Filiatrinos, the first hardfill dam with a geomembrane facing Alberto SCUERO Carpi Tech Balerna Switzerland Gabriella VASCHETTI Carpi Tech Balerna Switzerland Nikolaos I. MOUTAFIS Consulting Engineer Athens Greece Yannis THANOPOULOS Geotechnical Engineer Larissa Greece Introduction Hardfill (HF) dams belong to the family of Cemented Material Dams (CMD) that, having a low cementitious content, present a number of technical and cost advantages. The Hardfill Dam (HFD) is a dam constructed with aggregates of even poor quality and a low cement content (typically 50-80 kg/m 3 ). The HF mix is consequently of low strength, nevertheless sufficient for structural stability of the dam. Placement and compaction of the HF mix is done in sub-horizontal layers, following a methodology similar to RCC dams, but using thicker layers and without formation of transverse joints. Confinement of the HF mix on the faces of the dam, during placement and compaction, is achieved either by extruded lean concrete or by precast lean concrete elements. Lack of joints in the dam body and the use of extruded or precast concrete elements at the external faces of the dam, simplify and speed up construction. The hardfill dam, being semi-pervious due to the low content of cementitious materials, needs an upstream sealing element to fulfil watertightness requirements. Reinforced concrete slabs, constructed in continuous strips from the toe to the crest of the dam, similarly to CFRDs, have been applied as sealing elements, but unfortunately exhibit the inherent misalignment of the slab joints with the cracks that may develop in the dam body (due to the lack of joints). Such misalignments and the potential subsequent movements may cause undesirable cracking or dislocations in the slabs. In addition, the length of joints between the concrete strips, plus the length of the peripheral joint, considerably increase the risk of leakage. An alternative approach, which effectively addresses the disadvantages of potential and unforeseen cracking in the dam body, is the application of a geocomposite (geomembrane +geotextile) sealing system on the upstream face of the dam. Such sealing system must be equipped with an ample and thorough drainage underneath the geocomposite. The geocomposite is attached to the dam face through appropriately designed fixing components, anchored onto the extruded or precast lean concrete elements that form the upstream face of the dam. Effective drainage is provided by the porous lean concrete elements, in conjunction with a system for drainage collection and discharge. The technology of geomembrane + geotextile composites has advanced to such an extent that it offers long term reliability [5, 7, 9] and consequently can be used in dam construction. The assets of a geomembrane sealing system compared to a concrete slab, although of critical importance, are not the subject of this work. Nevertheless, it is believed that those assets are so important that a HF dam and its plinth that have been designed and constructed to accommodate a concrete slab, may and can be modified to accommodate a geomembrane sealing system. The geomembrane sealing system can be part of the dam s design, or be retrofitted to substitute a concrete facing, as it was the case at Filiatrinos dam in Greece, the world s first hardfill dam with an upstream geomembrane as sealing element.. Filiatrinos Dam Filiatrinos Dam is located near the town of Filiatra in western Peloponnisos, in Greece. It was designed in 2004 [3] as a ~55 m high, axisymmetric hardfill dam with the faces formed at slopes 0.8: (H:V), as shown in Fig..
