Construction of a water storage dam on permafrost: discharge and blocking up the river during construction of the Uilyui Hydro-Electric Power Station

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site

LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Technical Translation (National Research Council of Canada), 1969

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=13462ea2-e91b-4990-8538-2f606dd04c38 https://publications-cnrc.canada.ca/fra/voir/objet/?id=13462ea2-e91b-4990-8538-2f606dd04c38

NRC Publications Archive

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/20375843

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Construction of a water storage dam on permafrost: discharge and

blocking up the river during construction of the Uilyui Hydro-Electric

Power Station

Biyanov, G. F.; National Research Council of Canada. Division of Building

Research

NRC

TT -1353

NATIONAL RESEARCH COUNCIL OF CANADA

TECHNICAL TRANSLATION 1353

NRC

TT -1353

CONSTRUCTION OF A WATER STORAGE DAM ON PERMAFROST

GIDROTEKHNICHESKOE STROITEL'STVO. (10): t5 ·23, t965

AND

DISCHARGE AND BLOCKING UP THE RIVER DURING CONSTRUCTION

OF THE VILYUI HYDRO·ELECTRIC POWER STATION

GIDROTEKHNICHESKOE STROITEL'STVO. (2): t·5, t966

BY G.F. BIYANOV

TRANSLATED BY V. POPPE

THIS IS THE ONE HUNDRED AND SEVENTY - EIGHTH OF THE SERIES OF TRANSLATIONS PREPARED FOR THE DIVISION OF BUILDING RESEARCH

OTTAWA 1969

PREFACE

The design and construction of water-retaining structures, e.g. dams and dykes, in Arctic regions are of special interest to those engaged in northern engineering. Difficult problems of major concern include the thawing effect of water on structures founded on permafrost, and earth

moving, placing of ｭ｡ｴｾｲｩ｡ｬｳＬ and scheduling of construction in remote areas having a harsh climate.

Two Russian articles dealing with dam construction in northern areas of the U.S.S.R. are included in this publication. The first describes the design and construction of a water storage dam built on permafrost at

Mirnyi, in northwest Yakutia. The foundation and core of the darn are main-tained in a frozen condition by a duct system through which cold winter air is circulated. The second article describes the construction schedule and placing of materials under severe conditions for a large rock-fill dam associated with a hydro project at Chernishevskli on the Vilyui River in Siberia.

G.H. Johnston and R.J.E. Brown of the Northern Research Group of the Division of BUilding Research toured both these Projects during their visit to the Soviet Union in

1966.

The author, who was Chief Engineer and their guide and host during these visits, presented the articles to them. It is a special pleasure, therefore, to have these papers translated.

The Division wishes to record its thanks to Mr. V. Poppe, Translations Section, National Research Council, for translating these papers and to Mr. G.H. Johnston, of this Division, who checked the translations.

Ottawa

January,

1969

R.F. Legget Director

Title:

Author:

References:

Translator:

NATIONAL RESEARCH COUNCIL OF CANADA

Technical Translation 1353

Construction of a water storage dam on permafrost (Opyt stroitel'stva gidrouzla v usloviyakh vechnoi merzloty)

and

Discharge and blocking up the river during construction of the Vilyui hydro-electric power station

(Propusk stroitel'nykh raskhodov i perekrytie reki pri vozvedenii Vilyuiskoi gidroelektrostantsii)

G.F.

Biyanov

Gidrotekhnicheskoe Straitel' stvo, (10): 15-23, 1965 and

(2): 1-5, 1966

CONSTRUCTION OF A WATER STORAGE DAM ON PERMAFROST Gidrotekhnicheskoe Stroitel'stvoy(lO): 15-23, 1965

The Irelyakh water storage dam has been erected on the Irelyakh River, which flows into one of the tributaries of the Vilyui River, to create a reservoir for water supply and economic needs. The structural design and methods of construction were determined by specific environ-mental, climatic, engineering and geological conditions in the region.

The Irelyakh dam will be the largest among the dams with a frozen core and with retention of permafrost in the foundation, and will be the first such dam in the Soviet Far North.

Environmental, climatic, engineering, and geological conditions

The construction site is characterized by severe climatic conditions and the presence of continuous permafrost which exceeds 250 m in thickness. Characteristic features of the climate are long winters and short hot

summers. The air temperature drops to -63°C in winter and rises to 30 35°C in the summer months. The mean minimum temperature in January is -55°C. The mean annual temperature is _8.2oC, and there are only 56 frost-free days per year. The highest observed daily amplitude of temperature fluctuations in the middle of August 1956 was 33°c (+22.5°C during the day and -lO.50C at night).

The main source of supply of the Irelyakh River is the surface runoff resulting from the melting of ice and snow. Characteristic phenomena are spring floods (which last two or three weeks with a rise in the water level of up to 4.5 m and a maximum discharge of up to 160 m3 / s e c ) , _{low water in} the summer, and floods in the late summer and early fall resulting from heavy rains. The river runoff is distributed very unevenly throughout the year, and 50 - 99% of its annual volume coincides with the summer period

(May-June). In the winter period, beginning in October-November, the river flow stops and the river becomes completely frozen. However, a talik is retained beneath the river bed which serves as a channel for a groundwater stream. The heating effect of this stream is so great that the thickness of the talik may reach

8 -

10 m.

The dam is founded on bedrock as well as Quaternary deposits (Fig. 1). The bedrock is represented by greenish and bluish-grey marls and marl clays with intercalations of limestone, dolomite and dolomitized rocks 0.5 - 0.8 m in thickness. These rocks are weathered as a result of frost action to a depth of 40 m and the fractures are filled with ice to form ice veins 0.5 -1 cm and occasionally 6 - 8 cm thick. Bedrock (clay and marls weathered to clay) is in fact a heavy silty clay loam with a plasticity index of 10 and

unit weight of over 2 t/m3

• When frozen, the loam has a high bearing strength and is practically impermeable to water.

The unconsolidated Quaternary deposits in the river bed are re-presented by contemporary deposits on both river banks, by alluvial de-posits of the first terrace above the flood plain, and on valley slopes by eluvial clay loam and talus deposits. The alluvial deposits in the river bed are represented by silt with pebbles and gravel, silty clay loam, and sand with gravel and pebbles. The ground is perennially

frozen except for the talik beneath the river bed. The ice in permafrost is in the from of pockets and up to

20

mm thick veinlets. Since the ice content may reach

60%,

the ground is SUbject to settlement upon thawing. The alluvial deposits of the first terrace above the flood plain consist of silty clay loam and silt with a high content of plant remains. The ice content is high: up to

60%

or even

90%

in the silt. The ice in these soils forms uniform leanses and intercalations up to 0.5 m thick. Some sections of the flood plain reveal the presence of peat resting on buried ice lenses. It is quite obvious that the high ice content of these de-posits results in considerable subsidence on thawing, and when in a thawed state the 30ils are completely unsuitable as foundations.

