This article is part of Geosynthetica’s GeoAmericas series. Written by Natale Daly, Bertrand Breul, and Bernard Breul of Axter Coletanche and published originally in the GeoAmericas 2016 proceedings by Geosynthetica, the article takes a 40+ year view of bituminous geomembranes in dam engineering. Geosynthetic barriers have been shown to have exceptional service lives for dams, including with large dams. These barriers can help significantly extend the service life (and decrease maintenance) of the dams.
- BITUMINOUS GEOMEMBRANES IN DAMS
The dam of La Galaube completes the hydraulic system of the Montagne Noire area, in the south of France (hot and dry area in summer) to provide a total storage capacity in excess of 35 million cubic meters (1240 million ft3). In order to supply potable water to three counties and 185 municipalities in the south of France, it regulates the flow of the Canal du Midi, classified as UNESCO World Heritage site and produces electricity for ‘’Electricité de France’’. The dam of La Galaube has a capacity of 8 million cubic meters (280 million ft3) and the lake will cover an area of 68 hectares, at an altitude of more than 700 meters (2,300 ft). The cost of the whole project was in excess of 20 million US $.
Design and supervision of works for the La Galaube dam project were carried out by two French consulting engineers, I.S.L. and B.R.L, working on a worldwide level. The design of an upstream geomembrane for waterproofing on a rockfill embankment was selected, because it was the most economical and had the least environmental impact. A large amount of the work was performed within the footprint of the project. The mica schist excavated on the site was used to build the embankment and therefore minimal material needed to be imported. By contrast some of the other options considered for the embankment such as Roller-Compacted Concrete (RCC) or zoned embankment with cores and shells would have required large amounts of imported materials and truck traffic in a pristine area of southern France.
The La Galaube dam is a gravity embankment dam and its stability is ensured by the weight of the rocks, which consist of about 800,000 m3 (28,300,000 ft3) of mica schist. The embankment rests upstream on a reinforced concrete plinth, founded on fresh or slightly weathered granite. The La Galaube dam is 380 m (1250 ft) long at the crest with slopes inclined at 2H:1V. The maximum height above the foundations is 43 m (140 ft). A typical cross section of the dam is shown in Figure 1.
In addition, the project includes:
- Side spillway able to sustain a flow of 80 m3/sec (2825 ft3/sec)
- Upstream intake tower
- Under-embankment tunnel, including a hydraulic tunnel and a monitoring tunnel,
- Downstream outlet structure
Waterproofing of the la Galaube dam included lining the upstream face and grouting the foundation. The lining system installed on the upstream consists of:
- A 10 cm (4 in) layer of non-bounded material, with a 0/20 mm grading natural material, impregnated with bitumen emulsion
- A 10 cm (4 in) layer of cold asphalt mix, with a 0/10 mm grading natural material
- A bituminous geomembrane, ES3
- A 10 cm (4 in) layer of fibrous concrete laid upon a geotextile.
2. SUBGRADE PREPARATION
2.1 Compaction of the slope
The upstream slope was compacted with two 4 ton static rollers. Those compactors were pulled from the dam crest by two hydraulic excavators equipped with winches as shown in Figure 2.
2.2 Non-bounded material
A layer of non-bounded material consisting of a crushed limestone of 0/20 mm of grain size, was laid down as the subgrade. The minimum thickness of this layer was 20 cm (8 in). Applied directly on the rockfill, it filled up the voids in the coarse rock and provided for a smooth working surface with minimum sized protrusions. A total of 5,000 tons of the crushed limestone was imported by trucks from a nearby quarry.
The material was moisture conditioned, transported with a rear dump truck and unloaded at the crest along the slope while graded to the desired thickness using bulldozers as shown in Figure 3. The bulldozers were equipped with lasers. The material was compacted using a 4 ton roller. In areas difficult to reach (these areas are always badly compacted and subject of tear for the geomembrane), such as the foot of the intake tower, a vibrating beetle mounted on a hydraulic excavator was used.
2.3 Bitumen emulsion impregnation
An impregnation layer with bitumen emulsion was then hand spread, at a rate of 1.5 kg/m2. The purpose of the layer is to facilitate the laying of the asphalt mix and the intimate contact (glue) between non-bounded natural material and cold asphalt mix.
2.4 Cold asphalt mix
The 10 cm (4 in) thick layer cold asphalt mix, with a 0/10 cm aggregates was then laid on top of the bitumen emulsion impregnated crushed limestone. The purposes of the layer of asphalt are:
- To provide a smooth surface for laying and attaching the BGM
- To create a semi-impervious layer that will reduce leakage flow into the rockfill in case of accident of the geomembrane
A specific laboratory study was carried out to define the mix of the asphalt cold mix, in order to reach a permeability on the order of 10-6 m/s. Special care was given to the breaking behavior of the emulsion, so that the cold asphalt mix workability was ensured throughout the laying and compaction phases.
