11
208 data collection stations or to a central station. Instruments for direct reading, as a rule, are cheaper, but then they require person- nel to take the readings, to plot and to interpret them. Remote read- ing monitoring equipment, on the contrary, is more expensive and more sophisticated, so it may call for more frequent and intensive maintenance, but data can be acquired quickly and at any time, even automatically, to be fed into a computer that evaluates the data. The choice of instruments which are to continue monitoring certain parameters in a dam after the construction hence depends to a great deal on the type of processing the data received: Conclusions To monitor the behaviour of a dam during and after construction, a large variety of instruments is available and have proven useful, in many cases even indispensable. Based on the results of the engineer- ing geological field work, instrument systems to be installed both in dam and below it or in its abutments should be chosen carefully de- pending on the nature of the dam, on the specific geotechnical circumstances at the dam-site, and on the type of data processing. Instrumentation may be expensive, but during construction it allows a re-check of the initial assumptions for the design and to improve it for the sake of economy, while after construction it helps to assess the long-term stability and the safety of both the reservoir and the darn. References BRETH H. (1972): Der derzeitige Stand des Staudammbaue~ (The present state of dam construction; in German) Wasser- wirtsehafL 62, I[2. SCHOBER W. : Large scale application of Gloetzl type hydraulic stress cells at the Gepatsch rockfill dam, Austria. BaumetS- technik, Spezial-Informafionen fiber baustatische Messungen (Gloetzl, Forchheim, F. R. Germany). WET J. A. de - GROBBELAAR G. - ROUX W. J. Le (1976): The optimization of in-situ stress measurement. Proc. Symposium on Exploration for Rock Engineering, Johannesburg, Vol. 2, 75 - 89. Panel reports / Rapports des panelistes [ B U LLETI N of the International Association of ENGINEERING GEOLOGY ] de I'Association Internationale de GEOLOGIE DE L'INGENIEUR N~ 2011 -- 218 KREFELD 1979 HYDROGEOTECHNICAL CONTROL SYSTEM ON A HYDRO-ELECTRIC POWER PLANT WITH A BASALTIC FOUNDATION (SOUTHERN BRAZIL) SYSTEME DE CONTROLE HYDROGEOTECHNIQUE D'UNE CENTRALE HYDROELECTRIQUE SUR UNE FONDATION BASALTIQUE (SUD DE BRESIL) GUIDICINI G., CRUZ P. T. da, ANDRADE R. IVl. de, Engevix S. A., Estudos e Projetos de Engenharia, Rio de Janeiro, Brazil* Summary The hydroelectrical power plant of Itafiba, in Rio Grande do Sul, Southern Brazil, due to a favourable topographic situation, takes advantage of a large curve of the river, in a steep valley. The concrete structures are located on a topographic narrow ridge on the right side of the valley. The rock foundation is formed by a sequence of basalt flows, intercepted by a family of inclined faults. The nature of this rock mass made necessary an effective and large drainage system, and a careful control of the hydrotechnical behaviour was developed before, during and after the impounding of the reservoir. It was possible to observe the response of the various confined and independent aquifers and the importance of a fault plane as the dominant drainage feature of the rock mass has been confirmed. The paper includes data regarding the reduction of high uplift pressures developed under some blocks of the intake structure. The main problem was to identify the causes and the critical paths of water flow. Finally, emphasis is given to the fact that the low cost hydrogeotechnical instrumentation of a rock mass, using the drainage system itself, can represent the key to control of the complete behaviour of the structural foundations. More sophisticated instrumentation may be not necessary at all. * G. Guidicini (Engineering Geologist), P. Teixeira da Cruz (Geotechn. Engineer, M. So., Ph. D.) R. Monteiro de Andrade (Civil Engineer; Head, Civil Engng Department), ENGEVIX S. A., Estudos e Projetos de Engenharia, Rua Senador Pompeu 44 a 60, 20080 - Rio de Janeiro - RJ (Brazil)

Hydrogeotechnical control system on a hydro-electric power plant with a basaltic foundation (Southern Brazil)

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Page 1: Hydrogeotechnical control system on a hydro-electric power plant with a basaltic foundation (Southern Brazil)

2 0 8

data collection stations or to a central station. Instruments for direct reading, as a rule, are cheaper, but then they require person- nel to take the readings, to plot and to interpret them. Remote read- ing monitoring equipment, on the contrary, is more expensive and more sophisticated, so it may call for more frequent and intensive maintenance, but data can be acquired quickly and at any time, even automatically, to be fed into a computer that evaluates the data. The choice of instruments which are to continue monitoring certain parameters in a dam after the construction hence depends to a great deal on the type of processing the data received:

Conclusions

To monitor the behaviour of a dam during and after construction, a large variety of instruments is available and have proven useful, in many cases even indispensable. Based on the results of the engineer- ing geological field work, instrument systems to be installed both in dam and below it or in its abutments should be chosen carefully de- pending on the nature of the dam, on the specific geotechnical circumstances at the dam-site, and on the type of data processing.

Instrumentation may be expensive, but during construction it allows a re-check of the initial assumptions for the design and to improve

it for the sake of economy, while after construction it helps to assess the long-term stability and the safety of both the reservoir and the darn.

References

BRETH H. (1972): Der derzeitige Stand des Staudammbaue~ (The present state of dam construction; in German) Wasser- wirtsehafL 62, I[2.

SCHOBER W. : Large scale application of Gloetzl type hydraulic stress cells at the Gepatsch rockfill dam, Austria. BaumetS- technik, Spezial-Informafionen fiber baustatische Messungen (Gloetzl, Forchheim, F. R. Germany).

WET J. A. de - GROBBELAAR G. - ROUX W. J. Le (1976): The optimization of in-situ stress measurement. Proc. Symposium on Exploration for Rock Engineering, Johannesburg, Vol. 2, 75 - 89.

