When construction draws to a halt, and even after a dam has been inaugurated, we are perhaps all guilty of not fully appreciating the individual achievements of each hydro scheme. Suzanne Moxon decided to give members of the dam construction industry the opportunity to talk about their favourite project — one which they think should be considered a feat of construction. Other opinions are invited.
Located on the Caroni river in Venezuela is one of the largest operating hydroelectric projects in the world. The 10,300MW Guri hydro power scheme, according to Vito DiCosola, senior partner with Harza Engineering Company, should be included in the dam construction industry’s roll of honour. Built over a 23-year period, Guri was completed in 1986 after utilising a unique construction concept — development of the project was phased to minimise initial investment and to match power supply to the growth in electrical system demand.
‘The phased development allowed us to build this project economically,’ DiCosola explained. ‘Other concepts floated at the beginning were to build the dam to the final height at the first stage. This obviously would have been a much easier design but the region would not have absorbed all of the power, making it difficult to finance the scheme.
‘Harza proposed building a smaller project and then, when power demand increased, we planned to raise the dam accordingly. This was a unique concept when suggested over 40 years ago,’ says DiCosola. ‘It was also very difficult to do, and at the time had never been carried out on such a large scale before. We raised the dam by 52m.’ Planning for this project proved to be a very complex job. DiCosola explained that the design of Guri dam had to acknowledge the interface between the old and new concrete when increasing its height. ‘We needed to know exactly how to do this at the very start of the job. In 1963 the first construction phase started and by this time we had to have decided how to interface the future dam extensions. There was a lot of planning involved but it also meant that the final raising of the structure caused fewer problems.’ Innovative ideas continued to appear throughout the construction of Guri dam. A significant concept was developed which replaced post-cooled concrete. ‘Usually with post-cooled concrete,’ DiCosola explained, ‘a pipe is embedded in the concrete layers and circulates cooling water, which reduces the temperature of the concrete. As we were obviously very concerned about thermal cracking when raising the dam, we decided to use ice chips instead of the cooling water. This proved to be very successful and eliminated the need for cooling pipes.’ Other potential problems for Harza included river diversion and control during construction. Seasonal flow range varied from a low of 5000m3/sec to a wet season high of 12000m3/sec, and there was not a constant PMF. ‘One of our concerns was the control of the river,’ DiCosola says. ‘We had to give contractors early warning if a major flood was imminent so EDELCA, the project owner, invested money in weather forecasts and set up an automatic rainfall and gauging station in the upper drainage basin to monitor the situation. Fortunately, a major flood did not occur and the spillway performed very well.’ Designing the Guri spillway was an intricate operation. It was divided into three separate chutes to permit sequential raising of the crests with two chutes operational while one was under construction. Three floating bulkheads were used to close one spillway chute at a time when raising the spillway crests. At 20x36m, these were the largest bulkheads used for this purpose in the world. Indeed, according to DiCosola, during construction, many world records were established at Guri. These included:
•Highest raising of an entire dam in one step (by 52m).
•Largest amount of concrete placed in one month (200,000m3).
•Most fill placed in a 24-hour period (234,500m3).
•Largest construction trestle with eight 800ton whirley cranes for placing concrete with 6.1m3 buckets.
•Largest dead-end transmission tower supporting two 800kV transmission lines.
•Largest number of monitoring instru-ments installed at one site (1800).
As Ienan Prusza, chief civil design engineer at EDELCA, explains some of these records have now been broken but he still considers Guri to be a marvel of dam construction in the twentieth century. ‘The construction of the third and final stage was an accomplishment,’ he said. ‘It included raising the spillway and gravity dam by 52m, constructing the second powerhouse and excavating the second tailrace channel to increase the generation capacity from 2000-10,300MW.’ Notable construction methods worthy of mention, Prusza believes, include the pre-wetting of the borrow area by sprinklers which allowed fill placements of more than 200,000m3/day. The successful introduction of aeration in the spillway also acted as protection against cavitation and erosion of the concrete due to sustained high flow conditions (average annual flow of 5000m3/sec and PMF inflow of 48,100m3/sec).
‘Furthermore,’ Prusza went on to add, ‘the compact and incremental design of Guri resulted in a very economical power plant with power costing US$550 per kW.’ As DiCosola pointed out, the power was so inexpensive at the time that it attracted aluminium and iron ore smelting industries into the area.
Construction management and co-operation between all parties involved is a very important aspect of any project. DiCosola was keen to point out that there was excellent co-operation between Harza and EDELCA and that they worked very well together on many different aspects of the project. This resulted in the project being ‘pretty much’ on schedule and budget, and being hailed as a success by Harza and EDELCA.
Maintaining a focus on the Caroni river in Venezuela, Ienan Prusza also considers the Macagua project to be a construction achievement. Located about 10km upstream from the Caroni river’s confluence with the Orinoco river, the 2540MW project was completed in 1996.