There is a correlation between asphalt density and permeability, therefore the laboratory tests carried out on site that included compliance of the aggregates with the specified gradation curve of asphalt content and in-situ densities, allowed to reach the specified permeability. A total of 5,000 tons of cold asphalt mix was manufactured on site with a 200 ton per hour mixing plant. The aggregates were obtained from the same limestone quarry and to be used for the non-bounded material layer. Three gradation cut-offs of 0/2 mm, 2/6 mm and 6/10 mm were used during placement and compaction, and it was critical to keep the rollers wet so they would not stick to the asphalt mix.
The allowable tolerance of the finished asphalt surface that included the irregularities in the crushed limestone layer was set to plus or minus 2 cm (0.8 in) and was met.
The plant was installed on site to minimize transport cost and CO2 emissions for the respect of environment.
3. DEPLOYMENT OF THE BITUMINOUS GEOMEMBRANE
3.1 The bituminous geomembrane (BGM)
BGM is manufactured by impregnating a non-woven polyester geotextile with bitumen. In addition, the geomembrane is coated with sand on one side and an anti-root film on the other side. The sandy side is usually the exposed side, whereas the side with the anti-root film is placed against the subgrade. The difference between the two sides is essentially marked in the friction angle: 34° for the sandy face and 16° (like polymeric geomembranes) on the anti-root face. Note that in some applications where the designer would like the same angle of friction, the anti-root film is not included and the exposed bitumen face gives higher interface shear strength properties on both sides. This detail has been applied for a huge potable water reservoir for the Los Angeles Department of Water and Power. (LADWP). To support the geotextile width during the fabrication process, a glass fleece is impregnated in the product as well. A typical cross section of a bituminous geomembrane is shown in Figure 4.
An ES3 grade bituminous geomembrane was used. ES3 is manufactured in 5 meter (16.4 ft) wide rolls, with a mass per unit area of 5.5 kg/m² (171 oz/yd2), following ASTM D 3776 standard so easing the installation even with strong winds, and 4.8 mm (190 mils) thick. Typical length of the rolls is in the order of 65 meters (210 ft.) for a weight of 2 tons.
Notable properties of this geomembrane include:
- A high tensile strength in both directions: 28 KN/m (166 lbf/in) in longitudinal direction, 20 (137 lbf/in) in transverse direction, together with more than 90 % deformation at break (ASTM 4595 standard).
- A high resistance to static puncture (500 N or 128 lbf -ASTM D 4833)
- A good ageing behavior based on more than 40-year-old references, including observation on the upstream face of dams where the geomembrane had already been used. (Bianchi et al., 1979, and memo of French Ministry of Agriculture about Ospédale Dam, a 37 year old ICOLD dam in Corsica under very sunny and hot temperatures in summer with no loss of watertightness) Water permeability is always more than 10-13 m/sec.
Because transversal slope seams were not accepted, it was required to manufacture each individual roll with respect to its final position on the upstream face. This led to the fabrication of rolls longer than 100 m (330 ft), weighting in excess of 3 metric tons.
3.2 Laying and welding operations
Rolls were lifted and unrolled with a hydraulic beam carried by 20-ton track excavator. The hydraulic beam is a patented property of Colas Group and permits to roll or unroll rolls along the slope directly by the driver of the excavator.
BGM strips were laid side by side, with a 20-cm (8 in.) overlap of the seams. The high mass per unit area of the geomembrane reduced the risks of uplift by the wind and creation of wrinkles, which therefore eased up the welding operations.
3.3 Construction details
At the foot of the slope and along the whole periphery of the impervious face, the geomembrane was fastened to the reinforced concrete plinth, which is anchored into the rock foundation. The geomembrane was hot-welded on the concrete surface, which had been previously covered with a tack-primer mixture of bitumen and solvent, then anchored with stainless steel plates bolted into the plinth.
An anchorage by trench was placed at the top of the slope, of which the size and the distance from the slope were calculated to catch the total tangential effort all along the slope to not be taken by the interface shear strength.
3.4 Quality control
Quality control includes three operations:
- Visual control of the seams. Monitoring that the geomembrane overlap is at least 20 cm (8 inch). ES3 having a green indicator at this distance makes the visual control easy. Verifying the bitumen is melted enough to ooze out of the seam for at least 2 cm (0.8 inch) wide
- Monitoring the finish of the seams whereby a second crew melts the excess bitumen and smooths it with a trowel to ensure that the edge of the top geomembrane is welded
- Monitoring the seams with an ultrasound machine to verify that the weld is extended across the 20 cm (8 inch) of overlap. For this operation, two options are available:
- Option 1: spot control, an ultrasonic portable unit is dedicated to ultrasonic control the seam randomly.