Panel reports / Rapports des panelistes

[ B U LLETI N of the International Association of ENGINEERING GEOLOGY ] de I'Association Internationale de GEOLOGIE DE L'INGENIEUR N~ 2011 - - 218 K R E F E L D 1 9 7 9

HYDROGEOTECHNICAL CONTROL SYSTEM ON A HYDRO-ELECTRIC POWER PLANT WITH A BASALTIC FOUNDATION (SOUTHERN BRAZIL)

SYSTEME DE CONTROLE HYDROGEOTECHNIQUE D'UNE CENTRALE HYDROELECTRIQUE SUR UNE FONDATION BASALTIQUE (SUD DE BRESIL)

GUIDICINI G., CRUZ P. T. da, ANDRADE R. IVl. de, Engevix S. A., Estudos e Projetos de Engenharia, Rio de Janeiro, Brazil*

S u m m a r y

The hydroe lec t r i ca l power p l an t of Itafiba, in Rio Grande do Sul, S o u t h e r n Brazil, due to a favourab le topograph ic s i tua t ion , takes advantage o f a large curve of the river, in a steep valley. The concre te s t ruc tu res are located on a topograph ic na r row ridge on the r ight side o f the valley.

The rock f o u n d a t i o n is f o r m e d by a sequence of basalt flows, i n t e r cep t ed by a fami ly of incl ined faults . The na tu re of this rock mass m a d e necessary an ef fec t ive and large drainage sys tem, and a careful c o n t r o l o f the h y d r o t e c h n i c a l behav iour was developed before , dur ing and a f te r the i m p o u n d i n g o f the reservoir.

I t was possible to observe the response o f the var ious con f ined and i n d e p e n d e n t aquifers and the i m p o r t a n c e of a faul t p lane as the d o m i n a n t dra inage fea tu re of the rock mass has been conf i rmed . The pape r inc ludes data regarding the r educ t i on of h igh upl i f t pressures deve loped u n d e r some blocks o f the in take s t ructure . The ma in p r o b l e m was to iden t i fy the causes and the cri t ical p a t h s o f wa te r flow.

Final ly, emphas i s is given to the fact t ha t the low cost hyd rogeo t echn i ca l i n s t r u m e n t a t i o n of a rock mass, using the dra inage sys tem itself, can represen t the key to con t ro l of the comple te behav iour of the s t ruc tura l f o u n d a t i o n s . More soph is t i ca ted i n s t r u m e n t a t i o n may be no t necessary at all.

* G. Guidicini (Engineering Geologist), P. Teixeira da Cruz (Geotechn. Engineer, M. So., Ph. D.) R. Monteiro de Andrade (Civil Engineer; Head, Civil Engng Department), ENGEVIX S. A., Estudos e Projetos de Engenharia, Rua Senador Pompeu 44 a 60, 20080 - Rio de Janeiro - RJ (Brazil)

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R~sum~

La Centrale Hydro~lect r ique de l t a fba , dans l'l~tat du Rio Grande do Sul, gr~ce h une conf igura t ion topographique favorable, prof i t d 'une grande courbure de la rivi~re, dans une vall6e encaiss~e. Les s t ruc tures de b~ton occupen t un ~peron sur la rive

droite de la vall6e.

La fondat ion de cet 6peron est represent6 par une s~quence des couches basaltiques, sous-hor izontaux, in tercept6s par une famille de failles inclin6es. La s t ructure de ce t te fondat ion , associ~e h la perm~abilit~ tr~s elev~e des couches basaltiques, et en raison de la pr6sence de failles, on fait que s ' in t roduissent au pro je t un large syst6me de drainage. Un cont rb le tr~s soigneux du c o m p o r t e m e n t hydrog~otechn ique du massif a ~t~ r~alis~ avant, pendan t et apr~s le remplissage du reservoir.

On a observ6 ainsi, la r6ponse des diff6rents aquif~res, confin6s et ind~pendants , au remplissage du reservoir. On a confirm~ Faction d 'un plan de faille c o m m e chemin pr~pond6rante de drainage de la fondat ion.

On d6crit aussi les e f for t s h ~liminer les sous-pressions tr~s 616v~es q u ' o n t a t te in t la fonda t ion de quelques blocs de la prise d 'eau. La principale difficult6 a ~t~ l ' ident i f icat ion des chemins cri t iques de l ' inf i l t ra t ion d 'eau .

Enfin on fait ressort i r le fair que l 'auscul ta t ion hydrog~o techn ique d ' un massif rocheux , ef fectu6e par mesures peu couteuses , dans le m~me syst~me de drains, peut representer la c16 pour le cont rb le du c o m p o r t e m e n t global des s tructures, en per- me t t an t f r~quemment de se dispenser des formes plus sophist iqu6es d ' ins t rumenta t ion .

In t roduc t ion

The hydroelectric power plant of Itaflba, of CEEE, (Companhia Estadual de Energia El~trica do Estado do Rio Grande do Sul), is lo- cated at the centre of the State, in the middle portion of river Jacui, 200 kilometres from Porto Alegre, the Capital of the State.

The river runs, at the site, along a narrow valley, with steep slopes forming a large curve with intermittent falls, giving rise to 12 metres difference in elevation, in a stretch of 6.6 kin. The closure of the river was made with a rockfdl dam, with a clay core, 440 m crest length and a maximum height of 90 metres (see Figure 1).

The rockFfll dam is located about 1,000 m downstream of the topo- graphic narrow ridge at the right abutment, where the concrete structures were h u r l This ridge corresponds to the minimum width of the rock mass between the two sides of the river. At the base, the width is 350 m, and the top elevation is 120 m above the river level The slopes are quite steep, with an average of 40 ~ , including vertical stretches. The ridge contains the spillway and the intake structures. The first, with three chutes, allows a maximum discharge of 8000 m3/sec. Four penstocks start from the intake structures, towards the power house. Four turbines of 125 MW can generate a total of 500 MW. The normal water level of the reservoir is at an elevation 183 metres, and the river level, downstream of the tur- bines, is at elevation 94 m, for a discharge of 400 m 3/see.

