in mind that in the case under consideration, even

in nonrocky soils, steep slopes with angles up to 40 ~ and more often develop and deformation of such slopes after thawing is

almost always observed. For less steep slopes their stability, as follows from Fig. 4b, can be preserved. However, if before

freezing the slope was affected by slides and their stabilization occurred as a consequence of perennial freezing, then activation

of slide activity after thawing is inevitable.

Deformations can occur in slopes consisting even of slightly icy and weakly ice-saturated soils, for example, rocky

soils, but undercut by extended ice-saturated fractures or containing interlayers or large lenses of ice. As a result of melting

of the ice, local disturbances of stability accompanied by collapse of the soils, including frozen ones, occur.

In those cases where the formation of the slope was completed before freezing, which is not accompanied by inflow

of water, thawing does not affect slope stability. If the freezing process occurred with migration, then slope stability will

decrease after thawing.

Considerable difficulties for normal functioning of the geocryological environment during construction and operation

of hydraulic structures are created by its disturbances related to stone streams.

It is presently considered that movement of stone streams is due to cryogenic, thermogenic, and hydrogenic creep, flow

of the i ce - rock layer often composing the lower part of the body of stone steamscpiping and solifluction processes [7].

Cryogenic creep is a consequence of ascent of soil particles normal to the slope during freezing and their descent

vertically during subsequent thawing.

D. Smith (1960) observed the downslope movement of coarse-fragmental material with a steepness of 21 ~ To judge

from his data, the ratio of the value of downslope movement of the particles and their ascent is 0.40. In C. Davison's (1989) laboratory experiments with loose moist soil this ratio was, naturally, larger - - 0.58. Heaving of coarse-fragmental material

under quite unfavorable conditions did not exceed 0.05 m [1]. Consequently, in the case of one-time heaving and slope

4

TO

H t lll

5O ~ - 1 0 ~ 20 o 300 ~ 0 ~

25

O~, deg

a I

0,07 ~,MPa

l b

~5 ~, MPa

Fig. 4. Effect of strength characteristics on slope stability:

a) dependence of the height of slopes of various steepness on

the cohesion parameter for a thickness of the solifluction

layer of 2 m (1) and 3 m (2); b) dependence of the stability

of slopes of various steepness on soil strength.

steepness 10 ~ l = 0.01 m and for 30 ~ l = 0.03 m. Thus, for slope steepnesses characteristic for stone streams, a stone

stream will move 1-3 m during the standard service life of the structure as a result of cryogenic creep.

Thermogenic creep occurs due to a change in the volume of fragments composing the stone stream during a periodic

temperature change. According to the data of V. I. Tyurin, the depth of daily temperature fluctuations varies from 0.5 to 1.2

m [7]. If we assume that the process of cryogenic creep encompasses 3 m, the range of temperature fluctuations is 40 ~ and

the coefficient of linear expansion of the fragmental material is 10-5~ -1, then even for such overstated data an increase of

the soil layer is estimated to be 0.001 m. Consequently, the role of thermogenic creep in the movement of stone streams is

insignificant.

Hydrogenic creep occurs as a consequence of swelling of soils during moistening and shrinkage as a result of

desiccation (G. F. Gravis, 1969). With the dominance of coarse-fragmental material in the size distribution of stone streams,

the role of the given process, just as solifluction, in the movement of stone streams will be insignificant. The significance of hydrogenic creep can become more substantial if the filling in the lower part of the stone stream is represented by clay soils

of a montmorillonite composition, and its content exceeds 30% by volume. Such a combination is rarely encountered under

natural conditions.

There is still another cause of movement of stone streams -- flow of the i ce - rock layer. ;f we assume a thickness of

the stone stream of 5 m (it is usually less), then the tangent component from the soil weight for a slope steepness of 10 ~ will

be equal to 0.018 MPa and for 30 ~ 0.05 MPa, which is less than the strength of not only the i c e - ro ck material but also of

ice. An important, and in many cases the decisive, factor of movement of the upper horizons of stone streams is

solifluction. Stone streams can move as a result of seismic shocks, falls, snow avalanches, etc.

