A utilização de geomembranas na reabilitação e construção de novas barragens

A utilização de geomembranas na reabilitação e construção de novas barragens The use of geomembranes in rehabilitation and new construction of dams

 A.M. Scuero (1); G.L. Vaschetti (2); M. Gontijio (3) (1) Dr. Eng., Carpi Tech - [email protected] (2) Dr. Eng.,Carpi Tech - [email protected] (3) Dr. Eng., Carpi Brasil, [email protected]

Resumo Há mais de 50 anos, as geomembranas vêm sendo aplicadas em todo o mundo em uma variedade de estruturas hidráulicas, incluindo todos os tipos de barragens, canais, túneis hidráulicos e reservatórios. Seu uso em barragens expandiu de projetos pioneiros na construção de barragens de terra a projetos para a reabilitação a seco e subaquática de todos os tipos de barragens, e a construção de barragens de CCR. O artigo discute as características dos sistemas de geomembrana, sob os pontos de vista técnico e econômico, para cada tipo de aplicação, e, através de estudos de casos de experiências notáveis recentes, ilustra o estado-da-arte na tecnologia dos sistemas: Silvretta, barragem de gravidade com 80m de altura na  Áustria, como exemplo de um sistema de reabilitação com geomembrana instalada em duas campanhas distintas para minimizar o impacto sobre a produção de energia; Turimiquire, barragem de enrocamento com face de concreto (BEFC) com 113m de altura na Venezuela, como exemplo de reabilitação subaquática que atingiu uma profundidade de 65m em uma uma área crítica da face de montante da barragem, permitindo uma redução substancial dos vazamentos, sem impacto na exploração do reservatório; Sar Cheshmeh l, barragem de rejeitos no Irã, e Runcu, barragem de enrocamento na Romênia com 91m de altura, como exemplos de um novo conceito para a construção de barragens de terra, empregando uma geomembrana exposta a montante como elemento de estanqueidade; e por fim a barragem de CCR da UHE Mauá no Brasil, como exemplo do emprego de geomembrana exposta em uma barragem recém concluída como medida preventiva para evitar a infiltração de água no concreto. O trabalho apresenta os dados de desempenho disponíveis. Palavra-Chave: geomembranas impermeáveis, barragens de terra e enrocamento, barragens de concreto e de CCR

Abstract Since more than 50 years, geomembrane have been applied all over the world to a variety of hydraulic structures, including all types of dams, canals, hydraulic tunnels, reservoirs. Their use in dams expanded from pioneer projects in construction of new fill dams, to rehabilitation of all types of dams, in the dry and underwater, and to construction of new RCC dams. The paper discusses for each type of application the assets of geomembrane systems from the technical and economical point of view, and through case studies illustrates recent outstanding experiences of the state-of-the-art systems available: Silvretta 80m high gravity dam in Austria as example of a geomembrane rehabilitation system installed in two separate campaigns to minimise impact on power production, Turimiquire 113m CFRD in in Venezuela as example of underwater rehabilitation at depth reaching 65m on a critical area of the upstream face of the dam allowing substantial reduction of seepage with no impact on exploitation of the reservoir, Sar Cheshmeh tailings dam raising in Iran and the 91m high Runcu rockfill dam in Romania as examples of a new concept for construction of fill dams with an upstream exposed geomembrane as only watertight element, Mauá RCC dam in Brazil as example of an exposed geomembrane as preventive measure to avoid water infiltration in a dam just completed. The paper presents the available performance data. Keywords: impermeable geomembranes, earthfill and rockfill dams, concrete and RCC dams

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1

Introduction

1.1

Pioneer projects with geomembranes

The first geomembranes applications were carried out in new embankment dams because being too permeable they often required a separate impervious element. In many cases, geosynthetic barrier systems were more economical and easier to install than traditional impervious materials such as clay, concrete, bituminous concrete [Cazzuffi et al. 2010]. Europe was a pioneer in developing the use of geomembranes in dams: the first geomembrane installations were made in 1959 at Contrada Sabetta rockfill dam in Italy, and in 1960 at Dobsina earthfill dam in Slovakia. With only two exceptions (Terzaghi rockfill dam in Canada and Atbashinsk rockfill dam in Kirgikistan) up to the early 1980ies all geomembrane applications on dams were made in Europe. Also all first applications of geomembranes on concrete dams were made in Europe [ICOLD, 2010]: on some arch dams in Austria in the early 1980ies, on Heimbach gravity dam in Germany in 1974 and on Lago Miller gravity dam in Italy in 1976. In RCC dams, USA was the pioneer in the application of the covered geomembrane system (Carrol Ecton 1984), while Europe pioneered the application of the exposed geomembrane system, derived from the state-of-the-art solution for rehabilitation of concrete dams (Riou dam in France, 1990). Possibly, the credibility of impervious synthetic geomembranes has been established by the good performance of embedded PVC waterstops in a very large number of concrete dams worldwide. A geomembrane placed on the upstream face of a dam or inside a dam can be considered, from a conceptual viewpoint, as one huge waterstop sealed at the abutments, bottom and crest. Different from PVC waterstops, PVC geomembranes are engineered to resist environment exposure.