Local geological aspects

The normal sequence of basalt flows

The region is entirely covered by basaltic flows, to an estimated thickness of 350 metres, at the site. The thickness of each flow is extremely variable, from a few metres to more than 100 metres. The disposition is practically horizontal and the river cuts deeply this basaltic sequence. The slopes are covered by residual soils, coUuvium and "talus" accumulations-

The lithological predominant variations within each flow, from top to bottom, are the classical basaltic breccias of various types, veil- cular and amygdaloidal basalt and, f'mally, dense basalt, in various degrees of crystallization. Intertrap sandstones are also common. At the site of the dam and in the ridge area, eight different flows have been identified, inside the elevations of interest to the project, and they were numbered, from bottom to top. Figure 3 shows the above mentioned sequence.

Uncommon features in the foundation of the concrete structures'

The normal sequence of flows is intercepted in the subfoundation of the concrete structures, and also in the area of the rockfill dam, by a family of subvertical and inclined faults, oriented approxi- mately East-West, and representing planes of structural weakness of the rock mass. The direction East-West seems to identify a regional tectonism. The situation, considering the structural geological pattern in the area of the concrete structures, is quite complex, because these

faults are subjected to winding changes in dip, with bifurcations and interconnections one with another. In some of the figures of this paper, showing plane views and cuts, one can see how a se- quence of fault planes occurs, in the ridge area, with intervals of tenths of metres between them. Obviously, only the more super- ficial faults affect the project.

The project includes two superimposed drainage galleries within the rock mass, interconnected by a system of vertical drainage holes.

The opening of these galleries brought a most important cont~ bution in the amount and quality of the information on the spatial position of the fault planes.

It is supposed that the fault planes that daylight upslxeam of the concrete structures, within the intake channel and the substation, in the upper part of the ridge tend to incline in depth towards down- stream, daylighting again with much less steep inclinations along the low downstream side of the ridge (see Figure 4).

In summary, the geological model is composed of a series of con- choidal fault planes, parallel to each other, suggesting the denomi- nation of a "continous shell model", where the initially subvertical fault planes tend towards a subhorizontal attitude, leading to the formation of "shells". This concept resulted in the basic model used in the final stability analysis.

However, if these fault planes daylight and intercept the slope of the ridge they merge towards the inner side of the left abutment giving rise to a solid link of "wedges" formed by the fault inside the rock mass.

Hydraulic conduc t iv i ty of the geological fea tures

In the sequence of basaltic flows, the discontinuities of large lateral extension, such as the contact between flows and some major joints inside the flows, come immediately at site. These discontinuities, with the exception of other uncommon features, are dominant in the hydrogeotechnical behaviour of the rock foundation of any work in basalts, since the rock matrix can be considered as imperme- able. A synthesis of the hydraulic conductivity characteristics of the foundations of the rocld'dl dam and concrete structures of Itaflba plant can be seen in Figure 5. This information was obtained during the geological investigations at the site. It was elaborated more specifically for the rockffll dam area but, due to the regularity of the flows, the general considerations and data are valid for the foundations of the structures.

The eight basalt flows are presented at their average elevations. One can see immediately that larger values of permeability were found on the upper 25 to 30 metres. This more permeable layer results from a relief of the rock mass.

One can also observe that the hydraulic conductivity of the contacts between flows and large discontinuities tends to be smaller as the confinement increases, towards the inside of the rock mass. This behaviour is a normal characteristic of rock masses, mainly in the

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210

\

VERTEDOURO SPI [_ L W A Y ~

BARRAGEM \~,~, , DAM

OMADA D'AGUA ~~~;"NTAKE STRUCTURES I

C, ASA DE FORCA\ POWER HOUSE

\

0 10(3 200 300 400 f~-,m I r ~ f I

ESCALA GRAFICA

0Lo~=,A! t- ~,G6,4Z'A

= = , s , L / . ~ - --..,% ~ ' - ~ .. ~ 1 , 1 " . \

�9 ,,R,~s,L,,~ / . - /

~' " PARAGUA,q-.

1 ~ , . . . . . . . . . . . . . . . , , ,

2=~ .U+: I " " ' ~ . ~ . o

Fig. 1: Location map of the power plant of I t a lba

case of intrusive igneous rocks. Its validity in basaltic masses, as is b) the case of ItaOba, is l imited by the persistence of some preferential flow plane s at larger depths (Oliveira et al. 1976).

Considering specifically the foundat ion of the concrete structures, regarding its hydraulic conductivity characteristics, the following features are of interest:

a) among the various contacts between flows, the more evident are those at el. 144 and 117 metres. Along these two contacts, total water losses were frequent, as well as losses of water during c) drilling operations.

large discontinuities were identified inside the flows, two of which occurring at el. 160 m and 151 m to 153 m. A third dis- cont inui ty was observed inside the lower drainage gallery, at el. 110 to 114 m. These features represent preferential paths for seepage. The values of hydraulic conduct ivi ty of these disconti- nuities are very high, and the velocity o f water flow is typical of open joints, o f the order of cent imetres or decimetres per second, as a funct ion of the existing hydraulic gradients.

the shallow layer close to the excavations, as a consequence of blasting, shows a higher permeabil i ty than the rock at depth.

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211

Fig. 2: Itafiba power'plant. Intake structures and power house

0 I0 ZO 3o 40 ~Om

L " ~ , '~, GRAPHIC SCAt.E

�9 / ~ , [ �9 r iO3 O0.N A MA~ NOmMA L / ~ _ . . _ . . C ~ . T 0 ~ " \ . ; ~ ' ,

' N0l~MAt. MAX. W.L. / , ~ ~ ~ TOMAOA D'AGUA " ' = - - - - - - - - - / " : , " - - - - - - - L \ / ."-L ,"''"~ST"OCT'~"ES

I I CONTATO "z ~. ~ ~ N A T U R A L SURFACE OF TERRAIN

/~'.CORTI;~A OE FAMIt.IA O{ FAt.HAS INFERIOA~ lIP. ~ \ ~. ~, , P~WER HOUSE INJE~OES INFERRED FAMILY OF FAULTS ~ % r '