Max. Res. level +22.00 Dam axis +25.20 +20 +200 +90 0.8 0.794 Drainage pipes D200, @ 3m 0.8 Concrete slab HF +80 Drainage gallery +70 Collector PVC pipe at el. 70 - D630 +69.60 +60 Plinth Collector PVC pipe in the plinth - D630 Drainage holes Levelling concrete Fig.. Typical section of Filiatrinos dam [3] The face slab was foreseen to be concreted on the dam after completion of the dam body to crest elevation. Dam construction commenced in 20 and was completed in 205, due to a number of contractual disputes. In an attempt to complete the construction, the dam contractor proposed to the client the replacement of the foreseen concrete slab with a geomembrane sealing system, provided a number of modifications would be implemented to the dam and the plinth, to accommodate the placement and anchoring of the geomembrane. The proposal was approved and the prerequisite modifications on the dam were partially completed by June 205. The geomembrane installation works started in end of June and were completed in August 0, i.e. in 50 days. The original dam design provided for a conventional plinth (Fig. 2) with a conventional peripheral joint i.e. unnecessarily similar to the configuration of a CFRD plinth, since, contrary to CFRD, the relative movements between the plinth and the slab in HFDs are very small. Drainage was foreseen by a series of corrugated/perforated PVC pipes D200, engulfed in the HF, near the upstream face of the dam (Fig. 2). Those pipes discharge either into the D630 PVC collector pipe embedded in the plinth (Fig. 2), or into a similar collector pipe at el. +70 (Fig. ). Additional drainage is foreseen through a fan of holes discharging into a drainage gallery (Fig. ). Plinth 0.794 HF Collector PVC pipe in the plinth-d630 C/P drainage pipes D200, @ 3m Fig. 2. Section of the plinth and photo of drainage pipes wrapped in geotextile
2. Hardfill properties Crashed limestone was used as coarse aggregate for the HF, with a maximum size of 37.5 mm and content of fines <0%. Both cement and fly ash were used as cementitious material. The required characteristic strength (f c) of the HF material at 90 days was 4 MPa [3, 6]. The lower 20 meters of the dam were constructed with 58 kg/m 3 of cementitious materials (cement 5 kg/m 3 and fly ash 7 kg/m 3 ), while the upper 30 meters were constructed with 75 kg/m 3 of cementitious materials (cement 5 kg/m 3 and fly ash 23 kg/m 3 ) [6]. Compaction of the HF material near the upstream face of the dam was done manually (Fig. 3a), which resulted in the formation, of a triangular zone of HF material underneath the formwork, of reduced compaction, hence lower strength and higher permeability. The relatively low compaction of the triangular HF zone resulted in loose bonding of the HF materials, with the exposed coarse aggregates easily detachable from the HF mass. The presence of the detachable aggregates would not affect the construction of a concrete slab, but it could affect the performance of a geomembrane sealing system, so certain measures had to be taken to avoid accumulation of detached aggregates behind the geocomposite. The higher permeability of the HF was a positive side effect, enabling more effective drainage. Geotextile Geomembrane Backfill concrete Wire mesh Dowels Trimmed W/S HF Fig. 3a. Compaction of HF near the u/s face Fig. 3b. Waterstops trimming and concrete backfill 3. Modifications on the dam to accommodate the geocomposite The replacement of the upstream concrete slab with a geocomposite material necessitated the implementation of modifications in various parts of the dam, for the following reasons: The corrugated/perforated drainage pipes installed in line with the u/s face of the dam could not withstand the hydrostatic pressure exerted directly on them by the reservoir, so strengthening of the pipes was absolutely necessary. Placement and fixing of the geocomposite on the plinth required trimming of the copper and PVC waterstops and levelling off the groove between the concrete slab and the plinth with backfill concrete. Detachment and rolling down of aggregates on the dam face could result in accumulation of material behind the geocomposite and exertion of outward loads. Special anchoring of the geocomposite on the upstream face of the dam was necessary, due to the rather low compaction of the HF outer zone. Installation of the geocomposite from the crest of the dam would require modifications to the concrete crest walls. Strengthening of the drainage pipes to enable them to withstand the reservoir hydrostatic pressure was achieved by filling the pipes with pea gravel. Careful inspection of pipe filling was done by knocking tests and additional filling was done wherever necessary, from side openings along the pipes. In the lower part of the dam, in the area where placement of a backfill was planned, a layer of shotcrete was sprayed on the dam face to act as a stress distribution layer over the pipes, while measures were taken to ensure against uplift of the shotcrete layer. In the upper part of the dam above the shotcrete, wherever the drainage pipes were protruding from the hardfill surface, a smooth transition was provided by treatment with mortar.
