At Brazil's Itá plant, a new method of compacting the transition zone has been developed, drawing on experience from other projects
The 1450MW Itá hydro power project is currently under construction in southern Brazil. The plant is being built by CBPO, part of the Odebrecht organisation, under a turnkey contract placed by a private group of shareholders in the fields of energy, petrochemicals, steel milling and cement production.
The project site is on the Uruguay river, which forms the border between the states of Santa Catarina and Rio Grande do Sul. The project comprises the following structures:
•Five diversion tunnels. Two tunnels are located at a low elevation and are provided with an intake structure for final closure, while three are at a higher elevation and will be closed before the reservoir is filled, preferably during the dry season. The upper and lower tunnels have areas of 175m2 and 230m2, respectively, and all of them are more than 500m long.
•A concrete-faced rockfill dam, 125m high and containing 8.5M m3 of compacted rockfill.
•Two large spillways, capacity 49,940m3/sec, designed to cope with the probable maximum flow.
•The main water intake, a 36m-high concrete structure connected to five underground penstocks 7.5m in diameter.
•An external powerhouse containing five 290MW Francis turbines, and corresponding switchyards.
•Three earth and rockfill dykes, at the upper elevation of the reservoir, to confine water in areas of lower topography.
The Uruguay river region has a drainage area of around 45,000km2, which generates average flows of 1080m3/sec. The region is poor in vegetation but abounds in soil. Rainfall variation can result in extremely high water flows, occurring unexpectedly at any time of the year. However, there are generally two distinct periods: a dry season from November to April, and a wet season from May to October.
This past year has been atypical, as a result of the El Nino climatic phenomenon. In April of 1998 recorded flows were the highest for eighty four years, topping 16,500m3/sec.
Handling rivers of this size is accomplished by providing tunnels large enough to divert the flow, and using cofferdams to protect against floods with a recurrence of 20 years. In building the dam the aim is that by the second wet season after diversion, the elevation is high enough to provide protection against a flood with a recurrence of 500 years.
A rockfill dam
The Itá dam is the fourth concrete faced rockfill dam (CFRD) that has been built in Brazil. The choice of this method of construction is a result of the excellent performance of earlier Brazilian CFRDs — Foz do Areia (1980, 160m), Segredo (1992, 145m), and Xingo (1994, 150m) — and because it is simple to carry out and economically competitive.
In the construction, natural rockfill is placed in layers 0.8-1m thick, and compacted using a vibratory roller of 10t or more static weight. Depending on the type of rock, water is added using monitors at a rate of 150-200litres/m3. The areas of minor importance at the downstream of the axis are built in the same manner, with the same number of roller passes, although thicker layers (1.6-2m) are used, and no water is added.
The area immediately under the concrete slab, where there is transition material, is a very important zone. The behaviour of this area is closely associated with the degree of compaction of the transition material, when reservoir loads are applied and during normal operation of the dam. When the material size is smaller than 0.1m, then it is compacted in layers of 0.4-0.45m, which are often a submultiple of the contiguous rockfill layers.
Compaction is generally based on the technology developed at other dams and in basalt regions. The transition zone is compacted in layers of 0.4m, with four or more passes of the vibratory roller in the horizontal direction. The proximity of the upstream slope creates an uncompacted triangular area which requires additional compaction in the upslope direction. A normal procedure is to halt the execution of the transition, totally or partially, at intervals of elevation of 20-25m. Slope compaction is corrected as follows:
•The slope face is levelled manually, by cutting off higher parts and filling in depressions.
•The face is compacted by two or more passes of the 10t roller, with some water added but with no vibration.
•Asphalt emulsion is applied at a rate of 4 litres/min, and natural sand is added to break the emulsion.
•Four or more passes of the vibratory roller are again applied in the upstream direction, until a smooth and consistent surface is obtained.
In Brazil, as in other countries, 5-6t compaction rollers are also used with good results, along with vibrating plates in areas inaccessible to the roller.
