1.design and performance mazar dam simposio rio 2011

The Second International Symposium on Rockfill Dams Rio de Janeiro 27-28 Oct. 2011 1 / 9

DESIGN ISSUES AND PERFORMANCE OF MAZAR DAM: A 166mHIGH CFRD IN A NARROW CANYON (ECUADOR).

Etienne FROSSARD1, Ana Luca MOREIRA-YODA2, Cristian NIETO-GAMBO A3 , Secundo VANEGASVALENCIA4

1, Director Tecnico, TRACTEBEL Engineering-COYNE et BELLIER, 5-Rue du 19 Mars 1962 /92622-Gennevilliers-FRANCE , [email protected]

2, Ingeniero Principal, TRACTEBEL Engineering - LEME, Rua Guajajras 43- CEP30.180-909-Belo Horizonte-BRASIL, [email protected]

3, Ingeniero de Proyecto, TRACTEBEL Engineering-COYNE et BELLIER, [email protected], Jefe del Proyecto Mazar, CELEC EP HIDROPAUTE, Panamericana Norte Km7, Cuenca - ECUADOR

[email protected]

Abstract: Final design of Mazar Dam took place between 2005 and 2007, while some significant incidentswere observed at impounding various large CFRD worldwide, leading the profession to revise the designmethodology of this type of dams . The present paper details the specific difficulties inherent to Mazar dam

site configuration, the methods developped to evaluate their potential consequences, and the practicalremedial measures adopted in the design. The main features adressed are the design measures adopted to

cope with a narrow canyon, and a very steep right abutment.At the end of construction in 2009, the dam was impounded , and has been working satisfactorily so far.

Key words: Design, performance, steep abutments.

1 Background

Mazar Dam is part of the Paute-Mazar Hydroelectric Project, which entered in operation in 2010,and is located in the South East region of Ecuador, 100km from Cuenca City. This new scheme isthe upper step of the Paute River Hydroelectric cascade, owned by Ecuadorian utilityCELEC-HIDROPAUTE, including the existing 1075 MW Paute-Molinos, in operation since 1983and located just downstream of Paute-Mazar. Although the Paute Mazar installed capacity is only170 MW, the regulation provided by the size of its reservoir will increase the Paute Molinos annualgeneration by about 500 GWh. These two upstream steps of Paute River cascade will be completeddownstream by two other hydroelectric projects in the future: Sopladora, 487 MW, currently underconstruction, and Cardenillos, 327 MW, in design phase. In this eastern part of the Andes Cordillera, the Paute River has deeply incised its valley withina mass of metamorphic rocks including mainly quartzitic schists with intercalations of chloritic andsericit schists, which constitutes the foundation and the sources of construction materials for thedam body.


1.1 General-Basic features

Together with Mazar dam, the project includes a set of underground works for waterways andgeneration works, Figure 1.

The dam is a concrete face rockfill dam 166m high from the toe of concrete face, sloped at1,4h/1v upstream and 1,5h/1v downstream, with a rockfill volume of 5,35 hm3, and a 340m longcrest, its main section having been detailed in a previous publication [1].The main zones 3B and 3Cof dam body have been built with quartzitic schists with max size 500mm and 800 mm respectively,cautiously compacted and generously watered under strict quality control during construction [1].

Figure 1. Mazar hydroelectric project general layout

1.2 Specific challenges

The dam site configuration is constrained downstream by an affluent valley on left bank, andupstream by another one. The main dimensions given above, outline the very narrow proportions ofthe dam site, which included a very steep right abutment, with vertical cliffs displaying someoverhangs, Figure 2.

Figure 2. Mazar Dam narrow site configuration, with very steep right abutment.


This constrained location in a deeply incised valley, very narrow configuration and verticalityof the right abutment raised construction issues, particularly for accesses and earth moving, andspecific design issues [2] associated with the solution of a CFRD in such a site, which had been setin previous design stages.

2 Design Issues

For a flexible and deformable dam body in such a dam site, associated with a rigid concrete face,main design issues are aroused by the consequences of unavoidable settlements occuring winthinthe dam body below the concrete face.

2.1 Risks associated with settlements in a narrow canyon

The risks associated with settlement movements within the granular fill and their consequences, may beusefully schematized as follows [3]. In a typical section, Figure 3-a), the lines of principal stressesresulting from both impounding and self-weight forces, have the shape of line C-C on the figure.

Under the forces exerted by impounding, the settlements are resulting of small shear movementsdistributed within the granular fill mass, triggered by local breakage of stone or blocks. In a right bankto left bank section, transverse to valley axis and passing through line C-C Figure 3-b)-, the trace ofthese small shear movements are distributed within the rockfill mass, with orientations also widelyspread, but with some polarization on two characteristic directions.

