DAM ONTHEARPACHAI RIVER

G. O. Ovsepyau UDC 627.824.31

The reservoir on the Arpachai River in the Nakhichevan ASSR was accepted for operation at the end of 1977. The reservoir was created by an earth--rock dam* with a maximtzn height of 66 m (Fig. 1).

The structures of the hydroproject include also temporary and operating spillways, which combine in a tunnel driven on the right bank. A vertical loam core (bottom width 0.35H) is the cutoff element of the dam. The shoulders are made of alluvial gravel-~ebble soils with a surcharge of broken unsorted riprap. Between the core and the shoulders are transition (fil- ter) zones 2 m thick, made of a sand-gravel mixture consisting of fractions 0-40 mm (to the 50-m level) and 0-i0 mm (above the 50-m level). Placement of a fine-sand filter in the upper zone of the dam was recommended for providing self-healing of possible cracks in the core.

The volume of the dam was 4.3 million mS; the loams in the core amount to 9% and the rock fill to 23Z. The granulemetric composition of the soils suitable for placing in the body of the dam, taken according to the data of investigations of deposits of natural construction materials, is shown in Fig. 2. The filter material was obtained from the channnel gravel-- pebble soils by sorting out the large fractions. The valley of the Arpachai River in the vicinity of the reservoir intersects several large anticlinal folds composed of Devonian and Permian carbonate rocks consisting mainly of bedded limestones [I]. The geological struc- tures are broken by a system of faults into blocks of varying sizes. One of the large faults runs along the left abutment of the dam, intersecting the river channel at an acute angle. The region is characterized by seismicity with a scale intensity up to 9.

Alluvial gravel-~ebble deposits up to 40 m thick, underlain by steeply dipping beds of limestones that are tectonically strongly disturbed and fractured, form the foundation of the dam in the U-shaped river canyon (Fig. 3). These rocks compose also the side abutments of the dam, forming steep cliffs.

To block off routes of bypass seepage, the stratum of gravel--pebble soils in the dam foundation was penetrated by a bored concrete wall linked with the core soils. A grout cur- tain is provided for around the entire perimeter of the dam foundation in hard rocks, which intersects the regional fault.

The creation of the bored concrete wall and the grout curtain under it was accomplished through the 5-m thick loam layer of the core. The curtains on the flanks were made from tunnels driven from the base of the slope. The dam core was linked with the rock flanks by a leveling concrete slab placed in a trench after removing the weathered rock. The fractured rocks under the slab were areally grouted.

During the initial construction period experimental rolling of the soils in the embank- ments was carried out, which made it possible to select the optimal parameters for placing the soil in the dam. On the basis of the data obtained, the following soil densities were designated after consideration of the characteristics of the structure: in the core loams, up to the 5-m level, 1.70 g/cm ~, in the 5-60 m interval, 1.66 g/cm 3, and above the 60-m level, 1.57 g/cmS; in the gravel-~ebble soils of the shoulders, up to the 25-m level, 2.29 g/cm s and

*Constructed by the Azerbaldzhan Trust for the Construction and Assembly of Electric Power Stations (Azenergostroi) according to the design of the State Institute for the Design of Water-Management Structures, Ministry of Reclemation and Water Management of the Azerbaidzhan SSR (Azgiprovodkhoz); site explorations and geotechnical control were performed by the All- Union Planning, Surveying, and Scientific-Research Institute (Gidroproekt) (Baku and Moscow branches).

Translated from Gidrotekhnicheskoe Stroitel'stvo, No. 8, pp. 36-38, August, 1979.

0018-8220/79/0008-0795507.50 �9 1980 Plenum Publishing Corporation 795

796 G.O. OVSEPYAN

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Fig. 2

Fig. 1~ Profile of the dam in the channel part. 1) Core of loam soils; 2) fil- ter of sand-gravel mixture (fractions smaller than 40 ram); 3) shoulders of allu- vial gravel-pebble soils; 4) broken rock rlprap; 5) bored concrete wall in the stratum of gravel--pebble soils of the foundation; 6) grout curtain in fractured limestones of foundation.

Fig. 2. Granulometrlc composition of soils in the dam body. i) Loam soils in core; 2) gravel--pebble soils in shoulders; 3) rockfi11 materlal.

