Taking a synthetic approach12 January 2000
Synthetic geomembranes are widely used as a means of waterproofing and protecting dams. In recognition of this the International Committee on Large Dams has continued to assess the use of such materials
The use of synthetic geomembrane systems for waterproofing dams and related hydraulic structures is now widely accepted in the dam engineering community as a long-term alternative to traditional lining methods such as concrete or shotcrete. icold has addressed waterproofing geomembrane application with two Bulletins, (Bulletin 38 in 1981 and Bulletin 78 in 1991), but as the use of geomembranes has increased European engineers and the European National Committees of ICOLD felt the need to disseminate more information on the subject. In 1993, at the symposium organised by the French National Committee on Large Dams in Chambéry, a European Working Group for Geomembrane and Geosynthetics as Facing Materials was established. Its aims were defined during the 1994 ICOLD Congress in Durban. The Group hopes to increase the knowledge of geosynthetic applications for waterproofing and protecting hydraulic structures.
During 1994-8, the Group collected data on European dams which had been waterproofed with geomembranes. In 1999 it was entrusted by ICOLD with preparing a revised edition of Bulletin 78. The data collected so far document the increase in the application of geomem-branes: while 70 dams were listed in Bulletin 78, more than 150 applications were reported in 1999. It is expected that the new Bulletin will reveal improvements in design and installation techniques.
Geomembranes are synthetic materials, fabricated as thin sheets with very low permeability in the range of 1x10-12 to 1x10-13cm/sec. They are used in hydraulic and civil construction either to restore watertightness to deteriorated structures, or to provide an impermeable element in new structures.
The use of geomembranes on hydraulic structures dates back to the late 1950s. The first reported installations on dams were performed on embankment dams (Contrada Sabetta in Italy, 1959; Dobsina dam in Slovakia; and Terzaghi dam in British Columbia, Canada, 1960). In the 1970s, their use was extended to cover the sub-vertical facings of gravity dams (Heimbach dam in Germany, 1974; and Miller dam in Italy, 1976).
On dams, geomembranes are generally installed on the upstream face but on embankment dams can also be installed in a central position. They are generally applied as prefabricated sheets and fastened to the upstream face by mechanical anchorage. Geomembranes fabricated in situ, by the impregnation of synthetic materials such as geotextiles, or by directly spraying or painting the membrane material over the dam face, have had very limited application. It is difficult to maintain their thickness and quality and they have a limited efficiency over time. Polyvinylchloride (PVC) geomembranes are the materials most often used on all types of dams.
Prefabricated PVC geomembranes are reported to have various advantages over other lining materials:
•Their characteristics can be engineered to meet specific site requirements and they are not affected by adverse weather conditions.
• ‘State of the art’ PVC geomembrane systems construct a liner which is fully elastic (over 250% elongation at a break in three-dimensional deformation) and has no joints.
•They are thinner and lighter than traditional liners but provide the same impermeability, making transportation and installation easy in difficult to reach places (typical of old dams located in mountainous regions).
•Installation takes less time and requires simpler equipment and site organisation.
•Installation and operational costs are lower due to reduced outage time.
•No routine maintenance is required.
•Underwater installation is now possible.
The main concerns for dam engineers considering the use of geomembranes are their durability when exposed to the environment, in particular to ultra-violet (UV) rays, and their resistance to floating debris, ice impact and vandalism.
Resistance to UV has been improved by UV stabilisers included in the formu-lation, and by increasing the thickness of the geomembrane. An impermeable mem-brane barrier requires a PVC membrane of about 0.5mm thick. The thickness adopted on recent dam projects has been of a minimum 2.5mm. Furthermore, after the initial surface is affected by UV rays, migration of some plasticisers in the atmosphere makes the geomembrane slightly stiffer on the surface. As a consequence, the penetration of the UV rays is greatly inhibited so that the underlying membrane will be unaffected.
Geomembranes can be designed with adequate thickness and can be reinforced to aid resistance to ice impact and floating debris. While geomembranes cannot be designed to withstand vandalism, if damage occurs, the tear resistance of the geomembrane will avoid propagation of the puncture or tear, making repairs easier.
The following are examples of geomembrane installations carried out by Switzerland-based carpi.
