5
FULL-SCALE INVESTIGATIONS OF SEEPAGE IN AN EXPERIMENTAL BLAST-FILl, DAM V. P. Nedriga, G. [. Pokrovskii, V. F. Korehevski[, and G.N. Petrov UDC 624.131.6:827.824 A number of impounding structures have been constructed in our country by means of blasting. The largest of them are the mudflow-control dam at Medeo, the dam of the Baipaza hydrodevelopment on the Vakhsh River, the height of which exceed 60 m, the 90-m-high dam on the Akh-Su River in Dagestan, coefferdams of the Nurek and Chlrkey hydrodevelopments, and a number of others. Thus the Soviet Union can rightfully be considered the pioneer of this method of constructing dams. Construction experience shows that the use of blast-fill dams at hlgh-head hydrodevelopments will have a maximum economic effect, since their construction can be accomplished in a shorter time and with smaller capital investments. However, the construction of dams by the blasting method has nonetheless not gained wide use. The main reason for this is the lack of knowledge about the properties of the material in such struc- tures. An experimental 50-m-hlgh dam was constructed by the blasting method in February 1975 on the Burlyklya River* in the Kirgiz SSR (Fig. i) for the purpose of obtaining the neces- sary data for substantiating the blast-fill dam project of the Kambaratln hydrodevelopment. To conduct the investigations on the experimental site, 10 shafts were driven to depths of 21-47 m at five sites (Fig. 2). These shafts were used during sinking for determining the granulometrlc composition, unit weight, and permeability of the earth over the height of the structure and subsequently (after filling the reservoir) as plezometers for measuring the drawdown curve of seepage [i]. The seepage characteristics of the soll in the dam were determined at several water levels in the reservoir. The seepage drawdown curve for the maximum water level in the reservoir is shown in Fig. i. Table 1 gives the water-level elevations in the shafts at different water levels in the reservoir. The seepage properties of the soil composing the dam proved to be different in different zones. The central zone, located between shafts 1 and 3, has the maximum water-retalnlng capacity and the zones of the upstream and down- stream shoulders of the dam have the minimum. Analytic methods were used for determining the numerical values of the permeability co- efficients of the soll in the different zones of the dam. The permeability coefficient was determined by the equations of steady plane seepage for discharges and water levels in the shafts and upper pool recorded for 15-20 days. The values of the permeability coefficient were determined twice at the save level -- in the first case by Eq. (I) on the assumption that the flow was laminar with a linear law of resistance, and in the second by Eq. (2) on the assumption that the flow is turbulent; this is seen from the equations = 2LQ/~v(h~+ %) H (1) and = 2Ql~v(h, + h,)~77. (2) where K Z and K t are the permeability coefficients, respectively, in the case of linear and quadratic laws of motion of the flow, cm/sec; Q is the seepage through the dam, mS/sec; hl and h2 are the depths of the seepage in the initial and end cross sections of the investi- gated zone, m; H is the head, reckoned from the horizontal reference plane, m; I is the pressure gradient; L is the distance between sections, m; bay is the average width of the canyon at the selected zone, m. *See the articles published in Gidrotekh. Strolt., No. 5 (1977). Translated from Gidrotekhnlcheskoe Stroitel'stvo, No. 7, pp. 21-24, July, 1978. 0018-8220/78/0007-0679507.50 1979 Plenum Publishing Corporation 679

Full-scale investigations of seepage in an experimental blast-fill dam

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FULL-SCALE INVESTIGATIONS OF SEEPAGE IN AN EXPERIMENTAL

BLAST-FILl, DAM

V. P. Nedriga, G. [. Pokrovskii,

V. F. Korehevski[, and G.N. Petrov

UDC 624.131.6:827.824

A number of impounding structures have been constructed in our country by means of blasting. The largest of them are the mudflow-control dam at Medeo, the dam of the Baipaza hydrodevelopment on the Vakhsh River, the height of which exceed 60 m, the 90-m-high dam on the Akh-Su River in Dagestan, coefferdams of the Nurek and Chlrkey hydrodevelopments, and a number of others. Thus the Soviet Union can rightfully be considered the pioneer of this method of constructing dams. Construction experience shows that the use of blast-fill dams at hlgh-head hydrodevelopments will have a maximum economic effect, since their construction can be accomplished in a shorter time and with smaller capital investments. However, the construction of dams by the blasting method has nonetheless not gained wide use. The main reason for this is the lack of knowledge about the properties of the material in such struc- tures.