J ~ U I " RIYI[ l 110 O0 ~ " \ ' ' ~ / " . . . . . . o _~.o,oo I " ~ ~ /L , J \

- . - - . - OERRAME 4 ~LANO PRINCIPAl. TUNNEL \ \ ' L , ; ~ II L~ rr JACU{ t.AVA-Ft.OW 4 ~ OR[N/I.GEM ~{ ' ,~ ; 1 JACUI RIVER

. . . . . . ~'~=;'A~ . . . . . . . . . _v -~' --~--J, - _ ' ~

Fig. 3: Schematic section along the ridge (concrete structures area)

This layer has an approximate thickness of 3 metres. This upper superficial layer must contribute to the inflow of water towards the discontinuity detected at elevation 160 m, in the excavation for the diversion channel, and for the foundations of concrete structures.

d) the mentioned family of faults, even when partially filled with fine materials, tend to form preferential flow paths, due to their spatial configuration. The steep inclination on the upper port- ions facilitates a rapid transfer of high hydraulic heads, as well as volumes of water, from the higher elevations to the lower planes and drainage galleries. This occurs particularly in relation to the gallery at el. 110 m, that receives a large amount of water, through one of those inclined faults.

Implications of the geological features of the p r o j e c t

The purpose of the following considerations is to put in evidence the hydrogeological aspects of greater interest and to see how they

act on the drainage system. As mentioned before, the project in- cludes two superposed galleries, open inside the rock mass. These two galleries are interconnected with an upper gallery within the concrete structures. Continuous drains start from the upper gallery, towards the lower one, intercepted in between by the middle gal- lery at el. 144 m. This system forms what one can call a "large vertical drainage plane".

During the various phases of the project, it was perceived that the over-all stability of the structures and foundation should be sup- ported by a high efficiency of the drains. From such a conclusion the intense drainage system resulted, complemented by other foundation treatments and instrumentation. These complementary works included: the extension of a concrete slab on the diversion channel, to the positions of the grout curtain, located at the up- stream end of the slab; additional inclined drainage holes, directed upstream and open from the gallery at el. 144 m; the installation of extensometers, topo~aphic bench-marks, piaster seals and intense

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1 7 0 ~

CORTINA DE INJE(;OES GROUT COURTAIN

CANAL DE ADUCAO REVESTIDO POR LAJE ADDUCTION CHANNEL COVERED BY SLAB

GALERIA DE ORENAGEM "~ No .oc.oso (COTA 110) ~.

DRAINAGE TUNNEL. �9 INSIDE THE ROCK ..~',-~ MASS (EL. 110) ,"*-'7

zlC

~,PLANO DE FALHA FAULT PLANE

VERTEDOURO 41~ 6~176 L~/-:.4~. SPILLWAYS ,~

(" TOMADA DIA'GU A iL~ " , ,,...---1 INTAKEk STRUCTURES~,~Z~.,..

SUBSTATION J

Fig. 4:

,eo / , / /

17o . PLANOS DE FAIJ4 / FAULT PLAN ES~. 7

ACESSO A GALERIA DE DRENAGEM INFERIOR(10W ACCESS TO LOWER DRAINAGE TUNNEL(]04) ~l

o 20 i

Location plan of the main geological structures

ii /i i/

I I

ACESSX3 A GALERIA SUPERIOR(1441 ACCESS TO uPPER TUNNEL (144)

~160

~ 1 5 0

i ' ~F

d

"~'---. CASA DE FORCA POWER HOUSE

~t20

40 60

F. SCALA GaAFICA GRAPHICAL SCALE

80 I OC m i

8o

piezometry. In parallel, the reservoir Filling should be done in steps, and tests on the drainage system performed periodically.

Filling of the reservoir

The reservoir filling was divided into two stages. The rtrst stage, that lasted 20 days, brought the water level to el. 166, and it re- mained the same for 90 days. The second stage, that lasted 35 days, brought the water level to its maximum, at eL 184.

On the ridge area, before the reservoir fdling, the position of the

phreatic line was determined by means of a series of tests performed on the drainage system.

Plugging the drains at el. 1 I0 (the lower gallery), the piezometric level was observed. Three weU-dermed and differentiated regions, regarding hydrogeotechnical behaviour, were encountered: a) the portion of the foundation of the spillway up to the right chute, starting from the right side, results in a piezometric level at el. 140 m (see Figure 6); b) in the central portion, that includes the left chute on the spillway, the "rock nose" and the right block of the

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213

~70--

O M B R E I R A E S Q U E R O A L E F T A B U T M E N T

tlO - - ~7_~

GALERIA DE DESVIO DIVERSION GALLERY

| ~ ~"~ ~',-- :.:~::4' ; : z ! ~'~;'~:-'' PEROA D'~GUA [~PEC;FICA LOSS OF WATER oo - - - ~

] 0-1 b'mln, m.kg/cm ~ SO

] - - - - - - - - - Curva de =ovalore| o ~ ~ ~ 40 ~ 60 ro eO ~ i ~ . ] - 3 IIm~n.m. kg l cm z Same vo lue l Curve ESCALA G ~ F I C A

Contoto Qeoko'glCO GRAPHICA~ SC&L~ ] ] - ~ I / ~n m. kg /cm z GeolOglCO I con tac t

[ ] ~Q-~ N~ . . . . . . d . . . . . . ~> S I/m,n m kg/cm ~ Number ~f f l o ~

Fig. 5: Hydraulic conductivi ty of the rock mass at the dam site

L;NHA DE REFE~ENCtA

|

O M B R E I R A D I R E I T A

R I G H T A B U T M E N T

| IG leTAOUTMENT

FLOW t

SPILLWAY " R O C K N O S E q I N T A K E S T R U C T U R E

IO] , IB - W. L. AT OCT. 15, t ~ ' t l SAME AS JAN.Z~J, 11~1

~ ,EGEND:

PtEZOMETRIC LINE BEFORE THE RE c*ERVOIM FILLING

PIEZOMETRPC LINE AFTER TH E FIRST ~ ' ~ ~&GJE OF THE ~ESERVOIR rlI.LING

Iw . L. &T EL. IKI ; )

PIEZOME't3f lC LfNE AFTER THE / RESERVOIR COMPLETION ( W L AT

s 183)

OF $" I I I I I I I I I P~EZOMETRIC LEVELS IN THE MEASURED AT EL. 110 t i i

/ I l l FLOW $

CONTJCT(84 A 85)

Fig. 6:

GRAPHICAL SCALE

Section 'along the ~ e a t drainage curtain. Piezometric situation along various stages of the reservok filling

DRAINS O~"

intake structures, the piezometric level is controlled by a significant fault plane; finally, in the third portion, that includes the other blocks of the intake structure and the left rock mass, the piezo- metric level drops to el. 115, as can be seen in Figure 6.

