Concern for the environment in sensitive ground conditions

Subject: Foundation work in a sensitive environment 

Location: Stockholm Central Station, train yard area 

Description of the project:

The Citybanan rail link project is made up of a number of subprojects. One of these involves the construction of a work tunnel in the track area at Stockholm Central Station. The ground conditions there are extremely difficult. Old filling material, pollutants and high groundwater present special challenges in terms of management and implementation.

The fact that the main line passes at a distance of 5 meters means that no settling was permissible under any circumstances. The contractor, Züblin–Oden Constructors HB, began work in the fall of 2006 with jet grouting to seal the soil between the rock and the sheet piling, because of the strict requirements imposed on the leakage of groundwater. A total of 500 holes were drilled to a depth of between 10 and 20 meters.

What part did water drilling play in the best alternative for this project?

“By far the most important reason is that traditional pneumatic drilling in this environment is directly inappropriate. The ground consists of filling on top of loose clay with a thin layer of moraine. The risk of causing settling is always present when drilling into soils with traditional pneumatic hammers. This could result in serious damage to both the track area and surrounding buildings. That is why the Wassara system is the alternative for us. We carried out continuous measurements, and we failed to reveal any settling. The work progressed smoothly and easily, and the equipment functioned perfectly,” says Vesa Zajdel, site manager for Züblin-Oden Constructors HB.