1.2 Present geomembranes applications The range of geomembrane applications has dramatically increased, and all over the world they are now used not only in dams, but also in reservoirs, pumped storage schemes, canals, hydraulic tunnels, shafts, both for rehabilitation and for new construction, in the dry or underwater. 1.2.1 Rehabilitation In rehabilitation, geomembranes have been used on all types of dams. In concrete dams and RCC dams, there is a state-of-the-art system that is generally used for dry installation. The geomembrane is installed at the upstream face, it is mechanically anchored on the face, and watertight sealed at all peripheries. The state-of-the-art system has been adapted to installation on embankment dams, with the modifications required by a subgrade that can be less cohesive than concrete, and to underwater installation. Rehabilitation has been made over the entire upstream face of the dam, or over smaller 2  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

areas causing seepage, or to repair cracks. These aspects are addressed in the case histories of chapter 2. The assets of a geomembrane system in rehabilitation are that it is a long-term repair method (durability of well engineered PVC geomembranes is 50+ years to 100+ years depending on environment exposure), that it can be installed on any type of dam, quickly and in practically all weather conditions, and if necessary without dewatering the reservoir. The presence of a face drainage system allows continuous monitoring of the efficiency of the geomembrane liner, and leak localisation systems are available to spot the area of the leak if any. In case of alkali-aggregate reaction, the drained geomembrane system discussed in the paper has been proven to allow dehydrating the dam of already infiltrated water, contributing to slow the reaction [Liberal et al. 2003]. 1.2.2 New construction In new construction, geomembranes have been used in embankment dams and reservoirs, and in RCC dams. In embankment dams, the geomembrane can be installed at the upstream face in exposed position, to substitute concrete slabs or bituminous concrete facings and construct what is known as a Geomembrane Face Rockfill Dam (GFRD), such as described in chapter 3.1. When vandalism or excessive environmental aggression (falls of rocks or ice blocks) are feared, the geomembrane can be covered in the areas of concern. The central position is adopted to substitute clay cores when clay is not available in the needed quantity or in presence of unsuitable environmental conditions, or can be used in place of bituminous concrete cores to facilitate and speed up construction. The main assets of a geomembrane system in construction of new embankment dams is its capability of resisting settlements and differential movements that would destroy other types of impervious layers, the possibility of using for the fill materials of lower quality, of reducing construction times and constraints, and of avoiding the need for embedded waterstops. In RCC dams, the geomembrane is installed upstream. The exposed system adopts the same face anchorage used for rehabilitation and is discussed in chapter 3.2, while the covered system embeds the geomembrane in prefabricated concrete panels that are used as permanent formworks against which the RCC is placed [Cazzuffi et al. 2010]. The main assets of a geomembrane system in RCC dams are its capability of avoiding future concerns about the watertightness of the upstream face, including lift joints, contraction joints and joints between the RCC and the conventional concrete, of bridging any cracks that should form due to thermal constraints, avoiding the risk that water can hydro-jack the lifts, of reducing design uplift. Other design constraints, such as the need for bedding mixes or special paste treatment/waterstops at the joints, can also be significantly reduced. This leads to the possibility of placing one RCC mix over the entire cross section of the dam without the interference caused by placing bedding mix on 3  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

horizontal lift joints and conventional concrete mix at the upstream face, and by placing waterstops. 2

