Building plans24 September 2003
The 880MW Campos Novos hydroelectric scheme is now under construction through an EPC contract. When completed, it will be one of highest structures of it's type in the world
THE CAMPOS Novos hydroelectric project - one of the largest projects in construction in the South of Brazil - is located over the Canoas river, close to the Campos Novos and Celso Ramos towns in the Santa Catarina State. The project consists of: two diversion tunnels, arch rectangle section, 14m wide x 16m high, with an approximate length of 900m; a concrete face rockfill dam (CFRD) 202m high with a volume of 11,804,000m3. The length of the crest is 592m; a concrete intake structure, 32m high with a volume of 16,794m3; Three buried penstocks with a length of 385m each being 111m welded steel line. The internal diameter of the concrete section is 6.2m reducing to 5.5m in the steel lined length; an external power house, 48m high and 113m long. The volume of concrete is 60,806m3. This power house has an installed capacity of 880MW in three Francis unit; and an external spillway, 94m wide with four gates, 17.4m wide x 20m high and a capacity of 18.300m3/sec.
The approach channel to the spillway and power intake is the main source of rockfill for the dam. The project site is also characterised by very steep abutments which make construction of hauling roads for the dam difficult.
Figure 1 shows the layout and the main features of the project, while figure 2 is a cross section of the concrete faced dam showing the main zones. Table 1 shows the characteristics of the rockfill embankment, similar to that reported for Barra Grande dam in the September 2003 issue of IWP&DC. One important fact indicated in figure 1 is the location of an underground access, at the right abutment. This access was excavated by the main contractor, with dimensions 11m wide by 7.5m high, which permits the transportation of rockfill from upstream and downstream sources into the dam.
The EPC contract
Campos Novos, together with Barra Grande, are projects in which the same engineering, procurement and construction (EPC) civil contractor exerts the EPC leadership. This turnkey type of contract transfers the risks associated with design and construction to the EPC contractor, who is responsible for the overall management of the project. Table 2 presents the groups acting in the financing and construction of the Campos Novos Project.
Figure 3 shows the contractual schedule and the main target dates for generating power in 54 months. Due to construction strategies defined for the project, the development of the programme is ahead of time - it is feasible to complete the generation of the first unit six months ahead of the target date.
The diversion of the river was carried out on 29 September 2002, three months ahead of the contractual date.
The Campos Novos site has two periods of rains well defined: Dry period from November to April and Wet period from May to October.
The Hydrographic basin has an extension of 14,200km2 recording river flows in the order of 8300m3/sec during the wet season. The main cofferdam has been designed for a return period of 1:20 years, which corresponds to a flow of 6461m3/sec.
The philosophy of design in these concrete faced rockfill dams is to divert the river with a safe cofferdam, raising the priority section of the dam, during the dry period, to an elevation which will give a protection of 1:500 years.
The selected priority section in Campos Novos has a crest elevation of 570. This elevation is safe for floods up to 8000m3/sec. An underground fuse dyke has been constructed for protecting the cofferdam in case of a potential overtopping.
The spillway structure is designed for a return period of 1:10.000 years, which corresponds to a flood of 18,300m3/sec.
The CFRD dam
Campos Novos CFRD is the highest Rockfill in Brazil and one of the highest structures under construction of this type in the world.
Since the design of the dam was undertaken by the same engineering group acting in Barra Grande CFRD , the concepts discussed for the selection of the plinth, face slab dimensions, extruded curb, pre cast parapets and zoning are similar. The priority section of the dam was completed in May 2003 well ahead of the contractual date.
Starter slabs were built during the raising of the dam using timber protections in case of rockfill pieces falling from the embankment.
The construction of the embankment and face slab had been planned in five phases. In the first phase the rockfill commenced after dewatering of the riverbed raising it to approximately elevation 525. With the second phase, once the riverbed plinth was built, placing of processed materials 2A, 2B and 3A was initiated raising the upstream portion of the priority section to the specified elevation 570. Downstream rockfill was simultaneously placed. Rockfill production larger than 700,000m3 was obtained in January 2003.
