Sharing international experiences in CFRDs18 January 2013
Professor Bayardo Materon is the president of the International Society of CFRDs. Here he tells IWP&DC more about the role of the society, and takes a closer look at recent developments in this part of the dam construction industry.
To celebrate 20 years of Chinese construction of concrete face rockfill dams (CFRDs), an engineering symposium was organised inYichang, China in September 2005. Representatives from more than 20 countries participated including Australia, Brazil, Malaysia, Singapore, Sudan, Iran, France, India, US, Switzerland, Italy, Cyprus, Germany, Japan, Thailand and Turkey.
J Barry Cooke was invited to the meeting but could not attend and so asked me to present articles and participate in this important event. It was the proposal from Chen Qian of China that initiated the creation of an international club or society where designers and builders of CFRDs could share their experience and exchange information with other organisations in the study, research and knowledge of this type of dam.
The first preparatory committee of the International Society of CFRDs was nominated with the support of Chen Qian as the general secretary. Alberto Scuero (Italy), Cao Keming (China) and Paulo Cruz (Brazil) were vice-presidents, while John S Y Tan (Singapore) was appointed deputy secretary general.
I was nominated president in 2005. With the active cooperation of Qian, Cruz and Manoel Freitas from Brazil, we have been present in many seminars discussing the design and construction of CFRDs. We have investigated and participated in the understanding of the adequate design of this type of dam in narrow canyons and developed solutions for future design of higher structures.
The International Society of CFRDs participated in the 5th Dam Engineering Conference in Lisbon, Portugal in 2007. In this Conference Dr Humberto Marengo, from the Federal Commission of Electricity in Mexico, presented the experience of designing and constructing El Cajon CFRD in Mexico, one of the highest CFRDs built in the world with excellent performance.
In early 2008 the society was officially registered in California, US with the support of Hydrou, an Engineering firm from China.
The journal of the society CFRD World has been published from the beginning with articles by members detailing their experiences of new dams built in the world.
In May 2009, the society participated in the 23rd ICOLD conference in Brazil and society members Materon, Cruz and Freitas published the book Concrete Face Rockfill Dams edited in Portuguese and English. It presents real cases histories of 28 CFRDs including the highest in the world.
In this conference, one of the members of the society, Palmi Johannesson, gave a lecture about the design, construction and performance of Iceland's Kárahnjúkar CFRD - the highest CFRD built in Europe with excellent performance.
During the Second International Symposium on Rockfill Dams, held in late October 2011 in Rio de Janeiro and organised by the Brazilian Committee of Dams and CHINCOLD, the International Society of CFRDs updated articles on dam design in seismic areas. The use of the GIN method for grouting was widely discussed and recommended for future design of higher CFRDs.
Mexican Engineers from CFE, associated to our organization, presented data about La Yesca CFRD. Under construction in Mexico it is one of the highest dams in the world incorporating state of art in the design and construction of this type of dam.
A recent area of discussion for the International Society of CFRDs has been the performance of the Zipingpu CFRD. Built in Sichuan, China it was subject to an intensive seismic event in May 2008 and recommendations are being applied on new designs to prevent damage to high dams subject to seismic action.
An analysis of the design and construction of the 233m high Shuibuya CFRD has been reviewed by the society and some interesting articles have been shared between members to produce such safe structures. General Secretary Chen Qian is also preparing a book based on the successful construction of the Shuibuya.
The CFRD International Society has also worked with people involved with the design and construction of dams and, in conjunction with pre-stressed steel manufacturers, have tested preliminary innovative procedures to control face slab cracks in the face construction.
During 2005-6 three CFRDs presented high compression distresses with a sudden and noticeable increase in leakage through the central portion of the dam. The dams were: Barra Grande (185m) in Brazil; Campos Novos (202m) in Brazil; Mohale (145m) in Lesotho.
These dams were located in narrow valleys, with uniform graded basalt producing relative low modulus of compressibility (50-60MPa). During filling of the reservoir, friction forces between the rockfill and face slab were developed which resulted in high stresses breaking the slab and distressing the reinforcement bars. Analysis and observation of the Campos Novos dam showed the critical zone of the dam located at 30-40% of the maximum reservoir head. The correction of this phenomena has been solved by:
- Increasing the rockfill modulus of compressibility. This is obtained by selecting well graded materials, reducing the layer thickness for compaction, increasing the number of passes of the vibratory roller compactor and selecting heavier drum weights. ( >12t over the drum length).
- Using generous amounts of water during spreading and rockfill compaction. (200-300 l/m³).
- Using downstream layer thickness similar to upstream layers to avoid major differences in deformability.
- Using anti-spalling reinforcement in the central compression joints.
- Introducing compressible fillers in the central compression joints to mitigate excessive compression stresses.
- Incorporating additional vertical joints near abutments in narrow valleys.
- Using competent well designed water stops.
Following these rules recent CFRDs have been built successfully such as:
- Shuibuya (233m) in China.
- Bakun (205m) in Malaysia.
- El Cajón (188m) in Mexico.
- La Yesca (206m) in Mexico.
- Kárahnjúkar (196m) in Iceland.
- Sanbaxi (186m) in China.
- El Cercado (130m) and Porce III (150m) in Colombia.
As a member of the Board of Consultants, I had the opportunity to review, inspect and observe the performance of Bakun dam in Malaysia in May 2012, when the reservoir reached the maximum height.
Bakun CFRD is 205m high with a volume of compacted rockfill of almost 17Mm3. The design of the dam follows the state of art for structures located in a narrow valley and preventive measures to control excessive stresses in the central compression joints were adequately designed. The maximum settlement at the centre of the dam before impounding was 230cm and increased to 270cm after reaching the full supply level of almost 200m height.
Face slab deflection reached a maximum displacement of 92cm which compares very well with well compacted rockfill structures of this height.
The registered modulus of compressibility is close to 90MPa, typical of a very well compacted rockfill.
The vertical joints between the parapet wall opened close to abutments in a range of 10-20mm and the central joints shut as expected.
All joints were well treated by a mastic and a cover of EPDM. There are no signs of over stresses in the central portion of the dam since the central compression joints were treated with a local Borneo timber to mitigate high compression stresses.
Leakage is only 133l/sec which is a very low value for the height of the dam of 205m. In conclusion it must be considered that the performance of the dam is outstanding.
Progress in the design and construction of CFRDs, as Barry Cooke stated, is empirical and based on experience and precedent. Increasing the height of these structures has required better rockfill compaction and a reduction in layer thickness, aiming to obtain higher rockfill modulus of compressibility.
Very good progress has been obtained with the development of compactors with vibratory rollers. This equipment is fabricated to concentrate over the smooth drum pressures higher than 5t/m with a total weight over the drum exceeding 12t. The range of vibratory frequency produced by fabricants varies between 1400 to 2000 vpm.
Progress in the design of external water stops have been developed to allow movements up to 30cm. The pressure over the impervious element (PVC or EPDM) is controlled by geo-textiles with a capacity to support pressures equivalent to the height of the reservoir with a safety factor of 2. These designs allow the construction of dams in very steep abutments, such as the Mazar in Ecuador or Chaglla CFRD (205m) under construction in Peru.
The placing of transition material 2B has progressed by using a well graded material with equipment for pavement construction. In the Mexican CFRDs a sophisticated equipment places the pre compacted 2B material reducing losses of material and avoiding segregation.
The development of very light slipforms with productions up to 5-6m/hr have helped to design and build a faster and safe face slab and plinths.
All these design and construction considerations permit the construction of an efficient dam with adequate performance.