Rehabilitation

2.1 Rehabilitation in the dry: Silvretta, Austria 2010-2011 Silvretta is a gravity dam owned by Vorarlberger Illwerke and used for power production. The main dam, completed in 1948, is 80m high upon foundations and has 4 inspection galleries. The main dam and the saddle dam forming the 40,000,000m³ storage are located in the Alps at 2032m altitude, with annual thermal excursion ranging from -30°C to +30°C, frequent freeze-thaw cycles with layers of compact ice and snow/ice reaching maximum thicknesses of 0.9m and 2m respectively. Over the last decade, frequent exposure to freeze-thaw cycles in addition to the combined action of frost, ice and seeping water, caused severe damage to the dam concrete structure which rapidly deteriorated. Within the rehabilitation works that started in 2009 and ended in 2011, including waterproofing and placing new concrete on the crest, treating the foundations, grouting, renewing the instrumentation and conducting rehabilitation works at the outlet and inlet valves, Illwerke took the decision of reinstating imperviousness to the dam face by an upstream waterproofing system on the main dam and on the saddle dam. Silvretta is the first exposed PVC geomembrane project on an Austrian dam. The final design is the result of extensive research carried out by Illwerke on available rehabilitation systems, and of evaluation of performance data from previous similar applications. The selected system consists of an exposed PVC geomembrane system on both the main dam and on the saddle dam. The objectives of the system are to prevent infiltration of seepage water into the dam body, to protect the dam structure against frost and seepage water, to drain infiltration and condensation water, and to monitor the waterproofing system. Site specific peculiarities were the bad conditions of the surface concrete, the demanding climate (risk of snowfall also in summer) and the particularly tight window for installation (only three springtime months), which made it an extremely challenging project particularly with respect to planning and programming. The decision was taken to design the geomembrane system so as to follow the dewatering program foreseen for the general rehabilitation works. As a result the geomembrane system was installed in two separate campaigns. During the first campaign in 2010, water was lowered to 1995m and the saddle dam and top part of the main dam, from 2010m to crest, were lined, in total 12,020m2. During the second campaign in 2011, the reservoir was totally dewatered and the remaining lower part of the main dam, in total about 5,940m 2, was lined.  At Silvretta, reducing execution times was essential to successfully complete the project in the allotted time. The conditions of the concrete were very bad, but extensive surface preparation was not compatible with the tight time schedule. The use of geosynthetic materials instead of more traditional methods for surface preparation allowed minimising civil works and reducing the risk of delays due to bad weather. Thus, following surface 4  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

cleaning by hydro-jetting and minimal removal of loose concrete and re-profiling, to stabilise the shotcrete layer and avoid future detachments, a high-performance geogrid, type TENAX LBO 440, anchored with deep mechanical anchors into the sound concrete of the dam body, was placed on the entire upstream face to act as a containment layer for the shotcrete. The geogrid also acts as a support layer to the waterproofing system over the biggest cracks in the face. To create a protective cushion for the waterproofing layer against puncturing by the deteriorated concrete and excessive irregularities, a 2000 g/m 2 anti- puncture geotextile was placed on top of the geogrid. Installation was carried out from 8 travelling platforms suspended at crest and, in the ar eas not accessible by the platforms, i.e. at both abutments, from scaffolding that was equipped with a shelter to allow performing all tasks in winter conditions.

Figures 1 and 2 - Installation of support geogrid and anti-puncture geotextile under shelter at Silvretta dam

The waterproofing liner is Sibelon ® CNT 3750, a geocomposite consisting of a 2.5mm thick PVC geomembrane heat-laminated to a 500g/m 2  anti-puncture geotextile, and selected based on precedents having more than 20 years successful performance. The geocomposite is mechanically anchored to the dam face with Carpi patented tensioning system designed to resist wind velocity of 180km/h, and at submersible boundaries by perimeter seals watertight against water in pressure. The face anchorage system, which allows pre-tensioning the PVC geocomposite, consists of two stainless steel ribs (generally referred to as “profiles”), the first one, U-shaped, fastened to the dam upstream face, and the second one, omega-shaped, installed over the PVC geocomposite. The geometry of the two profiles is such that, when they are tightly connected, they secure the geocomposite to the upstream face and pre-tension it. Pre-tensioning prevents the geocomposite from becoming loosened or wrinkled during service. Pre-tensioning is preferable [ICOLD 2010] for the safety and durability of the system: if the geocomposite is not adequately tensioned, the repeated loads to which it will be subjected during its service life (waves and wind, varying water levels, etc.) will over time cause formation of slack areas and folds, which are places for potential concentration of stresses that can lead to faster ageing of the geomembrane. 5  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

Figures 3 and 4 - Pre-tensioning profiles, and works at the main Silvretta dam in spring 2010: from right to left, surface preparation, installation of vertical profiles, installation of support geogrid (the dark grey material), installation of anti-puncture geotextile (the white material), installation and fastening of the PVC geocomposite (the light grey material)