The bedrock on the river banks is covered with talus deposits: clay loam containing fragments and debris of carbonate rocks. The ice content in the deposits may reach

20 - 60%;

the ice is found in the form of pockets, veinlets and lenses. Within the ill-defined second terrace, above the

floodplain.on the slopes of the left bank, there is an ancient river bed in the bedrock filled with alluvial deposits (sandy loam, sand, gravel and pebbles).

At the level of the first terrace the width of the Irelyakh valley is about 235 m. The entire surface of the terrace is swampy and sparsely covered with brush. The soils on the construction site are perenially frozen and only the surface layers thaw in the summer. The depth of thaw is at its maximum in October, and freezing reaches its maximum depth in February. The temperature of permafrost at these times is as follows:

ｾ ｾ ｾ ｾ Ｎ +"'""f""ot.. re I o.pth, ... 5oi\ I I _I I °C I 5 I 8 10 I I _I

I

1-3,3 I

-O<.tob.., -0,1 I 0 3 I

1-'"

i-3.6 1 -3. 0 r"bruafi I -9.8 -6.0 , -3.5

i

-2.8 i-2.8 I I

Seasonal fluctuations of soil temperature resulting from changes in the air temperature are no longer observed at a depth of 10 - 12 m, and the soil temperature at this depth does not drop below

_2°C

or

_3°C.

-5-The hydrogeological regime in the area of construction is also

affected by permafrost and by the fact that the water-bearing soils remain frozen for a greater part of the year. In the regions of the Far North there are usually three water-bearing horizons: subpermafrost, supraperma-frost and intrapermasupraperma-frost. Among these, only the subpermafrost horizon, consisting of thawed soil, maintains a steady regime. The other water-bearing horizons are subject to both perennial and seasonal fluctuations. It was found on drilling that the piezometric level of the subpermafrost horizon is 70 m* below the foundation of the dam. It has no effect on the dam structures or the mode of construction since it is covered by a thick

(over 250 m)* layer of permafrost.

The suprapermafrost horizon lies in the seasonally thawed layer; the water-bearing soils are represented mainly by clay loam. This horizon usually freezes in winter since it is dependent on the amount of precipitation and the air temperature.

The intrapermafrost water occupies an intermediate position and is re-tained only in the taliks. In winter, during the seasonal freezing of upper soil layers and the reappearance of permafrost from below, the cross-section of the ground-water stream is narrowed and this generates pressure in the intra-permafrost horizon. Water bursts through the seasonally frozen cover as a result of this pressure and forms a naled. This group also includes the ground-water flowing through the talik beneath the river bed.

Structural complex

The main features of the development (Fig. 2) consist of the earth dam, the spillway and the ice-retaining wall supporting a highway bridge. The design of the dam calls for the retention of permafrost in its foundation. The earth dam (Fig.

3

and 4) has a maximum height of 20 m and a crest length of 320 m. The resulting storage basin has a useful capacity of 12 million m3• The somewhat flattened upstream slope is designed to allow for possible

settlement of the upstream section resulting from thawing of the foundation soils when the reservoir is filled. It is assumed that the foundation of the reservoir will thaw to a depth of 10 m in 50 years. The downstream section of the dam will freeze solid throughout its entire height in about 30 years due to the effect of the air temperature, the negative temperature of perma-frost in the foundation, and the permaperma-frost curtain. The central part of the dam is constructed of clay loam, the lateral surcharge sections consist of fine-grained sand. The slopes of the dam are reinforced with rock fill which * Translator's note: This is the correct translation but the depth and

one metre thick on top and 0.5 m thick at the bottom. An insulating layer of moss, one metre thick, has been placed along the crest and the upper part of the slopes.

The dam is built of thawed soil, like any other dam, but freezing will sUbsequently occur. Provisions have been made for artificial freezing of the central part of the dam to increase the rate of freezing and prevent a poss-ible seepage of water from the reservoir through the body of the dam (as a result of deformation on thawing of the frozen foundation soils). The frozen core of the dam must be joined to permafrost in the foundation.

The design calls for cooling and freezing the dam body by natural frost in the period between November and March with the help of forced circulation of cold air through pipes in the dam body. For this, 289 holes were drilled into the dam body and grouped into

7

freezing systems. Each section has a common header with a box-like cross-section made of sheet metal. The air is forced through ttle header by an EVR-4 fan. The holes are drilled into the bedrock to a depth of 3 m. The overall length of holes is 8.2 - 26 m and their spacing is 1.5 m. The freezing column consists of two pipes one inside the other and is shown in Figure

5.

The inner pipe, 139 mm in diameter, is inserted into a casing 219 mm in diameter. The bottom end of the casing is tightly plugged and the pipe is placed on the bottom of the hole. A space of 0.2 m is left between the plug in the casing and the bottom end of the inner pipe. The air from the ｯｵｾ･ｲ Dire enters the inner pipe through this space and returns to the surface (Fig.

6).

The amount of cold air entering each column from the header is regulated by a slide valve (see Fig.

5).

The spillway structure is located beyond the main dam on the gently sloping left bank, which made it possible to reduce the amount of excavation work to a minimum. The spillway represents an open, self-regulating canal which ends in a multi-step drop. The canal has a trapezoidal form and is 40 m wide at the bottom. Its banks slope at a ratio of 1 on 2, it is 700 m long, and its gradient is 0.0021. On a section 212 m long the banks of the canal are reinforced with concrete slabs to a height of 2 m placed on a two-layer filter. The canal has been designed for a calculated discharge of

166

m3 j s e c

in a flood with a frequency of 2%, and for a discharge of 356 m3 j s e c _{in a}

flood with a frequency of 0.1%.

The drop has four steps, 2.3 m high and 30 m long, which consist of re-inforced concrete walls erected on rere-inforced concrete piles driven into perma-frost to a depth of

5

m. The tailrace canal below the walls of the drop is re-inforced with concrete only in a section 30 m long.

It is not intended to discharge the ice through the spillway; it melts in the reservoir. The ice is retained at the entrance to the spillway canal by a

wall consisting of six reinforced concrete piers also erected on piles. The latter measures 35 x 30 cm and are driven into permafrost to a depth of 8 m. The piers support a highway bridge the span of which consists of 14 m long prefabricated, reinforced concrete T-beams.

The earth dam is filled with clay loam and fine-grained sand. In the Far North, in contrast to construction under normal conditions, allowances must be made both in the design and during the erection of structures on permafrost for possible changes in physico-mechanical and construction pro-perties of soils on thawing. These changes may affect the bearing proper-ties of soils, the extent, rate and nature of subsidence of frozen soil, and the seepage of water.

The reservoir accumulates an enormous amount of heat which will invari-ably result in the thawing of soil, at first beneath the bed of the reser-voir and later in the foundation of the dam. Furthermore, the depth of thaw-ing of foundation soils will be greatly affected by seepage of water unless steps are taken to prevent this. The most dangerous soils from the point of view of thawing and subsequent subsidence are the unconsolidated Quaternary deposits represented by eluvial, alluvial and talus deposits. On the other hand, the underlying marls, limestones and marl clays are a reliable founda-tion, since tneir subsidence on thawing is negligible.