An experimental 50-m-hlgh dam was constructed by the blasting method in February 1975 on the Burlyklya River* in the Kirgiz SSR (Fig. i) for the purpose of obtaining the neces- sary data for substantiating the blast-fill dam project of the Kambaratln hydrodevelopment. To conduct the investigations on the experimental site, 10 shafts were driven to depths of 21-47 m at five sites (Fig. 2). These shafts were used during sinking for determining the granulometrlc composition, unit weight, and permeability of the earth over the height of the structure and subsequently (after filling the reservoir) as plezometers for measuring the drawdown curve of seepage [i].

The seepage characteristics of the soll in the dam were determined at several water levels in the reservoir. The seepage drawdown curve for the maximum water level in the reservoir is shown in Fig. i. Table 1 gives the water-level elevations in the shafts at different water levels in the reservoir. The seepage properties of the soil composing the dam proved to be different in different zones. The central zone, located between shafts 1 and 3, has the maximum water-retalnlng capacity and the zones of the upstream and down- stream shoulders of the dam have the minimum.

Analytic methods were used for determining the numerical values of the permeability co- efficients of the soll in the different zones of the dam. The permeability coefficient was determined by the equations of steady plane seepage for discharges and water levels in the shafts and upper pool recorded for 15-20 days. The values of the permeability coefficient were determined twice at the save level -- in the first case by Eq. (I) on the assumption that the flow was laminar with a linear law of resistance, and in the second by Eq. (2) on the assumption that the flow is turbulent; this is seen from the equations

= 2LQ/~v(h ~ + %) H (1) and

= 2Ql~v(h, + h,) ~77. (2)

where K Z and K t are the permeability coefficients, respectively, in the case of linear and quadratic laws of motion of the flow, cm/sec; Q is the seepage through the dam, mS/sec; hl and h2 are the depths of the seepage in the initial and end cross sections of the investi- gated zone, m; H is the head, reckoned from the horizontal reference plane, m; I is the pressure gradient; L is the distance between sections, m; bay is the average width of the canyon at the selected zone, m.

*See the articles published in Gidrotekh. Strolt., No. 5 (1977).

Translated from Gidrotekhnlcheskoe Stroitel'stvo, No. 7, pp. 21-24, July, 1978.

0018-8220/78/0007-0679507.50 �9 1979 Plenum Publishing Corporation 679

680 V.P. NEDRIGA ET AL.

TABLE 1 Wa~er level Water level in shaft, m

reservoir, ! 3 6 ] 9 IO

I

I

45,0 40,0 35,0 30,3 27,2 .

44,0 38,8 31,8 27,2 24,2

29,9 26,7 24,4 22,8 21,5

27,0 I 25,5 24,1 22,0 21,5 20,5 19,9

23,4 19,7

Sh-3 Sh. ~$h~6~.~ Sh-9

_

# r / ....-/ /r162 r z~ ~.. r /~. ~.//z "/~/r162 .../r162162162162

Fig. i. Cross section of dam along axis of channel.

// ~m'~-~= ==---~=~ ====-----~-~ - "~-. \ % Z' [,%'~ ,~r "e.. "--'.. %

% .'/ zr "% V~ -[~-"-'_-.: _~.__.~ ~ r

N" r ~ " s", .... ___~_.-.'9.-~Butlvldva

Fig. 2. Seepage flow in experimental dam at 48-m water level in the reservoir. I) Blast- ing drifts; 2) contour of blasted mound of rock; 3) investigative shafts.

The results of the calculations by Eqs. (I) and (2) are presented in Table 2. We see from the data that the values of the permeability coefficients for different zones in the dam vary over wide limits. For example, in the central zone between shafts 1 and 3 they vary from 0.57 to 1.13 cm/sec for the linear and from 0.37 to 0.46 cm/sec for the quadratic law of motion. Such a change in the seepage properties of the soils in the dam is explained primarily by the difference in their physical and mechanical indices. A determination of the unit weight during sinking of the shafts showed that in the central zone (shaft 1-shaft 3) it is considerably higher than on the upstream and downstream slopes. In addition, the percentage content of fractions less than 5 mm is variable in these zones. For example, the content of fractions less than 5 mm in the central zone is considerably higher than in the others. The content of these fractions also increases with increase in depth (Fig. 3).