The groove between the plinth and the upstream face of the dam was filled with pea gravel concrete, reinforced with wire mesh, and anchored to the plinth by re-bars, as shown in Fig. 3b, to level off the surface, for geocomposite placement and anchoring. Detachment of HF aggregates from the upstream face of the dam was considered a high risk hazard, due to the potential vibrations of the geocomposite caused by wind and wave loads, in conjunction with the low compaction of the hardfill close to the upstream face of the dam. Potential detachment of aggregates, and their rolling down the face of the dam, could result in their accumulation behind the geocomposite and the exertion of an undesired outwards puncturing action. The detachment of aggregates was prevented by placing a strong, thick geotextile layer (~ cm thick), directly onto the upstream face of the dam, and by anchoring the layer with a dense grid of short anchors. This layer, consisting of a 2000 g/m 2 polypropylene geotextile, was laid in 2.0 m wide strips, and provided the necessary stiffness to retain the aggregates that tend to be detached. The length of the anchors was determined by pull out tests on the HF material. 4. Properties of the geocomposite The selection of the geocomposite was based on the loads that it would have to sustain during the service life of the dam: the stresses applied by the cracks that may develop in the dam body, and by uplift due to wind and waves, the puncturing action that possible residual irregularities could cause under the hydraulic load, and the weathering action of the environment. Such loads required on one side a flexible elastic material that could elongate on the cracks and accept the deformations caused by repeated loads, and on the other side a material with proven resistance to weathering. Regarding the parameters that characterize the tensile behaviour of a geomembrane (stiffness, yield strength and stress, ultimate stress and strength), in most typical situations the maximum strains induced in geomembranes in the field are in the order of 50-60%. Geomembranes that exhibit a peak or plateau at strains lower than 50-60% are prone to premature failure in case of local thickness reduction caused by a scratch []. Since geomembranes may be scratched in the field during installation, geomembranes in dams should have a tensional behaviour with tension increasing monotonically up to 50-60% strain, while a peak or a plateau in the tension-strain curve between 0 and 50-60% strain is unacceptable. The tension-strain curves of both PVC geomembranes and PVC-geotextile geocomposites are increasing from 0 to 200-400% strain for PVC geomembranes and 60-80% strain for PVCgeotextile geocomposites. In contrast, the tension-strain curve of High Density Polyethylene (HDPE) geomembranes exhibits a peak for a strain (elongation) of 2-5%, and Linear Low Density Polyethylene (LLDPE) a peak for a strain of 32-33% (Fig. 4). The tension-strain curve of PVC (geomembranes and geocomposites) is adequate for demanding applications such as dams, the tension-strain curve of HDPE and LLDPE geomembranes is inadequate. Fig. 4. Tension-strain curves of PVC and PE geomembranes
More flexible geomembranes such as PVC, additionally, exhibit a better puncture resistance and are more suitable to withstand long-term pressure, which is the case in field, than stiffer geomembranes such as HDPE. Furthermore, a geotextile may be added below a PVC geomembrane to improve the puncture strength. A PVC geocomposite (PVC geomembrane + anti-puncture geotextile) was selected for the project. PVC geomembranes have a formulation including, besides the PVC resin, other ingredients that are needed to facilitate manufacturing, and ensure flexibility and resistance to weathering. PVC geomembranes may therefore be quite different one from the other, and their formulation must be conceived in function of the type of application in which they will be used. In dams, the geomembrane most frequently adopted and with the longest documented experience is Sibelon, which was first installed on a dam in 976. The liner selected for Filiatrinos is Sibelon CNT 4400, a geocomposite formed by a 3.0 mm thick PVC geomembrane extruded in homogeneous mass by a flat die, formulated to be UV resistant, and heat- bonded during fabrication to a 500 g/m 2 anti-puncture polypropylene geotextile. To grant watertightness, the thickness of the PVC geomembrane could have been inferior. As a matter of fact, the PVC geomembranes typically used in the 980 s for rehabilitation of concrete dams at high elevation in the Italian Alps were 2.0 or 2.5 mm thick. In some of these dams, the performance of the Sibelon CNT geocomposite is being monitored by tests periodically carried out on samples that have been in operation for periods varying from 22 to 35 years. The results of testing testifies the long durability of this material: watertightness has increased and