The Nykirke railroad crossing

The Nykirke railroad crossing (1,5km) is a part of a project to modernize the railroad in Vestfold county, Norway. The project was implemented as a total enterprise after a target price contract with incentive.

The Vestfold railroad is crossing the Tangen creek on a 20 m high landfill on quick clay. The settlements were achieved during the construction phase by use of vertical drains and a landfill with extra height which was removed after 8 months. This solution resulted in construction savings of NOK 5-6 million compared to the design originally planned with piles down to bedrock.

The terrain along the tracks is very hilly and includes almost 40m of height difference. This factor combined with the fact that the railroad in use throughout the construction period was adjacent to the construction site, offered severe challenges when planning, designing and constructing the railroad crossing.

The track intersects the Tangen creek on a 20m high landfill. Strict requirements were given for settlements of the finished tracks and stability of the landfill, both during construction and in the lifetime for the track.

The soil conditions
The soil conditions in the area is dominated by marine clay deposits down to about 40 m depth. In meny parts, as along the Tangen creek, the clay is very sensitive (quick clay).

Fig. 1 Original solution with landfill resting on a foundation of
driven piles to bedrock.

Fig. 2 The contractor, Veidekke, solution with vertical drains
combined with temporary preloading to strengthen the clay.

The results from the soil investigations performed in the initial design phase were strongly influenced by sample disturbance, and did therefore not give a representative picture of the soil strength and deformation properties. This was revealed already in the tendering process from interpretation of two CPTU tests which were made in the bottom of the Tangen creek.

In the final design phase these assumptions were confirmed by soil sampling with a Ø250mm block test sampler and subsequent advanced laboratory testing on samples of very high quality. These tests confirmed the assumption that the ground elevation in the creek was eroded down from a higher original terrain level (seabed) and that the soil properties reflects this earlier level. The soil in the area is therefore overconsolidated.

The landfill
In the tendering process, the Norwegian National Rail Administration suggested a solution where the tracks would cross the Tangen creek by constructing a landfill on driven concrete piles down to bedrock. Fig.1.

With new upgraded geotechnical parameters, the analysis showed that the requirements on settlements and stability would be met by choosing a simpler and less expensive foundation design consisting of vertical drains combined with preloading. Prefabricated vertical drains were installed down to bedrock, see fig 3. Subsequently a rock landfill was constructed on top, with compaction in layers of 1.5 m, up to 3 m above the final level. After 8 months the overheight was removed and the superstructure added, followed by new tracks.

Great Savings
The development of settlements was continuously monitored. In addition measurements of the pore pressure in the clay underneath the landfill were also made. The total amount of settlement was a little less than what was expected from predictions, but the development over time was as expected. After removal of the overheight the settlements flattened out and stopped.

This project was good example of how important decisions concerning "correct" geotechnical parameters are for choosing for the optimum solutions both technically and economically. The final design represented 10-12 million NOK (20%) cost reduction of the entire project.

Vertical drains were installed with a special rig from a layer of fiber cloth and gravel, down to 20-22 m depth