15 years of experience in RCC dams

15 years of experience in RCC dam construction By Bernard BOUYGE Project Manager and Dam Expert, Bec Frères SA, France ABSTRACT Since 1986, our company has been awarded 12 RCC dams out of 22 bidden for; among these 12 dams, 9 were built and completed to date, whereas one was postponed and two were cancelled by the owner. These dams are located all over the world and show a wide range of techniques. RCC technique being relatively young, eight of these dams were built or given under the Design and Build principle, or based on extensive alternative design proposed by Contractors. All along this 15 years period, the principle which guided our company is the following one: RCC technique offers the opportunity to think about specifications which are strictly necessary for dam construction, for each project. It is the opportunity to remove the dust from specifications by getting rid of constraints which were piled without justification, and to carry out a detailed value analysis. It also allows to create dams which fit particularly with the conditions of each project. The result of this process is twofold: time and money saving with a quality at least the same if not better. In the main part of this article, we shall recall the whole of the projects we worked on, highlighting the specificities of each of them as regards their adaptation to particular site conditions. We insist upon the methods used in order to implement this objective. INTRODUCTION For the past 15 years, our company has tendered for 22 RCC dams and built 9 of them. (in addition, it gained 3 other ones, the construction of which was postponed or cancelled by the client). RCC technique being relatively young, eight of these dams were built or given under the Design and Build principle, or based on extensive alternative design proposed by our company. Indeed, the large and necessary contribution of Contractors during the development stage of this technique,in order to define and implement construction methods and therefore make design evolve, is one of the characteristics of RCC dams. OUR EXPERIENCE All the dams referred to hereafter are detailed in tables 1 and 2. These dams show a wide diversity: in their size, ranging from 1,800 m3 to 4,900,000 m3, and from 6 m to 120 m in height. Placing output ranged from a few hundreds of m3 per day to more than 13,000 m 3 per day (Tha Dan), giving hourly output ranging from a few tons per hour to more than 800 tons per hour. ● ● in their location: projects which are shown are located in all climates: temperate Western Europe climates as well as tropical Asian or South American zones, dry areas of Africa or Middle-East , or very high mountainous areas in South America. in their purpose: they usually are reservoir dams, but also happened to be built as a sill in a river (Durance sill) ● ● in their design: hard fill (Ribeiradio), material referring to Proctor density (Les Olivettes dam), medium and high paste materials (Platanovrissy, Kinta, Tannur dams) ● in their typical section: vertical or inclined (Wala dam) upstream facing, mixed facing (Tannur dam), smooth or stepped facing, ● in their watertightness design: either the materials themselves bring the watertightness or an internal (Angostura dam) or external (Le Riou dam) watertight membrane is placed , or a concrete facing is placed (Petit Saut dam, Ribeiradio dam); these various solutions are shown in figures 1 and 2 Table 1 Dams tendered or built: main characteristics of the design Comment (1) as built (2) tender * Stepped face 1 Traditional concrete against formwork 2 Rcc against formwork and external membrane 3 GERcc against formwork 4 Reinforced concrete wall cast before RCC starts 5 Slip-formed extruded facing elements 6 Rcc against precast concrete panels with internal membrane 7 Mechanically compacted unformed face of RCC 8 Rcc against precast concrete panels 9 Reinforced concrete facing cast on top of precast concrete units or slipformed facing elements 10 Reinforced conventional concrete cast after Rcc placement 11 Reinforced concrete face cast after completion of RCC Table 2 Dams tendered or built: main characteristics of the construction. Figure 1 Dams Section Figure 2 Dams Section , (contunue) ● in construction methods: ❚ RCC mixing in continuous or discontinuous (Kinta dam) plant ❚ RCC transporting and placing using trucks only (Les Olivettes dam, Petit Saut dam, Aoulouz dam, Tha Dan dam) or conveyors only (Angostura dam) or mixed ones (Rizzanèse dam, Toker dam, Tannur dam) ❚ Compacted upstream facing (Les Olivettes dam, Choldocogagna dam), slipform facing (Platanovrissy dam, Tha Dan dam), or precast elements placing for facing (Angostura dam) ❚ Use of GERCC (Grout enriched RCC) for facing and for contact between RCC and foundation ground or galleries (Tannur dam, Kinta dam) ❚ Sloped layers (Tannur dam, Kinta dam) ❚ RCC cooling (Tannur dam, Kinta dam) or RCC heating (Angostura dam) In their constituent materials: more (La Touch Poupard dam) or less elaborate aggregates, including raw alluvium materials (La Borie dam) or very bad quality weathered rocks (Ribeiradio dam) , as shown on figure n°3, cementitious materials including or not including Portland cement (Les Olivettes dam, Saint Martin de Londres dam, Villaumur dam) with or without admixtures. ● OUR CONCEPT ABOUT