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The Second International Symposium on Rockfill Dams Rio de
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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.
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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.
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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
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- 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.
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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
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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.
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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
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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.
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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.