THE ITAPEBI HYDROELECTRIC DEVELOPMENT IN THE JEQUITINHONHA RIVER

Transcription

1 THE ITAPEBI HYDROELECTRIC DEVELOPMENT IN THE JEQUITINHONHA RIVER Author: Silvano Custódio Albertoni

2 THE ITAPEBI HYDROELECTRIC DEVELOPMENT IN THE JEQUITINHONHA RIVER 1. INTRODUCTION The Itapebi HPP is located in the basin of the Jequitinhonha River, in the municipality of Itapebi, southern region of the state of Bahia (see Figure 1). The power plant has three Francis generator units of 150 MW each, totalling 450 MW of installed power. The small area flooded by its reservoir, of around 62 km 2, and the predominance of pastures, minimized the environmental impacts. Figure 1 - Location of the Itapebi Hydroelectric Power Plant The concession for the energy exploitation belongs to Itapebi Geração de Energia S.A., a firm of the Iberdrola Group. The works were initiated in October of 1999, and the beginning of commercial generation of the first unit took place in January of The civil works were executed by the contractors Norberto Odebrecht and Andrade Gutierrez. The electromechanical equipment was supplied by Alstom Power, and erected by the Construtora Norberto Odebrecht. The executive project was developed by Engevix and by Intertechne. 2. DESCRIPTION OF THE ENTERPRISE The river diversion was effected through three tunnels of arch-trapezoidal section, with maximum height of 16 m and an average length of 680 m located in the left bank of the river, under the site for the implantation of the future spillway. The diversion occurred at the end of October of 2000, six months in advance of the original schedule, thanks to the prior planning of the job and the strategy employed. The dewatering of the foundation area of the dam was carried out by means of soil and rockfill cofferdams, with a maximum height of 42 m, implanted upon a thick alluvial deposit. For waterproofing the alluvium, "jet-grouting" curtains and clayey blankets were executed. The infiltrations were controlled by a system of deep shafts for lowering the water table at the downstream toe of the cofferdams. The generation circuit, located in the right bank, is constituted by a water intake of the hollow gravity type, three pressure tunnels with a diameter of 7.40 m, an average length of 160 m, supplying the semi-indoor powerhouse, equipped with three vertical shaft Francis turbines. The spillway is of the surface type with a ski-jump, constituted by 6 spillway bays with radial-type gates, 17.4 m in width and 20.0 m in height, which discharge the ten thousand year flood of 20,915 m³/s. The dam is of the concrete face rockfill type (CFR), with crest at El , a length of 583 m and a maximum height of 121 m. Figure 2 presents the general layout of the enterprise and Figure 3 the cross and longitudinal sections of the CFR dam in the riverbed. 3. GEOLOGY, GEOTECHNOLOGY AND FOUNDATIONS The rock mass at the site of the Itapebi Hydroelectric Power Plant is inserted into the geological context of the ancient crystalline shield, of the Pre-Cambrian Era south of Bahia, constituted by metamorphic rocks of the Super Group Espinhaço. At the site of the development there are outcroppings of granitic gneisses intercalated with sub-horizontal lenses and layers of biotite-schist/ amphibolite (BX/AF). The regional direction of the deformation (foliation) structures is NW with a gentle dip towards the E quadrant. The structural framework is constituted by sub-vertical joints in the following directions: NS/85ºE, N85ºW/80ºSW, N50ºW/75ºSW and N42ºE/40ºSE. Relief joints are also observed and sheared joints in the NS and EW directions, and sub-vertical faults in the NS direction. The relief joints observed in the gneissic mass were 222

3 Figure 2 - General Layout of the Hydroelectric Power Plant Figure 3 - Cross and Longitudinal Sections of the Concrete Face Rockfill Dam in the Riverbed 223

