Speeding up construction3 September 2003
The privatisation of new hydroelectric projects have motivated the development of construction alternatives to speed up the completion of these structures in due time. In the case of the 185m high Barra Grande dam, the EPC leader developed construction strategies to complete target dates in a very tight schedule
LOCATED in the south of Brazil, close to the borderline between Santa Catarina and Rio Grande do Sul states, the Barra Grande project includes special technical characteristics, which pioneer the high concrete faced rockfill dams (CFRD) construction techniques recently applied in engineering, procurement and construction (EPC) contracts in the country.
• Two diversion tunnels located at the right abutment of the Pelotas river, having an arch-rectangle section, 15m wide x 17m high, and length varying between 862m and 933m.
• A concrete power intake, 50m high and 45m wide where three underground penstocks originate. Each penstock has a vertical shaft, 98m high and a sub-horizontal tunnel 290m long. Each concrete lined power tunnel is 7.2m in diameter reducing to 5.5m diameter where steel lined is required.
• A semi outdoor power house, with three Francis units totalling an installed capacity of 690MW. Volume of the power house is 60.550m3 of concrete.
• A spillway with six gates, 15m wide x 21m high each, a length
of 118m and a height of 34m. The spillway capacity is approximately 24,000m3/sec.
• A CRFD dam, 185m high with a volume of approximately 11.5 X 106 m3 and a crest length of 665m. The volume of the face slab and parapets is 92,900m3 of concrete.
An auxiliary tunnel for the rapid excavation of the diversion tunnels was excavated during the development of the diversion portals as also shown in Figure 1. The dam is of modern design, with internal slab plinth and new philosophy for dimensioning the face slab and parapets. Figure 2 shows a typical section of the dam and Table 1 the main characteristics of the fill.
The EPC contract adopted for the Barra Grande project is the typical one in which the main contractor is leading an entrepreneur group, with complete responsibility for execution of the civil construction, electromechanical assembling and construction of the transmission line. All management is concentrated in the leader who develops a turnkey solution.
It is observed that the time from mobilisation and diversion of the river is relatively critical, since many important activities have to be developed in a short period of time. Diversion of a river was designed, with a cofferdam protection of 1:50 years allowing the dam priority section to be constructed to elevation 560.The priority section is an integrated cofferdam within the main dam to prevent eventual overtopping, making the diversion scheme more economical. This elevation (560) is safe for river floods with a return period of 1:500 years. However, in accomplishing these construction stages a coordinated action of different activities is required, such as clearing abutments, process transition materials, plinth construction and so on.
The Barra Grande CFRD follows criteria recently adopted in Brazilian high dams, following experience of performance observed in other high international dams.
The upstream slope (1.3H:IV) and the downstream slope (1.4H: IV) were selected according to the strengths parameters
of the compacted basalt. Main characteristics are:
The plinth was designed and built leaving an external slab, which varies with the hydrostatic pressure and the foundation geology. An additional variable internal slab was provided to cope with the required hydraulic gradient imposed by the foundation and reservoir pressures. Consequently, the width of the plinth is variable internal and external giving an economical solution from the viewpoint of rock excavation and concrete.
The design of the face was carried out applying new slab criteria following the concept that the hydraulic gradient becomes critical when the hydrostatic pressure exceeds a certain dam height. The traditional formula T=0.30 +0.002H m was partially modified as a reconnaissance that the hydraulic gradient, for dams higher than 125m, increases at a more rapid rate than the traditional slab thickness computed by the formula. CFRD's lower than 125m have proven to work well with the application of the traditional formula. However, some higher dams have shown signs of exceeding the concrete tension strain. Increasing the slab thickness, concrete strength and reinforcement, at the lower portion of the face slab as shown in Fig. 5, provides control of the tensile strain.
The technique of using an extruded curb developed during the construction of the 125m Ita dam in Brazil nowadays applied in many international dams has been defined for Barra Grande. Placement of the transition material against this curb is carried out using an open bottom steel box, where the hauling units unload the material. The same truck pushes the box ahead, spreading the material and eliminating use of graders or dozers, which is more cost effective.
Two precast parapets as shown on Figure 5 have been selected for the dam. The reason for this design is that the adoption of a parapet wall saves a volume of quarried rockfill, providing convenient space at the crest for the face slab construction. The pre-cast solution was used for the first time in the 125m high Machadinho dam in Brazil and since then this crest arrangement has been adopted in some recent CFRD dams with moderate seismic activity.
