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  • THE IRAP DAM ANDHYDROELECTRIC PROJECT

    This paper was written by Brasil Pinheiro Machado based on papers indicated in the references.Some figures and the technical features were based from documents furnished by Waldaisy S. Abreu Sifuentes from Leme.

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    THE IRAP DAM ANDHYDROELECTRIC PROJECT

    1. INTRODUCTIONThe Irap Project, is a 360 MW hydroelectric project,

    built in the upper reach of the Jequitinhonha River, in theState of Minas Gerais, Brazil. The main purpose of theproject, besides local power generation, is to create alarge reservoir to regulate the river flow for a series ofdownstream hydro-projects programmed to be built inthe near future and including the already built 450-MWItapebi Project. The reservoir, with a total volume of6 billion cubic meters and flooding an area of 137 km2, iscreated by an earth-core rockfill dam, 208-m high, thehighest in Brazil.

    The Project is owned and operated by CEMIG -Companhia Energtica de Minas Gerais, the power utilityfor the State of Minas Gerais. The Project was built fromMay 2002 to August 2005, with a strict and very tightschedule of 40 months from start of site activities to initialpower generation. It was designed by a consortium oftwo Brazilian engineering firms, INTERTECHNEConsultores and LEME Engenharia acting as engineersin an EPC joint-venture led by ANDRADE GUTIERREZand including construction companies ODEBRECHT,HOCHTIEF and IVAI and with Voith-Siemens Hydro for

    equipment supply. CEMIG technical team acted asOwner's Engineer and managed as well the administrativeand commercial issues of the realisation of the Project.A general view of the Project is depicted in Photo 1.

    2. BACKGROUND

    The Jequitinhonha River Basin is located mainly inthe Northern part of Brazilian State of Minas Gerais witha minor part in the State of Bahia, as illustrated in Figure1. The river basin was initially surveyed for itshydroelectric resources during the sixties as part of themajor Brazilian Inventory of Hydroelectric Resources,covering the South-Central area of the country. Thesestudies identified 20 feasible sites for power projects inthe basin. A later study, carried out between 1984 and1987, revised the conclusions of the previous survey andreduced the number of feasible sites to 14, in which theIrap site was identified, and the project considered as amajor upstream reservoir for regulating the river flowsbesides local electric power generation.

    Comprehensive site topographical, geological andhydrological investigations were carried out by CEMIG

    Photo 1 - General View of Irap Hydroelectric Project

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    Figure 1 - Location of Irap HPP

    during the nineties, associated with project feasibilitystudies. These studies considered the possibility of aRCC dam, but ended up by discarding this alternativebecause of the questionable quality of the economicallocally available concrete aggregate. Therefore anembankment dam was the chosen alternative, andadditional studies were concentrated between an earth-core and a concrete-face rockfill dam. After detailedstudies of either type of dam and different project layouts,an earth-core embankment dam was selected.

    In 1998 CEMIG obtained from the Federal Governmentthe official concession for exploring hydroelectric powerfrom the site, and in 1999 contracted the EPC Consortiumwhich included the designers, INTERTECHNE and LEME,who carried out the basic design of the project. This basicdesign was submitted to ANEEL the Federal Governmentelectric power regulating agency in January 2002 andapproved for construction. Construction started in May2002 and the first unit started power generation in theend of August 2005, a 40-month construction schedule.

    3. SITE CHARACTERISTICS

    The region of the project site presents a flat erosionsurface deeply entrenched by river channel. The siteproperly is formed by a narrow canyon where the riverruns between very steep abutments, about 80 to 100 mwide and 80 m deep, a narrow flat bottom and an additionalriver channel about 30 m deep and 20 m wide.

    The rock mass at the site is a graphitic quartz-mica-schist with disseminated sulphide minerals, with goodfoundation and excavation conditions. The rock is usuallyfine grained, dark grey coloured, with a metamorphicmilonitic foliation dipping around 11 to SE.Stratigraphically, it belongs to the pre-Cambrian SalinasFormation, of the Macabas Group. The soil cover ismoderate to thin, but the zone of weathered rock is quitethick and irregular. Besides stress relief fractures and

    the ones related with the foliation, the rock mass hastwo main sub-vertical joints systems. Near the surfaceand the steep scarps, these structures were found open,weathered, or filled by soil-like or lateritic materials.