In the first port ion o f the rock mass, there is no communica t ion be- tween drains, and the inflow of water is mainly along the permeable contact at elevation 144 to 140 m. In the central portion, the hy- draulic behaviour is controlled by the geological fault that is much more permeable than the rock mass. The communica t ion between drains occurs through the fault. The left por t ion of the foundat ion is hydraufically controlled by the discontinuity at el. 110 to 114 m. The rock mass above this elevation remains practically free of water percolation.

Observations during the first stage of reservoir filling

In the first stage of the reservoir filling, the flow through the drains and walls of the lower gallery (el. 110) increased substantially. The upper gallery (el. 144), however, remained dry, even when the re- servoir level reached el. 166, as can be seen in Figure 6. The piezo-

metric level remained at elevation 144, on the right port ion of the rock mass, just as before. On the central portion, there were some changes, resulting in a more well stablished phreatic line, probably due to the saturat ion of the rock mass. On the left portion, the piezometric level remained as low as before. Some of the drains be- longing to the vertical drainage plane presented a high flow of water, but the majori ty remained dry. When the reservoir level was at el. 166 m, 17 drains carried more than 1 l i tre/minute each, with a total flow of 360 l/rain. A sole drain, however, was responsible for 200 l/rain, on the floor of the lower gallery and it is supposed that it intercepted the inclined fault plane ment ioned before. The flow through the walls of this gallery reached 100 l/min.

During the t ime in which the water level remained at el. 160 m, a tendency was observed of increasing flow of water along the walls, and a reduction through the drains. The total flow remained con- stant. This could be thought of as a progressive saturation of the rock mass.

-['he above observations led to the drilling of seven additional drains, intermediate to those of high flow, or intermediate to areas of

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214

0 5 I0 I'~ 20 25 30 0 J U N E , 1 9 7 8

~ , I ~ ~ T ITA F RAT F

REe, ERVOIR WL. �9 EL. | 84M

_ . ~ . ~,

FLOW RATE BELOW INTAKE ', TRuCTuRE5 (

FLOW RATE THROUGH BOTTOM I RAINAGE

~TA -~'- ~'~'~i = = L-- FLOW RATE THROUGH X ! THE WALLS --!/~'~.. L,/ FLOW g~ATE "I'M ROUGH ROOF

I I I [ 10 1'5 20 25

S E P T E M B E R , 1 9 7 8

I O U O

9 0 0

8 0 0

700

600

Z

In

500 W

4 0 0

300

200

I 0 0

Fig. 7: Drainage tunnel at eL 110. Comparison of water inflow at different steps of reservoir filling

high piezometric head.

In many occasions the interconnection between drains was ob- served, although erratically, because in some cases the communica- tion occurred between adjacent drains, but in other cases in drains some tenths of metres apart.

The uplift pressures close to the foundation level of the concrete structures were measured through piezometers instaUed in the con- crete gallery at el. 157 m, as shown in Figure 7. The uplift pressures below the spillway were practically nil, in the first stage of the re- servoir frilling, but under the "rock nose" and the intake structures they reached high values. Under block 4 of the intake structure (left side), the uplift pressures were again small From the sLx piezo- meters that gave higher pressures, three are short (only 2 metres in rock), positioned for the concrete-rock contact. They showed the uplift pressures practically reaching the reservoir level (see Figure 8). If one considers the extensive trealznent on the upstream area, that included a concrete slab, and the grout curtain, much smaller pressures were expected. It was concluded that a free entrance of water had to exist, close to the intake structures, possibly close to block 1 (right side). It is important also to mention that the drains that go from the gallery at el. 157 m to the gallery at eL 144 m re- mained almost dry. So, the high pressure zone was somehow con-

fined on the upstream area, and did not reach the vertical drainage plane.

During this stage, seven extensometers, starting from the gallery at el. 144 m, were observed. Only one showed a small displacement of the rock towards the downstream site, of 0.3 mm. The rest read zero displacement.

Observations af ter t h e final filling of the reservoir

The second stage of reservoir filling took 35 days, at a daily rate of half a metre. The water level went from el. 166 m to el. 184 metres.

All the attention was concentrated on the foundation area of the intake structures and the "rock nose". In particular, the response of piezometers 6A, 7A and 8A, that had given high uplift pressures at the First stage, would be of great interest.

The response was immediate, that is, the piezometric level followed very closely the reservoir level. The final stablished values were one to two metres below the reservoir level, as shown in Figure 8. ]buring this second stage of reservoir filling, tests of pressure and flow were performed in piezometers 6A, 7A and 8A. The tests con- sisted in opening separately or simultaneously the piezometers,

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making the water drain through. Flows were measured and the test results axe summarized on the following table:

1 st test W.L. = 170.90 m

PZ-6A P = 1.05 P = 0.46 P = 0,033

PZ-7A P = 1.05 P = 0.46 Q = 9 2

PZ-8A P = 1.05 Q = 9 2 Q =55

Total flow from piezometers 92 147

110 m) reached 1000 l/mm. The upper gal lery(el . 144 m) drained 1201/min.