THE CONTRACTOR’S ROLE: EXAMPLES When tendering for a project, a Contractor faces two situations: ● Either the specifications dictate the construction methods Or construction methods are left under his responsibility. In some cases, alternative design is allowed or even asked for. ● When the Contractor has an initiative margin, he is able to propose and implement solutions designed to optimise the project, construction wise: this process can lead him to propose complete alternative design; thus Les Olivettes dam was built in RCC following a proposal by Contractor as an alternative to a basic rockfill design. Without reaching such extreme, there are numerous examples of solutions which were proposed by the Contractor and adopted by the Client. These solutions may then either be developed by the Contractor himself, or Figure 3 tendered or built dams: RCC grading curves take advantage of proven solutions or solutions which were broadly outlined in other projects, thus implementing a constant research and development process. Some such examples are given hereafter. Reinforced concrete wall built first concept was finalised by the Contractor for La Borie dam (see figure n°1): the target was to find a costeffective solution using the alluvium available on site: these alluvium, which included a high content of shale, did not enable to work out a sufficient quality concrete which would guarantee the dam watertightness. It was therefore decided to dissociate the watertightness function from the stability function, the first one being fulfilled by a reinforced concrete wall anchored in the RCC. In order to take maximum benefit from this solution, it was decided that the wall would be built far ahead of the RCC in order to ease placing of it. Such a solution was also used in Sep dam and Villaumur dam, to save costs on aggregates. Last, it was applied to build Petit Saut dam in French Guyana: in this case, the technical capability prevailed over the gain in cost, as it showed the most suitable in order to stand temporary acidity of reservoir water due to increased content of carbonic dioxide (due to the fact that the tropical forest was not cleared in the reservoir). This solution is applicable subject to the following limiting factors: ● The sizing of the wall which has to stand the RCC thrust, as it was noted that such a thrust was not negligible and could cause movements of the wall during building, ● The sizing and durability of anchorages of the wall into RCC in order to resist rapid drawdown The advantages of this solution are: ● To dissociate facing works and main dam construction building works. ● Possible use of low quality materials in the dam body. Downstream slope compaction technique was launched during the construction of Les Olivettes dam (see figure n°2) in order to save costs when building the downstream slope. The idea came from the fact that the downstream slope of a dam (apart in the weir section) is not subject to any significant stress and only forms a geometric boundary to the structure. Any solution which would guarantee a sufficient stability of the downstream slope under weather related stresses and giving a density similar to the dam density is therefore acceptable (which is indeed the added value of a reinforced concrete facing made of 15 or 20 Mpa concrete !). Compacting the downstream slope proved interesting and was confirmed by 15 years of experience. This process was also used for Choldocogagna dam, Le Sep dam and Villaumur dam. This process can be used within the following limits: Facing is sensitive to frost : trial tests showed that frost resistance of compacted RCC slope is moderate. This process can therefore be applied only in temperate climates. ● ● Aesthetic look of a compacted slope is similar to a fill slope: before choosing compacted slope, possible client’s reservations should be overcome. This method has the following advantages: ● The method is cost-effective and fast ● It improves the insertion from an environmental point of view, by reducing the concrete aspect of the downstream face. Sloping layers concept was set up in China ; it was used for Tannur dam and proposed for Kinta dam (with however different layers thickness). This method consists in laying horizontal lifts of the dam, n times 30 cm high (up to 3 m) so that, in any of these lifts, the time between two RCC layers, 30 cm distant vertically) doesn’t exceed 1 to 2 hours. This method gives very good bonding conditions between RCC layers. The following difficulties may be encountered: ● Interface between the dam body and the facing ● Additional stresses to formwork: along a piece of formwork, the elevation of RCC (and therefore of concrete facing or GERCC) may reach 3 meters within a few hours. Resulting thrust on formworks therefore far exceed the ones usually encountered and formworks and their anchorages should be sized accordingly. ● Cold joint treatment (every 1.2 m in Tannur dam, and every 3 m in Kinta dam): whereas the number of cold joints is significantly reduced, these surfaces are uneven and require additional work which can however be carried out in parallel to main works. This method brings the following advantages: ● Time limit criteria for overlapping layers are successfully met Surfaces to be treated (cleaning, mortar laying) are reduced: within a 3 m slice (for