The aforesaid shows that movement of stone streams occurs as a consequence of the joint action of many factors, the

establishment and isolation of the leading ones of which are difficult. Thus, for many types of structures stone streams are not

a great danger if during construction and operation the slopes covered by stone streams are not undercut or overloaded. As for

the hydraulic structures, the extent of their effect and the quite substantial thermal influence lead to a comparatively rapid

increase on large territories of the temperature of the soils composing the stone streams with the formation of surfaces of

weakening at the boundary of the thawed and frozen layers, thereby creating the prerequisites for active movement of stone

streams. Substantial disturbances in the geocryological environment are caused by thermokarst. The thermokarst process

develops as a result of melting of ice and removal from the soil stratum of the water that formed in this case. Consequently,

for the occurrence of thermokarst depressions it is necessary that the soil have a high ice content, and after thawing have a

permeability providing seepage of water from them. Furthermore, a load creating pressure gradients for which seepage occurs

should exist. In a natural setting a thermokarst forms, if natural water bodies are not considered, within the active layer, the load

from which usually does not exceed 0.1 MPa. In the case of a permeability coefficient of the soil of 10 .4 m/day and less and

loads up 0.4 MPa, the development of thermokarst will be quite limited.

For reservoirs the situation will be substantially different because, first, they are a constant source of a large amount

of heat, which in many cases leads to an increase of the active layer and degradation of frozen ground, in particular, to

expansion of the talik zones, second, their interaction encompasses large territories, and, third, water exerts an additional load

on the thawing soils. In connection with this, thermokarst is the most widespread process in the region of influence of

reservoirs if the banks are composed of icy and severely icy swollen soils, especially with inclusions of monomineralic

underground ice deposits. The forms of manifestation of thermokarst are different in the zone of the direct water- thermal and

microclimatic influence of the hydro development. The thermokarst process often occurs jointly with subirrigation, thermal

erosion, and other geodynamic processes, causing disturbances of the surface and subsurface runoff, decrease of slope stability,

and continuity of the vegetation cover, which has a negative effect on the vital activity of flora and fauna.

Construction and operation of hydraulic structures under harsh climatic conditions are necessarily accompanied by

aufeis which in many cases is detected as a consequence of disturbance of the geocryological environment.

It is known that most favorable for the formation of aufeis are stretches of development of water-bearing noncohesive

soils in the case of their nonuniform freezing, which is quite characteristic for alluvial deposits. Furthermore, excavation of

foundation pits and other construction cuts promotes the approach of aquifers toward the surface, leads to disruption of natural

paths of subsurface water movement and to a change in the places of unloading and often to penetration of aquifers, which

increases the probability of their nonuniform freezing with the formation of aufeis.

In cases where the depth of freezing hfr exceeds the depth of the bottom of the aquifer h a, comparatively small aufeis

forms, the dimensions of which depend, other conditions being equal, on the amount of precipitation. When hfr < h a

replenishment of the aufeis can occur, and considerably larger aufeis with a more stable regime than in the first case forms.

A reservoir has the greatest influence on the geocryological environment by means of all aforementioned effects,

especially the thermal effect. The rate of reworking of the banks is determined by the character of the permafrost strata (continuous, discontinuous,

etc.), soil properties acquired after thawing, and characteristics of the climate and reservoir regime.

Generally speaking, with respect to strength perennially frozen soils are such that they could withstand the impact action

of waves for a long time. Therefore, in most cases reworking of the banks begins after thawing of the soils, when their strength

decreases so much that they lose the capacity to resist wave action. If the soils are slightly icy and, consequently, the ice only

insignificantly strengthens them, which is characteristic, for example, for sands, then the role of waves can become

determining, although the significance of the heat arriving from the reservoir remains substantial. The greatest influence of heat

is noted in the zone of contact of water and permafrost, in particular, at the waterline of the reservoir, where niches form in

the bank ledges. Such niches weaken slope stability, leading to its collapse.