The unconsolidated Quaternary deposits with an ice content of up to 60% cannot be regarded as a suitable foundation for a dam. The dam engineers, therefore, suggested that the body of the dam be joined to the foundation by means of a cut-off, cutting right through the unconsolidated and ice-satura-ted Quaternary deposits and extending into the bedrock to a depth of 0.5 m.

The design called for a bottom width of 12 m for the cut-off so that an

ex-cavator could be used but was later reduced to

6

m. The joining of the dam body with the bedrock in the foundation by means of the cut-off ensures com-plete protection of the dam body from seepage and creates more favourable conditions for the freezing of the dam body and the foundation during the operation of the dam.

Construction period

The extremely difficult environmental, climatic, engineering and geologi-cal conditions, and consequently special requirements with respect to quality and reliability of structures, made it absolutely essential to work strictly according to specifications. As was mentioned earlier, the dam will consist of thawed ground but the foundation soils will be retained in a frozen state. According to specifications, the foundation had to be prepared in the fall and winter and then covered by a layer of frozen .loam 1.5 m thick to prevent it from thawing during the spring flood. The dam had to be constructed of clay

o

ＭＭＨｉｾ

loam in the summer. The river bed was to be cleared of alluvial deposits by dredging.

Construction was started in October 1961 and the bulk of the work was completed in November 1963. The freezing system was installed in the winter of 1964 when the freezing of the dam core was started. The entire construc-tion period may be divided into five main stages: I - winter and spring 1962; II - summer and fall 1962; III - winter 1962/63 and spring 1963; IV - summer and fall 1963; V - winter 1963/64.

In the first stage of construction the work was started by cutting the trees and removing the stumps, vegetation and soil. On the banks this was done by bulldozers beginning at the end of October. The dam foundation on the flood plain section was laid only after the arrival of cold weather and freeZing of the entire swampy section. The removal of unsuitable material on the left bank flood plain was carried out after preliminary loosening of the soil by drilling and blasting. Drilling and blasting could be done successfully only after complete merging of the seasonally frozen layer with permafrost. After blasting, the soil was loaded into trucks by £-1252

excavators and some bulldozers.

The excavation of the cut-off, which had a maximum depth of 8 m, was done by excavators after prelimitlary loosening of soil by drilling and blasting. The bulk of ttle work on the foundation and the excavation of the cut-off trench on the left bank flood plain was completed in the winter 1961/62. However, the excavation of the cut-off trench in the section adjacent to the river bed was delayed and could not be finished during this period since rapid thawing of soil set in with the rise in the air temperature. The water-saturated soil began to lose its bearing strength and the movement of trucks and use of equip-ment became impossible; the ice-saturated soil became unstable and the banks of the cut-off trench began to cave in. To prevent further damage, the bed and the slopes of the cut-off trench were covered by a 1.5 m thick layer of marl loam, having a lower ice content, taken from the construction site. The cut-off trench in this section could not be completed before the winter of

1962/63. In the completed section, clay loam was successfully placed in the dam body and the cut-off in the spring and summer. The foundation was covered

with a layer of clay loam 1.5 m thick to protect it from thawing.

During preparation of the dam foundation, a hole 2 m deep was formed beneath the downstream section where silt and peat had been removed. The hole was separated from the river bed by a longitudinal cofferdam of natural ice-saturated soil. In this section the loam was not placed in the dam body on time. With the onset of rapid thawing of soil and the rise in the water level

-9-in the river, the cofferdam failed and the hole was immediately flooded. To prevent the thawing of foundation in this section, the loam was placed by the method of underwater filling. The loam was brought in from the drying site by a bulldozer and was compacted by a tractor. The control samples taken in

the fall showed that ｴｾ･ ､･ｾＸｾｾｾＧ &lld ｾｯｩｳｴｵＭｾＩ･ content of loam placpd jn this

way ｣ｯｾｰｬ･ｴ･ｬｹ ｳ｡ｴｩｳｦｾ･､ the specifications. To ensure complete freezirlS

in winter, the snow in this section was continuously removed.

During this period, the cut-off and the body of the dam on the left bank flood plain were filled with soil taken from the spillway canal which was ex-cavated in talus deposits consisting of clay loam with inclusions (up to 20 -30%) of rock waste, gravel, pebbles, and occasional sand lenses. Since the canal was excavated along a high slope of southern exposure, the soil had a relatively low ice content and its water content on thawing was close to op-timal. Therefore, this soil was, on the Whole, suitable for use in the dam without any special drying. In fact, having been loaded by excavators and transported by bulldozers and trucks in warm spring weather, these soils gave up excess moisture relatively quickly and were ready for use. On days when there was not enough sunlight and wind, the soil was dumped on drying sites along the banks of the cut-off trench. Here the soil thawed and was aired, so that its moisture content was reduced to 16 - 20%. Dried soil was pushed down the slope of the cut-off trench by a bulldozer. A second bull-dozer spread the soil in layers 0.3 m thick which were compacted by running a C-IOO tractor over them

8

or 10 times. This method of compacting proved quite adequate and resulted in a unit weight of soil of 1.60 -

1.75

t/m3 _as

compared with a specified value of

1.50

t/m3

• Odd frozen lumps of soil were

pushed aside and after thawing were used in the dam. In the summer the soil was compacted by loaded

MAZ-205

and

MAZ-525

dump trucks, which proved to be quite adequate. The unit weight of soil placed during this period was

1.55

-1.70 t/m3•

The frozen soil in the spillway canal was removed by excavators and bull-dozers after preliminary loosening by drilling and blasting. The soil was transported to the filling site by trucks and later by bulldozers when the construction site and the pits became impassable for trucks, as a result of rapid thawing of the ground.

The spring flood in the first year of construction was discharged through the natural unconstricted river bed. To protect the cut-off trench on the left bank from flooding, a dyke made of clay loam was erected along the river

channel on the crest of the first terrace above the flood plain. In the second stage, in the summer of 1962, work was continued on placing loam in the cut-off and the dam body on the left bank. The surface layer of soil on the right bank

-10-was removed as it thawed in the early summer of

1962

by bulldozers moving on frozen ground. This part of the slope was then immediately covered by a 1 - 1.5 m thick layer of loam to retain its foundation in a frozen state. The excavation of poor quality soil (silt, peat, etc.) on the flood plain on the right bank was done by an

E-302

excavator with a reversed shovel. It was equipped with wider tracks to improve its mobility.

At the end of September, cofferdams were erected in the river channel above and below the dam and the ｣ｵｴＮｾＭｯｦｦ trench was drained. The flow was di-verted to the tailrace by a pipeline and a pumping station. In the third stage, the cut-off trench was completed, the river bed was cleared of silt, alluvial

deposits and talus, and the river channel section was filled with loam. The unstable soils in the talik beneath the river bed were removed layer by layer as they froze, by means of an

E-652

excavator after preliminary loosening by drilling and blasting. The ice and snow were removed to speed up the freezing process. About 29,000 m3 of soil were excavated in this way. In places where it was not possible to achieve complete freezing of the talik, groundwater burst through the thin ice crust broken by excavators and trucks. In some cases the inflow of groundwater could be stopped by a layer of thawed loam

0.5

m thick. If the inflow was insignificant, the water was allowed to accumulate and was then removed by tank trucks equipped with pumps.