The change in the physical and mechanical properties of soils in the dam is related to the technology of blasting, namely: position of the blasting drifts in plan and height, change in the power of the linearly distributed charges, and sequence of their explosion.

EXPERII~ENTAL BLAST-FILL DAM 681

+ I I ~'~- .rumBa ; l i d

7z

E

=. e~

8~

28

Y8 Sh-I

Fig. 3.

TABLE 2

Zones of [.~ determ- ! > ining the ~ r m e - I ~

coeff. I ~ ,E Entry

sec . s shaft 1

Betwee~ shafts 1 and

B e t w e e n shafts 3 and 6

Between shafts and 9

B e t w e e n shafts 9 and 1(]

Entry see,, shaft I0

E E

48.l 29.65 29.6 54.25 45.0 [ 26.4 26.1 47.5 40 0 21.0 20.9 41.0 35,0 [ 16,5 11.9 ] 35,8 3o.3 11o,3 9,35 30,5 ~,3 ~l 0,9 __6'2613~176 48.1 ! 29.6 14.9 i 44.5 45,0 j 26,1 12,6 40,5 40,0 20,9 9.,3 35.9 35,0 13,9 7,0 32,9 : 30.3 9.35 i 5.4 26.65 27.3 6.26 4.15 29.75

48.1 14.9 12,5 38,25 46.0 12.5 10.3 34.95 40.0 9.3 7.4 32.5 35,0 7.0 4.8.5 30,0 3oo 5,4 3.8 ~ 8 27,3 4,15 3,25 26:5 l 46,1 12,6 12,0 33.5 4.%0 ;o,3 11,o 31,9 40.0 7.4 7.5 30.75

48,1 : 12,6 14.0 i 30,5 45,0 j II .0 11,2 27,6 40,0 ! 7,6 7,5 i 25,8

i 48.1 29.65 14.0 45.0 45.0 26.4 11.2 48.5 40.0 21.0 7.5 45.5

0.6 3.0 1,0 2.3 1,2 I ,IE 3.~ 0.5 r 3.0 0.3~ 3.1 0.1 r .

15,2 3,0 14,1 12,1 7,4 4,4~ 2,61

3,1 3,0 2,9 2,3 2,6 2,~ 0.59 2.3 0.38 1.6 i 0.19

2.1 3.0 ! ,5 2.3 2,1 1.18

i~9 8.0 2.1 2,3 2.6 1.18

22.9 3.0 21,8 2,3 20.3 1.18

3.0 6.59 5.35

1.18 5,15 0.59 2.23 0.38 3.42 0.19 3.41

0.57 2,3 0,73 1,18 0.63 0,59 0,81 0,38 1,1~ 0.19 1,05

6.65 7,18

1,18 6,02 0,59 2,45

2,63 4,36

14.95 19.86 LO.79

19.46 [7,94

[,18 i [3,28

2.61 2,27 1.89

1.12 0.99 0.64 0.50 0.65 0.54

0.37 O. 47 0.37 0.37 0.49 0.46

1.95 2.03 1.62 1,18 1,10 0,92

'81~ 3.21 2.36

3.79 3.68 2.86

0,89 0,76 O. 59

Content of fine fractions < 5 mm, 9c

Or I0 20 I0 ZO 30 ~0 78 20

8: ~ i<~ ~, "

,z: I'-- " . - r _ , . , r ( ;"

�9 i rH ' 14" ~,

r 9 _/

l rz l - - - ~ - - = ~ l '5: ~ T "

~1--- ~ ~-,.-..,..._ ~01 l " "

Sh-Z Sh -3 S h ' 6

IIBIKI ll~-'.~-i

311 Sh- 9

Graphs of the change in the content of fine fractions with depth in the experimental dam.

Figure 2 shows the relation between the direction of action of the bulk of the left-bank charge and the zone of the dam with smaller permeability due to a marked drop in pressure of the seepage flow. The permeability of the soil in the dam increases toward the lower pool owing to a decrease in the height of the blasting drifts and, consequently, in the height of falling of the rock, which on the end zone of the dam does not exceed i0 m.