4 produced due to the intense erosive process in the valley, which created vertical cuts more than 100 m in depth, opening, in this manner, the existing joints oriented in the NS and EW directions. These openings permitted the infiltration of water to depths of more than 50 m, a fact that produced an alteration in the layers of BX/AF, principally at the contacts with the granite gneiss. The commencement of the works produced the exposure and permitted access to the diverse layers of BX/AF, as exemplified in the Photos 1 and 2, which depict the layers present in the right abutment of the dam. The diverse sub-horizontal lenses and layers of BX/ AF were mapped and designated for differentiation concerning each bank or structure. This is due to the lack of correlation between the left and the right banks resulting from the erosive discontinuity of the river valley, aggravated by the existence of bifurcations and ramifications of the layers. In the right bank, between the dam and the powerhouse, the layers are truncated by a fault in the NS direction, impeding their correlation in the upstream/downstream direction. In July of 2001, during the excavation for the spillway, a large rock slide occurred in the left abutment. This rock slide took place along one of the layers of biotite-schist present, with a mean inclination of about 13º and along a length of approximately 210 m, involving a rock volume of about 170,000 m³. Photo 3 shows the site of the slide. Photo 1 - General View of the Right Abutment with the Presence of Various Planes of BX/AF Photo 3 - General View of the Left Abutment straight after the Slide of the Rock Mass over the Layer of Biotite-Schist. Photo 2 - Detail of a Layer of BX/AF of Metric Thickness In the left hillside the erosive relief of the valley promoted the release of large blocks of rock according to the reticulation of the fractures, released at the base by the planes of the weathered BX/AF. The layers and lenses of BX/AF present undulations in the scales of decametres, metres and centimetres, and attain lengths greater than 500 m. Their thickness varies between 0.20 and 3.00 m, and the granite gneiss package between the layers varies from 5.00 m to m. The planes formed by the layers and lenses of BX/AF dips between 10º and 20º to the NE (downstream), being liable to present localized inflections of the dip of around 30º. The uncommon and unforeseeable nature of a rock slide of this magnitude led to a minute evaluation of its causes, requiring a re-evaluation of the shear-resistance parameters of the weathered layers of BX/AF, whose adopted values had been obtained by triaxial and direct shear tests on blocks of undisturbed samples withdrawn from the excavations at the beginning of the job. A reevaluation was also made of the geomechanical model in order to dimension the type and quantity of the reinforcements to be utilized in the foundations of the structures of the development after the slide, without affecting the contractual commitments for the start of generation. Different intervention approaches were initially considered for treating the layers of BX/AF present in the foundations of the main structures of the project. Meanwhile, the underground interventions were granted priority since they permitted the simultaneous execution of the reinforcements to the foundation and of the superstructures themselves, with a minimum of interference between these jobs, thus allowing maintaining the time schedule of the contracted services. 224

5 To this effect, the solutions that resulted most appropriate both from the construction aspect as also regarding the questions of job safety were: Reinforcements of the foundations through the construction of concrete shear keys implanted along the tunnels and trenches excavated in the rock, in order to intercept the layers of BX/AF Excavation of complementary drainage tunnels in the abutments to assist in relieving the uplift pressures imposed by the reservoir; Placement of tendons through the shallow BX/AF layers permitting safe excavation and concrete placement of shear-keys and structures. Subsequently, these tendons were incorporated into the final containment elements and considered in the global safety analysis of the structures. Figures 4 and 5 show the typical shear-key section and the foundation reinforcement of the dam in the area of the abutments. In addition to the adverse conditions in the abutments, the Jequitinhonha riverbed at the Itapebi dam-site is constituted by a large deposit of sand that covers the bedrock over the entire width of the river. The investigations carried out to subsidize the project of the dam and the cofferdams included boreholes effected by percussion, rotary and piezocone drills, as well as geophysical soundings by the geo-radar method. These investigations indicated an alluvial layer with a maximum thickness of around 28 m. During the removal of the alluvium from the riverbed in the region of the foundation plinth, the existence was verified, furthermore, of a deep paleochannel that had not been detected in the investigations. Figure 4 - Foundation Treatment of CFR Dam - Typical Shear-Key Figure 5 - Typical Section of Foundation Treatment - Area of the Abutments 225

