The renovation project for the City Hall block along Linnankatu in downtown Turku is one of the most significant construction projects in the area. Renovation and repair work on a building stock that is over 200 years old presents both technical challenges and a unique opportunity to gain experience in foundation work at an archaeologically significant site. The City of Turku’s facility services is the project’s client and developer.
We are carrying out foundation reinforcement work on a total of five buildings in the Turku City Hall block using bored piles and driven piles, grouting of stone foundations, and many load transfers via concrete and steel beams as well as cast-in-place structures.
The old foundations will be rebuilt using driven and bored piles
The foundations of Turku City Hall and the buildings in its courtyard are over 200 years old in some places. The drop in groundwater levels has contributed to rot damage in wooden piles and log structures, making it necessary to replace the foundations.
The new foundations will be constructed using bored and driven piles, with pile lengths that are exceptionally long—up to 45–65 meters. In bored pile construction, the piles are driven into the bedrock and welded together in sections of approximately 1.2 meters. It takes an average of two days to install a single pile.
There are approximately 180 piles in the main building of the reinforcement site, including the basement; approximately 150 in the two courtyard buildings; approximately 50 in the side building; and approximately 50 in the small side house to be constructed in the final phase. The total number of piles will thus exceed 400, and the total length of the piles will approach 20 kilometres.
Caption: The bored piles used in basements are mainly of the RD140-10 size. The pile sections are 120 cm long and are welded together as drilling progresses, resulting in a total length of 45–65 meters.
In addition to pile length, pile driving presents additional challenges, such as slurry management. At the construction site, slurry pits are used for slurry management, where solids are separated from the water before the water is discharged.
–Piling has gotten off to a good start, and the piles have been completed on schedule. The excavation work for the foundation walls required for the load-transferring structures is well ahead of the piling work, and some of the load-transferring structures have already been reinforced, says Aatu Hento, construction manager for KFS Finland on the project.
Caption: The first bored piles were installed in the basement of the annex on March 11, 2026. A total of approximately 41 piles will be installed in the basement.
Structural surprises are ordinary
The demolition and renovation work on the old building keeps revealing surprises. The structures and elevation levels differ from the original plans, and, for example, the depths of the foundations do not always match the assumptions. This highlights the site’s ability to respond quickly to changing situations.
–Settlement during construction is closely monitored, as pile driving can affect the delicate, historic building stock. That’s why we use continuous monitoring and expert supervision, Hento explains.
The client is responsible for monitoring settlement and conducting vibration measurements at the site, while the contractor performs daily levelling checks during construction and conducts pile quality inspections, such as straightness checks and inspections of joint welds, as usual.
–As a contractor, we are always prepared for any unexpected settlement by having various support structures in place, and we are able to respond quickly by taking the necessary measures, Hento summarizes.
Working in basement spaces brings its own unique challenges to the project. Among other things, water management plays a key role in bored pile construction and when working in basement areas, as water and clay are constantly brought to the surface during drilling.
Approximately 41 piles will be installed in the basement of the building on Linnankatu, and the aim is to position them as precisely as possible within the wall lines. This ensures that the interior spaces remain functional and prevents unnecessary stress on the structures.
Construction site logistics are challenging overall, especially in the main building, where work is carried out at depth and in confined spaces.
–We carefully plan how to transport the pile-driving equipment into tight spaces. Walls and other delicate surfaces will be protected, and walkways will be built for the machines in the narrowest spaces, Hento continues.
Injections to reinforce the overall structure
In addition to piling, grouting work will be carried out at the site to reinforce old stone foundations in particular prior to piling. The work phases overlap and require careful planning and execution.
–Based on the structure of the old stone foundations, we can also use the method of casting the foundation in a single pour—known as ‘mantling’—using structural concrete, which allows us to transform foundations with large voids into a structure that is as solid as possible, explains Olli Salviander, site manager at KFS Finland.
Caption: Aatu Hento (left) serves as the construction manager for KFS Finland’s work on the Turku City Hall project, and Olli Salviander as the site manager for KFS Finland’s work.
Working together to create a complete and exceptionally strong neighborhood
The project emphasizes collaboration among various stakeholders—seamless cooperation between the client, contractors, archaeologists, and experts is essential for a successful outcome.
The staff working on-site are committed to a long-term project in which each phase lays the groundwork for the next.
–With an old building, you always have to be prepared for surprises, Hento says concisely.
Good teamwork is evident on the construction site, for example, in the form of quick decisions when the situation calls for them. Everyone on the construction site is strongly committed to working together to stay on schedule, deliver high-quality construction, and take pride in their work.
–Our collaboration with KFS runs smoothly, and we are very pleased to have been able to bring them on board for this challenging project. The schedule is tight, and many surprises have come to light in the structures during demolition and excavation work. Close cooperation among the project partners has enabled a cost-effective solution to the problems without compromising on quality, and the project has remained on schedule. KFS has demonstrated a strong commitment to advancing the project and has played a key role in developing structural solutions. Cooperation plays a crucial role in ensuring a successful outcome in a demanding renovation project like this, commented Michael Pettersson, project manager at Rakennustoimisto Laamo Oy.
Mari Virtanen, the project manager from the City of Turku, is also pleased to have secured a local and skilled contractor for the foundation reinforcement work on this exceptionally demanding project. Weekly meetings and discussions among the project team are moving the project forward step by step. Although we have occasionally had to make changes to our procedures and plans on short notice, these changes are always based on a joint decision supported by sound reasoning. The project also involves a third party, the plan reviewers, whose involvement is a requirement for obtaining a building permit.
Caption: The work phases are coordinated, and once the archaeologist has visited the site, work can begin.
Construction in a historic setting also requires close collaboration with archaeologists. There is a team of 10–15 archaeologists working at the site, and progress is determined by the findings of their research: first, the structures are demolished; then archaeologists examine the site; after that, permission is granted to continue construction. Archaeological supervision also extends to excavation work, and the project requires constant coordination among the various parties involved.
Rakennustoimisto Laamo Oy is the client for KFS Finland, and the project is proceeding in phases. The contract for KFS Finland began in December 2025.