The submersible perimeter seals are made with 80x8mm stainless steel batten strips compressing the PVC geocomposite on a regularizing resin, with the aid of rubber gaskets and stainless steel splice plates to distribute the compression and achieve watertightness against water in pressure. The depth of the anchor bolts for the tensioning profiles and for the watertight perimeter anchorage was based on the results of the pull-out resistance tests carried out on the face concrete. The seal at crest was designed not to be air-tight, so as to establish atmospheric pressure in the drainage system and avoid suction. The drainage system is divided in 6 separate vertical compartments to allow monitoring the performance in sections. Drained water is discharged in the bottom gallery by transverse discharge pipes, one pipe for each compartment. Waterproofing works of the 2010 campaign started on March 29 and were completed on June 29. The waterproofing liner installed was temporarily sealed at about 2001m elevation with a horizontal seal made by 50x3mm stainless steel batten strips tied with mechanical anchors at 0.20m spacing. This seal was dismantled in the 2011 campaign, when the PVC geocomposite on the upper part of the dam was connected to the PVC geocomposite installed in 2011 in the lower part of the dam. In 2011, the reservoir was totally dewatered to allow lining the main dam, from about 2001m elevation down to the bottom. A thicker layer of mud and debris had to be removed to expose the foundation. Installation of the waterproofing system proceeded in the same way as described for the upper section. At left of the intake structure a sheltered scaffolding was installed, while at right of the intake structure installaiton was carried out from travelling platforms. As for the 2010 campaign, this arrangement allowed organising the works so as to minimise idle time due to bad weather. Waterproofing works of the 2011 campaign started on March 16 and were completed on May 12. In total 17,960m2 in the two campaigns. 6  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

Figures 5 to 7 - At left works preparation at the main Silvretta dam in spring 2011, at middle waterproofing works ongoing under shelter at the left of the intake and from travelling platforms at the right of the intake, at right works completed

Illwerke recognized to Carpi a bonus of 40,000€ for completing the works   in both

campaigns before the contractual deadline [Scuero et al. 2012]. In autumn 2011, with reservoir at full water level, seepage at the drains was only drops, and stands unchanged until the present day.

Figure 8 - Silvretta main dam and saddle dam at impounded reservoir

2.2 Rehabilitation underwater: Turimiquire, Venezuela 2010-2011  A milestone in the application of a PVC geomembrane system was reached on Las Canalitas, better known as Turimiquire dam, a 113 m high CFRD in Venezuela. Owned by Ministerio del Poder Popular para el Ambiente, the dam was designed by Barry Cooke and is used for potable water supply. The dam was impounded in 1988 and just after a year, leakages were observed. In 1989, 1994, 1996, 1999, and 2000, repeated repairs were carried out with clay material and granular material of different sizes, plus a geomembrane over 450m2. The result of repairs was very poor, leakage increased again after each repair, finally attaining 9800l/s. Investigations carried out with sonar multi-beam scanning showed that the leak was not at the plinth, contrary to what originally believed, that there were two epicentres, a crater of 7  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

4.1m2 in slab 24 and one at el. 270m in slab 9, and a large zone with fissures and cracks without a definite orientation and up to 7m in length, believed to be caused by settlement detected at the crest of the deep slabs. The concrete presented scales and loss of cementitious material (honeycombs) in several places. In addition there was a permeable area below the geomembrane placed in 2000, which due to insufficient anchorage had been locally displaced and was not performing as expected. Accumulation of sediments and debris was found, and bathymetric measures carried out in 2008 suggested sediments thickness in the range of 2 to 3m on top of the plinth and > 5 m at the upstream toe. Following the very poor outcome of all repair measures adopted until then, in May 2008 the decision was taken to install an impervious polyvinylchloride (PVC) geocomposite at the upstream face as permanent repair measure. The geocomposite system was to be installed at first over the most critical areas, in total about 14 930m 2, from el. +304 down to el. + 245, with total acceptable residual see page ≤ 3,000l/s. Less crucial areas to be lined in a subsequent phase, under a separate contract. Since the reservoir is critical for water supply and the water level can be lowered but must meet the demands of the population, the owner decided to perform the works on the most critical area with the water level at +295. Most of the repair work was therefore to be carried out underwater. A contract for the design of the waterproofing system was awarded to Carpi based on the experience acquired by the company in both dry and underwater application of geomembrane systems on dams. The geomembrane system was designed to be installed over the easily accessible areas obviating the immediate need for expensive sediment removal. A design choice has been to have the same conceptual exposed geomembrane system for the dry and underwater parts. Another design choice has been to minimize as much as possible surface preparation (fissures, loss of cementitious material, large cavities, and severe roughness were present) through the extensive use of synthetic materials, like at Silvretta. A support layer consisting of a geogrid with characteristics such as to provide the required strength on the cavities, and of an anti-puncture layer consisting of a 2000g/m 2 polyester geotextile on top of the geogrid, were installed over the entire surface. Two different geogrids were used for the face slabs, a standard one for the smaller cracks and a heavy duty one f or the larger cracks and the cavities. The waterproofing liner is a geocomposite consisting of a 3mm thick PVC geomembrane laminated during fabrication to a 700g/m 2  nonwoven geotextile. A robust geocomposite was deemed necessary due to such high water head. The geocomposite has been supplied in 2.10m wide sheets. For the underwater part, four sheets were pre-welded at site to prefabricate 7.7 m wide panels that allowed minimizing expensive underwater operations. The face anchorage system of the dry section is the same patented tensioning system of Silvretta. In the underwater section, the geometry of the profiles was modified to adapt it to the underwater environment: in dry conditions the tensioning profiles are waterproofed by a PVC geomembrane strip welded over them, in underwater conditions since welding is not feasible the profiles are modified to be intrinsically watertight. The underwater anchorage lines consist of a trapezoidal stainless steel profile fastened to the 8  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