6 The Itapebi CFRD was the sixth concrete face rockfill dam to be built in Brazil and presents some peculiar characteristics, such as, the incorporation into the embankment of part of the riverbed alluvial deposit, and the declivity of the upstream slope being equal to 1.25H: 1V. Other relevant aspects related to the foundations were the presence of sub-horizontal layers of biotite-schist/ amphibolite, with thicknesses ranging from centimetres to metres cutting the gneissic rock in the abutments, and the presence of a deep paleochannel in the riverbed. Photo 4 - View of the Alluvial Layer in the Riverbed The principal dam data are listed below: Maximum height in the plinth of the paleochannel region: 121 m Length of the crest: 583 m Volume of rockfill and transitions: 3,841,000 m³ Area of the face: 66,500 m 2 Photo 6 presents a view of the dam from upstream before the filling of the reservoir. 4. HYDROLOGY Photo 5 - Paleochannel in the Riverbed The drainage area of the undertaking is 68,100 km 2, with a flooded area of km 2. The mean long term flow MLT is 406 m 3 /s, the maximum monthly mean flow is 4,140 m 3 /s and the minimum is 48 m 3 /s. The diversion of the river was sized for a flow of 9,300 m 3 /s and the ten thousand year flood passing through the spillway is 20,915 m 3 /s. 5. DAM Three alternative arrangements were contemplated during the feasibility studies, one of which demonstrated the technical and economical advantage of a concrete face rockfill dam - CFRD. The other two alternatives contemplated dams of roller compacted concrete - RCC, which were at a disadvantage due to the balance of the materials available and the risks imposed by the local geology, in addition to the higher costs required for the execution of this type of dam. Photo 6 - View of the Dam and Upstream Cofferdam before the Filling of the Reservoir 6. SPILLWAY The spillway, located in the left abutment, was designed for a ten thousand year flood of 20,150 m 3 /s. Due to the unforeseen geological condition that caused a large rock slide during the excavation, there was the need to modify the design of the geometry of the chute, executing a block of RCC at its end, in the area already excavated for the dissipation basin, in order to stabilize the excavation and launch the jet of water further away, thus also avoiding the risk of regressive erosions of the rock. In order to permit the adaptation of the ski jump to the already excavated local conditions, the solution presented was a flip bucket inclined 45º with respect to the axis of the chute. This unusual solution of a jet not orthogonal to the flow was studied on the reduced scale model, and its operation after various years qualifies it as an adequate solution. The Photos 7 and 8 show the spillway after its conclusion and in operation. 226

7 Photo 7 - Spillway at the End of Construction Photo 8 - Spillway in Operation Figure 6 - Plan View of Location of Monitoring Instruments installed in the CFRD 227

8 7. PERFORMANCE OF THE DEVELOPMENT The plan of the monitoring instrumentation of the dam considered the installation of instruments within the rockfill embankment and in its concrete face as also in the foundations of the abutments and drainage galleries installed in both abutments of the dam. Figure 6 presents the plan view of the dam with the positions of the instrumented key sections and the location of the other instruments in the concrete face and the rockfill embankment. More than a 100 monitoring instruments were installed in the CFRD, all of them with the purpose of measuring the displacements suffered both in the rockfill and in the concrete face. To monitor the behaviour of the abutments, in which were placed shear-keys filled with concrete and intercepting the layers of biotite-schist, the instrument types installed were load cells, multiple extensometers and inclinometers. The drainage galleries received manometer piezometers, triorthogonal jointmeters and flowmeters. During filling of the reservoir, when the water level reached El. 102 m, eight metres below the normal water level, a significant increase in percolation rate in the dam was observed, as well as an increase in the deformation of the rockfill embankment in the right abutment, due to its saturation. This increase in deformation of the embankment led to a major deflection of the slabs, mainly at the crest, beside the parapet wall. Inspections in the foundation drainage galleries of the dam failed to indicate an increase in seepage through the rock mass. The leakage of the dam occurred through the slabs of the upstream facing, as confirmed by the underwater inspection which detected the existence of horizontal cracks in the concrete. After mapping the cracks, a process was begun of launchig fine silty sand from the crest of the dam, to silt up the cracks in the facing. This procedure proved effective, reducing the maximum measured flow from 902 l/s to 127 l/s, at the beginning, and at present the measured percolation flow has stabilized at 50 l/s. The instruments installed in the foundations of the abutments don't indicate increases in piezometric levels nor significant deformations or displacements in the rockfill embankment, all of which indicates that the treatments executed in the foundations have proven adequate. 8. TECHNICAL FEATURES General Location Rio Jequitinhonha - Itapebi - South of Bahia State Year job initiated 1999 Year of conclusion 2003 Owner Itapebi Geração de Energia S.A. Designer Engevix Engenharia S.A. - Intertechne Consultores Ltda. Civil Contractors Construtora Norberto Odebrecht and Andrade Gutierrez Erection Construtora Norberto Odebrecht Electromechanical equipment supplier Alstom Power Basic data Area of the hydrographic basin 68,100 km 2 Average annual precipitation 877 mm Reservoir Area at the maximum normal level km 2 Total storage volume 1, hm 3 Maximum normal water level m Maximum flood water level m Minimum water level m Tailrace channel Maximum normal water level Maximum flood water level Minimum water level m m m Flows Average incoming flow 406 m 3 /s Maximum recorded flow 4,140 m 3 /s Minimum daily flow recorded 48 m 3 /s Maximum diversion flow and period of recurrence 9,300 m 3 /s, PR=25 years Maximum Probable Flood MPF or ten thousand year flood 20,915 m 3 /s Dam Concrete Face Rockfill Dam - CFRD Length 583 m Maximum Height 121 m Crest elevation El m Width at the crest 7 m Spillway Surface with Ski Jump Length of the Structure m Height of the Structure m Discharge capacity 20,915 m 3 /s 228

