Design and Construction of Cerro Del Águila Gravity Dam in Peru

Transcription

1 Design and Construction of Cerro Del Águila Gravity Dam in Peru Sayah S. M. 1, Bianco V. 2, Ravelli M. 1, and Bonanni S. 2 1, Lombardi Engineering Limited, 6648 Minusio, Switzerland, selim.sayah@lombardi.ch 2, Astaldi SpA, Via G.V.Bona 65, Roma, s.bonanni@astaldi.com Abstract: Cerro del Águila project in Peru represents the final step of the Mantaro River major hydropower scheme cascade development. This 520 MW hydropower scheme, presently under construction, will include an 88 m high and 270 m long RCC gravity-arch dam equipped with 6 mobile gates providing a total capacity of the surface spillway of around m 3 /s. The bottom outlets increase the total discharge capacity of the dam up to m 3 /s. The volume of the dam is approx. 0.5 Mm 3 with a relatively high percentage of vibrated concrete due to the fact that the 6 bottom outlets are large and require high strength concrete. The bottom outlets are in fact designed to allow an easy annual flushing of the reservoir knowing that the sediment yield is around 3.5 Mm 3 /year. A special attention was also given to the design of the upstream face of the dam in order to confirm its behavior during earthquakes as the site is considered seismically very active. This dam is equipped with galleries for the foundation consolidation grouting. These galleries were designed in order to accelerate the construction process allowing consequently the treatment of the base of the dam without interfering with the construction activities. Several other concepts were also applied that allowed rapid placement of the RCC paste. Concrete placement is mainly carried out using a high capacity blondin combined with transport trucks and steel chute. The mix design of the RCC was optimized using full scale test sections. With a low percentage of pozzolanic content up to around 20%, temperature rising during the hydration of the concrete was a concern. Extensive investigations were thus considered in order to reduce the cement content while respecting the design strength. In this paper an outlook of the main design features of the dam is provided together with some major highlights regarding the optimized mix design, the construction process, and concrete placement. Key words: Gravity Dam, RCC mix design, bottom outlet. 1 Project background Cerro del Águila project in Peru represents the third step of the Mantaro river major hydropower scheme cascade development. It is situated downstream of SAM/Restitución Hydroelectric Plants. Originally, another project was planned to develop the second curve practically down to its confluence with the Apurimac river. This project would have included a long low pressure tunnel and a 250 m high dam situated downstream of the Colcabamba river confluence in order to include the discharge provided by the additional catchment area and the additional head between the SAM HPP tailwater level and the Mantaro river.

2 Taking into consideration the very large landslide in the Mayunmarca area that occurred in 1974 and the apparent vulnerability of the valley side stability, the idea of implementing a very high dam was abandoned and partially substituted by the following: - The Restitución Project, developing the 250 m remaining head between the SAM HPP tailrace and the Mantaro river, and - The Guitarra Project, developing at short distance downstream of the denominated Guitarra curve. After almost 25 years of consideration, this Guitarra Project (1983) has been redefined and renamed Cerro del Águila Project. Figure 1. The 77 m Tablachaca gravity dam, situated at around 100 km upstream of Cerro del Águila Dam on the Mantaro River, constructed in the 70 s. The project is being developed by Kallpa Generación S.A., a subsidiary of IC Power. It falls under an Engineering Procurement and Construction (EPC) scheme. In November 2011, Astaldi S.p.A and their joint venture partner Graña y Montero SAA won the contract. Lombardi SA was chosen later as the project designer. In the beginning of 2012 the excavation of the access roads started. In the mid of 2014 the dam foundation preparation ended allowing the start of the dam construction. 2 Dam geology Cerro del Águila Dam is located in a higher mountainous environment with steep valley slopes (average 30, locally exceeding 60 ), some 50 km upstream of the Amazonian area, and located in the main bedrock lithologies formed of granites/granodiorites (Villa Azul batholiths). Mantaro River has eroded its current river bed directly into the bedrock. The left-hand and the right-hand side slope shows different quaternary deposits: The left-hand side is steeper and directly shows bedrock under a thin cover of colluvium deposits and locally present rockfall/debris flow deposits (Figure 2). On the right-hand slope the dam site is on the terrace of a fan of mixed origin and

3 terraced recent alluvial/moraine deposits. Due the erosive environment of Mantaro River, only very little current alluvial deposits are present around the river bed. Figure 2. Geology of Cerro del Águila dam site. 3 Design of Cerro del Águila Dam 3.1 Dam characteristics The new dam main characteristics are given below and illustrated in Figure 3. Hydrology and geomorphology: - Catchment area: km 2 ; Drainage at intake: 9.04 l/s/km 2 - Project flood (at the dam site): Q 1000 = m 3 /s - Assumed sediment yield in the new reservoir: 1 to 3-4 Mm 3 /yr Artificial reservoir and dam: - Dam type: RCC gravity dam (arch form) - Dam height: 88 m from foundation (El El m asl) - Dam crest length: 270 m; Dam crest elevation: El m asl - Exceptional operating water level: EL m asl - Normal operating water level: EL m asl - Total impoundment volume: ~37 Mm 3 - Bottom outlet: 6 x 2 slide gates b x h = 4.60 x 6.00 m; sill level: El m asl - Spillway: 4 x radial gates b x h = 12.40x m; 2 flap gates b x h = x Spillway crest sill level: El m asl (radial gates); El m asl (flap gates) - River diversion: 340 m long pressure tunnel; capacity of 715 m 3 /s. 3.2 Dam layout and typical section A typical section of the new dam is illustrated in Figure 4. The dam is an RCC concrete gravity dam with 18 independent blocks, each around 16 m long. The 270 m long dam has a slightly curved planimetric axis (R=400 m), with a crest elevation of m asl. At the lowest point, the base elevation of the upstream toe of the dam is m asl. The maximum dam height measured at

