In 2017 the excavation of 5 large rock caverns including an access tunnel started in Oslo, Norway. The caverns are 15–25 m wide, 18–20 m high and 127–198 m long. In total 300.000 m3 rock are excavated. This paper describes the planning and execution of pre-grouting at this site.

The rock caverns are located in the Oslo Field. The rock mass consists mainly of syenite (nordmarkite) with several intrusions of diabase and rhomb-porphyry including presence of swelling clay of variable thicknesses. Generally, the syenite is moderately jointed and has a low permeability. The properties of the diabase and rhomb-porphyry intrusions varies greatly between moderate to densely jointed with moderate to high permeability. A major weakness zone consisting of diabase and rhomb-porphyry is crossing four of the five rock caverns. The caverns are situated below the ground water level, and the main concern was lowering of the water level in nearby ponds and wet-area. Originally, a total leakage of 100 l/min was considered acceptable and a systematic pre-grouting of the largest caverns was planned. Later on, early on in the construction period, hydrogeological modelling was carried out. The modelling indicated little hydrological communication between the ponds/wet-area and the caverns and the level of acceptable leakage was increased to 300 l/min. Hence, systematic probe drilling ahead of tunnel face was performed in order to determine if pre-grouting should be executed.

Pre-grouting in the project has proved to be challenging for several reasons. The geology varied greatly and was hard to predict from MWD-data (Measurement While Drilling). Moreover, the geometry was complex with large cross-sections and several tunnel intersections. At times the contractor was working at 5 tunnel-faces at once which demanded frequent adaption of the pre-grouting design. In addition, the site is located near existing rock caverns, which have not been pre-grouted. The possibility of increased leakages here had to be considered when planning the pre-grouting in the new caverns.

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