The geology of the foundation of the dam and the canal was recorded during construction. It was found that the talik beneath the river bed was consider-ably larger than was originally thought. The true position of the zero isotherm was determined in early November

1963

by drilling an observation hole in the river bed 70 m below the dam. It was found that the boundary of the talik was

6

m below that given in the plan. Therefore, in order to arrest the water flow beneath the river bed completely, the holes for the permafrost curtain were made correspondingly deeper. Later, observations of the temperature regime in the foundation made in holes drilled through the permafrost curtain along the axis of the dam, confirmed that the findings concerning the true position of the talik were correct.

By the beginning of spring

1963,

the river bed was completely cleared down to bedrock and was covered with a layer of loam 5.5 m thick. The spring flood was discharged through the river bed. In the left bank portion of the cut-off, the loam was placed there at the time of the flood.

In the summer, loam was placed in the dam on both sides of the river. In September, after construction of upstream and downstream cofferdams, the trench was drained and cleared, and a start was made on the river bed portion of the dam. As was the case in the first stage of construction, the discharge was diverted to the tailrace by a pumping station. By the middle of October the

-11-loam core of the dam was completed. The sand surcharge on the slopes was placpd at the same time. The upstream slope was reinforced with crushed rock in the winter 1963/64. To ensure complete freezing, the downstream slope was continuously cleared of snow and the crushed stone cover was placed here only in April. It was found that this resulted in freezing of the

downstream slope to a depth of over Sm. Due to a lack of water it was not possible to place a cover of ice and sawdust on the downstream slope in the winter of 1964/65.

The loam and sand for the dam body were excavated from borrow pits situated on the right bank at a distance of 1 - 1.5 km from the dam site. The layer of suitable loam did not exceed 1 - 1.8 km in thickness. The soil in the borrow pits was perennially frozen and thawed only to a depth of 1 - 1.2 m by early fall. The loam was excavated in the summer and fall by a bulldozer which removed the layers as they thawed. To accelerate the thawing by solar radiation, the trees in the borrow pits were cut down and the tree stumps removed during the winter. The surface soil layer was re-moved in the spring. The excavated loam was heaped in mounds up to 12 m in height for winter storage. No special covers were provided for the mounds. The soil in such mounds froze to a depth of 3 m but remained un-frozen in the middle. In the spring the mounds were broken apart by exca-vators after preliminary loosening of the surface layer by drilling and blasting. Large lumps of frozen soil were kept off the drying site. In the early spring of 1963 the frozen loam in the borrow pits was also removed by excavators after preliminary loosening by drilling and blasting.

Most of the loam was placed in the dam in the summer and fall after some preliminary drying. The soil on drying sites was dried by wind and the sun. The moisture content of soil dried in this way made it possible to compact it to the required density when placed.

In the winter 1962/63 a series of experiments were carried out on the construction site to determine the procedures for bulk thawing of frozen loam prior to its use in the dam, and for placing frozen loam in the dam with sub-sequent thawing and compacting.

A drying oven measuring 20 x 20 m and consisting of a system of pipes was constructed for thawing and drying the loam. The heat was supplied by an engine from a TU-I04 turbojet plane mounted on a special stand. The exhaust gases from the jet engine were brought to the oven by a duct 1200 mm in dia-meter. Frozen loam was placed over the pipes of the oven. However, it was not possible to achieve the desired effect. The heat supplied by an enormous amount of exhaust gases having a temperature of more than 300°C was wasted. The efficiency of the oven was very low and the consumption of fuel very high.

-12-Furthermore, the loam placed immediately on the pipes or close to them thawed and became too dry, while loam away from the pipes remained frozen.

Experiments were carried out on methods of placing frozen loam on the right bank of the cut-off and thawing it by heating under tarpaulin covers developed by the Orgenergostroi aided by heating electrodes, prior to com-paction. Frozen loam was spread in 0.4 - 0.5 m thick layers, and compacted by a bulldozer and a loaded truck. The electrodes were then placed in the loam which was saturated with a calcium chloride solution to improve its electrical conductivity. This made it possible to thaw loam on a site measuring 10 x 50 m2 in the course of one shift. However, on subsequent compacting the high water content of thawed, ice-saturated loam made it impossible to obtain the required density. This method gives satisfactory results in the case of frozen loam with a normal water content. A large scale application of Orgenergostroi heaters is fully dependent on the avail-ability of an efficient heat source. A somewhat modified high pressure nozzle* may be used for this purpose. Experiments are being continued.

The placing of loam in the dam was supervised by a field party from the All-Union Research Institute of Hydro-engineering and the geotechnical labo-ratory on the site, which determined the water content and the unit weight of the soil skeleton. A system of measuring instruments was placed in the dam body to keep a check on the temperature and seepage regimes of the dam.

On drilling the holes for the permafrost curtain on the right bank flood plain and in the river bed portion of the dam in October-November 1963, ground-water under pressure was found in more than 20 boreholes. Water was detected either along the base of the cut-off trench at the contact between the fill and the bedrock, or in the bedrock at a depth of 1.5 - 2 m. Water was present only in some holes and, in addition, the piezometric levels were not the same: in some holes water rose to a height of 12 m, and in others to 5 - 6 m, although the depth of the holes was the same and there was no water in neighbouring holes. Such unexpected appearance of groundwater in the foundation of the dam could not but cause great concern. In addition to design engineers, the members of the B. E. Vedeneev All-Union Research Institute of Hydro-engineering and the Yakutsk Institute of Permafrost Studies were also asked to help with this

problem. Various suggestions were made but there was no common agreement on the reason for the appearance of water. In our opinion the explanation lies in the presence of intrapermafrost water on the construction site and the slopes of the river valley.

The holes for the permafrost curtain were drilled as various sections of the * Designed by F. G. Lyutov (Vilyuigesstroi).

-13-dam were filled to the required height. The bulk of this work was completed by the end of 1963. The holes were drilled and the pipes of the freezing

system installed by BU-20-2 cable drills under exceptionally severe conditions: the air temperatures dropped to -45°C or -50o

e

and the wind was very strong at times. Because of extreme cold, some drill rods broke, casing pipes failed, and a number of ､ｲｬＱＱｩｮｾ rigs broke down. Use W2S made of warm water which was heated on the site in large holes provided with primitive stoves.

To improve the efficiency of the fans, the freezing columns were grouped into systems consisting of 36 to 47 columns each. Each system was provided with its own fan which forced the cold air into the header. The exception was

system no. 6 installed within the river bed, where the header was divided and provided with two fans to increase the rate of freezing. Two freezing systems were installed in the section where the dam joined the spillway canal. These systems formed a permafrost curtain along the right bank of the canal to pre-vent the plug separating the canal from the dam from thawing during discharge of the spring flood. A perlnafrost curtain was also formed along the bottom of the spillway canal by a system of five 273 mm pipes placed horizontally one on top of the other (Fig. 7).

