The 600-MW Kyogoku pumped-storage project is under construction on the Bihinai and Pepenai rivers in Japan. To ensure sealing of the upper reservoir of this project, owner Hokkaido Electric Power Co. is using an asphaltic lining. While asphaltic lining of reservoirs is not a new concept, this is the first time the company has used this type of lining for a dam or reservoir.
During installation of the asphaltic lining, Hokkaido Electric Power is employing two innovations that will save time and money. First, a foamed asphalt mixture is being used for the base layer of the lining, rather than regular asphalt. This represents the first time such a mixture has been applied at a dam. This mixture can be placed one and a half times faster than regular asphalt. For the second innovation, the upper impervious layer of lining will be placed in one thick layer to seal the reservoir, instead of the more typical approach in Japan of placing multiple layers. This is expected to cut construction time for placing this layer in half.
Choosing an asphaltic lining
The upper reservoir for the Kyogoku project is being built with three sides embanked with rock excavated from the site. To ensure adequate sealing of all surfaces inside the 180,000-square-meter reservoir, Hokkaido Electric Power needed to use a lining system.
In 2002, the company signed a contract with Kajima Corp. of Japan to construct the upper reservoir. Together with personnel from Kajima, Hokkaido Electric Power began investigating options for lining the upper reservoir. Both asphaltic and concrete linings were considered. Concrete lining is less expensive and is more durable under certain conditions than asphaltic lining. However, there are no design criteria in Japan for concrete lining of a reservoir. In this country, many reservoirs are lined with asphalt. For example, the 90.5-meter-high Yashio Dam, which impounds the upper reservoir for the 900-MW Shiobara pumped-storage project, is the highest dam in the world with an asphaltic lining. Because of established success with using this type of lining in Japan, it is easier to get legal permission to use an asphaltic lining than a concrete lining.
In addition, the results of seismic analysis at Kyogoku verified that an asphaltic lining could follow deformation during an earthquake, even at low temperatures in the winter. The seismic analyses indicated that a concrete lining would have to be very thick and contain a large amount of reinforcement. This would make the concrete lining far more expensive than the asphaltic lining.
For these reasons, Hokkaido Electric Power chose an asphaltic lining for the upper reservoir at Kyogoku.
In 2002, Hokkaido Electric Power and Kajima performed full-scale field trials of the proposed asphaltic lining to verify its mechanical properties, construction specifications, and placing procedures. Based on the results of these trials, the companies decided to proceed with placement of asphaltic lining at Kyogoku.
Placing the lining
The lining for the upper reservoir at Kyogoku is made of six layers. The first, and deepest, is a transition layer of crushed rock. The next layer, the base layer, is the foamed asphalt mixture. On top of that is a lower impervious layer of a dense graded asphalt mixture. The next layer is an intermediate drainage layer of an open graded asphalt mixture. On top of that is the upper impervious layer of a dense graded asphalt mixture, placed in one thick layer instead of two or three layers. The top layer is a protective layer of mastic asphalt.
In placing the lining, Kajima is using two innovations.
First, the company is using a foamed asphalt mixture, rather than regular asphalt, for the base layer during the paving work. The base layer is designed to adjust the roughness of the transition layer to obtain a uniformly flat finish, to maintain the thickness of the upper layers to be placed later, and to protect the transition layer against erosion due to avalanches or melting snow.
Foamed asphalt is made by spraying air and water into asphalt at a high temperature (about 150 degrees Celsius), which gives it a volume ten to 15 times greater than regular asphalt. It also has a lower viscosity than regular asphalt, which allows it to mix easily with wet aggregate and ensure the homogeneity of the mixture. Foamed asphalt is used in the base layer for roads but this is the first time it has been used at a dam.
During placement, the foamed asphalt does not fully cover coarse aggregate but instead adheres to the fine aggregate, which then is integrated with the coarse aggregate after compaction of the lining. This property of fully adhering to the aggregate means locally available materials can be used as aggregate, avoiding the extra cost and time involved in buying and transporting aggregate from a distant quarry. The aggregate for this work is being produced at an on-site plant using excavated rocks.
Kajima began placing the foamed asphalt mixture in 2005 as part of the initial excavation and embankment work. At the end of 2008, placement work for the foamed asphalt base layer was about 25 percent complete. On the area where the foamed asphalt has been placed, placement accuracy is within a vertical error of 25 millimeters. Placement of the remaining layers of the lining is scheduled to begin in 2010 and be complete in 2013.
For the second innovation, Hokkaido Electric Power and Kajima chose to place a single 8-centimeter-thick layer of asphalt for the upper impervious layer, rather than the more traditional approach of placing two or three layers of asphalt, each 5 centimeters thick. Hokkaido Electric Power chose the 8-centimeter thickness based on results of a finite element method analysis of the seismic behavior and safety of the embankment and asphaltic lining. Results showed that strain in the single layer during an assumed earthquake is lower than the tensile yield strain obtained in a laboratory indirect tensile test. These results confirm the safety of the upper impervious layer under the most severe design conditions.
Since the mid-1970s, a single thick layer of asphaltic lining has been used for reservoirs in Europe with a lining area of more than 100,000 square meters. However, the practice in Japan has been to place two or three 4- to 6-centimeter-thick layers of lining using road paving equipment. Recently, asphalt plants and equipment needed for asphaltic lining in the country have been improved to the extent where it is possible to place a single thick layer. This is the main reason why Hokkaido Electric Power chose to use the single thick layer for the upper impervious layer at Kyogoku.
Placement is proceeding on the asphaltic lining for the upper reservoir at the 600-MW Kyogoku pumped-storage project. Innovations being employed during placement of this lining will save time and money over the typical lining placement approach used in Japan.
To facilitate the work of placing this single thick layer, Kajima altered its traditional paving equipment. For example, instead of using a single asphalt tamper on the finishing equipment, the company will use double tampers to provide better compaction. This also will reduce compaction time for the layer. In addition, the company reduced the wheel load of the vehicle that will dump the asphalt for this layer, to avoid damaging the surface of the lower layer during placement. Use of this equipment was tested during a full-scale field trial at the site in 2008.
By opting for placement of this single thick layer, Hokkaido Electric Power will realize several benefits:
- Reduced construction costs by decreasing the amount of asphalt used. Cost for a single thick layer is about 80 percent of the cost for the traditional approach;
- Shortened construction time. Time for construction will be reduced by nearly half. The time savings will come from decreasing the time involved in placing two or more layers of asphalt for the upper impervious layer;
- Better placement quality. Quality will improve because temperature will drop more slowly in the single thick upper impervious layer. This allows the mixture to stay longer within the temperature range required for roller compaction, providing a higher quality of work; and
- Avoidance of the potential problem of blistering between layers, which could result in a penetrating hole or crack that would cause leakage.
Hokkaido Electric Power initially estimated it would cost 7.9 billion yen (US$85 million) to install the asphaltic lining for the upper reservoir at Kyogoku. Using the two innovations mentioned above, the company was able to reduce costs by about 2.9 billion yen (US$31 million) — a 37 percent cost savings!
— By Jun Takano, director, Hokkaido Electric Power Co. Inc., Kyogoku Project Office