100

Fig. 3. Geologic section through site of the Arpachal dam. 1) Pebble bed with gravel and boulders with sand and sand-- clay filler; 2) dlabase sheet intrusions; 3) bedded bituminous limestones with thin interlayers of paper shales; 4) thin- bedded limestones with Interlayers of shales (layer 7); 5) dlfferently bedded limestones, here and there massive, with interlayers of shales, rarely sandstones and marls (layers 1-6); 6) fault zones; 7) groundwater level; 8) contour of trim- ming for the dam core; 9) contour of the grout curtain; i0) tunnel for the tem- porary and operating spillways. DM = distance mark.

above 25 m, 2.26 glcmS; in the sand--gravel mixture of the filter zones, to the 50-m level, 2.00 g/cm s and above the 50-m level, 1.90 g/cm=; and in the rock fill, 1.80 g/cm ~. The opti- mal moisture content of the los~ns was 17-20~ To provide plasticity in the core so as to eliminate disturbance of its continuity by the formation of cracks during deformation, a moisture content of the loams 2-3X above the optimal was specified for the flank abutments and crest of the dam (above the 60-m level).

The dam was constructed by the method of layer-by-layer filling with rolling by earth- compacting equipment (multlwheel rollers, loaded vehicles, and tamping plates). The thick- nesses of the soil layers were selected on the basis of the results of experimental rolling with consideration of the change in soil density over the height of the dam and were: in the core loams, 30 c~n up to the 57-cm level, 50 cm in the 57-60-m interval, and 100 cm above the 60-m level; in the gravel--pebble soils of the shoulders, 0.5 m to the 57-m level and 1.0 m above 57 m; and 1.5 m in the rock fill.

DAM ON THE ARPACHAI RIVER 797

TABLE 1

Indices

Volume.of fill, m"

No. of deter- minatiom of demitv of soil skeleton

No. of analy-

ooInI~)s io fion

Year of comtrucUon

1972

0.2

462

44

1973 1974 1975 1976

0,5 0,3 1,0 I ,I

1446 1054 2882 6060

73 79 137 198

19"/7

,2

733

294

TABLE 2

rotal Proba- bility

4,3

19 6,37 0o% S0% 1

NOo o~176176 825 deter.

Abut- Intervals of levels, m merits

TABLE 3. Density of the G r a v e l - - P e b b l e Sol1 S k e l e t o n in Dam S h o u l d e r s , g/oms

Probability

90% 5o% tOo/o

No. of deter- m~t~m

I~e~,als of levels, m

up .ea rn shoulder

2,27 2,28 2,30 2,30 2,32 2,31

5313

0-25 I dow mtream should.

2,28 I 2,28 2,30 2,30 2,32 2,32

4360

D u r i n g t h e e n t i r e t ~ n e o f dmu c o n s t r u c t i o n g e o t e c h n i c a I c o n t r o l was c a r r i e d o u t , t h e ma in t a s k s o f w h i c h c o n s i s t e d i n c h e c k i n g t h e f i l l i n g t e c h n o l o g y , t h e q u a l i t y o f t h e s o i l s p l a c e d i n t h e dam, and t h e d e n s i t y a t t a i n e d i n so d o i n g i n c o n f o r m i t y w i t h t h e r e q u i r e m e n t s [2 , 3 ] . T a b l e 1 shows t h e number o f d e t e r m i n a t i o n s o f t h e m a i n i n d i c e s o f t h e s o i l s a s c r i - t e r i a o f t h e q u a l i t y o f dam c o n s t r u c t i o n .

I n t h i s c a s e t h e v o l u m e o f p l a c e d s o i l p e r d e t e r m i n a t i o n o f t h e d e n s i t y o f t h e s o i l skeleton was: in the core loans, from 60 to 125 m s (average 75 mS); in the gravel--pebble soils of the shoulders, from 240 to 320 m 3 (average 265 m. s) ; in the sand--gravel soils of the filter zones, from 25 to 75 m s (average 35 m s ) ; in the rock fill, from 3000 to 7000 m s (ave- rage 4500 m s) . The volume of gravel--pebble soll per analysis of the granulometric composition was greater than the recommended [2, 3] and averaged 8200 m s, varying within the range, 6000- 12,000 m = . Such a divergence is explained primarily by the difficulty of manual screening of a large amount of soil, especially in the case of a high rate of dam construction. How- aver, the stability of the granulometric composition of the borrow material with quite small deviations of the limiting curves from the mean made it possible to evaluate with confidence the quality of the gravel--pebble soil placed in the dam as corresponding to the design re- quirements. It is necessary to note that the requirements of the standards [2, 3] with re- spect to the number of determinations of the granulometric composition of sand-gravel soils are ~n need of scientific substantiation [4] in which the properties of the soils (especially the variability of the granulometric composition) should be taken into account. Statistical treatment of the data obtained showed that the density of placing the losm in the core is characterized by the data given in Table 2. These iwlices were attained mainly after 8-10 passes of the D-551 roller.