The cold air passed through the annular gap between che inner pipe and the casing in each column at the rate of 150 m]/hour (300 m3 / h o u r _{in system no. 6)}

and escaped to the surface through the inner pipe. The difference between the temperature of the air enetering and leaving the freezing systems reached 10 -12o

e .

The removal of heat from the body of the dam resulted in the formation of frozen soil envelopes around the freezing columns which eventually merged to form a continuous permafrost curtain.

The jndividual freezing systems began to operate immediately after instal-lation which had been accomplished in the period between December 1963 and 20 February 1964.

The presence of seepage water created serious difficulties in the period prior to the formation of a continuous permafrost curtain. As was mentioned earlier, the talik beneath the river bed turned out to be considerably larger than was originally thought. It appeared unlikely that intrapermafrost water in the talik had considerable discharge and high seepage velocity. To be on the safe side, however, several possibilities of arresting the seepage flow beneath the river bed were examined in order to create normal conditions for the forma-tion of a permafrost curtain first of all within the talik itself. To achieve this freezing system no. 6a consisting of 22 columns was installed in the down-stream slope. Since these relatively shallow holes were drilled mainly in the bedrock, they were completed very rapidly and the system became operational in a very short time. Two weeks after the start of operation of this system it was

-14-found that there was a difference in the groundwater levels above and be-low this permafrost curtain, which indicated that the detrimental effect of water beneath the river bed had been eliminated. SUbsequent observations confirmed this also.

At this stage, the Yakutsk Permafrost Institute was asked to help orga-nize a programme for observing freezing progress in the dam. The amount of cold entering the core of the dam was recorded by measuring the amount of air forced into each column, its temperature on entering and leaving the column, and the rate of its escape from the inner pipe. This indirect method of mea-suring the amount of heat removed from the dam core made it possible to obtain preliminary estimates of changes in the temperature regime of the dam core based on heat balance considerations allowing for the water content of soil. As has been shown by direct measurements carried out by the All-Union Research Institute of Hydro-engineering with the help of RTI* in test holes drilled at distances of 0.80 - 0.95 m from the freezing columns, the preliminary estimates were close to actual values. These measurements showed that by the spring of 1964 (the end of operation of the freezing system), the core and the foundation throughout the entire length of the dam were almost entirely frozen and the frozen envelopes had merged to form a frozen curtain with a thickness of over 2 m. However,

observations and calculations carried out by the Hydro-engineering Institute revealed the presence of two "windows" (unfrozen sections) in the permafrost curtain.

As the air temperature rose to -15°C, the freezing system was closed off for the summer. The spring flood of 1964 filled the reservoir, the dam was ex-posed to design pressure, and the flood was discharged through the spillway canal. Throughout the entire operational period no undesirable seepage or other phenomena have been recorded. At the end of October 1964 the water sto-rage dam was approved for operation by the State Commission. In November 1964 the freezing system was turned on and the creation of a permafrost curtain in the dam body and along the banks and beneath the bed of the spillway canal was continued.

Conclusions

1. During construction in permafrost the r.ost vulnerable and important place in the entire underground profile of the dam is the talik beneath the river bed, through which intrapermafrost water continues to move. The heating effect of subsurface water which is held under pressure, after flooding of the reservoir results in the growth of the thawed zone and may cause complete thawing of the *Translator's note: The only available explanation for this abbreviation is

-15-dam foundation. To prevent this, the talik should be entirely removed by excavation and the resulting trench filled with good quality, impermeable soil. Alternatively, the subsurface water should be frozen, etc.

2. To avoid possible errors in design and construction, special atten-tion should be given 1;0 the determination of the true boundary between the tllawed and the frozen zones when determining the extent of the talik beneath the river bed. It has been found that there is no clearly defined boundary between the thawed zone and permafrost. For a considerable interval of

depth

(3 - 3.5

m) there is a zone of semi-permafrost, within which the tempe-ratures range from _O.loC to _0.2oC. Here the ice is in a semicrystalline state and water is retained in some fractures. Very sensitive temperature measuring instruments are required to determine ttle true positions of the zero isotherm and the permafrost table.

3.

On warm spring days, frozen clay loam containing ice thaws very quickly (on excavation, transportation and spreading prior to compaction), can readily be used as fill, and is easily compacted, even if thawing has not been complete. ｾ｡ｲｬ clay loam containing up to 40% gravel and carbonate rock fragments is easily compacted if used as fill. Individual frozen lumps in the loam have no effect on the density of the fill.

4. On ･ｲ･｣ｴｩｮｾ a dam on ice-saturated soil, the core of the dam must be

joined to norl-subsiding soil in the founrtation by a cut-off which must extend right through these unreliable soils which lose their bearing strength on thawing. This protects the dam from seepage and ensures its stability on possible deformation as a result of differential settlement of the foundation. It also eliminates the necessity of replacing strongly subsiding soils beneath the entire dam foundation by high quality material.

In the absence of seepage the subsiding soils beneath the downstream section are retained in a frozen state. The soils beneath the upstream section thaw due to the heat accumulated by the reservoir and their subsidence is inevitable. Therefore, the upstream section should be made flexible.

-16-" _{... , .L),}ｾ ... -. / /20 ,IS ｬｾＺ［ f ..ＺＮＮＮＨＡＯｾｾｾ ［ｦ［ｾｾ［ＺＺＺＬｾｔ［［Ｂ［＿ｩＺＮｾｾｾＭＭ ＭＭＮｾＭ｟ＷｃＭＭｾ］＿Ｍｾ］

__

Ｎｾ｟Ｌｾ］｟］ＺＭ

-__

ｾｾＺｾ

.. _-

ｔｾ ＭＮ｟ＬＭｾｾ ＭＧＭＭＭＭｾ

..

<.:-.... -ｾｊＺｻＩ＿ｾ 1q ｟ｦＮｾＧｾＭＭｲＺＺＮＺＢＧＺＢＺＢＮＢＯ＼ＧＭｾＺＺＬＩＮＬ 10 _ .. _....- __ ＬＬ｟Ｌ｟ＢｏＮｾ｟ＢＢＢ｟ｗＴｔ･ｲＮ､Ｎ･ＺｾＧ｟ _ " ' , _ Ｏｾ｟｜Ｌ＿ｲ .. ; --t.1j - -17 - i,Q' to Fig. 1

Longitudinal section along the axis of the dam: 1 - top soil and vegetation; 2 - natural ground surface; 3 - boundaries of maximum seasonal thawing (hachures point in the direction of permafrost);

4 - peat excavation;

5 -

silt excavation;

6 -

silty clay loam, ice content 60%; 7 - clay loam with rock waste and fragments of

bedrock, ice content 20 - 60%;

8 -

dense clay (bedrock);

9 -

outline of the cut-off trench; 10 - fractured dolomites; 11 - fractured marls; 12 - thin plates of limestone; 13 - old river bed filled with gravel and pebbles; 14 - spillway canal; 15 - crest

of the dam; 16 - talik beneath the river bed; 17 - revised boundary of the talik; 18 - excavation of alluvial deposits in the

river bed; 19 - boundary of the talik according to the plan; 20 - permafrost curtain along the side of the spillway canal

-17-c

t

....

