The western metro extension in Helsinki (known locally as Lansimetro) is a 7km addition to the 35km Helsinki metro in the Finnish capital. Stretching from Matinkylä to Kivenlahti it will include five new underground stations: Finnoo, Kaitaa, Soukka, Espoonlahti and Kivenlahti and a new underground train depot at Sammalvuori.
The new stretch of line will be completely underground as a twin tube tunnel with one train track per each tunnel. This new section is scheduled to be complete in 2023 at a cost of EUR 1.1bn (USD 1.25bn).
This link builds on an earlier 14km extension with eight new stations from Ruoholahti to Matinkylä.
Construction for the West Metro’s phase 2, the Matinkylä–Kivenlahti section, began in 2014 and the excavation works are now fully complete. Other construction works started early this year within the stations and tunnels including the construction of platforms, installations of HVAC, safety, electrical, railway equipment, testing and commissioning.
GRK is executing the construction works at Finnoo station and associated tunnels, while YIT and ARE are doing the construction works at Kaitaa station and Soukka station. Espoonlahti metro station’s construction works are being executed by SRV. Skanska has the contract for the Kivenlahti metro station’s construction. Contractors will hand over the work in 2022–2023, followed by joint use tests and official inspections. The operator, Helsinki City Transport, will take over the stations and the track in 2023 in preparation for launching passenger transport.
Pöyry’s principal designer of rock engineering Jukka Salminen explains that they have carried out a number of geological investigations.
The consortium responsible for geological investigations involved engineers from Taratest, SMOY, Pöyry Finland and City of Espoo.
The rock quality was assessed with 60 core samples along 5.5km within 7km of the tunnel. At each station the rock stress has been measured in addition to making 3D rock mechanical simulations. The composition of rock is mainly granite or granodiorite, in some sections there is also quartz-feldspargneiss.
There are also fault zones along the metro line. The twin tube tunnel is 6.2m wide, 6.37m high and the cross section is 36.2sqm and the rock pillar between tunnels is 10-11m. The excavation method adopted is drill-and-blast, the staple of Finland. “This method is good for projects with typical Finnish rock quality, with underground structures of different dimensions, as well as in the presence of cross-sections, and with a limited schedule,” says Salminen.
The drilling depth at the tunnel excavation is 5.0m but in case of poor rock quality it is 3.0m. The advance rate at each blast is approximately the same as the drilling depth. An example of drilling jumbo used at the tunnel excavation is Atlas Copco XE3C-30.
Before excavation, the rock was pre-grouted with a cementwater mix and then supported with VikØrsta CT-bolts M22 and sprayed shotcrete.
In some sections wire mesh has also been used, and in others the rock was supported before excavation with spiling bolts. Temporary structures were also needed to guarantee a stable and safe working environment in other areas.
B5000B rock bolts with a diameter of 25mm and a length between 3.0 and 6.0m were grouted to bore hole with concrete.
They were also zinc coated and plated with epoxy to ensure a structural lifetime of 100 years. The shotcrete was reinforced with polymer fibres for the same reason. Sometimes CT-bolts are also used for final rock support. In some cases of poor rock quality, also self-drilling anchors are used for rock support like Titan Ischebeck 40/16 or equivalent length varies between 3.0m and 16.0m.
The ground support is based on the Q system. “Before the tender phase we estimated the rock quality, based on core samples and geological mappings,” Salminen says. “During excavation, a re-assesment of the rock quality along the tunnel was done on site. The bolt spacing varies from between 1m and 2.5m and shotcrete thickness varies from 40mm to 200mm accordingly. No final lining and no thermal isolation was needed as the whole section was underground and the temperature is above 5°C.
“We didn’t use any segments, only shotcrete and rockbolts. However, we didn’t meet any issues during the excavation, as the rock quality was quite similar as found during investigations so we didn’t have any surprises. We were well prepared on the geological conditions. Geological conditions encountered were quite similar to the existing line.”
Salminen explains that they had to do some open cut excavations in areas with thick soil layers of clay at technical shafts and metro station entrances. “These were supported by bored piles,” he says. “It was a good solution because bored piles were also used for the permanent structure.”
All the excavated material was transported through the access tunnel at each station. Some rock masses were stored in a landfill, and some were crushed and used on bearing course. Several stations, entrances, shafts and metro line are located in a dense urban area. Existing buildings, structures and wells were mapped and taken into account during the design phase.
Approximately 200 groundwater pipes were installed along the metro line to monitor ground water levels in soil layers and in rock. Some 1,200 settlement measure points were installed to existing buildings located near metro’s underground spaces. During the excavation phase existing settlements and ground water levels were measured regularly. In some sections the measurement was made weekly, others monthly or four times of year, depending on the location and phase of the excavation works. The results of these measurements were taken into account on excavation works. For example, supplementary grouting rounds were performed to improve tunnel’s water tightness in certain areas. Groundwater recharging systems were installed to ensure certain level of groundwater table to avoid any change at settlement levels.
Salminen adds: “We also have a ground monitoring system in every station, largely involving extensometers installed in rock and monitoring points in concrete structures. Noise and vibrations restrictions were present in several areas. Buildings and equipment, such as computers, were investigated and taken into account in the blasting and excavation design to prevent any damages. Several devices were isolated against vibrations. In some places blasting and excavation works were carried out at that time, when sensitive equipment was not in use.
The waterproofing is based on pre-grouting, post-grouting, shotcrete and shotcrete drainages, which are installed under the actual shotcrete layer, and will lead the possible water away from there. This is a commonly used method at drill-and-blast tunnels in Finland. Before excavation rock is pre-grouted with cement, water or a mixture of them with polyurethane.
If leakages are found after excavation, either they are post-grouted with fine cement and polyurethane, or shotcrete drainages are installed to each leaking point. With shotcrete drainages water leakages are lead to drainage pipes at tunnel floor.
In the final phase tunnel crown and walls are covered with shotcrete. In some sections watertightness-improving admixtures were added at the batching plant.