face of the dam with mechanical anchors, and by a flat stainless steel batten strip compressing the two adjacent and overlapping PVC geocomposite sheets on the trapezoidal profile. Adequate gaskets assure even compression all along the profiles, achieving an intrinsically watertight fastening line.

Figures 9 to 11 - Turimiquire: at left the face anchorage profiles and support geogrid, at middle and right the 7.7m wide panels of geocomposite are unrolled over the geotextile for underwater placement

The face anchorage system in the section that will be mostly covered by water has anchorage lines at 7.4m spacing, while in the areas above water level, based on the higher suction deriving from the possibility of these areas being exposed to wind, the anchorage lines are at 3.7m spacing, to assure an easy and efficient transition between the dry and the underwater parts, which can be achieved either using the same spacing, or a sub-multiple of 7.4 m. The geocomposite is sealed along all peripheries by a perimeter seal to avoid water infiltrating behind it. The peripheral seals are of the mechanical type, sealing against water in pressure. The seal is achieved by compressing the geocomposite with flat stainless steel batten strips, 80x8mm section. In the dry they are bolted to the dam surface with chemical anchors at 0.15m spacing, underwater, due to the different requirements of the working environment, they are bolted to the dam surface with mechanical anchors at the same spacing. The final design at peripheries envisaged a system that will in the future allow lining the longitudinal joint face slabs/plinth, should the owner decide to remove the sediments. The PVC geocomposite stops 2m shy of the sediments line, so that if for future removal of sediments a drag-flow equipment is used, the 2m distance will avoid the geomembrane being affected by suction by the drag flow equipment. Additionally, along the bottom perimeter of the waterproofed area an additional width of PVC geocomposite has been left beyond the perimeter seal; this additional PVC geocomposite will be watertight connected to the PVC geocomposite that will be installed over the areas where sediments will be removed. The same principle has been adopted along all the peripheries to allow connecting to the waterproofed area. Water elevation at 295 (resulting in maximum diving depth of 50m) originally established could not be always maintained, and diving depth exceeding at times 60m, and some slight modifications of the lined area were necessary [Scuero and Vaschetti 2011]. 9  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

Figures 12 and 13 - The areas lined at Turimiquire, and view of the dam during underwater works

 At completed underwater works, with about 1/4 of the total upstream face lined, seepage has been reduced from 9800l/ to 2400l/s, well below the target leakage of 3000l/s. 3

