The design of Länsimetro, the westward extension of the Helsinki–Espoo metro line, presented a wide range of challenging features. These special characteristics required numerical three-dimensional calculations to predict the rock mechanical behaviour of the rock mass. Special attention has been paid to the Keilaniemi station, designed with spans up to 25 m and located next to an existing building and below four yet to be built towers up to 120 m high. Furthermore, in some wide sections of the station, the estimated rock roof is as low as 3 m at its maximum. The calculation took into account the complex geological structures in the area, consisting mainly of blocky hard rock mass with overall Q' value up to 15, but with some weakness zones in which Q' is less than 1. The 3-D elastoplastic model was created considering also the contacts that allow slip and separation, in order to obtain the overall behaviour of the excavations. To find the most likely behaviour the calculation was carried out with multiple excavation stages. For the most problematic areas 2-D FEM simulations were used to simulate the behaviour of the rock mass with a joint pattern almost corresponding to the real one.

1.1 The Länsimetro project

The Länsimetro extension connects to the Helsinki metro system in Ruoholahti and continues through Lauttasaari, Koivusaari, Keilaniemi, Otaniemi, Tapiola, and Urheilupuisto to Matinkylä (Fig. 1). In the current construction stage, the planned provision for a station in Niittykumpu has been decided to be constructed. In total, Länsimetro will be 13.9 kilometres long with the stations in locations with a sufficient amount of users. As Länsimetro is completed, two lines will be in use: Matinkylä–Mellunmäki and Tapiola–Vuosaari. During rush-hours, both lines will be run at five-minute intervals; otherwise, at ten-minute intervals.

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