The expansion of Lippulaiva is a construction project located in Espoonlahti, Espoo. Existing Lippulaiva shopping center will be demolished and the land area will be excavated simultaneously with the extension of Helsinki metro line – west metro. Espoonlahti metro station will be connected to the new shopping center with two shafts and the station hall is located below the shopping center with minimum seven meters of rock overburden. In September 2017 excavation and reinforcement of the metro station are ongoing. The surface excavation on top of metro station hall starts after demolition of the existing shopping center.
Locating two major excavation and construction projects in the same small land property simultaneously requires a lot of cooperation and good will from engaged parties. Several construction concepts were studied and negotiated to integrate the visions and goals of both parties. Rock mechanical simulations were used to identify viable concepts for future development and discard the ones that either were technically too risky or did not produce sufficient value for the investment involved.
Characteristic to the areal geology is igneous granite with very little cleavage and amphibolite with some cleavage. The rock mass is mainly jointed non-systematically. The geological conditions at the site play a major role in the design process as the area encompasses several weakness zones, some of which pass through critical sections of the excavations, such as the shaft openings. These geological features have dictated some design choices during the property development as some excavation combinations were not considered feasible after detailed investigations and rock mechanical simulations.
Most of west metro tunnels will be excavated and reinforced prior to the excavations of the shopping center. This means that significant measures are taken to ensure that the shopping center excavations or foundation loads do not damage the existing underground spaces of the metro line or the installed reinforcement structures. To increase the challenge, the in situ- stress measurements done at the site were inconclusive – there is a big variation in the results of the major horizontal stress component magnitude at the metro station level varying from 1-12 MPa. Also, the orientation of the stress field is unclear. This uncertainty was mitigated by running several rock mechanical simulations with varying stress fields to find out the best and worst case scenarios.