The station had a large span of 16–22 m without any pillar. At both ends of the station the overburden was only 6–8 m of partly poor rock. In order to achieve a safe construction of the cavern an observation system based on deformation measurement was installed. The results from the measurements and finite element calculations were the base for the choice of support. The measured high horizontal stress field around 10 times higher than the vertical overburden was found to have a stabilizing effect of the cavern. The deformation pattern showed an uplift of the roof and inward movements of the walls of around 1 mm. The results indicated that the rock mass behaviour could be interpreted as an elastic response. The rock support was therefore carried out with single bolts and a 10 cm layer of ''shotcrete instead of the planned concrete arches.
Die U-Bahn Station hat eine Spannweite von 16–22 m ohne Zwischenpfeiler. An beiden Enden der Station betrug die Überdeckung aus teilweise schlechtem Fels nur 6–8 m. Um einen sicheren Bau der Kaverne zu gewahrleisten, wurde ein Messprogramm auf der Grundlage von Verformungsmessungen eingerichtet. Die Ergebnisse der Messungen und einer FEM Berechnung wurden der Wahl der Sicherungen zugrundegelegt. Das gemessene horizon tale Spannungsfeld, das den 10 fachen vertikalen Spannungen entsprach, hatte einen stabilisierenden Effekt auf die Kaverne. Das Verformungsbild zeigte eine Hebung der Firste und eine nach innen gerichtete Verformung der Ulmen von ungefahr 1mm. Die Ergebnisse zeigten, dass das Gebirgsverhalten als elastisch angesehen werden konnte. Infolgedessen wurde die Gebirgssicherung mit Einzelankern und einer 10 cm erndicken Spritzbetonschale durchgefuehrt anstatt der geplanten Betonbögen.
Sans les piliers, la station a une grande portee de 16 à 22m. Aux deux extremites de la station Ie terrain de recouvrement, constitue de roche en partie pauvre, etait seulement de 6 à 8 m. Afin de proceder à une construction sûre de la caverne, un systeme d''observation base sur Ie mesurage de la deformation y etait installe. Les resultats des''mesurages et des calculs d''elements finis etaient à la base du choix du renforcement. Le champ d''une haute contrainte horizontale mesure, environ 10 fois plus eleve que Ie terrain de recouvrement vertical, à ete adopte pour produire un effet de stabilisation de la caverne. Le modele de la deformation montrait une elevation du toit et des deplacements internes de 1 mm environ. Les resultats indiquaient que Ie comportement de la masse rocheuse pourrait être interprete comme une reaction elastique. Par consequent, au lieu des arches prevues, Ie renforcement de la roche a ete execute à l''aide de simples boulons et d''une couche de 10 cm de beton projete.
One part of the Stockholms Metro system called "Jarvabanans södra gren" is recently built with underground stations in rock with a single cavern with a span between 16–22 m and without any pillar. The older stations were built with twin cavern with a row of pillars between (Benedik 1982).
The stations are built as close as possible to the rock surface in order to reduce the cost for the elevator shafts. The rock mechanics problem is the stability of a tunnel with a large span in a jointed hard rock with a small rock overburden. At one of the stations, Huvudsta, the rock overburden was only 6–8 m and the rock was also poor in some parts of station.
The Stockholms City Streets and Traffic Administration had two choices in handling the routine way of using the problem. One was more or less heavy concrete arches immediately following after the blasting and the other one was to install an observation system in order to guide the choice of the rock support and, if possible, reduce the support to bolting and shotcreting.
The last system was chosen and the measuring system and the measurement. results with the rock mechanical discussions will be presented here. A more detailed report will be found in Stille, Olsson, Delin (1981).
The underground station Huvudsta had a "trumpet" shape with a span of 14 to 22 m narrowing to the east. The rock overburden is small, around 8 m at the both ends of the station. At the eastern part there is a gas station with tanks buried in the soil and vulnerable to settlements and at the western part there is a road bridge founded on the rock surface (see fig. 1).