4 foundation is 88 m. The normal operating water level is at El m asl, allowing a total storage volume of around 37 Mm 3. The typical cross section of the dam is designed with inclined faces in order to ensure the required stability during a seismic event (MCE=0.4.g; MDE=0.25.g). The upstream dam face has an inclination of 1:0.1 (V:H), and the downstream face correspond to an inclination of 1:0.75 (V:H). The maximum width at foundation is around 70 m. The typical width of the crown blocks is 6.2 m. The dam crest is 6.5 m wide and equipped with a concrete parapet at the upstream face. The total volume of concrete of the dam is around m 3. Figure 3. General layout of Cerro del Águila Dam. The release of extreme flood events is guaranteed by a gated surface spillway equipped with 4 radial gates and 2 flap gates, as well as 6 bottom outlets equipped with slide gates. With a total capacity of m 3 /s for the gated surface spillway and m 3 /s for the bottom outlet, the combined total flood discharge capacity of the scheme amounts to m 3 /s. The floods are discharged downstream at the dam-integrated 45 m long chute spillway equipped with a ski jump. A series of 3 m wide and 3 m high deflectors are foreseen at the end of the ski jump in order to open the hydraulic vein and facilitate the air entrainment, favoring the energy dissipation of the water jet before impacting the plunge pool. 3.3 Big bottom outlets for sediment flushing The operational conditions of the Cerro del Águila Dam in the long term will significantly depend on the proper management of the sediments yield carried by the flows of the Mantaro River and deposited at least partially in the reservoir during the flood season. And average annual sediment yield of around 2 to 4 Mm 3 entering the reservoir was estimated. Therefore an annual flushing will be required. In order to optimize the flushing procedure and make it shorter in time thus avoiding significant loss in energy generation, 6 big bottom outlets were proposed (Figure 5).

5 Figure 4. Typical section of the central blocks. Each is equipped with two slide gates that can be operated partially for partial sediment flushing. When they are completely opened it is possible to empty the reservoir and carry out a complete flushing of all the deposited sediments. Based on physical and numerical modeling, a flushing period of several days it was estimated as sufficient. All the bottom outlets are steel lined and completely aerated allowing a free surface flow during the emptying of the reservoir (Figure 5). Figure 5. Steel liner concept (ATB-Riva Calzioni S.p.A.) (left) Bottom outlet design (right). 3.4 Dam RCC-CVC zoning The 3D stress analysis of the typical central block of the dam is illustrated in Figure 6. This analysis carried out for typical seismic events (in the present case an MDE event), showed that the

6 upstream face of the dam exhibits positive tension up to around MPa while at the toe of this face tensile stresses might rise to values as high as 6-7 MPa. In order to cope with these high values, it was decided to apply a high resistance conventional vibrated concrete with a compressive strength equal to 25 MPa. An RCC concrete was applied to the remaining sections of the dam. Two different RCC strength were applied depending on the stress distribution inside the dam body. In the core of the dam body a strength of 12 MPa was adopted, and, at the downstream toe and below the slide gates, a 15 MPa compression strength was applied. Applying the zoning concept it was possible to avoid thermal treatment of the main massive concrete (RCC 12 MPa). Furthermore, this optimization allowed a significant reduction of the cement content of the concrete. Figure 6. Stress Analysis of the typical central block for the MDE earthquake (left) and zoning of the Dam body as a function of the concrete type and strength (right) [1]. 4. Construction of the Dam 4.1 Mix design and full scale test sections In the beginning of the 2014, a comprehensive study of the mix design for both the CVC and RCC concrete started. Figure 7 illustrates the full scale test section of the RCC concrete (left) which was around 4 meters wide and 10 meter long. This figure also illustrates the core extraction procedure carried out for the systematic and followed-up laboratory analyses. Typical laboratory tests were carried out, such as compressive and tensile strength (on the main body concrete and lift joints), permeability, density, vebe number, thermal conductivity, heat of hydration, etc. In Table 1 the main characteristics of the different mix design as it was adopted for the construction of Cerro del Águila Dam are given.