- Option 2: continuous control using the CAC 94. [Breul et al., 1998] machine. The machine is based upon a measuring wheel, which includes 24 ultrasonic sensors that can detect gaps in the weld as little as 0.8 x 0.5 cm at the interface between the two geomembranes.
If a section of welds shows defects, a patch extending at least 20 cm (8 in) beyond the limits of defect is welded and checked again. Continuous checking of the seams were performed at the La Galaube Dam.
4. MECHANICAL COVERING OF THE GEOMEMBRANE
A mechanical covering was deemed necessary to protect the geomembrane against damages that could be caused by floating tree trunks (a lot of forests in this part of France).This reservoir is used for producing electricity while quickly changing the water level on the total height of the slope. For another dam, only part of the slope was covered. This protection consisted of cast in-place rectangular concrete slabs of 5m wide x 10m long x 10 cm thick (16 ft x 33 ft x 4 in). The concrete was reinforced with polypropylene fibers to prevent cracking.
The slabs were cast in-place on the slope in aluminum forms. The concrete was pumped into the forms with a maximum pumping distance set at 60 m (200 ft). Fluidity of the concrete was checked frequently to keep a compromise between its behavior on the slope, and its ability to be pumped. The concrete was vibrated and compacted using reciprocating vibrating drum pulled from the top of the slope. The joints between the concrete slabs within the range of the water level in the reservoir were filled with an elastomeric binder.
Other dams waterproofed with a bituminous geomembrane:
- In France
- Ortolo (37 m or 122 ft high) in Corsica waterproofed with a bituminous geomembrane. Challenges on site with strong winds and flooding during construction were overcome (Figure 7).
- Ospédale (25 m or 82 ft high) is a 38 year old ICOLD dam, which was the first high dam waterproofed with a geomembrane. The Ministry of Agriculture did a complete survey of this dam in 2008, after 30 years of life as it is required by ICOLD Organization. The results were good and very encouraging for using BGM for waterproofing the dam. There was no loss of imperviousness in almost 40 years (Figure 8).
- In Chile
- El Mauro dam in 2006, an RCC dam with an upstream face using a BGM on a slope of 0.7 H to 1 V. Geomembrane is exposed since 2006.
- In Peru
- Cerro Lindo was subjected to and survived an earthquake of 8.1 magnitude. The results of the audit done by Golder Associates on Cerro Lindo after the earthquake are presented.
The 2007 Chincha (Peru) earthquake that hit the central coast of Peru was characterized by a magnitude of 8.1 and resulted in ground shaking for about three minutes. The Peruvian government stated that 519 people were killed by the earthquake. The epicenter was approximately 20 km (13 miles) from the Cerro Lindo mine which is located east of Chincha. The peak ground acceleration recorded at Ica was 0.27 g. Though mining operations were stopped due to a power outage, no failures were observed immediately after the earthquake at the mine.
A review of the dam carried out by Golder Associates in 2012 showed that the BGM liner survived the earthquake without damage or change in properties. The performance of the BGM during dynamic loading was also confirmed recently in tests done at Precision Laboratories (Group TRI) in Los Angles. This positive behavior of the BGM under seismic loading is linked to the high strength of the geomembrane and also to the high interface shear strength that develops between the geomembrane and the adjacent material. This high interface shear strength was observed in recent laboratory testing conducted for a project of the City of Los Angeles Department of Water and Power (the City).The City was looking for a high strength geomembrane that would also exhibit high interface shear strength for inclusion into the foundation fill under two large buried concrete reservoirs. The reservoirs could be subjected to earthquake loadings during their economic life. Los Angeles is located in one of the highest seismic zones in the world.
5.1 La Galaube dam
Despite difficult site conditions due to autumn rains, the short five-month construction schedule was respected to carry out the project. The waterproofing structure was delivered in November 2000, which allowed the Owner to start filling the reservoir before winter, as shown in Figure 10. The La Galaube Dam is one of the tallest dams in the world, in which the upstream impervious face is based upon a bituminous geomembrane.
5.2 Other dams
These BGM installations were done at the satisfaction of the consultants (Golder Associates Santiago and Lima, and SRK Yellowknife) and the clients (Milpo and Canadian Government) on time and without any delay due to bad weather or strong winds.
In spite of the large earthquake at the Cerro Lindo mine, the BGM maintained its core function of waterproofing and there were no defects of seepage as a result of this. The BGM has maintained the reservoir in the dam at full capacity for the past eight years and there has been no loss of activity at the mine due to seepage.
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