As we see from Table 2, the density the loam placed in the core is sufficiently high, the difference between the extreme values of the soil skeleton density (90 and 10% probabili- ties) being only 0.03-0.06 g/cm s. The moisture content of the soil during its placement was mainly within the optimal values; however, the design requirement that the moisture content of the loam be 2-3% higher in the zones of the flank abutments and above the 60-m level (to increase plasticity) was for all practical purposes not met.

The density of the gravel--pebble soll placed in the shoulders of the dam is rather high and is characterized by the data in Table 3.

798 G.O. OVSEPYAN

The indicated density of the gravel--pebble soil was reached after 6-8 passes of the D-551 roller in combination with rolling by loaded vehicles. The filter zone was entirely filled with a sand--gravel mixture consisting of fractions 0-40 mm. The density of the placed soll averaged 2.02 g/cm 3 (50% probability) with extreme values of 1.99-2.06 g/cm 3 (90 and 10% probabillties). Placement of the filter material was carried out jointly with the gravel-- pebble soil.

The limestone rock was packed layer-by-layer by multlwheel rollers and loaded vehicles. Control samples for determining the placement density were taken from pits through the entire depth of the layer (1.5 m), the volume of which was 1.5-2.0 m 3. The density of the material placed in the rock fill, expressed as the dry unit weight, averaged 1.86-1.87 g/ems (50% probability) with extreme values of 1.83 and 1.90 g/cm 5, respectively, and 90 and 10% p r o b a b i l i t i e s .

During c o n s t r u c t i o n the seepage p r o p e r t i e s of the s o i l s in e lements of the dam were de - termined s y s t e m a t i c a l l y . The p e r m e a b i l i t y of the core s o i l s a cco rd ing to the d a t a of i n v e s - t i g a t i o n s of m o n o l i t h s i n which the d e n s i t i e s of loam a c t u a l l y a t t a i n e d was e s t ima ted by the p e r m e a b i l i t y c o e f f i c i e n t to be 3 - i 0 - s cm/sec . I n the dam shou lde r s the p e r m e a b i l i t y c o e f f i - c i e n t , de te rmined by pour ing wate r i n t o the g r a v e l - p e b b l e s o i l s , averaged I i m/day, v a r y i n g from 7 to 15 m/day. In t he f i l t e r zones t h i s c o e f f i c i e n t averaged 9 .0 m/day wi th v a r i a t i o n s w i t h i n 6 -10 .3 m/day.

CONCLUSIONS

i. The earth--rock dam is suited to the engineerlng-geologic conditions of the Arpachai valley, which are characterized by a complex tectonic structure, presence of a deep alluvial incision in the flood-plain of the river, and high seismlcity of the region.

2. Geotechnical control carefully performed during construction made it possible to achieve the indices of the soils in the dam stipulated by the design. It should be ac- knowledged that the requirements of the standards [2, 3] with respect to the number of deter- minations of the granulometric composition of sand-gravel soils (one determination per 1000- 2000 m 3 of fill), in our opinion, are markedly overstated. It apparently should be based on the specific properties of the soil with consideration of its granulometric composition.

LITERATURE CITED

i. G.O. Ovsepyan, "Engineering-geologic conditions of constructing dams in the valley of the Eastern Arpachai River," in: Summaries of Reports of the Fourth Conference in Sur- veyors of Gidroproekt, Eng. Sec. [in Russian], No. i, Informenergo, Moscow (1972).

2. SNiP III-B.I-71. Earth Structures. Rules of Performing and Accepting the Works [in Russian], Stroiizdat, Moscow (1972).

3. Guide to Quality Control of the Construction of Earth Dams. II 42-75 [in Russian], VNIIG, Leningrad (1976).

4. G.O. Ovsepyan, "Quality control during construction of earth dams," Gidrotekh. Strolt., No. 8 (1973).