The third phase was initiated simultaneously with the construction of the first stage of the concrete slab, levelling the dam to elevation 570. This phase is under construction.
In the fourth phase the dam will be raised to the foundation of the lower precast parapet. Then the second slab phase will be constructed. The final phase will be executed during the installation of both parapets at the crest dam.
The phase I to elevation 568 was completed in early July 2003. Two light slipforms operated by syncronised hoists were available. The copper waterstops for the vertical joints were fabricated directly at the dam by using a machine installed on the crest. The slip forms, crews and complementary equipment have been mobilised to the Barra Grande project for construction of the face slab first stage.
The Phase II from elevation 568 to elevation 656 will be built after completion of the embankment to that level. Phase III is the upper complement to elevation 660. Precast parapets will then be installed.
The construction strategies of the project were defined as follows:
• Complete the priority section of the dam to elevation 570 ahead of schedule.
• Accelerate construction of the diversion tunnels by using an
• Clearing of the abutments and plinth construction in both abutments simultaneously with construction of the diversion tunnel.
• Locate a transportation tunnel at the right abutment of the river for reducing the construction distances for placing rockfill. The same tunnel will be used as a riparian outlet for discharging water during the filling of the reservoir as an environmental requisite.
• Use of the auxiliary tunnel as a fuse dyke for preventing destruction of the main cofferdam by an eventual overtopping.
• Adoption of an extruded curb 0.5m high for increasing productivity.
• Planning the face slab construction in coordinated sequence with Barra Grande slab minimising costs.
• Complete excavation of the power intake to simplify construction of the penstock shafts by raise boring.
• Using auxiliary tunnels to introduce steel lining without interference with the power house.
The auxiliary tunnel referred here is shown schematically on Fig. 1. This access avoids crossing the plinth communicating upstream and downstream rockfill sources with the dam. The hauling distances are substantially reduced increasing productivity. The same access is providing riparian water during the reservoir filling.
At the moment of closing this article the construction status is as follows: The dam is 45% built, with a total of 5,700,000m3 of rockfill having been placed. The priority section to elevation is complete including the construction of the first stage of the face slab. The downstream portion of the rockfill is at elevation 540 approximately.
The power house is also 100% excavated, with 60% concrete executed and installation of mechanical equipment is around 10%. The penstocks are 75% excavated and some pieces of the steel lining are already placed. Excavation of the Spillway is at 50%.
The power intake is completely excavated and some preliminary work has started for placing concrete. Open cut excavation is at 62 and underground excavation is 99% complete.
A total of 96500m3 of concrete is built, which represents 27% of the total to be executed. The scheme is ahead of schedule and generation of the first unit is expected six months ahead of the contractual date.
The flexibility of the CFRD construction is one the major advantages for selection of this type of dam, even with the very tight schedules of Barra Grande and Campos Novos hydroelectric projects. It is important however, that design and construction strategies are defined, which will permit critical activities to be completed on time.
Some important ways to increase the speed of construction include building the diversion tunnels developing multiple excavation fronts. This may require using auxiliary galleries leaving the intake structure free for rapid construction. Another way is to build the plinth above river water level simultaneously with the construction of the diversion tunnels. If the tunnels are already built always give priority to the plinth construction and abutments clearing.
Also, select an upstream priority section, providing an incorporated cofferdam into the dam, giving additional protection against eventual floods and optimising the diversion lay out; and select the diversion date as the end of the rainy period permitting during the dry season raising safely the fill. Study the possibility of incorporating underground solutions minimising hauling access and increasing productivity; and define the use of higher extruded curb compatible with the vibratory roller increasing productivity and keeping the dam levelled.
These decisions resulted in increased dam progress as reported for Barra Grande and Campos Novos CFRDs.
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