9 Spillway Gates radial Number 6 Dimensions Width m Height m Manufacturer Alstom Water intake Hollow Gravity Structure Length m Maximum height m Turbine Francis Number of units 3 Rated power MW Reference Head 76.7 m Maximum discharge 213 m 3 /s Generator Rated Power 160 MVA Rated voltage 13.8 kv Power factor 95% Rotation clockwise Intake gates Dimensions Width Height Manufacturer 3 Fixed Wheel Gates and 1 Stoplog 5.50 m 7.20 m Alstom Step-up transformers Number 3 Three-phase Rated power 160 MVA Rated Voltage (Primary) 13.8 kv Rated Voltage (Secondary) 230 kv Diversion Diversion gates Height Width Tunnels 3 Sliding Gates m 4.20 m Diversion tunnel Number 3 and dimensions of section Arch-rectangle 16x14 m Length 579/683/727 Penstock Pressure Tunnels Number 3 Internal diameter 6.70 m Length 175 m of Tunnel and 45 m of Armour Plating Manufacturer Alstom Powerhouse Height Length Installed capacity Indoor 34 m 112 m 450 MW 9. BIBLIOGRAPHY [1] ANTUNES, José Sobrinho; ALBERTONI, Silvano C.: "Escolha da Barragem Tipo EFC para a UHE Itapebi" - II Simpósio sobre Barragens de Enrocamento com Face de Concreto - Florianópolis [2] RESENDE, Fernando Dias: "Novo Método Executivo para Barragens de Enrocamento com Face de Concreto" - XXIII SNGB - Belo Horizonte [3] CONCRETE FACE ROCKFILL DAMS:"J. Barry Cooke Volume" - CFRD Beijing, China [4] FERNANDES, Rafael; BELITARDO, Gustavo: "Projeto e Instalação da Instrumentação da BEFC Itapebi" - XXIV SNGB - Fortaleza [5] RESENDE, Fernando Dias, ALBERTONI, Silvano C, FERNANDES, Rafael: "Aspectos do Projeto da Barragem EFC no AHE Itapebi - XXIV SNGB - Fortaleza [6] RESENDE, Fernando Dias; BELITARTO, Gustavo; ALBERTONI, Silvano C, Moraes Roberto B.; FERNANDES, Rafael: "AHE Itapebi - Tratamentos Especiais das Fundações" - XXV SNGB - Salvador [7] ALBERTONI, Silvano C, FERNANDES, Rafael; ANTUNES, Jaqueline: " Instrumentation and Auscultation of Itapebi CFRD During and After Filling the Reservoir" - Seminário BEFC - Florianopolis

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