the decrease in tensile properties (flexibility, elasticity, etc.) is such as to present no criticality in terms of effectiveness of the geomembrane as water barrier [9]. The relatively high 3.0 mm thickness was selected for Filiatrinos dam to further extend the durability of the system, in consideration of the semi-perviousness of the hardfill. Recent research has indicated that the durability of the exposed 3.0 mm thick Sibelon CNT 4400 geocomposite material largely exceeds 50 years, as shown in accelerated ageing tests performed in specialized laboratories [5]. Based on the same tests, and on the use of the Arrhenius model, the functional service life of the geomembrane exposed in a certain environment can be in the range of 00 years [7]. 5. Drainage system The dam has two drainage collectors consisting of a PVC pipe. As mentioned in chapter above, one collector is peripheral and embedded in the plinth, and one is horizontal and embedded in a concrete beam at elevation +70. The drainage system for the geocomposite is conceived to avoid unbalanced backpressure due to presence of water behind the geocomposite. Drained water travels by gravity down to elevation + 75, where it is collected by a horizontal collector consisting of a triple m high band of high-transmissivity drainage geonets (two Tenax CE 750 + one Tenax CE 200), and from there discharges via five transverse pipes drilled to reach the existing horizontal collector that discharges into the drainage gallery (Fig. ). An anti-intrusion stainless steel plate is positioned in front of each transverse pipe. The position of the drainage collector required installing the waterproofing geocomposite in two separate horizontal sections, i.e. from elevation + 70 to plinth, and from elevation + 24.20 (at parapet wall) to el. +70 (Fig. 5). Fig. 5. Installation of anti-puncture geotextile and of PVC geocomposite under (left) and above (right) elevation +70
The geocomposite of the bottom section is anchored at top by a 50 x 3 mm flat stainless steel profile secured with expansion anchors, the geocomposite of the top section overlaps it and is watertight seamed to it. The overlapping is covered by a SIBELON C 3900 PVC geomembrane strip, of the same composition as the Sibelon CNT geocomposite, but without the geotextile, heat-seamed unto the geocomposites. 6. Geocomposite anchoring An exposed geocomposite must be anchored to the face of the dam against uplift by wind and waves, and sealed at all peripheries to avoid water seeping underneath the geocomposite and infiltrating in the dam body. 6. Face anchorage For the face anchorage system, two zones have been identified: Above el. +85, the geocomposite is exposed to wind & varying water level, and a systematicc face anchorage has been adopted. Below el. +85 the geocomposite is covered by backfill, as it was in the original design with the concrete slabs, and face anchorage is provided by the ballasting action of the fill. Above el. +85, since there were no provisions in the constructed HF for geocomposite anchorage (in dams where the geomembrane system is part of the design, extruded curbs or precast concrete elements are used to provide a subgrade where the face anchorage system can be embedded), alternative anchorage techniques were necessary. Installation of deep point anchors seemed appropriate, and an extensive pull out testing program of anchors was carried out to determine anchor details. Anchors of various lengths were tested and, and even for the shortest anchoring length tested of 0.5 m, there was no de-bonding from the HF. Following the pull out tests, and in favour of safety, it was decided to use.0 m long stainless steel anchor rods, placed in a regular pattern. The anchor rods were grouted with cement grout into boreholes drilled in the HF material. Assuming a design wind velocity of 25 km/h, and an anchor spacing of 4 m, the geocomposite will load each anchor with a force of 5.3 kn, well below the achieved resistance of the tested anchors, which in all cases tested was higher than 35 kn. The grouted anchors have an end stainless steel washer (Fig. 6a) that distributes the load exerted by the wind force and limits stress concentrations on the geocomposite. The steel washers are waterproofed by a Sibelon C 3900 geomembrane washer, installed on a 000 g/m 2 anti-puncture polypropylene geotextile, and seamed unto the geocomposite along the perimeter of the anchor. The system has been developed and patented by Carpi Tech. From elevation +85 down to the plinth, the ballasting fine graded backfill was placed by the main contractor over a 000 g/m 2 protection geotextile (Fig. 6b). Fig. 6a. Washer of deep point anchor Fig. 7b. Placement of the fine graded backfill below el. +70 All intermediate fixations at changes in inclination consists of 50 x 3 mm flat stainless steel profiles secured with expansion anchors and waterproofed by SIBELON C 3900 PVC geomembrane strips.
