As early as the 1950s, developers in Iceland proposed that three glacial rivers in the east of the country be harnessed for electricity. In 2002, the Icelandic Parliament approved a project to generate hydroelectric power from two of the rivers.
Beginning in 2003, three dams were constructed to create the Hálslón Reservoir. With a surface area about the same as Manhattan, the reservoir provides seasonal storage of 74 billion cubic feet of water (2.1 billion cubic meters). Water from Hálslón and two smaller reservoirs passes through 45 miles (72 kilometers) of underground water tunnels and down a 1,380-foot (421-meter) vertical penstock.
As lead consultant for a joint venture, Mott MacDonald directed an integrated team of 50 people providing expertise in engineering and construction supervision. A challenging combination of volcanic and glacial geology required daily mapping by tunnel engineers.
Mott MacDonald developed an advanced database to manage the large amount of data generated by up to seven simultaneous tunnel boring and drill-and-blast drives.
About 31 miles (50 kilometers) of tunnel were drilled by three tunnel boring machines, a method never used before in Iceland. The remaining 14 miles (22 kilometers) were completed with conventional drilling and blasting. Geologic faults more than 30 feet (9 meters) wide required stabilizing measures including grout injections, foam and concrete in front of the cutterhead, and steel support ribs behind the head.
The Kárahnjúkar Hydropower Plant now supplies 690 megawatts of power to a single customer, an aluminum smelting plant on the eastern coast of Iceland.