----

-Fig. 2

Plan of the Irelyakh Dam: I - highway bridge supported by the piers of the ice-retaining wall; 2 - spillway canal; 3 - earth dam; 4 - axis of the permafrost curtain in the dam; 5 - reinforced concrete steps;

6 -

axis of the permafrost curtain along the right side of the spillway canal; 7 - permafrost curtain beneath

-18-Fig. 3

Cross-section of the dam (left bank flood plain) 1 - rockfill; 2 - heat-insulating layer of peat and moss;

3 -

sand surcharge; 4 - loam core of the dam;

5 -

permafrost boundary; 6 - cut-off filled with loam; 7 - [rost-heaved clay loam, ice content up to 60%; 8 - clay loam with marl fragments;

9 - silty sandy loam with gravel and pebbles; 10 - sand with plant remains; 12 - holes of the permafrost curtain; 13 - natural

ground surface; 14 - depth to which silt, peat and top soil have been removed; 15 - upper limit of bedrock

<PY ＱﾷＮｾ __ｾＮＬｾｾＺＭｾｾ｟ 2

6 12 5 IS

Fig. 4

Cross-section of the dam (river bed portion). For explanation see Figure

3;

11 - axis of temporary permafrost curtain no. 6a;

16-17 - alluvial river bed deposits consisting of sand, gravel and silt

-19-Fig.

5

Design of a freezing column: 1 - casing;

2 -

inner pipe; 3 - plug;

4 -

header;

5 - slide valve

Fig.

6

General view of freezing columns: 1 - inner pipe; 2 - casing; 3 - slide valve;

-20-A-r;

Fig.

7

Design of permafrost curtain beneath the spillway canal 1 - canal bottom; 2 - pipes of the freezing system;

-21-DISCHARGE AND BLOCKING UP THE RIVER DURING CONSTRUCTION OF THE VILYUI HYDRO-ELECTRIC POWER STATION

Gidrotekhnicheskoe Stroitel'stvo, (2): 1-5, 1966

The Vilyui hydro-electric power station is being built on the

Vilyui River (a tributary of the Lena River) which has its source in the Krasnoyarsk Krai, in the central part of the Middle-Siberian platform, and flows through the vast territory of the Western Yakut region. T1le Vilyui is a mountain river with many rapids and is 2,435 km long. It has not been adequately studied, since systematic observations of its regime have been carried out over the last thirty years at only one gauge in Suntar, 747 km from its mouth. The feeding of the river and

its regime depend on the fact that the Vilyui is one of the rivers of the polar basin which are c1laracterlzed by the presence of continuous permafrost (on the construction site permafrost is 250 m thick) and very severe continental climate. The amplitude of annual temperature fluctuations reaches almost 100°C, the summer is short and hot, and the winter cold. According to periodic observations, the maximum tempera-ture is 3SoC and the absolute minimum temperatempera-ture is -6IoC. The mean annual temperature of the air is _8°c. Precipitation is low and the mean annual precipitation does not exceed 350 mm. The drainage area of the river at the power station is 141,000 sq m.

The river is fed mainly by surface runoff. The annual distribution of the runoff is most uneven: about 84% of the volume is discharged in the spring, 14% in the summer and fall, and only 2% in winter. Freezing usually sets in in the middle of October when the level of autumn floods falls but the water level in the river is still high. The ice may be 1.5 to 1.8 m thick. Since the river flows in the zone of continuous perma-frost, its source of supply is completely cut off in winter and its dis-charge is sharply reduced. In some dry years and in especially severe winters the river may freeze solid. During construction the winter dis-charge did not exceed 1.5 m3 / s e c while the discharge at the time of spring floods reached 12,600 m3 / s e c , _{which exceeded the calculated discharge with} a frequency of 1%. The ratio of minimum and maximum discharge 1 : 8,400. During a flood the water level rises very rapidly. Under natural con-ditions the water level fluctuates as much as 15 m. In the period between the end of the spring flood and freeze-up, several floods due to rain usually occur during which the maximum discharge may reach 2,300 m3 / s e c . The maximum spring flood discharge at the dam is as follows:

Sf"'''.:J \:\oocl dis-c\-'nrj<2

"",,3/

se. c.

-22-I

O.J

I

I

I

5

I

10

I

20 113YlO

!

II1M19 400

i

3540J"75: • I I

The mean annual discharge with a frequency of

95%

is about 400 m3 j s e c . The dam is situated in a relatively narrow canyon which cuts into a trappean massif to a depth of 100 - 150 m. Main components of the hydro-electric power dam are as follows: a rockfill dam

64

m high and 600 m long along the crest with a facing of local loam, rubble and de-bris; a powerhouse of a semi-underground type located in the right bank of the river - the water is brought to the turbines by a headrace canal; a spillway canal about 1500 m long, the initial section of which (before the intake) simultaneously serves.as the headrace canal of the station; below the intake there is a spillway with a concrete sill and segmental 16 m high lift gate with a span of 40 m.

Extremely irregular annual runoff of the river was the governing factor in determining the order of er'ection of principal structures, es-pecially the rockfill dam, and the rate of djscharge during the construc-tion period. In the initial stages of construction the discharge was passed through an opening in the rockfill dam, and, after blocking of

the ｲｩｶｾｲＬ through the construction trench excavated in the rock on the

left bank of the river. The trench is about 650 m long and 20 - 22 m wide (16 m in the deepest part). The shape of the trench, its dimensions and discharge capacity have been determined by a series of investigations carried out at the hydraulic laboratory of the B. E. Vedeneev All-Union Research Institute of engineering and at the Department of Hydro-engineering, M. I. Kalinin Polytechnic Institute in Leningrad. The ini-tial section of the trench has a flat bottom and is 10 m deep. There is a trough-like depression 17 m deep in the middle section of the trench with the horizontal part of the trough being 17 m long. The horizontal sect jon, which forms a baffle in the trench, is joined with the bottom of the trough to form a rapid-flow section having an angle of slope exceeding the critical angle. At a rate of discharge e x c e e di ng 2,700 m3 j s e c ,

the baffle acts as a free-fall weir. The trough is designed to create a hy-draulic jump in order to increase the discharge capacity of the trench. According to laboratory investigations, the trough with the rapid-flow section makes it possible to lower the level of the forebay by four metres when discharging the rated flood. To dissipate the stream energy still further, the end portion of the trench is provided with dissipators in the form of trenches 4 m deep and 80 - 110 m long, the axes of which are

-23-at an angle of 1460 to the direction of the stream.

The design of the construction trench was somewhat modified in accordance with suggestions made by engineers on the site. A channel

5 m deep and 12 m wide (measurements dictated by local conditions) was cut in the sill, which in this section is 10 m above the river bed. This made it possible to reduce by 5 m the height of the upstream cofferdam in the river bed portion of the dam and hence lower the pressure on the cofferdam when passing the winter discharge through the construction trench.

In the first year of construction, the discharge was passed through the natural river bed narrowed down somewhat by rock debris from the river banks.