New construction

3.1

Fill dams: Sar Cheshmeh, Iran 2008, and Runcu, Romania 2011-2013

Sar Cheshmeh and Runcu are examples of exposed upstream geomembrane systems adopted to substitute a concrete facing. Originally designed as CFRDs, when construction had already started they both underwent a change in design and became GFRDs (Geomembrane Face Rockfill Dams). In both dams, an exposed PVC geocomposite substitutes the concrete slabs originally planned at the upstream face. Construction is dramatically simplified, since no reinforced concrete works and no waterstops are needed, and the PVC geocomposite is placed directly over the extruded curbs that from the upstream face (Iota method), and is anchored to the dam by welding it to PVC anchor strips embedded in the curbs. PVC, with elongation at break > 230%, can exceed by large any possible deformation of the embankment. Additionally, the PVC anchorage system is already constructed when the fill is completed, so installation and anchorage of the PVC geocomposite is much quicker than with more traditional anchorage methods. The upstream geomembrane allows reducing constraints in respect to the type and grading of fill material, to the type and thickness of drainage layer. The dam body can be constructed with a large range of materials locally available. The waterproofing system is “more forgiving” in respect to conventional systems as far as construction skills are

concerned. Companies with limited previous experience in embankment dams can achieve construction of a safe dam, while a specialised contractor constructs the waterproofing system. Being prefabricated in the controlled environment of a factory, under ISO procedures, all components of the waterproofing system have pre-established and constant properties. 10  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

Properties are verified upon arrival of materials at site, weather conditions at installation do not alter the materials, joints and perimeter seals are verified for watertightness with standard methods. Hence the characteristics and the final quality of the waterproofing system are pre-established and remain unaltered during the entire installation process.

Figure 14 - Scheme of face anchorage at Sar Cheshmeh and Runcu dams. The upstream exposed PVC geocomposite is welded over PVC anchor strips embedded in the curbs that also provide the filter/transition/drainage layer

3.1.1 Sar Cheshmeh tailings dam, Iran 2008 Sar Cheshmeh is a 75m high tailings dam owned by National Iranian Copper Industries Company. The dam was designed and constructed in the late 1970s. An upgrading project aiming to a production escalation involving almost 1 billion tonnes of tailings over 31 years was initiated in 2001. Designers are ATC Williams (previously Australian Tailings Consultants). The upgrading project included raising the Main Embankment and Saddle Dam No.1 in four stages, for a total of 40m of heightening. The existing dam, whose internal zoning included an inclined clay core and an upstream colluvial gravel shell, suffered from seepage, settlement and cracking relating to high pond levels. Iran being one of the more seismically active regions in the world, seismic resistance was critical. The conventional “downstream” design required exte nding the existing inclined clay core; however, the raised core would have a thin rockfill cover to act as a surcharge during seismic loading. Stability analyses showed that the resultant seismic factor of safety was unacceptable. Further issues were a shortage of suitable clay-based soils in the semi-arid Sar Cheshmeh area, and that the borrow sources used in the original embankment construction had been covered by tailings. It was concluded that clay materials could not be available for the heightening, and a detailed study of alternative water retaining elements was conducted. An upstream geomembrane sealing system on a rockfill embankment was preferred to an asphaltic core and to an asphalt concrete facing on the basis of it being a more stable, efficient and buildable arrangement. It became clear that from a construction, performance and cost perspective, an exposed geomembrane should be preferred, and that a PVC geocomposite represented the only geomembrane 11  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

solution which would meet the necessary requirements for Sar Cheshmeh [Noske 2010]. The concept was to provide imperviousness to the dam by a totally deformable system that could withstand seismic events, and construct the dam with a quite simple layering. Stage II B and Stage II C consist of a zoned rockfill placed against extruded porous concrete curbs (Itá method) at a slope of 1V:1.5H, on which with the upstream exposed PVC geocomposite system is placed. Total length of Stage II C dam was 1,000 m and the total installed geocomposite 20,500 m 2 for Stage II B and 18,000 m 2 for Stage II C. TYPICAL SECTION ON ANCHOR STRIPS  Anchor strips w=0.50 PVC geocomposite Sibelon CNT 4400

Interlock keys between curbs 4 m long centered between two anchor strips.    5 1   0.

 

  m   l d   m  e   0  4   t  w  a  e   H

Temporary ballast (concrete, gravel, sand, steel bar, etc.)

Porous concrete curb



  

0.30

0.15

        0         1   .         0

Figure 15 - Cross section of Sar Cheshmeh curbs

 An exposed PVC geocomposite system was installed on Stage IIB and Stage IIC, from the existing crest at elevation 2175.5m to elevation 2194m, in total 18.5m of raising. The PVC geocomposite is anchored to the dam by PVC anchor strips that are fastened to each of the curbs by the ballasting action of the fill. Each strip overlaps the strip placed on the curb underneath, and is heat-welded over it at the overlapping, resulting in one continuous vertical PVC band on the upstream face. The PVC vertical bands, placed at about 6m spacing, constitute the anchorage lines for the waterproofing PVC geocomposite Sibelon ® CNT 4400, formed by an impervious 3mm thick PVC geomembrane laminated to a 500g/m2 geotextile. The PVC geocomposite was temporarily fastened at crest, deployed over the curbs, and permanently secured to the dam by heat-welding it to the PVC strips, made of the same Sibelon ® CNT 4400 material.

