More and more people are drawn to the cities. The Norwegian capital Oslo is no exception. However, the city only has one main source of drinking water.
Lake Maridalsvannet currently supplies 90 percent of Oslo Municipality’s population with drinking water. If there are problems with this source, the supply situation will no longer be adequate for the city and its growing population. A second source of supply is intended to prevent this: to secure the water supply, the city of Oslo is making its largest investment to date with the New Water Supply project.
The project consists of an untreated water tunnel, several galleries and caverns for the water treatment plant and a clean water tunnel. The untreated water tunnel and the clean water tunnel, almost 30 kilometers in total, will be excavated using three Herrenknecht Double Shields.
Oslo Municipality has commissioned two sub projects with two different contracts for the overall New Water Supply Oslo project: Skanska Norge AS handles the raw water tunnel, and JV AF Ghella ANS the clean water one. The particular challenge in this project lies above all in the hard Norwegian geology.
On the section between Holsfjorden and Huseby, two Double Shields are driving the 19-kilometer-long tunnel toward each other for contractor Skanska Norge AS. One of the machines starts from the west at Lake Holsfjorden, the other from the east in Huseby in Oslo. In the Oslo urban area, the AF Ghella joint venture is using a Herrenknecht Double Shield to construct the 11-kilometer-long freshwater tunnel from the new water treatment plant in Huseby to the city center of Oslo.
Both projects have long excavation distances, with a limited number of potential access points from the surface. Proximity to protected nature and recreation areas at Lake Holsfjorden and the high building density in the city of Oslo allow only for a small number of access tunnels. The minimal impact on the environment and the high level of occupational safety thanks to segmental lining carried out during the advance, coupled with good performance, convinced the contractors to use mechanized tunnelling technology.
In Scandinavia, very hard rock meets little topsoil, which can lead to groundwater being drawn into the tunnel during tunnelling. There is a risk of dewatering of the soil. For this reason there are strict regulations that prescribe regular injection drilling in the rock.
To meet these requirements, all three Double Shield TBMs were equipped with injection drilling equipment. While this is a solvable challenge for a Double Shield with a diameter of 7.02 meters, the machines with a diameter of 5.15 meters were a completely different story.
In close consultation with the customer, the Herrenknecht engineers found a solution: an additional boring shield was placed between the front shield and the gripper shield, integrating the injection drilling equipment. It injects cement into the rock to prevent the groundwater from flowing away. The result of this development: performance and safety go hand in hand.
Read more in All Dimensions.