An embankment was built within the upstream cofferdam in the river bed portion of the dam in order to carry out the necessary foundation work for the dam and erecting the concrete grouting gallery. This temporary blocking of the river was carried out on the 19th of March 1964 when the discharge did not exceed 6 m3 j s e c . During this time the river runoff was stored in the forebay. After the 27th of March, when the water level was raised by

3

m, the discharge was passed through the unfinished con-struction trench. The lowering of the sill in the trench greatly simpli-fied the construction of the spillway canal; the inflow of ground water was insignificant and this made it possible to erect the grouting gallery in a short time on a section 200 m long in the river bed portion of the dam (Fig. 1) prior to the beginning of the spring flood of 1964.

The concrete grouting gallery is crossed by the construction trench in section no.

33.

The bottom of the gallery is 2 m below the bottom of the cut-off trench at the sill. For the temperary blocking of the river, concrete in section no. 33 was laid only to the height of the first level and in adjacent sections to the design level. Concrete plugs were provided on the side of the keyed section no. 33. In this way a deep opening 13 m wide and 5m was formed in the concrete grouting gallery for the passage of the winter discharge (Fig. 2). Concrete was laid in the keyed section to the design level prior to the onset of spring floods of 1965.

By the onset of the spring flood of 1964, the rockfill dam on the right bank was built up to a height of 30 m and on the left bank to a height of 37 m. The river bed in the dam section was narrowed down to 90 m. According to the design, this opening must be at least 120 m wide to discharge the ice and the 10% spring flood (8,540 m3 j s e c ) . _On

-24-some of the rock could have been washed away. This risk was justi-fied to a certain extent: about 15,000 m3 _{of rock was washed away on}

discharging the flood but the quarrying went on without interruption and the excavated rock was used for rockfill.

By the onset of the spring flood of 1964 the grouting gallery with a concrete cover in the river bed portion was completed. This gave rise to a threshold 3 m high in front of the dam. A second threshold was for-med upstream from the gallery by the upstream cofferdam which was par-tially washed away by the flood. Themaximum discharge of the 1964 spring flood was 6400 m3 / s e c ;

the drop in the levels within the main structures reached 5 m. The rate of flow in the opening came to 4.5 - 6 m/sec. After the spring flood, cofferdams were erected at the entrance to and the exit from the construction trench, and the latter was drained to clear the rocks which fell from the left bank. The summer and autumn floods of 1964 were discharged through the opening in the rockfill dam, after which the river bed was blocked in late October 1964 and the dis-charge was diverted to the construction trench.

The entrance to the trench was again temporarily blocked on the 25th of March 1965 to place concrete for the second level of keyed section no. 33 and to complete the work on the construction trench

(Widening of the left bank, ､･･ｰ･ｮｩｮｾ of the trough, and to finish drilling the energy-dissipating channels). The discharge was stored in the fore-bay. Since the discharge during this period was only 4 m3 / s e c , _{the water} level in the temporary storage basin rose by 2.14 m in 21 days. At

the same time, clay loam was placed above the gallery, and work was done on installing the inverted filter and placing the surcharge and rock toe. The clay loam was protected from above by prefabricated reinforced con-crete plates. Hence a step was formed in the construction trench prior to discharging the spring flood. The height of the step was determined by the height of the crest of the gallery.

The discharge during the second stage of construction was of greatest interest. According to calculations, the construction trench was cap-able of discharging the spring flood of 1965 at a rate of 9,400 m3 / s e c _(5%

frequency) by raising the water level in the [orebay by approximately 25 m. Because of this, the unfinished dam had to be raised to a height capable of withstanding a head of up to 35 m. By the beginning of the flood the rockfill dam in the river bed portion was raised to a height of 40 m and the loam facing to a height of 25 m (Fig. 3). It was expected that under such conditions a powerful filtration stream would flow through the upper

-25-part of the rockfill with seepage in the lower -25-part. To prevent this, the downstream slope was flattened and reinforced with coarse rock, the volume of which exceeded 150,000 m3• The antiseepage structure on the upstream slope consisted of a layer of rubble and a second layer of the filter. It was expected that the dam would settle by 10% of its height as a result of loss of ｦｩｮ･Ｍｾｲ｡ｩｮ･､ material from the rockfill. The right bank of the construction trench was protected to a height of 41.6 m by a concrete wall. The calculated rate of flow at the entrance to the construction trench was 4.5 m/sec, and therefore the surface of the rockfill dam in this section was reinforced by concrete plates. Various other Ineasures were adopted to strengthen this section of the dam.

The pumping station, the concrete plant, the construction labora-tory and other buildings, as well as the I'oad leading from the quarries to the dam, al'e allan the right bank below the dam. A dyke 180 m long was erected to protect this section of the right bank from erosion by placing 41,000 m3 _{of rubble and debris and 14,500 m}3 _{of large rocks} weighing

5

to 15 tons (extra large rocks were placed by a powerful crane and held together by a net of reinforced steel). Additional measures were taken to protect the concrete plant and the pumping station from possible ice damage at high water levels.

The spring discharge was considerably below the estimates and the crest of the flood was lowered due to the presence of the storage basin. The actual discharge during the spring flood of 1965 did not exceed 4,620 m3 / s e c .

The largest drop in the levels within the dam structures, equal to 14.9 m, did not coincide in time with the maximum discharge. The stream, which was leaving the trench with a velocity of 18 - 19 m/sec, was joined to the tailrace by a hydraulic jump, beyond which waves four to five metres high were observed initially for a considerable distance. The direction of the stream during this period was well defined. It followed the axis of the exit section of the trench and was directed to-wards the dyke protecting the right bank. On hitting ttle dyke it was divided into two streams; the main stream with a velocity of 16-18 m/sec turned and proceeded downstream along the bank, the second formed a low wave with considerable eddy velocities immediately behind the dam. The dyke was substantially destroyed within two days, while the wave eroded the slope of the ｾｯ｣ｫＭｦｩｬｬ･､ section accommodating the workshops. On further increase in the discharge and the rate of flow, the loosened rocks in the channels which were to act as energy dissipators, were washed out and the latter became operational. The right bank of the trench in the

-26-exit section was exposed to rapid erosion. Because of this the exit section was widened and the stream was flattened. A redistribution of velocities took place and the erosion of the dyke stopped, but the erosion of the exit section still occurred. The rock banks of the trench were undercut in places to a depth of 2 m, but there were no cave-ins or rock slides. Low quality rockfill on the left bank was substantially eroded also. The eroded rocks were transported by the stream for 250 - 300 m and deposited in the river bed opposite the cement plant in the form of a flattened ridge over 2 m in height and with a total volume of over 40,000 m3

•

The ice passed through the trench at low water levels without complications. A channel 50 m wide and 1,600 m long was blasted in the ice above the dam, and thus played a positive role in the initial ice push. On blasting, the ice was fractured across the entire width of the river and therefore entered the narrow trench relatively easily. The difference in water levels at the sill formed by the grouting gallery at the entrance to the trench was retained, and the ice was broken up there (Fig. 4).

