NGI has long experience in the anchoring of foundations and bridge structures in rock. An exciting project in this field of expertise is the construction of a stressed ribbon bridge over Fantesteinsvatn at the Sognefjellshytta in Jotunheimen. The bridge has been built to provide access to the surrounding mountain area for wheelchair users and others with reduced mobility.

The client for the bridge is the National Park Administration in Oppland municipality, subordinate to the Norwegian Environment Agency. NGI has provided civil engineering consultancy and design/dimensioning of the anchorage chambers.

Bridge design

The stressed ribbon bridge across Fantesteinsvatn is in many ways a simple suspension bridge. There are only 50 or so such bridges in the world, most of them walkway and bicycle bridges. The bridge has a total length of 52 m, with a span of 47 m. The bridge must withstand an enormous tension force. With maximum snowfall and lowest temperature, the tensile force on each foundation is equal to 11,000 kN, which equals 1100 tons!

The extreme horizontal forces acting on the anchor chambers require that each of these be solidly anchored. This is achieved using six strand anchors. The anchors pass through the chambers, slope backwards and down into the bedrock through boreholes with a diameter of 200 mm. Each anchor has 24 strands with a cross-section area of 150 mm2. The total length is 18 m, the free length is 12 m and the bonded length is 6 m. Each anchor is subjected to an acceptance test load and locked off at a load of 4000 kN.

NGI Bru over Fantesteinsvatn2

 
Follow-up during tensioning

NGI participated in the follow-up of the construction work. Before installing the anchors, the bedrock was injected with grout to seal open cracks through an independent set of boreholes. The grouting revealed several open joints in the rock mass. As a result, grout was leaking out in the lake.

After installation of the anchors, mortar used for bonding, was left to harden for three weeks prior to testing and final tensioning. After the anchors were fully anchored in the underlying rock mass, work began on tensioning the bridge deck. Horizontal displacement of the anchor chambers was measured continuously throughout the process. The displacements were about 0.6 mm on the east side and about 1.0 mm on the west side. The rock mass on the west side is more jointed and more massive than on the east side.

A lot of heavy equipment was used during construction work. To minimize the external environmental impact on this pristine area, equipment and other materials were either transported to the site on the snow and ice-covered lake during the winter or rafted from a ramp at Sognefjellshytta to the two anchor chambers.