Those movements are predominantly clockwise shear on rockfill above left abutment, predominantlyanticlockwise shear on rockfill above right bank, and mixed directions on rockfill in the center of valley.At vicinity of perimetric joints, at junctions between the plinth and the concrete facing:- if the abutment slope is sufficiently smooth Detail A on Figure 3-b)-, the slip lines resulting from

those small shear movement, are intercepted at short distance by the foundation, so the associatedshear cannot extend over a long distance, and the deflection line of the concrete facing near theperimetric joint, will be regular and progressive;

Figure 3- Settlementsmicromechanismswithin the dam bodyand its consequences


- if the abutment slope is excessively steep- Detail B on Figure 3-b)-, then the slip lines resultingfrom the shear movements are no longer intercepted at short distance by the foundation, candevelop over long distance, and then develop a localized shear zone at abutment contact, whichresults in a concentrated differential settlement of concrete facing relatively to the plinth, localizedat the perimetric joint, resulting in a step on the deflexion line of the facing , right at theperimetric joint.For typical value of the physical friction angle between rock pieces in rockfill, the

corresponding critical abutment slope is about 60 to 65 on horizontal (without safety margin). Basicmitigation measures on steep abutments, can be either to provide a smoother slope by excavations, or tobuild a zone of low compressibility fill at contact with steep abutment. Another risk is associated with the consequences of these settlements in dam body in the centerof the valley, under the concrete facing. These consequences are the horizontal contraction strains,resulting from wedging of dam body between the abutments during impounding. Horizontalcompression strains are then induced in facing, which may reach failure in its central part, as in casesrecently reported in Brazil, Lesotho, and China. For steep abutments inducing significant contact shear, the order of magnitude of these strainsin the rockfill, may be evaluated at mid-height (Fig. 4) on the basis of simple kinematics. This leads tothe practical relation of Fig. 4 a), which links this compression strain during impounding to twoadimensional factors: a dam deformability ratio, and a valley shape ratio.

For given geometric site conditions and dam height, this simple relation sets the rockfill rigiditymodulus magnitude required in order to keep these strains within acceptable limits; reversely, for givengeometric site proportions and given rockfill rigidity modulus, this relation sets the maximum heightallowable. Knowing that reinforced concrete threshold for damages under compression strains is about

Figure 4 Horizontal contraction strainsinduced in the center of concrete facingat impounding, and its consequences:concepts and rationales.


0,2%, this simple formula leads also to a diagram of risks for concrete facing failure by horizontalcompressions (Fig. 4b). Damaged and undamaged dams recently commissioned do locate quite well inthis diagram.

The specific conditions for Mazar dam, also displayed in the diagram, showed that, for usual rangeof rigidity modulus at end of construction, between 40 and 80 MPa, significant risks of failure in thefacing by horizontal compressions were to be considered.

2.2 Detailed 3D numerical analysis of dam behavior

A detailed numerical analysis by non-linear finite elements model was then developed, in order tosustend these design issues, and first of all, the question of reshaping the upper part of right bankabutment with excavations, although limited by the presence of the spillway (see figure 2).The Figure 5 displays the concentrations of shear strains at contact between the dam and the rightabutment (foundation removed, upstream face on left), construction settlements on pictures at left,impounding ones at right, original foundation surface at top, reshaped one at bottom. The reshapedfoundation shape, designed according to the concept of Figure 3, permitting to avoid shearconcentrations just below the concrete facing (compare upper and lower pictures).

Figure 5. Expected shear concentrations at contact with RB foundation: FEM study.

2.3 Specific measures implemented

The right abutment reshaping adopted by this way consisted in smoothing the rock slope byexcavations, to 2h /3v over the first 50 m in height under the concrete facing, in order to provide


better contact conditions for the rockfill on this right abutment just below the facing, and reducestrongly the intensity of differential movements to be anticipated between the plinth and the facing. As previously outlined, the presence of the spillway on this right bank was a heavylimitation for this reshaping (see Figure 2), so significant residual movements were to beanticipated at perimetral joint. In order to withstand these differential movements and securethe watertightness, specific perimetral joint features were then designed, on the basis ofChinese practices, Figure 6.

Figure 6. Specific perimetral joint features for RB: principles and practical implementation.