7 Figure 7. Full scale test section for the study of the dam concrete mix design. Concrete typology RCC 1 RCC 2 CVC 1 CVC 2 Construction methodology Roller compacted Roller compacted Conventional massive Conventional massive Specified compressive strength of concrete [MPa] Mean compressive controlled strength of concrete [MPa] <fcj<19 12<fcj<14 15<fcj<19 25<fcj<30 Age [days] Consistency / slump Vebe 20±5 sec. Vebe 20±5 sec. Slump 100±25 mm Slump 100±25 mm Cement [kg/m 3 ] Water [kg/m 3 ] 130±2 132±2 190±5 185±5 Sand [kg/m 3 ] Aggregate (25-5 mm) [kg/m 3 ] Aggregate (50-25 mm) [kg/m 3 ] Admixtures Set retarder/water reducer Set retarder/water reducer Set retarder/water reducer + Superplasticizer Set retarder/water reducer + Superplasticizer Theoretical density [kg/m 3 ] 2430± ± ± ±20 Table 1. Mix design of the Roller compacted concrete (RCC) and the conventional vibrated concrete (CVC) used for the construction of Cerro del Águila Dam [2]. 4.2 Concrete transportation, placement and compacting Concrete placement was carried out using mainly a blondin device designed by the Italian company Agudio S.p.A with a transportation capacity of around 9 m 3 (Figure 8). The average placement volume registered during the construction of the central blocks of the dam varied between 100 and 120 m 3 /hour knowing that the geometry of the dam galleries is quite complex. After its placement and extension, a 12 t vibrating roller drum carried out the compaction of the RCC with an average of 7 passes. In order to increase the transportation and placement rate, a steel chute was also mounted on the right abutment and used as a complementary device to the blondin. This allowed reaching on several occasions a placement rate of around 210 m 3 /h. While a layer of 30 cm thickness was applied for the RCC, it was considered sufficient to adopt a 60 cm thickness layer for the CVC. As illustrated in Figures 4 and 8, a 1 m wide perimetral vibrated concrete layer (CVC 15

8 MPa) was applied around all the galleries of the dam and the RCC was placed later against this layer. This prevents having the RCC in direct contact with air and provides a better finish of the walls and base of the galleries. In order to avoid placing formworks for the construction of the roof of the galleries, several types of prefabricated reinforced beams (Figure 8) were used depending on the width of each gallery (the largest roof beam was 9 m long placed at the bottom outlets). By this manner it was possible to avoid long construction delays. Figure 8. Concrete placement using the blondin for the placement of the perimetral CVC of the diagonal access gallery to the gates chamber. 4.3 Lift Joint treatment procedure A special attention was made for the treatment of lift joints between two consecutive layers. Due to the complex dam design and the concentration of galleries having sometimes a complex geometry, it was unavoidable at several occasions to bring to a halt the concrete placement for more than 24 hours. Since the lift joints are considered very critical for RCC dams regarding the tensile and cohesion strength, it was considered wise to apply a specially designed bedding mortar after a sound cleaning with high pressure water/air of the old layer surface before placement of the consecutive layer (Figure 9). This procedure was judged successful, specially after carrying out concluding laboratory tests made on drilled cores extracted from several locations of the dam.

9 Figure 9. Special treatment of the RCC lift joints. 4.4 Consolidation grouting and grout curtain All the works regarding the consolidation and contact grouting of the dam foundation were carried out using special galleries foreseen in each block of the dam in the upstream-downstream direction. These galleries have a section of 3x3m (see Figure 10). With this manner it was possible to carry out the grouting works independently of the construction and concrete placement of the dam body thus avoiding any delay. Figure 10. GIN curve applied for the grout curtain (left); consolidation and contact grouting galleries foreseen in each dam block (right). In Figure 11 it is illustrated the grout curtain of the dam for primary and secondary grouting. All the grouting works follow the GIN curve (Figure 10) with a GIN number of around 1500 [3]. This was considered sufficient taking into account the average rock quality of the dam foundation. The grouting pressure applied in each hole varies between a maximum of 30 bar (around 40 m below the dam base) and a minimum of 5 bar (close to the dam base). 5 Conclusions Presently the dam construction is at around 30% from completion (Figure 12). Consolidation and contact grouting of the foundation are almost terminated. The grouting curtain works are ongoing. The first filling of the dam is foreseen by the end of 2015.

10 Figure 11. Grout curtain of Cerro del Águila Dam. Figure 12. Cerro del Águila Dam under construction (photo taken on July 2015) Acknowledgements The authors acknowledge the work of G. Rotundo head of the technical on site office of Astaldi S.p.A, the contribution of M. D arrigo with regards to the construction procedures, the participation of F. Andriolo in the definition of the mix design, the strong involvement of A. Ricciardi, F. Tognola and J. Arbolí in the design work of the Dam, the support of M. Braghini, head of the hydraulic section of Lombardi Eng. Ltd, and last but not least the support of J. Monaco, owner s project manager. References [1] Lombardi SA, Dam safety evaluation, General Stability and Stress Analysis, [2] CRM SA, Informe técnico características de los concretos colocados en la presa, [3] G. Lombardi, and D. Deere, Grouting design and control using the GIN principle, 1993.