6.2 Peripheral anchorage Anchorage at submersible peripheries (plinth, intake, spillway inlet structure) is designed to be watertight against water in pressure and consists of 80 x 8 mm flat stainless steel profiles, secured with chemical anchors at 0.5 m spacing. A regularising epoxy resin bedding, rubber gaskets, and stainless steel splice plates at abutting profiles, ensure the stress distribution that is necessary to achieve a watertight seal. The top seal at the parapet wall is watertight against rain, waves and snowmelt only, and consists of 50 x 3 mm flat stainless steel profiles secured with expansion anchors. 7. Conclusions Replacing the foreseen in the initial design reinforced concrete slab with a geocomposite implemented at Filiatrinos dam in Greece, improved the functionality and reliability of the facing watertight element, with a simultaneous reduction of construction time and cost. Construction procedures were greatly simplified by eliminating the use of a slip form for the construction of the slab, eliminating the requirement for a mix design, omitting the construction of multiple joints in the slab and the foreseen installation of elaborated waterstops and minimising potential sources of leakage. References. Giroud J. P., 984, Analysis of stresses and elongations in geomembranes, International Conference on Geomembranes, Denver, USA 2. ICOLD Bulletin 7, 2000, The gravity dam, a dam for the future 3. Hydrosistima Ltd., 2004, Design of Filiatrinos Dam, Prefecture of Peloponissos, Department of Public Works 4. ICOLD Bulletin 35, 200, Geomembrane sealing systems for dams 5. Koerner R.M., 203, Lifetime prediction of 3mm Carpi PVC Geomembrane in Panama canal-reservoir, Report on laboratory research 6. Gouvas X., Orfanos C., 203, Determination of factors affecting compressive strength of hardfill mixtures. The experience of Filiatrinos Dam, 2nd Conference on Greek Dams, Athens, Greece 7. Giroud J.P., 203, Functional Service Life of SIBELON Geomembrane for Panama Canal WSB, USA 8. Moutafis N.I., Thanopoulos Y. 205, The geomembrane faced hardfill dam, Hydro 205, Bordeaux, France 9. Daniele Cazzuffi et al., 205, Long-time behaviour of exposed geomembranes in rehabilitation of the upstream face of dams in Italy, 0th Rencontres Géosynthétiques, La Rochelle, France. The Authors A. SCUERO graduated in Hydraulic Civil Engineering at Turin Polytechnic in Italy. After working for major civil engineering construction companies in Italy and Africa, in 986 he joined CARPI, a private Dutch group that works in the field of waterproofing with geosynthetics. He has been involved in research on geomembrane technologies, for which he invented and holds several patents, in design and application of waterproofing geomembrane systems to all types of hydraulic structures, including >20 large dams. He was the coordinator of the ICOLD European Working Group who prepared ICOLD Bulletin 35 on Geomembrane Sealing Systems for Dams. G. VASCHETTI graduated in Civil Engineering at Turin Polytechnic in Italy. After three years as assistant teacher to practical lessons in the same University, she worked as registered professional engineer for private consultants. In 992, she joined CARPI, a private Dutch group that works in the field of waterproofing with geosynthetics. She has been involved in design, tendering, research and development of waterproofing geomembrane systems to all types of hydraulic structures. She was the secretary of the ICOLD European Working Group who prepared ICOLD Bulletin 35 on Geomembrane Sealing Systems for Dams. N. I. MOUTAFIS, B.Sc. in Civil Engineering from London University, UK, M.Sc. in Soil Mechanics from the University of Aston in Birmingham, UK and Ph.D. on anisotropic elasticity from the University of Aston in Birmingham, UK. Head of Geotechnical Sector in the Hydroelectric Development Dept., PPC, Greece, Director of Design Branch, Hydroelectric Development Dept., PPC, Greece. Lecturer at National Technical University of Athens, School of Civil Engineering, Dept. of Hydraulic Works and Environmental Engineering. Since 997, consultant on dams, hydro and underground structures to engineering firms, contractors and dam/hydro owners. Y. THANOPOULOS graduated in Civil Engineering from the National Technical University of Athens and specialized in Hydraulics and Geotechnical Engineering in Grenoble, France (Ph.D. in Soil Mechanics). He worked for 30 years with the Public Power Corporation, Hydroelectric Schemes Development Department in Greece, managing the owner s supervision unit. He is presently working as a consultant in dam and hydro schemes in Greece and abroad.