•Rehabilitation of gravity dams
Illsee dam is a 25m high gravity dam, including a section with very rough masonry facing, situated at 2361m elevation in the Swiss Alps. Temperatures there range from -30oC to +35oC, and freeze-thaw cycles are in the order of 100 per year. The dam is subject to alkali-aggregate reaction, and the synthetic geomembrane system was installed as a remedial measure to limit water circulation in the dam, with the aim of slowing, and gradually stopping, the expansion phenomenon.
The adopted system uses a composite membrane, consisting of a 2.5mm thick PVC geomembrane, heat-coupled during fabrication to a 500g/m2 polyester geo-textile. The geocomposite was attached to the dam face by stainless steel profile assemblies. It was drained behind to begin the dehydration process that the owner and their consultants had required to reduce water content in the dam body. The drainage system was divided into separate compartments to refine mon-itoring, and to allow a separate measurement of water drained from the foundation and from the upstream face. The rough stones in the masonry section of the facing, which would have been too aggressive on the geomembrane, were covered with a very thick geotextile to act as additional anti-puncture protection.
•Rehabilitation of embankment dams
Built in 1966, Moravka dam is a 39m high earthfill dam located in the Czech Republic. The deterioration of the original upstream impervious bituminous concrete facing required the placement of additional layers of bituminous concrete, which tended to slide over the layers underneath, aggravating the infiltration.
In 1999, after removing the additional bituminous concrete layers in the upper part of the dam, a new PVC geocom-posite liner was placed directly over the existing bituminous concrete on the entire dam face. The geocomposite, which remains exposed, is secured by mechanical linear anchorage fastened by anchor bars embedded in grouting mortar. Large prefabricated panels, anchored at 6m interaxis, allowed the installation of 26,000m2 of PVC geocomposite to be completed in 15 weeks.
Lost Creek dam is a 36m high arch dam, built in 1924 in the mountains of northern California, US. Deterioration due to freeze-thaw had resulted in spalling and loss of concrete at the downstream face. The main constraint for rehabilitation was that it was not possible to draw down the reservoir completely. Among all considered alternatives, a PVC geocomposite system that had been developed by Carpi for underwater installation was chosen as having the best technical and cost benefits.
The waterproofing system was installed while the reservoir was partially dewatered to allow rewinding of the generator. Installation in the dry and underwater was performed at the same time, and was completed without any loss of hydro power production.
•Construction of embankment dams
Bovilla dam is a 91m high earthfill dam located in a seismic region in Albania. The original design of a CFRD was changed during construction to a design incorporating a waterproofing geo-composite, to allow quicker and easier construction.
The system as designed has a flexible membrane liner that can accommodate the movements that are expected to occur, while maintaining the liner’s impermeability.
•Waterproofing joints and cracks
Platanovrissi dam is a 124m high RCC dam in Greece. Instead of using traditional PVC waterstops, the induced joints of the dam have been waterproofed with a new system installed over the upstream face. The system consists of a PVC geocomposite covering the joints and sealed along its perimeter. An anti-intrusion system is placed under the geocomposite to avoid intrusion in the active joints under hydraulic load. The new waterproofing system, designed so that it is able to follow the movements of the active joints without affecting impermeability, allows inspection and easy repair if needed.
•Construction of RCC dams
Balambano dam is a 93m high RCC dam in Indonesia. The installation of a PVC geocomposite providing watertightness on the upstream face has reduced construc-tion times, allowing much less stringent requirements for concrete mix design. The geomembrane provides watertightness in case new fissures open in the dam surface, mainly due to thermal constraints or seismic events.
To reduce construction times, the PVC geocomposite was installed on the lower part of the dam while construction was still in progress. When construction was completed, the upper part of the dam was waterproofed and the two sections joined with a watertight seal. The total surface of the dam was covered in 15 weeks and impounding could start a couple of weeks after concreting work had been completed.
The ability to waterproof a dam as it is being raised has been developed and will also make possible improvements in the construction of embankment dams. Adequate fastening of the geomembrane allows the dam to be overspilled, eliminating the need to construct an independent spillway structure.
A first pilot installation has already been completed on a small dike in Turkey, which was visited by delegates at the ICOLD executive meeting in Antalya in 1999. The system is at present being considered for several other dam projects.