The construction trench will be completely dammed during the third stage of construction in the winter of 1965-1966 when the river discharge is 20 m3 / s e c .

The entire discharge will be stored in the forebay and the filling of the water reservoir will be started. A rockfill dam will be erected in this section to a height of 55 m prior to the spring flood, i.e. over a five-month period; 140,000 m3 _{of clay loam will be placed} under severe winter conditions. The work will have to be kept ahead of the rise in the water level in the reservoir. Special attention will have to be paid to joining this part of the dam to the river bed portion which is under pressure. In this stage of construction the spring flood will be passed through the permanent spillway canal.

Based on hydrological considerations, it was decided to block the bed of the Vilyui River after the summer-autumn flood. The date of blocking (the end of October 1964) depended on the state of readiness of the construction trench and the erection of the dam to a height which would permit discharge of the 1965 spring flood through the trench.

Inadequate knowledge of the geological structure in the construction trench area resulted in extensive rock slides on the slopes of the trench which were constructed according to the design (slope 10 : 1). This took place in the summer of 1964 and the clearing operations lasted until the

-27-30th of October. The final date for blocking was set for the 31st of October*.

The design** ･ｮｶｩｳ｡ｾ･､ the blocking of the Vilyui River by a pioneering method involving a wide-profile dyke, ､ｵｾｰｩｮｧ rock into the

opening in the ｾ｡ｩｮ body of the rockfill ､｡ｾＬ subsequently blocking

t:je ｵｰｳｴｲ･Ｒｾ cofferdam, and diverting the river into the construction

trench. 7he rock was to be placed from the right ｢｡ｮｾ along a section

60 m wide. The width of the opening along the water edge was to be 14 m

at ｴｾＱ･ start of blocking operations when the calculated rate of

dis-cnarge would be 200 m3 / s e c . The maximum velocity in the opening was ex-pected to be 7.5 m/sec and the maximum difference in levels on closing the dyke ＵＮＹｾＮ ｾｯ significant filtration through the dyke was expected since it was assumed i t would be made water tight by slush ice and

fine-Grained rock.

blocking operation

volume of rockfill required for final stages of the was ･ｳｴｩｾ｡ｴ･､ at 53,000 m3

including over 48,000 m3 of quarry run rock and 4,500 ｾＳ of large rocks (weighing between 5 and 25 tons).

ｾｨ･ preparations for the blocking operation were conducted in

con-ｪｵｮ｣ｴｩｯｾ with the ｣ｬ･｡ｾｩｮｧ _{of the} ｣ｯｮｳｴｾｵ｣ｴｩｯｮ _{trench. About 4,500 m}3

of large rocks were sorted and placed in three piles (rocks weighing

5

to 10 tons in the first pile, 10 to 15 tons in the second, and 15 to 25 tons in the third). Because of a considerable drop in the river dis-charge which occurred in the second ten days of October, the rocks of standard and non-standard sizes were filled along the entire right bank and the opening was gradually narrowed. The left bank of the opening was reinforced by large rocks weighing 5 tons to protect the rockfill ｦｲｯｾ erosion.

The plan of the blocking operation was then somewhat modified. The

*

7he date set for the ｣ｯｾｰｬ･ｴ･ blocking of the Vilyui River (end of October 1964) cannot be considered a good choice, since the final stages of dam erection had to be carried out under extremely severe conditions. A large proportion of work on the dam and placing the

ｬｯ｡Ａｾ facing had to be done in a very short time under harsh winter

｣ｯｮ､ｩｴｩｯｾｳＮ

** The design was developed by the Leningrad section of Gidroproekt and the Vilyuigesstroi and was based on model studies carried out at the

ｊ･ｰ｡ｲｴｾ･ｮｴ of Hydraulics, Moscow Institute of Energy, under the

-28-blocking front was reduced to 30 m, and as a result of a narrowed river bed it was found that in the final stage of blocking it would be sufficient to fill 10,000 m3 of quarry run rock and 1,500 m3 of non-standard rock. The quarry run rock was obtained from the exca-vated spillway canal, supply dumps and quarries with the help of EKG-4 excavators. The total volume of rockfill prepared for blocking came to 40,000 m3• Two excavators (E-1252 and EKG-4) were modified to act as cranes in the loading of large rocks. Forty-five dump trucks (MAZ-525 and KRAZ-222 types) were provided for the transportation of quarry run rock, and in addition 11 trucks of the same types were specially equipped for the transportation of large rocks. Fourteen KRAZ-222 dump trucl{s were held in reserve. Fifteen bulldozers were made available for work in the quarries and for the damming operation. Three emergency dump trucks of the MAZ-205 type loaded with salt and rock flour from the crushers were made available to repair steep sections of the roads and to keep them free from ice.

Blocking of the river bed was started on the 31st October at 1 p.m., when the discharge was 77 m3 / s e c _{and the difference in level 5.0 m. A} part of this difference, equal to 0.5 m, was concentrated at the upstream cofferdam and at the rock toe of the dam. The maximum rate of flow in the opening did not exceed 4.5 m/sec. At the time of blocking, the river was completely frozen. (except for an air hole in the vicinity of the power plant). Slush ice was practically absent.

Thanks to good performance of equipment and good state of the roads, the average rate of the rock filling operation reached 500 m3 / h o u r .

The opening in the rockfill dam was 15 m deep. Under such conditions the crest of the blocking plug was 10 m above the diversion trench, which made it necessary to provide a large volume of rockfill. At the same time, however, the great height of the plug created more favourable hy-draulic conditions: when placed, large rocks came to rest in the lower part of the plug as a result of segregation of material, which improved the filtration properties of this part of the plug without reducing its stability. When the blocking was complete, the filtration discharge through this plug was 43 m3 / s e c ,

or 56% of the river discharge.

As a result of favourable conditions under which the blocking opera-tion was taking place and due to the fact that rocks were not scattered, the width of the plug was reduced to 10 m on top, although at the bottom it still exceeded 50 m.

The blocking of the river bed was completed at 4.45 p.m. on the same day when the first trucks drove across the plug to the left bank. Work

-29-was t h en started on blocking the upstream cofferdam, placing its clay l o a m and r ock sur charge and pumping the basin. Th e discharge of the Vi lyu i Riv er was diverted to the construction trench with partial sto-rage i n the stosto-rage basin formed by that time.

Fig . 1

Con s t r u c t i o n of the river bed portion of the grouting gallery

Fig. 2

Di s c ha r g e through the keyed section of the g r o ut i ng gallery

-30-'11.5

Fig.

3

The state of readiness of the river bed portion of the rockfill dam at the beginning of the spring flood

of 1965: 1 - rockfill; 2 - large rocks reinforcing the downstream slope : 3 - levelling layer; 4 - second

layer of filter (graip size : 0 - 150 mm); 5 - first layer of filter (grain size: 0 - 40 mm) ;

6 -

loam

facing;

7 -

filter of sand and gravel;

8 -

rock surcharge; 9 - rock toe ; 10 - grouting gallery ; 11 - concrete layer on rock at the base of loam facing

Fig .

4