Figure 2 and Table 1 summarise the zoning of the dam. The priority section and the upper portion of the dam are well compacted. Field compaction tests, using four passes of the 10t vibratory roller, indicated that zones 2B and 3A could be placed in 0.5m layers with adequate density and moderate percentage of deformability. Zone 3B is compacted in 1m layers using heavier rollers. This zone was tested with a vibratory roller of 12.5t, six passes and addition of water in a proportion of 20% of the compacted volume.
Downstream zones are being compacted with the same vibratory roller in layers of 1.6m without water.
The tight schedule for the diversion of the river and the high rockfill production demand for completion of the priority section of the dam, before the rainy period (May-October, 2003), made it necessary to impose the following construction strategies:
Speed up of diversion tunnels construction
Excavation of both portals, upstream and downstream, was executed simultaneously. An auxiliary tunnel, located between the main cofferdam and the upstream portion of the dam, was also excavated.
Underground and downstream working fronts allowed for completion of the tunnel excavation while the intake structure was built without interferences. Both tunnels were completed between November 01 and August 02 and the intake structure, 3m high, between February-August 02. Since the upstream cofferdam was designed for 1:50 years return period, a fuse dyke was built at the right abutment to provide protection to the main cofferdam in case of an eventual overtopping of the cofferdam.
Complete diversion of the river was carried out on 17 October 2002 before the scheduled date of 31 October 2002.
Abutments Clearing and Plinth Construction
Common excavation, in both abutments, was executed simultaneously with excavation of the diversion tunnels. This allowed placing of rockfill before diversion at the right abutment. A volume of 1,340,000m3 was placed before diversion.
An important decision was constructing the plinth, above river water level, before diversion of the river. Excavation and concrete placing of the plinth were executed between February-December 02. After dewatering the riverbed, construction of the remaining plinth was completed in December 02 allowing placing of materials 2A, 2B and 3A for the priority section.
Extruding curb and transition placing in layers of 0.5m
The extruded curb is in general 0.4m high in other Brazilian dams. Since the available time for transition placing to meet the priority section was only five months (Dec 02 April 03), the curb was built 0.5m high. Tests were conducted for proving adequate compaction of the materials 2B and 3A against the extruded curb .Two layers of transition material 2A - 3B matched well with the 3B layer of 1m.
This allowed high productions, keeping the upstream zone completely levelled. It is important to emphasise that rockfill productions higher than 900,000m3/month were obtained in March 03 and May 03. The priority section of the dam was complete on 30 April 2003 on the contractual date.
Auxiliary tunnels for power tunnels and power house
Additional access tunnels to the power house and to upper and lower penstocks are helping to maintain the power house works on schedule. These accesses will help in the assembly of the steel lined sections avoiding interference with the construction of the power house.
Phase I of construction was started after diversion of the river, leaving a space of 30m between the river plinth and the rockfill. During the placing of this rockfill the plinth at the riverbed was completed.
With Phase II, once the plinth was built on the riverbed, the priority section of the dam was started by placing the transition material 2A, 2B and 3A and some 3B rockfill. The 560 elevation was reached on schedule on 30 April 03. The upstream portion of the dam was raised to elevation 580 in June 03 reaching productions higher than 900,000m3/month. Phase III is currently under construction. Here, the downstream rockfill will be levelled at elevation 580. Simultaneously the first stage of the face slab to elevation 578 will be initiated.
In Phase IV, during completion of the slab first stage the rockfill will be raised to elevation 625, leaving a working platform of 40m for the concrete activities. Phase V will be the completion of the fill to the upstream parapet foundation. Then the second stage of the face slab will be built. The final stage Phase VI will see the installation of the pre-cast parapets and completion of the crest rockfill.
The construction of the project is currently ahead of schedule. 80% of common excavation is complete and rock excavation is 60% executed. 26% of the concrete is complete.
The construction of the first stage of face slab, which was originally planned to be at elevation 560 has been transferred to elevation 580.The same equipment and slipforming crews from Campos Novos CFRD, located over the Canoas river, close to the Campos Novos and Celso Ramos towns in the Santa Catarina State, are being mobilised to Barra Grande after completion of the first stage there.
This coordinated procedure, apart from offering better utilisation of available resources, is also cost effective.
A volume of 7,890,000m3 of rockfill has been placed in the dam and cofferdams, representing 60 % of the total volume.
Face slab concrete operations are being prepared. Starter slabs for setting the slipforms are almost complete. Two slipsforms, operated by hydraulic jacks, will be used for the dam face, following the successful experience during the slab construction of Machadinho dam by the same contractor.
Progress in the remaining structures is as follows: the power house is 65% complete with 10% electro-mechanical equipment installed; the spillway is 100% excavated, with 10% of concrete having been placed; the Power intake is 95% excavation ready to initiate concrete; and the project is four months ahead of schedule.
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