The strong heat effect of the reservoir leads to thawing of soils in a rather extensive zone, in which their properties

markedly worsen, which leads to deformations of the bank slopes, including to spreading of the thawing soils, exposure of the

frozen horizons, etc., right up to the formation of a dynamic equilibrium profile.

Worsening of soil properties as a result of thawing intensifies the development of cryogeodynamic process, which in

turn facilitates reworking of the reservoir banks. Problems of predicting reworking of banks complicated by such processes

are elucidated in detail in [3].

In the literature are described more than 40 eases of an increase of seismic activity in regions of hydraulic structures,

whereas several thousand dams have been constructed worldwide, many of which are located in zones with high seismicity.

In the overwhelming number of cases, data on the increased frequency of earthquakes in regions of hydrostation construction

are absent, which lessens confidence in the reliability of information. Thus, the Nurek hydrostation is described in [2] as an

example of intensification of seismic activity as a consequence of filling a reservoir. However, a detailed analysis by A. A.

Nikonov showed a general decrease of the seismic energy released during the time of existence of the hydrostation and not its

increase [6]. The causes leading to intensification of seismic manifestations also have not been established. The existing

hypotheses (the trigger action of a reservoir, increase of pore pressure) are far from always confirmed by data. For example,

W. Meegy established that only in 10 of the 58 cases of operation of hydro developments with high dams analyzed by him is

a relation between seismicity and reservoir filling possible [2]. Induced seismicity can be manifested only under such structural-

geological conditions in which even a small change in the stress state of the mass leads to discharge of seismic energy.

It seems that presently there are no grounds to consider that hydraulic structures necessarily worsen the seismic

situation in the region of their location. The problem of the influence of reservoirs on seismic manifestations has not been studied under the effect of hydraulic

structures in regions of development of permafrost. It is obvious that the effect of seismic loads on perennially frozen soils wiI1

be less effective than on their thawed analogues as a consequence of the greater strength and lesser deformability of the former.

However, thawing unconditionally leads to substantial activation of this effect, especially in the zone of influence of the

reservoir, and, accordingly, to activation of slope processes and reworking of banks.

An analysis of the influence of a specific external effect on a specific geocryological environment carried out on the

basis of the characteristics of such an effect described above permits proceeding to a predictive evaluation of the size of the

region of influence of the latter. Prediction of the indicated size is based on an examination of the regularities inherent to the

geocryological environment, since there are no grounds to fear the occurrence of geodynamic processes in stretches of

nonswollen soils and their changes under the influence of hydraulic structures, and on calculations, for example, of the

stress-strain state of the dam or region of thermal influence of the reservoir.

Prediction of changes in the geocryological environment is carried out with the use of data on the aforementioned

characteristics within the region of influence of the external effect. Having established the character of changes in the

geocryological environment and taking into account its behavior under the influence of the planned structure, we can evaluate

the significance and the extent of changes in it. For instance, thawing of slightly icy coarse sands occurring in the foundation

of the powerhouse of a diversion-type hydrostation does not affect the operation of this powerhouse and, consequently, the

changes will be insignificant. As for the extent of the changes, it is usually related to the size of that element of the

geocryologicat environment which is subjected to the changes.

The next stage in drawing up the geocryological environmental protection model consists in predicting the economic

and social consequences of changes in it. Here the responsibilities of the engineering geologist includes giving recommendations

on minimizing the negative consequences of the indicated changes if they are significant.

Preservation of the perennially frozen ground is a radical means of protecting the geocryological environment. This

is achieved comparatively simply for soils with a temperature not higher than minus 3~ by means of frozen curtains and in

rocky soils, also grout curtains. Under such temperature conditions in nonrocky varieties good results can be obtained by

constructing thermal piles and heat-insulating fills, coverings, etc. In sand and coarse-fragmental soils it is recommended to use heat-insulating coverings.

Subirrigation protection measures in territories located within and outside the permafrost region hardly differ, although

in the first case measures aimed at not allowing thawing, which were mentioned above, can impede subirrigation.