Figures 16 to 18 - Sar Cheshmeh. Overlapping PVC anchor strips are welded on each other, to construct parallel vertical anchor bands on the upstream face

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The watertight seal of the PVC sheets of Stage II B to the clay core of the existing dam was made at a 10m wide area excavated in the upper part of the existing clay core, by placing the PVC sheets over the clay, and by covering them with compacted clay. The watertight seal at the abutments was of the tie-down type, made on a concrete plinth constructed on the abutments’ rock. The top anchorage of the PVC sheets of Stage II B, at

elevation 2185m, was made with a flat stainless steel batten strip fastened to a to conventional concrete curb. Installation of the waterproofing system of Stage II B started on 28 June 2008 and was completed on 14 August 2008. Construction of Stage II C started shortly afterwards, in September 2008. The waterproofing system of Stage II C is identical to the one of Stage II B, except for the top and bottom perimeter seals: top anchorage at Stage II C was made in a trench excavated along the crest and backfilled with a mass concrete anchor beam, bottom anchorage was made by welding the PVC geocomposite of Stage II C unto the PVC geocomposite of Stage II B and further waterproofing the junction with a PVC cover strip.

Figures 19 to 21: Sar Cheshmeh. At left placement of the clay layer over the PVC sheets at the clay core of the existing dam, at middle and right placement of the PVC sheets of Stage II C, and watertight connection to the PVC sheets of Stage II B

Installation of the waterproofing system of Stage II C started on October 12 2008 and was completed on November 26 2008. The designer [Noske 2010] reports “From the

perspective of both the designer and the owner, the selected GSS has resulted in an efficient and economic waterproofing solution ... anchor strip installation became a simple, routine process…. installation was fast…. The overall construction period was also significantly reduced…… Impoundment of water against the toe of raised embankment

has recently commenced, and seepage measurements downstream of the embankment have not changed from their steady-state levels. It is concluded that the geocomposite faced rockfill approach is a viable, effective means of tailings dam construction …. where

natural materials are either not available, or are unable to be used from a technical perspective”.

3.1.2 Runcu rockfill dam, Romania Runcu is a 91m high rockfill dam under construction in Romania, which will be used for water supply and irrigation. When construction had already started, the designer 13  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

 Aquaproiect modified the CFRD design, incorporating an exposed geomembrane instead of the concrete facing, to a construct a GFRD. The reasons for changing the design where related to particularly long construction times required for the support layer of the slabs and for the slabs themselves, and to the high costs involved with the CFRD design: the solution with concrete slabs would have cost about 130% more than the solution with the geomembrane facing, because it entailed constructing three different zones of granular material made on site as support for the slabs. The final design of the waterproofing system was developed by SC Sembenelli Consulting of Italy in association with Carpi. The dam is a homogeneous rockfill placed in 1.5 m high compacted lifts, with quite steep (1.4H / 1V) upstream face. In the original design a Zone 2 (5 to 250 mm) layer is placed on the fill, followed by a Zone 1 (5 to 90 mm) layer; both layers, each about 5 m thick, are covered by a porous concrete layer before placement of the concrete slabs. The dam is constructed in phases, a first phase up to elevation +681m, a second phase up to +710m that will allow partial operation of the dam to a water level of +700m, and the third phase up to crest elevation of 736m. When the design was changed, the rockfill had been constructed up to elevation +681m, with a 0.2m thick layer of sand and gravel placed on top of Zone 1. The design of the waterproofing system had to adapt to the existing situation. Up to elevation +686.50m the dam will be constructed according to the original design and the PVC anchor strips will be embedded in vertical trenches excavated in the existing surface at about 6m spacing and filled with concrete. From elevation +686.50 m up to crest, the Zone 1 and Zone 2 layers will be eliminated and the face will be formed by porous concrete curbs extruded by a curb extruder as done also at Sar Cheshmeh, and the PVC anchor strips will be embedded in the curbs. The waterproofing liner of the three sections is a PVC geocomposite with different thickness depending on the water head: in the bottom and intermediate section the PVC geomembrane will be 3.5mm thick, in the top section 2.5mm thick. At the time of preparation of this paper, the second phase, including construction of the curbs and embedment of the PVC anchor strips in the curbs, is ongoing. When completed, Runcu will be the highest fill dam in the world having an exposed geomembrane facing as the only waterproofing element.