Da te : Ju ly 28 ,1978

Q = 4 6 Q = 6 3 Q = 29 Q = 61

P = 0.35 P = 0.77 Q = 1 2 0 Q = 65

Q = 84 P = 0.82 P = 0.25 Q = 52

130 63 149 158

P = piezometric head in kg/cm ~ ; Q = flow rate in l i t res/minute

At full reservoir level, the piezometric line showed a peak in tile posit ion of piezometer 7A, dropping at both, sides, as shown in Figure 8. It was observed that a similar behaviour occurred at lower elevations (see Figure 10) within the rock mass, detected through the seven extensometers of the upper gallery (el. 144 m), used as

In the lower gallery, the rise in flow was quite homogeneous. Figure 7 shows this performance, during two months of the reservoir ftll- ing. No singular points of excessive flow were detected in any part of the gallery, or in any drain. The same behaviour was observed in the upper gallery (el. 144 m), where the rise in flow was uniformly

160-

! ~ '20-

b.{l

N ~ o

B O -

40-

I ...." ............. .-

180 . . . . . . . . [ . . . . . . . .," -- /

t N ~ Do RESERWT6R,O T I E R~SERVO,R W.L

~ k t l : > j ~: I613 . . . . . ~} :'

r u e E 8 O >

'.,', VAZ,~O DA GALERIA IIO "I ~"i FLOW RATE IN TUNNEL AT EL I I0

120 . . . . . . . . . . . Z

VAZAO DA GALERIA 144 FLOW RATE IN TUNNEL AT E L I 4 4 L ~

I l ~ I ABRIL 78 MAIO --JUNHO JULHO AGOSTO SETEMBRO

. . . . . . . L . . . . . .

-~--VER--- SIGNIFICADO DOS EPISC)DIOS NO TEXTO ( SEE MEANING IN THE TEXT

OUTBBRO NOVEMBRO OEZEMBRO JANEIRO 79

Fig. 8: Flow rate of water in the foundat ion of the concrete structures

piezometers. These extensometers, in fact, had three functions: its main one of the reading of displacements, a second as drains, and a third as piezometers, to which it was necessary to modify slightly the frontal plate. These piezometric readings tend to indicate the pressure of independent aquifers, that are common in basaltic for- mations.

At the same time as the occurrence of high uplift pressure close to the structure foundation, the water flow in the lower gallery (el.

distributed among the drains and the walls. A curious piece of in- formation is the low piezometric head measured on the 40 drains starting from the gallery at el. 144 m, with inclinations of 5 ~ and 20 ~ with the hor~onta l , and directed to upstream. The explanat ion is that they did not reach any impor tan t aquifer. After the filling of the reservoir, a new test of pressure and flow was performed in piezometers 6A, 7A and 8A, using the same procedure described above. The results are shown in the table below:

2 nd test W.L. = 183.000 m

PZ-6A P =2.20 P = 1.45 P = 0.32

PZ-7A P = 2.20 P = 1.45 Q = 161

PZ-8A P =2 .20 Q = 144 Q = 98

Total flow from piezometers 144 259

Date: September 19, 1978

Q = 64 Q = 84 Q = 35 P = 0.50 Q = 26

P = 1.00 P = 1.69 Q = 189 Q = 214 Q = 126

Q = 127 P = 2.00 P = 1.30 P = 1.30 Q = 106

191 84 224 214 258

P = piezometric head in kg/cm 2 ; Q = flow rate in l i t res/minute

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As in the previous zest, one can see the large level of communication among the three piezometers. This pressure redistribution extends for over 60 metres, immediately upstream of the foundation of the intake structure. In "all tests and conditions the PZ-7 shows the highest flow.

All the data from the test seem to indicate that the shortest flow path should be closest to the PZ-7A. It was confirmed that the high water pressure area was confined, and did not reach either the verti- cal plane of drains between the gallery of el. 157 m and the upper rock gallery at el. 144 m, or the line of drains directed towards up- stream, opened from the gallery at elevation 144 m. The idea that a significant geological discontinuity was responsible for the high hy- draulic conductivity observed in the foundation looked sound, al- though the connection of this feature with the reservoir was un- known. The discontinuity at elevation 160 m, visible during the con- struction period, practically in all the area of the diversion channel, close to the foundation level, could have remained, through its rami- fications, below the foundation level of the intake structure. Once the high piezometric levels were confirmed in the foundation of the intake structure, considering that although acceptable they were un- desirable, action was taken to achieve a better understanding of its nature and the means to avoid such high pressures.

Works p e r f o r m e d to reduce f low and up l i f t p r e s su re

Once it was clear that there was free communication between the reservoir and the piezometers, two hypotheses were raised: a) the communication occurred between adjacent blocks, upstream from the "fugenbands"; or b) the communication occurred through an opening between the intake structure and the upstream concrete slab on the diversion channel.

Cement grout in the joints between the blocks

The first hypothesis seemed reasonable, due to the tact that the "fugenbands" were located at some distance from the upstream face. Vertical and subvertical drill holes were open from the crest of the dam. Six holes, with lengths varying from 26 to 31 metres, were drilled along such joints, with 0.5 to 5.0 metres into rock founda- tion, and then grouted.

ttigh cement takes were observed, both in the rock, and within the joints then,selves, but no reductions either of water flow or pres- sures were detected in piezometers 6A, 7A and 8A. Some reduction of flow was observed in the two rock drainage galleries. In the lower gallery the flow dropped from 1000 l/rain to 900 l]min, and in the upper gallery., from 120 1/min to 80 I/min (see Figure 9). Several activities still had to be developed until the sealing of the water paths of percolation was achieved. These events are described in the following pages, and are also summarized in Figures 8 and 9. Each one ot the numbers represents a step given as an attempt to reduce the high uplift pressures found in the foundation of the in- take structures. The related activities consisted of:

1 to 5 - different cement grouting operations along the struc- tural joints between the blocks of the intake struc- ture~

6 - air injection in piezometers 6A, 7A and 8A, to deter- mine the path of water seepage,

7 and 8 - cement .grouting operations along the joint at the up- stream side of the intake structures (as described in item 6.3),

9 - repetition of air pressure test in piezometer 8A,

10 - tentative sealing the fissures along the upstream joint by placing clay in it,

11 - t-mat cement grouting along the same fissures (as des- cribed in item 6.6).