instance), layers follow each other fast, and the treatments are minimum or even no more necessary. ● Quality of the contact between successive layers is therefore increased significantly. ● GERCC concept, set up in China and Australia, was applied to Tannur dam in order to improve the bonding between RCC dam body and the facing. In this process, there is no more usual standard facing concrete : RCC is laid against the formwork and, near the formwork, in an area about 40 cm wide, cement grout is added and compacted with a vibrating poker. Difficulties in implementing this process are: Avoiding any previous compaction of GERCC area which would hamper grout penetration into the material, ● ● Limiting vibration to the minimum to avoid resurfacing of grout, which would consequently increase the work of preparation of surfaces and reduce the RCC placing output. ● GERCC is implemented just before RCC placing, and any delay to this task would slow down RCC placing. ● GERCC has a lower or equal compressive strength compared to RCC This method brings the following advantages: There is a continuity between the dam body and the facing, as can be seen from numerous core samples taken in Tannur dam showing continuity ● ● Quality surface finish , as can be noted on site ● No need of a conventional concrete team when placing layers. The use of industrially manufactured hydraulic binders without OPC (mainly made of fly ashes and slag) already used for roadworks is due to the possibility ,by using such binders (having a low hydration heat), to reduce the time constraints in overlapping layers. Using these products may prove difficult as regards: Certification of the product: as these binders are not standardised products, they require preliminary trial tests for each project, prior to their use. ● On the other hand, benefits when using these products are the following ones: ● Reduced heat release ● Guarantee of a constant quality as these products are manufa c t u r e d industrially. ● Easier operations on site The use of continuous mixing plant came from experience gained in mixing soil cement for roadworks, using reliable plant equipped with high quality weighing devices. Their use may however be limited due to the difficulty to include a cooling system in the RCC production line. “Hard fill” concept was set up by Pierre Londe and was proposed for Ribeiradio dam (see figure n°1) in order to adapt the dam to bad and uncertain foundation conditions, and due to the need to use site materials the quality of which did not allow to reach 10 Mpa compressive strength for RCC. Limits in using this concept may be: ●A ●A high increase of the dam volume necessary facing to obtain watertightness On the other hand, such structures may: Suit poor foundation conditions Enable the use of poor quality aggregates ● ● The preferred use of trucks to deliver RCC to the dam, when this is feasible taking into account topographical data, as this method is simple and can be adapted easily. Moreover, such a system never breaks down totally, and as such is not a critical point . The use of trucks may however be limited due to : ● The necessary cleaning of wheels when the system is “all trucks” ● The need to build tracks which evolve when the dam rises. On the other hand, in the case of “all trucks” solution, the tracks which are built give a permanent access for all other operations, and are widely used on sites. CONCLUSION All along these 15 years of experience, we were guided by the following principle in defining the RCC process: RCC technique gives the opportunity to think about specifications which are strictly necessary for dam construction, for each project. We seized the opportunity to improve the specifications by deleting contraints which are not necessary. It allowed to define structures which fit particularly with the conditions of each project, built within a short time frame, and cost-effective. During this period, the ideas came out during brainstorming, meeting, which were explored and then either abandoned or adopted. REFERENCES Bouyge, B., and Martin, J.P. (1988) “An economical process for constructing roller compacted concrete embankments” Proc., 16th International Congress on Large Dams, . San Francisco. Bouyge, B., Gourdin, R., and Jensen, A. (1988). “Le barrage des Olivettes sur la Peyne” Revue Travaux. N° 684, Paris Bouyge, B., and Jensen, A. (1989). “La Fissuration des Barrages en Béton Compacté au Rouleau. Exemple du Barrage des Olivettes.” Bulletin de Liaison des Laboratoires des Ponts et Chaussées N° 162, Paris Bouyge, B., and Martin, J.P. (1991). “Economy and Quality in RCC Dams.” Proc., International Symposium on RCC Dams, Beijin Bouyge, B., Frouin, A., and Jensen, A. (1991). “Adapting a RCC dam to site conditions : La Borie Dam project.” Proc., 17th International Congress on Large Dams, Vienne Forbes, B.A. (1999). “Grout Enriched RCC : a history and future.” B.A.Forbes. International Water Power and Dam Construction, London Bouyge, B. (2000). “Construction du Barrage de Tannur en Jordanie.” Revue Travaux. N°765, Paris Wagner, C. (2001). “Tannur dam : paving the way for RCC in Jordan.” International Water Power and Dam Construction. London Forbes, B.A., Iskander, M.M., and Husein Malkawi, A.I. (2001). “High RCC standards achieved at Jordan’s Tannur dam.” Hydropower and Dams. London