Figures 22 and 23 - At left the bottom section of Runcu dam completed up to elevation +681m, at right the curbs and embedded PVC anchor strips under construction above elevation +681m

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3.2

RCC dams

3.2.1 Background The database of Bulletin 135 [ICOLD 2010] reports 40 RCC dams incorporating a geomembrane for watertightness. In year 2011at least 3 more RCC dams have used a geomembrane. In almost all cases the geomembrane system was included in the design and installed at construction. The geomembrane has been installed in exposed position (21 cases) and in covered position (19 cases). The adopted geomembrane, with only 3 exceptions, is always PVC [Cazzuffi et al. 2010]. The existing 4 cases of repair include repair of cracks and failing joints in the dry and underwater, and repair of the entire upstream face. The PVC geomembrane is always exposed. 3.2.2 Mauá RCC dam Owned by Consórcio Energético Cruzeiro do Sul and built by J.Malucelli Construtora de Obras from Brazil, Mauá is an 83m high RCC dam that will be used for hydropower. The RCC mix has 85kg/m 3  of cement. As prudential precaution measure, it was decided to waterproof the upstream face of the dam with a drained geomembrane system, to impede any water infiltration into the concrete of the dam. Mauá is the first example of geomembrane system not included since the design stage and adopted when t he dam had already been completed without a geomembrane. The use of a waterproofing system for Mauá RCC dam was decided just a few weeks before the planned impounding, and its design had to be adapted to a site-specific situation. The requirements were that the waterproofing system should be installed under “emergency schedule” so as to allow starting power production according to the schedule

originally planned for the operation of the dam. The design was based on the fact that in the lower part of the dam the geomembrane system would permanently be covered by water, and taking into account the fact that the lower and intermediate parts of the dam had to be waterproofed in the shorter possible time, to allow impounding the reservoir to a level adequate to start power production, while installation of the waterproofing system in the upper part of the dam could be completed while the units were already running, and the water was kept at a level allowing performing the installation totally in the dry. The waterproofing system has therefore been divided in 3 separate sections, with different face anchorage depending on the elevation and on the time of execution: a) from the plinth to elevation 572.70m, area that will be always below water level during operation of the dam and where there is no drainage discharge possibility: double geomembrane, face anchorage provided by the ballasting action of water; b) from elevation 572.70m to elevation 628m: face anchorage with flat profiles , which can be installed at a much faster rate than the tensioning profiles; c) from elevation 628m to crest, area where the waterproofing system is to be installed during early generation of electricity: face anchorage with tensioning profiles, in the same external configurations adopted at Silvretta. The PVC geocomposite Sibelon ® CNT 3750 was installed over a drainage geonet that will act as full face drainage layer capturing any infiltration water at the dam face, impeding 15  ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC2012 – 54CBC

that it enters the dam body.

Figures 24 to 26 - Mauá: at left the PVC geocomposite under installation on the drainage geonet, at middle the 3 sections can be seen (double geomembrane at bottom, flat profiles at intermediate section, tensioning profiles at top section. At right, the waterproofing works under completion

Waterproofing works started on August 10, 2011 and were completed on December 4, 2011. Total surface lined was 33.468 m 2. 4

References

CAZZUFFI, D.; GIROUD, J.P; SCUERO, A.; VASCHETTI, G.. Geosynthetic barriers systems for dams, Keynote lecture 9th International Conference on Geosynthetics. Guarujá, 2010. ICOLD, International Commission on Large Dams.  Bulletin 135. Geomembrane sealing systems for dams - Design principles and review of experience.  ICOLD-CIGB, Paris, 2010. LIBERAL, O., et al.. Observed behaviour and deterioration assessment of Pracana dam. ICOLD 21st International Congress, Montréal, 2003. NOSKE, C. Geocomposite faced rockfill —  an innovative means of water-proofing tailings storages. Mine Waste 2010, A.B. Fourie and R.J. Jewell, Perth, 2010. SCUERO, A.; VASCHETTI, G.. Underwater rehabilitation of a 113 m high CFRD: experiences from Turimiquire. XXVIII National Seminar On Large Dams, Symposium, Rio de Janeiro, 2011. SCUERO, A.; VASCHETTI, G.; BACCHELLI M.. Geomembrane systems for repair of dams: rehabilitation of Silvretta gravity dam, Austria 2010-2011. Hydrovision Russia, Moscow, 2012.

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