C o m p r e s s e d a ir t e s t

Since the grout between blocks did not give any result, the second hypothesis was investigated. Compressed air was injected through piezometers 6A, 7A and 8A, trying to follow the inverted path of the water flow. The attempt was effective, once air bubbles were seen in the reservoir, immediately upstream of the intake structure, exactly in the joint between the concrete structure and the concrete slab. A diver was able to locate the fissures of air leaks, upstream of the first block of the intake structure, on the right side. It was an

3 rd test

PZ-6A P =2.40

PZ-7A P = 2.40

PZ-8A P = 2.40

Total flow from piezometers

open joint due to displacement, in a place where there were no "fugenbands". The sealing was done by "l-gas" plastic material covered by mortar, and by a layer of asphalt. The diver identified two fissures along the joint, with a length of about one metre each, and an opening of 15 mm and 8 ram. This compressed air test gave important information on the one hand, but on the other seems to have cleaned out the water flow paths, because the flow towards the piezometers increased. The air pressure used in the tests was 0.5 kg/cm" higher than the water pressure at reservoir level. A third pressure and flow test was then performed, with the results shown below:

W. L = 182.15 m Date: October 29, 1978

P = 0 .65- Q = 26

Q = 169 Q = 165

P = 1.95 Q = 193

169 384

P = piezometric head in kg/em 2 ; Q = flow rate in litres/minute The increase in total flow was of the order of 50 per cent.

Cement grouting in the fissures

In each of the two fissures located by the diver, between the intake structure and the upstream concrete slab, gravity cement grouting was done from the crest. The first fissure had an absorption of 2350 kg of cement and the second one took 2000 kg, corresponding to two cubic metres of solid material in each fissure. The grout was stopped without exhausting the absorption capacity of the fissures, because no one knew where the grout was moving to, with a good chance of it flowing inside the reservoir. No reduction of pressure or flow was detected in piezometers 6A, 7A and 8A (see Figure 8). The extensometer n ~ 3, however, at the gallery at el. 144 m, was plugged by cement. This information o f great hydrological interest, shows the influence of the geolo~c',fl features in the hydrogeo- technical behaviour of the foundation rock mass of the ridge area. The extensometer was located at some 15 metres from the grouting point, and it should intercept a fault plane, of high dip, one of the faults that appear in the gallery at el. 144 m (see Figure 6). The cement grout must have gone through the fault plane to the exten- someter hole. The four tons of cement did not reduce the uplift pressures and flows in the foundation of the structure, but it re- duced the flow into the galleries. In the lower gallery, at el. 110 m, the flow of water dropped from 900 l/rain to 750 1/min, and in the gallery at eL 144 m, from 80 to 60 l/rain (see Figure 9).

After 30 days, a new cement grout operation was accomplished at the two fissures. On this occasion, a total of 2800 kg were grouted, by gravity. As result, a sharp decrease was observed in the flow rate of piezometers 6A and 7A, that dropped to 6 and 3 litres/ minute respectively. PZ-8A continued at a flow rate of about 140 l/min. Uplift pressures remained high (as shown in Figure 8) due to the free flow towards PZ-8A. Again, a reduction of flow was observed in the gallery at eL 110 m, that dropped from 750 to 650 I/min. At the gallery of eL 144, the flow dropped from 60 to 30 lJmin. The problem, however could not be considered exhausted, and a new reduction in the uplift pressures was necessary to con- sider the situation as satisfactory.

New air p r e s s u r e t e s t and clay sealing

Next step consisted in a new air pressure test, applied to PZ-8A. A diver was able, again, to detect two fissures at the toe of the pier between blocks 2 and 3 and the diversion channel concrete slab. Again, the air pressure injection caused an increase in the water flow towards PZ-8A, that rose f rom 130 l/min to 170 l/min (see Figure 9). A trial of placing clay directly in the detected fis- sures between the concrete slab and the main structure was done. Initially, fourteen bags of clay were placed on the fissures without any result. Another fourteen bags, with equal volumes of clay and sand, were placed on the fissures later. A slow response to these works was then observed, that seemed to be able to block gradually the seepage paths and, after 10 days, the total flow dropped from 185 to 115 1/min. After a few days, however, it increased again to 130 l/min. The uplift pressures remained high, and were not affect- ed during this procedure. It was then decided to try cement grouting in the foundation, just below the structures, oriented to those

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217

200

160-

-? z :!

J 120-

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80~

4 0 .

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Fig. 9:

200 -

120-

SOMAT(~RIA DE VAZC~ES NOS /Ir--------------~/(~ PZ-6A.7A E 8A / TOTAL AMOUNT OF FLOW AT

. / . . . . . . . . ~ ~ PZ-6A.7A AND 8A

NIVEL PlEZOMETRICO ~/ PIEZOMETRIC LINE / \

o P Z - 7A / II -I- P z - 6A / / ...... \ \

i

/

"~LL"'" - -

P~EZOMETRO5

- - I - - - ~ N t EL PiEZOMETRICO (PZ 6A, 7A E SA) PIEZOMETRIC LINE AT PZ6A,7A AND 8A

~VER SIGNIFICADO DQs EPISC)DIOS NO

IOO J t ABRIL- 78 MAIO " JGNHO JL.~,IC AGO'STO

Behaviour of piezometers in the foundation of the intake structures

SPILLWAY INTAKE STRUCTURES JAN.AT 0CT. 13,29, 19791978 SAME

L E G E N D : - -~ PIEZOMETRIC LINE MEASURED

THROUGH PIEZOMETERS INSTALLED ALONG THE UPPER CONCRET GALLERY ( P Z - ) AND THROUGH EXTENSOMETERS SPECIALLY ADAPTED ( E X - ) , A T T H E SAME DATE(OCT. 13 ,T8 )

- - ~ -4" - - -RESIDUAL PIEZOMETRIC LINE, MEASURED AT THE SAME POINTS ABOVE AFTER THE TREATMENT CARRIED OUT (JAN. 29, 79)

r 1 5 6

. , ,? tKtU -v-.. E• ~ ~ ~ ' ~ x ~ ~

~ , F . , N r t F - - - - - ~

; ~ FAMILY OF FAULTS_

--PIEZOMETRIC LEVELS IN THE FOUNDATION l""ql / MEASURED AT EL.I]0 ~ .