C

Fig. 5. Maps of the influence of a reservoir on the geocryological environment and

evaluation of measures for its protection: a) stretches of development of geodynamic

processes after filling the reservoir; b) measures for protecting the geocryological

environment; c) relative cost of geocryological environmental protection measures (the numerals are the cost of the given measure relative to the cost of the structure

in percent); 1) line of ske; 2) populated areas; 3) road; 4) position of normal pool

level; stretches of development of: 5) thermokarst, 6) slides, 7) subirrigation, 8) solifluction; 9) protective embankment; 10) retaining wall; 11) drainage; !2) heat- insulating fill; 13) forest-harvesting zones; 14) zones of disturbance of pastureland.

Movement of stone streams can be prevented by constructing wall buttresses, counterberms, or cutting off the upper

horizons of the stone streams and in the case of small thickness -- by removing them. It is necessary to keep in mind that such removal can change the thermal regime of the base of the stone stream.

Retaining structures, additional loading of the lower and cutting off the upper parts of slopes are used for increasing slope stability. Retaining structures can be recommended for minimizing the harmful effect of solifluction flows, just as terracing slopes, construction of ditches along roads, and increase of the width of the road bed.

Heating-insulating coverings can be used to control thermokarst, and if thermokarst development is presumed on a stretch limited in size, then devices preventing seepage of water from the thawing soil are used.

Measures against aufeis are widely used in industrial, civil, and road construction. They include frozen-ground belts of various types and designs and nonfreezing drainage on the path of movement of subsurface flows, which can be used when constructing and operating hydraulic structures.

A general measure is the maximum possible decrease of the volume of removal of the vegetation cover.

It is obvious that all aforementioned measures are feasible only locally, mainly in stretches of the structures, and

thereby their significance in solving the problem of protecting the geocryological environment is limited. A greater contribution

can be made by reducing the size of the region of influence of the external effect, which for hydraulic structures is achieved only by reducing the head on the dam, i.e., by decreasing the capacity of the hydrostation.

Further, by evaluating the economic and social consequences of realizing measures on minimizing the negative

consequences of changes in the geocryological environment, decisions are made about selecting the most favorable stretch of siting the structure, its parameters and design, methods of performing the works, etc., and in certain cases about reducing the production of electricity and even about the advisability of constructing the hydro development.

It is useful to depict changes in the geological environment, types of measures, and their economic evaluation by

constructing for this purpose special (e.g., according to each type of effect or engineering decision) and general (e.g., according

to cost of realizing the measures) maps. Aia example of such maps is shown in Fig. 5. The proposed scheme of constructing the geological environmental protection model can be used under any engineering-

geological conditions with consideration of factors determining its mutual influence with a hydraulic structure.

REFERENCES

.

.

3.

.

5.

.

.

B. V. Arkhangel'skii, "Heaving of soi!s in operating practice of hydraulic structures," Izv. NII Gidrotekhniki, 24

(1939). K. Gupta and B. Rastorgi, Dams and Earthquakes [Russian translation], Mir, Moscow (1979). A. A. Kagan and N. F. Krivonogova, "Prediction of reworking of reservoir banks in the permafrost region,"

Gidrotekh. Stroit., No_ 4 (1991). T. N. Kaplina, Cryogenic Slope Processes [in Russian], Nauka, Moscow (1965). Ya. A. Kronik and T. S. Onikienko, "Effect of thermal abrasion of banks and zone of the Khantaika reservoir on an increase of its volume," in: Proceedings of Conferences and Meetings on Hydraulic Engineering [in Russian],

Leningrad (1980). A. A. Nikonov, "Induced seismicity during filling of reservoirs (two examples in mountains of Tadjikistan),"

Gidrotekh. Stroit., No. 3 (1993). A. I. Tyurin, N. N. Romanovskii, and N. F. Poltev, Frozen-Ground Facies Analysis of Stone Streams [in Russian],

Nauka, Moscow (1982).

9