In the methods for determining the permeability coefficients of the dam soil used above, it was tentatively assumed that the flow is laminar with preservation of a linear re-

682 V.P. NEDRIGA ET AL.

=I-'

18L

~L--- jl f"

/

/ / I,

0 a'~ U r.2 1.6 Z/ z~m: q ~ec

J I I I ,,IIIIIII

o ~ 8 Iz 16 2o ~ z8 Jz days

Fig. 4 Fig. 5

Fig. 4. Seepage through dam as a function of the water level in the reservoir.

Fig. 5. Change in the seepage through the dam at a constant water level in the reservoir of 45 m. i) 1976 (15 days); 2) 1977 (40 days).

sistance law or turbulent with a quadratic resistance law. However, of great interest is a determination of the actual regime of the seepage flow and law of resistance. For this pur- pose we used electrochemical and calorimetric methods of determining the true flow velocity in the core of the rock mass. The velocity of the tracer was measured more accurately in the zone between shaft 6 and the lower pool. The true velocity of the seepage flow in the given zone was determined for two values of the pressure gradient I (0.061 and 0.092). The velocities for these two cases were, respectively, 545 and 761 m/day.

To determine the resistance law we used the well-known Smreker equation

v s = K I r k , (3)

where v s is the seepage velocity; K is the permeability coefficient; m is a parameter (I < m < 2 ) .

Knowing the true velocity of flow in the soil pores and the value of porosity in the given zone, we found the unknown quantities K and i/m by solving a system of two equations with two unknowns

nv, = Kl~/m; 1 .v, = rt~/~, �9 (4)

where n = 0.27 is porosity; v, and v2 are the true velocity of the flow.

Solution of the system of equations (4) for two values of the pressure gradient and seepage velocity gives the following values of the sought quantities: K = 1240 m/day and m = 1.23. These data agree with the results of laboratory seepage investigations of model mixtures in vertical "Darcy" type instruments. For all investigated values of density (Pdry = 1.75-1.95 tons[m s ) and content of fractions less than 5 mm from 23 to 48% the pa- rameter m in Eq. (3) varied within 1.25-1.43.

Figure 1 shows the drawdown surface of the seepage through the experimental dam plotted from the observation data. As we see from Fig. I, the seepage pattern is complex spatially. Therefore, the values of the permeability coefficients calculated by the equations of motion of a two-dimenslonal (in section) flow can be regarded only as approximate, though they give a sufficiently complete idea about the seepage characteristlcs of the dam soil.

The conduction of observations of piping and silting processes in the dam and on the upstream slope was one of the most important problems of the full-scale seepage investiga- tions. Daily visual inspection of the dam and taking samples for turbidity were carried out for this purpose. The solid sediment in the seepage outflow did not exceed 6 g/m s , on average, and the natural turbidity of the Burlykiya River at this time reached 300 g/m s. The chemical composition of the water in the upper and lower pools remained practically un- changed. These data indicate the absence of piping in the dam at the investigated pressure gradients.

EXPERIMENTAL BLAST-FILL DAM 683

The seepage discharges were determined by means of a hydrometric flume. Figure 4 pre- sents a curve showing the change in the seepage through the experimental dam as a function of the water level in the reservoir. Current flowmeter observations made during 1976-1977 established the change in the seepage with time (Fig. 5). As we see from Fig. 5, the total seepage during 40 days of observations in 1977 decreased 30%. The discharge decreased most rapidly in the first half of the observation period. Thereafter, it diminished. The de- crease in seepage with time through the dam along with a relatively high content of fine fractions in the soll is apparently related to internal mixing of particles and their denser repacking.

CONCLUSIONS

i. The seepage flow through the experimental dam is of a complex nature spatially. The law of motion of the seepage flows is nonlinear, i.e., it does not follow Darcy's law. The permeability of the soils composing the structure varies over a large range.

2. A characteristic feature of blast-fill dams is the zonal variation in the granulo- metric composition and, consequently, the permeability coefficient. In the central zone the material contains a greater quantity of fine fractions and is placed more compactly than in the shoulders.

I.

LITERATURE CITED

V. F. Korchevskli and G. N. Petrov, "Geotechnical investigations of an experimental blast-fill dam on the Burlykiya River," Gidrotekh. Stroit., No. 5 (1977).