    The occurrence sulphides in the rock mass - mainlypyrite and pyrrhotite - in percentages higher than normalwere a key preoccupation of CEMIG and of the designerswho carried out extensive studies to evaluate the impactof such a feature in the performance of the different projectstructures. This was considered to affect unprotectedconcrete as a result of oxidation of sulphites producingsulphuric acid, and could be deleterious to the rockfill. Adetailed description of the problem and the solutions usedfor the Project is presented by Marques Filho et al. (2009).

    The Jequitinhonha River, at the Irap site, has a meanflow of 158.1 m/s and well defined dry and rainy seasons.The average annual precipitation in the river basin is1,300 mm, with a very clear distinction between the dryseason (April through October) and the rainy season(November through March). The flood pattern at the Irapsite shows, correspondently, a clear seasonality:5,950 m/s during the rainy period versus 2,525 m/s duringthe dry period, for a 1,000-year recurrence time. The PMFhydrograph has a peak of 11,446 m3/s and an averageflow of 7,946 m3/s. As mentioned further down in this report,the spillway structure was sized with consideration of theflood routing in the reservoir, which of course, reducessubstantially the peak of the outflow flood hydrographs.

    4. PROJECT DESCRIPTION

    The characteristics of the site and the schedulerequirements set forth by the Owner, made it very difficultto develop a project layout capable of allowing theconstruction to overcome successfully these obstacles.In fact the topographic features made it difficult andexpensive to provide access to the various constructionfronts. On the other hand, for the Owner, the feasibility ofthe Project, based on economical and financialconsiderations, could only be achieved if the constructiontime for dam and other key structures could be reducedto allow initial power generation in an extremely tightperiod (for the size and type of structures) of 40 months.To accomplish this, a number of innovative design andconstruction features were adopted, some of which forthe first time in Brazil. These allowed the achievement ofthis goal and an early reservoir impounding (previous todam completion) permitting initial power generation, asmentioned, 40 months after start-up of contractormobilisation. In relation to the original conventionalplanning there was an advance of seven months whichwas achieved by an engineering design in strictassociation with the construction planning and of courseby providing construction equipment and manpowercoherent with the production rates required. A detaileddescription of the challenges and solutions encounteredis presented by Kamel et al. (2006).

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    The Project layout is depicted in Figure 2.As seen in this figure, the Project is developed across

    a rather tight river curve which has its inner side in theleft margin, and ending in an almost 90 angle to theright of the incoming stretch of the river. The dam axis isplaced upstream of the "vertex" of the river curve in asection which was considered, topographically andgeologically best fitted for it. The power waterways andthe spillway structure are routed in the left margin,bypassing the dam, but with features and locationsespecially designed to reduce construction quantities andto expedite construction activities.

    4.1. River DiversionRiver diversion for construction was done through

    two diversion tunnels designed with a cofferdamheight compatible with a 50-year recurrence period duringthe first wet period and subsequently for higherfloods and different crest dam elevations. The tunnelswere built on the right bank, contrary to what seemedto be the logic design solution, because thereexisted a public road on the right margin of the river,from which access to the initial construction areawould be easier and faster. This impliedlonger diversion tunnels as compared with thealternative location in the left abutment, but allowedachievement of river diversion 11 months afterinitial construction mobilisation, that is three monthsbefore originally scheduled.

    One of the tunnels was designed and builtwith its entrance at a higher elevation than the other one,and without a flow control structure. This tunnel was1060 m long. The other tunnel, was 1230 m long, had anentrance elevation lower than the other and had anunderground gate controlled structure, accessible by avertical shaft. This concept of one of the tunnels with ahigher entrance without control structure is now a commonpractice in Brazil and is intended to save inconcrete, equipment and of course in time because thelogic is that it is closed with a soil embankment at thebeginning of the dry season when the lower tunnel iscapable to handle the river flows. This concept was firstused at the Segredo Powerplant in the Iguau River, in1992.

    The diversion tunnels had a mushroom rectangularsection, 13,0 m high by 13.2 m width, excavatedin a crown and bench