Citybanen Stockholm

Stockholm City Line is a 6 km-long double-track commuter train tunnel beneath central Stockholm, Sweden. Since the line opened end of 2017 long distance trains and commuter trains operate from different tracks so that the capacity is doubled and rail traffic congestion is reduced significantly. Stockholm City Line passes for the most part through rock, but in the waters of Riddarfjärden between Riddarholmen and Söder Mälarstrand it runs through a submerged concrete tunnel.

The Stockholm City Line was one of the biggest projects Rockdelta did in Sweden. In the extensive tender phase the calculation and prediction tools Rockdelta has developed proved extremely helpful in providing the necessary data and verifications. The attenuation of ground-borne vibration had a high priority in the project as the granite through which the tunnel was blasted (using dynamite) transmits much more vibration than sand, lime or other types of soil. The comfort of the people in and above the tunnel had to be considered. Also, a famous historical church could be affected by the ground-borne vibration.

One of the challenges was to find a happy medium between the demand of the engineers for minimal deflection of the rails and the acousticians demand for the required vibration attenuation.

The building site was situated in the busy center of Stockholm. All building materials had to be delivered (JIT) to a storage site 50 km south of the city from where they were distributed further. Construction materials were often stored on this open-air site for months, undergoing the harsh Swedish weather conditions – including frost, heavy rain and snow storms.

In the meanwhile, the conditions inside the tunnel were sometimes quite extreme. Large fans were used to refresh the air, meaning that in winter times temperatures sunk to well below zero. In view of the tight schedule it was important to work with construction materials that could be installed under these conditions without special requirements.

A challenge on another level was the fact that projects of this kind invariably are confronted with delays, however strict the planning and schedule may be. Rockdelta staff visits construction sites regularly to give advise or tips for the installation of the mats and to be available to answer any questions that may arise during the process. Timing these visits proved challenging sometimes.

Installing Rockdelta resilient mats vibration attenuation of 20 dB was achieved in the mean frequency between 63 and 125 Hz. In the main part of the tunnel mats with a thickness of 50 mm were installed at the base of the rail track superstructure. In the section underneath the church mats with a thickness of 85 mm were applied. The implementation of the Rockdelta mats resulted in effective protection of the historic church from structural damage by ground borne vibration. The comfort of passengers and personnel is guaranteed as well as that of the people in the buildings above the tunnel.

The Rockdelta solution brought with it several advantages for this project. The 60.000 m² of natural stone wool mats that were used are made from fire-safe, non-flammable stone wool.

Fire safety

Fire-safety was a major issue, especially in view of the use of dynamite to blast the tunnel. Sometimes the rubber elements with which the tunnel is protected near the blasting area catch fire and may cause tunnel fires. The non-flammable Rockdelta mats were safe to store and use inside the tunnel at any moment.

Ease of installation

The Rockdelta mats are very easy to handle and easy to install, no special tools are needed, and no welding, gluing or taping is required. This proved a big advantage in the confined tunnel space and sometimes difficult working conditions. Thanks to this ease of installation, the use of Rockdelta was never cause of delays and put no additional pressure on the tight schedule.

Rockdelta resilient mats can be installed on an uneven surface without the risk of the material being punctured by stones or sharp edges. Therefore, the granite bottom did not have to be flattened completely, saving much time and money.

Robust Product-Weather proof

The stone wool mats are insensitive to weather conditions. During storage and installing sub-zero or high ambient temperatures and moisture do not influence the intrinsic properties or the processability of the mats.

Robustness-Mechanical resilience

Once installed and covered with ballast stones the mats were required to be able to bear the load of trucks and lorries for internal logistics. Rockdelta met this requirement easily as the mats can bear up to 20 tons per m².

Rockdelta mats are maintenance free and their tested and certified life expectancy of more than 50 years amply exceeds the 40 years required for this project.