The design conception for Itá considered the possibility of cofferdam overtopping, and specified a priority section up to el348m to ensure protection against a 500-year flood. Co-ordinated action is required between placement of the transition and the rockfill in order to keep the dam level. In addition, it is convenient to have a stable and solid surface in case the cofferdam is overtopped. Experience with cofferdam overtopping in other dams shows that although it is possible to preserve the cofferdam, large flows over the upstream slope can create erosion, affecting the construction schedule and increasing costs.
Several problems have been noted in dams in areas of intense rainfall:
•Grooves in the slope caused by concentrated flow during heavy rainfall.
•Deep erosion in the contact areas of the plinth and face that require side-retaining to prevent concentrated flows from reaching high speeds. Erosion as deep as 4-5m has been observed.
•Treatment with emulsion or cutback is needed. As well as labour, this requires the use of additional equipment such as cranes, shotcrete machines for sand application and asphalt spraying machines.
•Simultaneous treatment of the slope face and plinth grouting requires special protection to retain material that comes loose in the inclined compaction process.
•In high dams similar to Itá, it is normal procedure to provide additional protective asphalt treatments, before beginning to place reinforcements for the main slab.
•Construction deformations have caused dams treated with shotcrete to be seriously damaged during heavy rainfall.
These factors, and the contractor’s experience in roller compacted concrete, have been the cause of a search to find a more productive method of placing the transition material. The aim is to aggregate a portion of cement to the transition material, to achieve a compound with cohesion similar to the material placed next to the perimeter joint and to ensure stability.
Two alternative methods were developed to stabilise the upstream slope.
In the first, a lateral hydraulic cylinder was used, drawing on CBPO’s experience at the Rosana dam in Brazil. At Rosana, a soil–cement riprap was compacted laterally via coupling to the main cylinder. This was used to develop and assemble a rigid auxiliary cylinder with a horizontal arm that allowed simultaneous lateral and horizontal compaction.
This is placed as follows:
•The transition material is placed, leaving a 0.4m space near the upstream slope.
•The empty space is filled with transition placed with cement already mixed at the concrete plant, using 6m3 mixer trucks. Levelling is performed manually. Some material still falls on the upstream slope.
•The material is compacted near the edge by lowering the cylinder laterally and then horizontally.
Although the results obtained were better than those obtained with the traditional method, there were still material losses and a risk of accident because the compaction equipment remained close to the slope as the dam increased in height. Operating the heavy machines near the slope was made still more dangerous by the frequent foggy days.
In the second method, a low cement curb extruded wall was constructed. In this method an extruding machine was used similar to that employed in road construction for fabricating the road curbs known as ‘New Jersey median’.
After several tests, the following procedure was adopted:
•The transition layer was levelled and the extruder was aligned, either topographically or by means of a laser beam.
•A small extruded-curb wall was built using a steel form with the design specification 0.4m high, 1.3 H:1V upstream slope. The mixture composition was optimised as: cement 75kg/m3; aggregate (3/4 – No4), 1173kg/m3; sand < No 4, 1173kg/m3; water 125litres/m3.
•The extruded curb wall was produced at a speed up to 60m/hr, keeping alignment under control. After about 1hr, the complementary layer of transition material was placed, but its width was reduced to 3m. This layer was placed using an open-bottom steel dispenser towed by a D6 tractor.
•The transition material was compacted, using a 10t vibratory roller, in four passes. It was then levelled using a grader, before work on the next layer was started.
Using this method four workers are required to produce the concrete wall: one for levelling and to operate the machine; one for material transfer from the mixing truck to the extruder; and two for material control inside the extruder. The equipment required was:
•Concrete plant (also used for other activities).
•Mixer truck, 6m3 capacity.
•Extruder, assembled for the project.
•Partial survey team, with optional laser equipment.
•Water tank truck.
•Vibratory roller, 10t.
Productivity was 40-60m/hr of curb wall and 35,000m3/month of transition. These results were better than were experienced in traditional construction. This was because of the elimination of losses caused by transition material falling over the upstream slope and the reduction in transition width. What is more, interference with the crews working on grouting and slab starters was reduced.
This simple and economical technology can be applied in the future to other high CFRDs.
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