These features having been adopted to provide mitigations measures as regardsdifferential settlement effects on right bank, the question of horizontal contraction strains andrisks of failure in the central part of concrete facing was addressed through the same 3Dnumerical analysis. A map of computed horizontal contraction strains to be expected under thefacing was prepared on the basis of the numerical analysis. Over the whole area where morethan 0,2% of contraction (onset of damages in concrete under unconfined compression ) was tobe expected, then the following specific measures were adopted, Figure 7:- reducing the slab width by a factor 2 (7,5m width slabs instead of 15m);- provide vertical compression joints between these narrow slabs in order to absorb the

compression movements, these compression joints were designed with 3,2 cm width voids,filled with a compressible special wood (copal), able to accept more than 50%compression strain, with a compression stress well below the concrete strength;

- extend these compression joints within the concrete curb under the facing by sawing thecurb before concreting the face slabs, in order to avoid curb buckling below the facing.


Figure 7-Horizontal contraction strains expected under the facing at impounding-Adaptationsof joint spacing within the facing, and provision of specific features on RB side.

Between this zone and the right bank plinth, as the 3D numerical model displayed heavystrain gradients, a reinforced border slab was included in the design, to provide some bridgingcapacity in case of heterogeneous movements in this area. The Figure 8 displays the nearlyfinished concrete facing.

Figure 8 Finition works on slab joints on RB side


3 Performance

3.1 General dam behaviour at impounding

The dam has been impounded in the fall of 2009, Figure 9.A method of inverse numerical analysis [5] has been used to assess the rigidity modulus finally

achieved within the dam, on the basis of observed settlements. The values found are about 50 MPafor 3B and 3C zones, in spite of the cautious placement and compaction procedures, which wereimplemented targeting a more rigid compacted rockfill.

Nevertheless, no significant cracking has been observed so far on the facing, except capillaryhorizontal fine cracks in the upper part.

Figure 9 . Commissioned Mazar Dam and reservoir

3.2 Performance of specific measures implemented

Although significant differential settlement were observed on the right bank side , about 10 cmover the upper 90 m of the dam, up to 11 cm at crest, the specific features implemented on the RBperimetric joint appear to have worked adequately.The absence so far of vertical cracking by compression which has occurred in the past in variousCFRD, [6], outline that the features implemented to release these compressions appear also to haveworked properly.


4 Conclusions

After a period of design based on empirism of past examples, leading to some incidents, the designof CFRD is heading at a more rational approach. In the case of Mazar Dam, it is thought that theuse of detailed 3D numerical modelling with non linear constitutive model for rockfill has broughta real design tool. However, in the opinion of the authors, progression is still needed in masteringthe mechanical behavior of rockfill, and specially the mechanical behavior of the large sizerockfills used currently in rockfill dams, marked by significant size effects [7].

Acknowledgements

CELEC-HIDROPAUTE is gratefully acknowledged for the permission to publish the present paper.

References

[1] C.A. Ramrez Orejuela, Mazar Dam: a 166m high CFRD in an assymetric canyon Ecuador ,Proceedings of III Symposium on CFRD Dams Honoring J.Barry Cooke- CBDB-ICOLDFlorianopolis, Brazil, Oct 2007.

[2] Consorcio Gerencia Mazar, Memorias de Clculo e Informes de Diseo del Proyecto Mazar- Hidropaute, 2005-2006

[3] E. Frossard, On the structural safety of large rockfill dams . Transactions of XXIII InternationalCongress on Large Dams, Q.91 R.39, Brasilia, May 2009

[4] Consorcio Gerencia Mazar Analisis del Comportamiento 3D de la presa Incidencias sobre elDiseo Ejecutivo Hidropaute Julio 2006

[5] C.Nieto-Gamboa, Mechanical behaviour of Rockfill Applications to Rockfill Dams-PhD - Thesis Ecole Centrale Paris March 2011-

[6] N.L. Pinto, Very high CFRD Dams-Behaviour and design features, Proceedings of III Symposiumon CFRD Dams Honoring J.Barry Cooke- CBDB-ICOLD Florianopolis, Brazil, Oct 2007.

[7] E.Frossard, W.Hu, C.Dano, P.Y. Hicher - Rockfill shear strength evaluation: a rational methodbased on size effects Gotechnique 62, N5, 415-427, London, May 2012.

Note: More elements may be found in detailed conferences performed later

E.Frossard- El diseo de la presa Mazar y su comportamiento a la puesta en servicio InvitedConference-Seminario International Experiencias en Construccin de Proyectos Hidroelectricos-CELEC-Cuenca (Ecuador) Oct 2012

E.Frossard, C. Nieto Conception du barrage de Mazar-Comportement en service SymposiumTechnique du 31 Jan 2013- Grenoble (France), Comite Franais des Barrages et Reservoirs.

1.design and performance mazar dam simposio rio 2011

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