‘At this site the river widens to about 4km and divides into many channels, cascading down to the level of the Orinoco,’ Prusza said. ‘Some of these channels terminate in the spectacular falls of Cachamay and La Llovizna, one of Venezuela’s most scenic attractions. Also, because of the project’s location within the Ciudad Guayana urban zone (an industrial city with a growing population of 700,000 inhabitants) the project represented a construction challenge from an environmental point of view. The cost of the mitigation measures implemented to address human and environmental impacts of the project represented 14% of the total cost of the scheme.’ The environmental construction characteristics of Macagua are summarised below:
•A low head powerhouse was constructed for the sole purpose of providing a permanent flow of 660m3/sec to the Cachamay and La Llovizna waterfalls. Additionally, this plant generates 160MW of power.
•Artificial lakes, covering an area of 1.4km2, were constructed to stabilise the water level in the surrounding parks.
•An eight-lane highway was incorporated into the project to connect the cities of Ciudad Guayana, Puerto Ordaz and San Felix. It provides access for visitors to the surrounding park areas.
•The reservoir has controlled beach areas for use by the population.
A clean design
Dr Lombardi from Lombardi Engineering named the Contra dam on the Verzrasca river in Switzerland in his dam construction roll of honour. Commenting on the outstanding cleanness of the design of this structure, Lombardi explained that the 220m high, thin arch dam, which has a hydroelectric generating capacity of 105MW, was completed in 1965.
A special feature at the time was the high slenderness factor incorporated into the design. This is described as a very slender dam, which reached the highest value possible, reducing the volume of concrete required for construction. The result was a very economic design. ‘Since Contra people have tried to design even more slender dams but have seldom been successful,’ Lombardi said.
Lombardi described the excellent management of this project and the outstanding co-operation between owner, designer and contractors. ‘There were no problems,’ he said. ‘It was a very simple organisation. At the time there was not quality control or modern project management so it was possible for people to concentrate on the job in hand.’ Lombardi admits that this is a rather nostalgic way of viewing construction but he believed that it worked. The schedule was very tight but was respected, and the overall construction period was less than five years.
Problems, however, were encountered at Contra. It was necessary to deepen the excavation of the right bank due to weathering of rock. ‘This only took 15 days to adapt the design to the new conditions,’ Lombardi said. ‘There was no bureaucracy then. Now it would entail endless meetings and could take up to one year to achieve.’ During impounding earthquake tremors, as many as 25 a day, were experienced. This only happened during filling and not drawdown and after six years disappeared completely. ‘This was due to the initial weather pressure induced settlement of tectonic shear zones,’ Lombardi explained.
Reflecting on why the Contra dam should be considered a feat of construction, Lombardi again spoke of its aesthetic and clean design and the way that the elements of the power plant, ancillary works and dam are integrated in an ‘organic fashion’.
Good international management
Vince Zipparro, member of the board of directors and managing partner-client sponsor of Harza Engineering, considers another arch dam to be a construction achievement. The 240m high, 774m long Ertan dam is the third highest operating double curvature arch dam in the world. Construction of the 3300MW hydroelectric dam on the Yalong river in southwest China, one of the world’s largest hydro power projects currently being built, will be completed in December 1999.
Zipparro, project manager for Ertan, explained that Harza Engineering has been working on the scheme since 1979, being involved in the feasibility studies, the preliminary and final design, through to inspection and construction manage-ment. So does a 20-year involvement in a project present any problems from a construction management perspective? ‘Construction has only been under way for the past seven years,’ Zipparro explained, ‘so from a construction consis-tency viewpoint this is not really a prob-lem. But how do we ensure consistency throughout the design and pre-planning stages?,’ he mused. ‘Well, to be honest it’s not really anything that we have thought about, it just kind of happens.’ Zipparro is perhaps being modest, as he goes on to add that from a construction perspective dam building is quite unique. ‘When considering the design of a dam we do not really have to keep abreast of recent developments,’ he said. ‘This is not really the nature of hydro power development or dam construction. There are not new advances being constantly made, unlike information technology when things can become obsolete in about three years. There are standard techniques and designs in dam building which are pretty much well known.’ When designing Ertan the concept of a double curvature arch dam was a relatively recent development in the geometry of a dam. Zipparro explained how Harza suggested this to its Chinese colleagues. ‘With an arch dam you can model the structural behaviour of a dam to save concrete,’ he said. ‘Our contribution was to help the Chinese institutes to incorporate this newer design into the project which helped to reduce concrete volume, and so the amount of construction required.’ Dr CH Yeh, senior partner of Harza Engineering, explained that Ertan is a parabolic double curvature arch dam, one of the most recent designs. ‘This is a difficult design,’ he said. ‘It is a very complex structure with a very large flood release capacity (21,000m3/sec) which also made construction very difficult.’ He explained that the dam has three flood release facilities:
•A seven-bay crest spillway.
