THE FIRST RCC ARCH DAM OUTSIDE SOUTH AFRICA & CHINA
September 2011 saw first power generation at the 230 MW Changuinola 1 hydroelectric power project in north-east Panama.
With delays in the project start caused by re-settlement problems, the definitive notice to proceed for Changuinola 1 was only finally issued towards the end of 2007 and work on the dam site started in earnest in early 2008. As a result of significant excavation and earthworks requirements on the main access road, the road to the power station and at the power station itself, priority was not initially given to the dam foundation excavations and it was December 2009 before the excavations and progress on the diversion culvert allowed the placement of the first RCC for the 105 m high dam.
The consistently high river flows meant that it was necessary to construct the dam with a diversion sized for a low recurrence interval flood and to phase construction to make best use of the short duration lower flow periods. At Changuinola a “dry” month contains 15 days on which rainfall is recorded and this increases to 26 days for a “wet” month and rainfall was undoubtedly the most significant challenge to RCC placement. With over 4 m of rain recorded during the period of RCC placement, it was not so much the intensity as the unpredictability that was the greatest problem. On many occasions, the surface clean-up would be just about complete when the next rain shower arrived.
RCC placement in the river section began in early May 2010 and the last of the 860 000 m3 of RCC was placed on 26th April 2011. With construction, post-cooling and grouting of the conventional concrete spillway crest required before impoundment could be initiated, the two 9 m x 9 m diversion culverts were closed on 22nd May using a steel stoplog and support truss arrangement designed to carry the full reservoir head. With an average inflow of 127 m3/s, the 388 million m3 impoundment subsequently filled over the following 33 days and first spillage occurred at midday on 24th June. While the achievement of a full reservoir was marked by the arrival of a small flood, the dam subsequently spilled continuously until the first of the two main turbines was brought on line in September.
While a RCC arch/gravity dam is currently under construction at Portugues in Puerto Rico and Pakistan’s recently completed Gomal Zam Dam is described as an arched gravity-type dam, Changuinola 1 is the first arch/gravity dam to be brought into operation outside China and South Africa and most importantly, the first to be designed in accordance with the findings of recent South African research into the early behaviour of high workability, high cementitious materials content, fly ash-rich RCC.
With a comprehensive system of instrumentation, data was reported to the designers at ARQ regularly during construction and first filling and the behaviour of the dam and its constituent RCC was monitored and compared with expectations through Finite Element analyses on a real-time basis. In the absence at the design stage of complete certainty as to the final RCC behaviour, a system of groutable induced joints was developed and strategically installed in the dam, while a clever arrangement was applied in the spillway crest cap to ensure that any creep that might develop would allow the induced joints to open and be grouted while the dam remains under load.
Comparing the instrumentation results and the analysis predictions, however, demonstrated the RCC to be behaving as predicted and confirmed that it should never become necessary to grout the induced joints at Changuinola 1 Dam. While high cementitious, fly ash-rich RCC undoubtedly suffers very significantly less stress relaxation creep during the hydration cycle, compared to conventional mass concrete and lower strength RCCs, it is as much the consequential behaviour of the RCC within the overall dam structure during construction as the minimal creep that gives rise to the overall behaviour that is so beneficial for an arch dam.
While this technology now offers significant benefits for RCC arch dams, it also implies that an additional factor now exists for consideration in RCC mix design and that the associated behaviour must receive appropriate attention during the Thermal Analysis for the construction phase.
Even on a site that could not be considered topographically, or geologically ideal for an arch dam, ARQ was able to develop a 20% saving in concrete volume through the applied arch/gravity structure at Changuinola 1. At sites recently reviewed in southern Africa and Turkey with lower crest length/height ratios, a concrete saving of approximately 50% has been demonstrated to be typically possible.
On a significant hydropower scheme, it is obviously advantageous to reduce the implementation period and to bring forward the date at which the first return on the capital investment is realised. For a 200 MW project selling electricity at US$ 10 cents/kWh, a month’s worth of generation would have a value of approximately US$ 10 million. Accordingly, while the cost savings associated with a concrete volume saving of 500 000 m3 for an RCC arch dam may have a direct benefit of US$ 50 million, the benefit associated with early income generation for a capacity of 200 MW might well exceed that amount. Consequently, the true advantage of an RCC arch dam can only be realised with impoundment immediately on RCC completion and correspondingly, a good deal of the benefit would be lost should post-cooling and induced joint grouting be allowed to delay impoundment.
With the findings of ARQ’s research, a very significant step forward has been taken and RCC can now be designed for the construction of arch dams that do not require post-cooling and joint grouting in all but extreme climatic conditions, thereby allowing the maximum economic benefits to be achieved. |