" / DISCONTINUITY

p I L I' [ /

- - ~ DISCONTINUITY

J D R A I N S OF 3 "

I ! I

0 $ 10 20 30 40 50m k ~ i , i I I

GRAPHICAL SCALE

Fig. 10: Piezometric situation in the foundation of the intake structure. Uplift pressures before and after the treatment

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218

points where the fissures had been observed (four points in all).

Cement grout at the upstream toe of the intake structure

The grouting of the foundat ion was programmed to be done through holes, open from the top of the structure, by drilling, oriented in such a way to intercept the vertical joint between the main structure and the concrete slab. The holes should be close to vertical, going through 23 metres of water, a concrete slab of one metre, and then the foundat ion rock.

The first drill hole was located in the neighbourhood of the fissures detected at the upst ream toe to the structure, between blocks 2 and 3. As the borehole reached the foundat ion rock, a tree outlet o f rock dust was observed from drilling through the PZ-8. The hole was drilled up to three metres in rock.

Grouting was started by gravity, with a water: cement ratio of 0.7 : 1, in weight. Cement grout was leaking at PZ-8A, that remain- ed open through the drilling operations and grouting. After a take of 1,100 kg of cement, a marked decrease in flow (with cement) was observed at PZ-8. The grout followed with the addition o f sand in the ratio 0.7 : 1 : 1. After 900 kg o f cement and another 900 kg of sand the flow through PZ-8A stopped. The grout was considered completed. After grouting, PZ-8A was plugged and a quite small water flow was registered in piezometers PZ-6A and 7A. A definitive reduction on the uplift pressures in the foundation at the area of interest was confirmed later (see Figure 10), due to the opening of new piezometers aligned with the existing and lost ones. A new line of drains was also drilled toward the upst ream side, from the gallery at elevation 157.

After this grout operation, the flow at the lower gallery (el. 110) dropped to 640 l/rain, and at the upper gallery (el. 144) dropped to 16 1/min. The grout hole seemed to have reached in full the remaining principal seepage path. The flow started, for sure, in the fissure along tile poorly sealed contact between the main structure of the intake power and the concrete slab of the diversion chan- ne l

T h e geo log ica l i n f l u e n c e a n d c o n d i t i o n s o n t h e deve lop - m e n t o f u p l i f t p r e s s u r e s

The work's done at the Itaflba site, here described and performed from September 78 to January 79, did not have the effect of making the rock mass foundat ion of the intake structure imper- meable. They succeeded, however, in sealing satisfactorily the entrance paths of water to a combinat ion of geological features of high hydraulic conductivi ty. These features were responsible for the development of the high uplift pressures. The fissures between the main structure and the concrete slabs were responsible for the in- flow of water. ,

In summary, from the history of the construction, one could point ou t the geological conditions, that had a fundamenta l role in the development of such pressures. The combinat ion of the following geological features occurred in the ridge area:

i) a plane of discontinuity, of subhorizontal att i tude, at elevation 159 - 160, with many bifurcations, was only partially excavated in the foundat ion of the concrete slab. Part o f this plane may have remained in the foundat ion of the intake structure;

ii) a vertical fault, that occurred immediate ly upstream of the in- take structure, but intercepting the foundat ion of the first block, on the right side (block I), and the ,,rock nose", between the intake structure and the spillway. The fault plane was also clear- ly identified and observed during the excavations of the concrete slab on the division channel, in the vicinity of the intake struc- ture;

iii) a superficial zone of tile rock mass, shaken by blasting ope- rations, resulted in a high hydraulic conduct ivi ty zone.

Final c o n s i d e r a t i o n s

The control system of flow rates and uplif t pressures installed in the foundat ion of the concrete structures, const i tu ted by a concrete slab at the upstream side of the s tructures and by a wide drainage net, proved to be efficient` The drainage system, complemented by few additional elements of hydrogeotechnical monitoring, and adequately used in the detection of upl if t pressures, was able to bring, alone, the a m o u n t and quality o f in lbrmat ion judged to be necessary to evaluate the behaviour o f the structures. Thus, the further monitoring elements installed had a complementary and secondary role. The validity could, therefore, be observed of a reliable hydrogeotechnical monitor ing system that, in the case of Itauba, has the additional funct ion of assessing the efficiency of works carried out along the foundat ion of the intake structures, where uplift pressures reached very high values, exceeding the max imum rates admit ted in the design.

The hydrogeotechnical con t ro l beside that, confirmed the high level of independency present inside the basaltic flows by the different confined horizons of water percolat ion in the foundat ion of the concrete structures. Each prefenrential path of water see- page (contacts between lava-flows, or large discontinuities inside each flow) acts independent ly from the other. They only interact when intercepted by some tectonic feature or where a drainage net

is installed, eliminating this independency, such as happened in the case of Itafiba.

Acknowledgements The authors are indebted to Companhia Estadual de Energia El6trica (CEEE) and the ENGEVIX S. A. tbr the oppor tuni ty of developing this work.

R e f e r e n c e s

CRUZ P. T. - SILVA R. F. (1978): Uplift pressures at the base and the rock basaltic foundat ion of gravity concrete dam. Proc. Intern. Sympos ium on rock mech. related to Dam Found. , ISRM, Rio de Janeiro, III-1-26.

OLIVEIRA A. Ivl. S. - SILVA R. F. - GUIDiCINI G. (1976): Compor tamento hidrogeol6gico dos basaltos em fundaq~es de barragens. Proc. First National Congr. on Engin. Geol. ABGE, Rio de Janeiro, vol. 2 ,413 -429 (in Portuguese).