•Six middle level outlets. The gate has an operating head of 100m and so these were a challenge to construct.
•Two tunnel spillways.
‘Past experience has shown that their design is not simple,’ Yeh said. ‘For ex-ample, Glen Canyon on the Colorado river in the US experienced erosion in the spillway tunnel as it was not in frequent use. Big problems occurred when the spillway was used in the 1980s. However,’ he added, ‘we anticipate using the tunnel spillways at Ertan every year with large quantities of water. We used them successfully last year and also anticipate using them this year.’ Physical construction techniques were not the only things which Yeh believes the Ertan project should be applauded for. ‘This was a significant project as it was the first time where international bidding was used for dam design and construction in China. This was a new step for China. Yes the country does build a lot of dams but this was the first international venture. It opened a new chapter for Chinese dam building,’ Yeh says. ‘It also proved to be a good learning experience for both Chinese and international engineers as they were able to share their knowledge.’ Zipparro says that because of the international aspect of the project it was very complex to manage. Indepen-dent contractors and equipment suppliers had to be co-ordinated effectively to ensure that the work remained on schedule and equipment was delivered on time. ‘It was a difficult and challenging endeavour,’ he commented.
Perhaps one of the greatest challenges was communication between all the parties involved at Ertan. Drawing on his experience, Zipparro said that the key to successful communication was regular meetings. ‘We recognised that communication was going to be a big problem,’ he said. ‘We got all the key people from organisations, limiting it to the most important ones, and ensured that communication lines were open from the beginning of the project. We arranged weekly meetings and then, when the work stabilised and communication was flowing freely, we extended them to monthly sessions.’ All major construction has now been completed at Ertan and all units will soon be running in the powerhouse. The one thing that remains left to do is the punch list (ie items that contractors need to do to finish, such as clean up debris).
Reflecting on the construction work which has already taken place, Zipparro spoke about the steep course of the Yalong river, with its abundant run-off, which could have been problematic. ‘The diversion tunnels were designed so that they were generously sized,’ he explained. ‘They worked very well. There was a major spring run-off but the tunnels handled this well.’ The Ertan project also has an underground powerhouse with six 550MW Francis units. During excavation Harza, who worked in a joint venture with AGN of Norway, experienced in situ high forces in the rock. ‘With major excavation’, Zipparro explained, ‘in situ stresses want to redistribute around the cavern which results in high concentrations of forces and rock explosions, cracks and rock falls etc. Several such incidents occurred at Ertan. They were studied and corrected by rock bolting and post-tensioning anchoring to redistribute the stresses. This achieved the equilibrium needed for construction to continue.’ The problem had been anticipated but it was more a case of when and where it would occur. It was not catastrophic or unforeseen and was a problem the construction workforce could handle.
Zipparro is very confident that the Ertan project will be ready in December 1999, although he admits that there were contractor claims on the project. ‘With a project of this size it is really unavoidable,’ he said. ‘Unfortunately you do not know everything you need to know when planning the project. We did have difficulties in acquiring land rights for the project and contractors made claims as they could not access the land and so delayed construction.
‘Our schedule is very important. We needed to accelerate the construction and contractors offered alternatives to achieve this but, obviously, these were not free. However,’ he concluded, ‘the project has done well in relation to both cost and schedule.’
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| Location: Caroni river, Venezuela Construction details: The first stage of Macagua was completed in 1961 with an installed capacity of 370MW. The second stage, which comprises a dam, a spillway and two hydroelectric power plants, generates 2540MW and was completed in 1996.
Construction included a 2800m long concrete faced rockfill dam, with a total volume of 1.6M m3 of rockfill.
| Location: Caroni river, Venezuela Construction details: Construction on Guri’s first phase started in 1963 and was completed in 1968. The work included construction of a 110m high, 511m long concrete gravity dam composed of 31 monoliths to allow for future raising, a 540MW powerhouse, rockfill wing embankment dams and a 30,000m3/sec spillway.
Stage two was completed in 1977 and consisted of powerhouse no 1. This required construction of a cellular cofferdam between powerhouse unit 3 and the spillway, to allow for expansion of the powerhouse and erecting the penstocks for units 4-10.
The final stage included:
Raising the existing main dam and spillway.
Construction of new concrete gravity dams on the left and right banks of the Caroni river (built as extensions to the existing concrete dam).
Construction of earth and rockfill dams between the concrete dam and the left and right abutments.
Construction of powerhouse 2 at the foot of the new dam on the right bank of the river to accommodate ten new generating units.
Excavation of a new tailrace channel.
Expansion of the existing switchyard.