Hailuoto Causeway

Image courtesy: Finnish Transport Infrastructure Agency

Hailuoto is an island in the northern Baltic Sea close to city of Oulu in Finland. There are about 1000 inhabitants in Hailuoto. Current access to Hailuoto is arranged with road ferries. The ferry route is about 6.8 km long and it takes about 25 minutes.

Road access to Hailuoto has been studied since the early 1980s. Current general plan was made in 2016 and the HAIKI-alliance for the project was formed in 2022. The construction began in May 2024 and it will be completed in 2026. Economically the road access in quite expensive, but on the other hand both of the ferries are getting old and they need to be replaced with new ones. Also the continuous operating costs of the ferries are not low.

The Hailuoto causeway will have 8.4 km of new road, of which 7.7 km will be on sea. Mainly the causeway is made by making an embankment of blasted and crushed rock, but there will be also two new bridges. Bridge S1 is called Huikun silta, it will be a 9-span continuous steel-concrete composite girder bridge with total length of 767 m and a headroom of 18 m. Bridge S2 is called Riutun silta, it will be a 10-span continuous prestressed concrete girder bridge with total length of 737 m and a headroom of 5 m.

Geotechnical investigations for the bridge locations were made from the ice during the winter. Image courtesy: AFRY. 

Installation of piles for bridge S1 from temporary work platform. The ferry at Hailuoto end of the route at the back.  Image courtesy: AFRY.

Both bridges are founded on driven steel piles. The piles are driven through soft sediment layers at the seabed into loose/medium dense sand layer. The corrosion conditions of the sediment layer are unusual. The chloride content, sulphate content and electrical conductivity are above the Finland’s national permissible limits for bridges. Due to this the steel piles are functioning only as formwork and the final situation is designed as reinforced concrete structure. The design service life of the bridges is 100 years.

Bridge S1 has pile sizes RR600/12.5 at the abutments and RR900/14.2 at the other supports. both of the abutments have 12 piles and the other supports have 8 to 10 piles each. The piles were installed from a temporary work platform. The installation was done with 9 ton hydraulic hammer and the PDA measurements were taken with a 10 ton free-fall hammer. The maximum pile loads are P_d = 7.63 MN compression and T_d = 1.48 MN tension.

Before the actual work, two test piles were installed to mainland side shore of the designed causeway. The aim was to determine the penetration rate of the piles and also the resistance of the piles by PDA measurements. Despite the experiences gained from the test piling, the actual piling was found extremely slow.  From the beginning of the installation, the penetration of the piles into the layer interpreted as medium dense was difficult. The number of blows was up to thousands per meter. RR1200 piles consistently fell short of the original target level. Few different methods were tested to improve the penetration of the piles, for example pre-drilling. Use of pre-drilling made the penetration of the piles little faster, but still the number of impacts remained high.

After challenges with pile penetration, the piles were designed to be shorter. Based on the compression resistance (PDA) and tensile resistance (CAPWAP) as well as the horizontal stiffness of the foundation, the number of piles was increased by 2-4 additional piles/support. According to the geotechnical investigations made after the piling, the subsoil (sand) compacted significantly more than estimated from the test piling.

Installation of piles for bridge S2 from temporary embankment. Between the piled supports the formwork for the bridge deck was also made on the embankment. Image courtesy: AFRY.