Hamburger Börs hotel in the city of Turku will be founded on SSAB’s high strength RRs220/12.5 piles made of S550J2H steel grade. Piling work with pile lengths up to 50 m has now been accomplished with good results.

Hamburger Börs hotel is traditional and famous hotel in the heart of city of Turku. Hotel consists of old building which has been built already in 1895 and it’s now a listed building. Hotel also had a newer building which had been built in 1970’s. This newer building has now been demolished and totally new hotel will rise to same location as the old one.

Confined space and surrounding buildings defined pile type

Location of the construction site is in city center and it’s surrounded by old buildings. Also old foundation structures of the demolished old building have influence on selection of suitable piling system and pile type.

Even though the old hotel building and its structures were demolished, old piles were left to the ground. These old piles together with foundations of surrounding buildings defined the new pile type. Old piles are cast-in-place concrete piles and they extend all the way to the bedrock. Also some even older wooden piles were found from the ground. Most of the surrounding buildings are piled, but with several different pile types and one old building nearby is founded with timber grillage on clay. Pile types of surrounding buildings include driven concrete piles, drilled steel pipe piles, cohesive wooden piles and jacked steel pipe piles. Some of the buildings have original piles and some of them have been underpinned. In addition to all these surrounding old buildings, the construction site and excavation of new parking facility, Turun Toriparkki, is just across the street.

Use of drilled piles might have a risk to lower the end bearing resistance of old driven piles, if the piles would be installed close to the toe levels of the existing piles. Due to this risk the designer chose driven steel piles.

In dense urban areas the displacement of soil due to piling can cause damages to surrounding buildings and structures. This can be eliminated by using slender piles with high resistances. To eliminate these risks designer decided to use SSAB’s RRs220/12.5 piles made of high strength S550J2H steel grade. RRs220/12.5 piles have mechanical splices which are huge benefit when piles are long like in this project.

Versatile monitoring through piling work

Demanding surroundings also induced a need for versatile monitoring during piling work. Surrounding buildings naturally were monitored for vibrations and displacements, but also monitoring for pore pressures was utilized.

Monitoring showed no results worth mentioning, for example pore pressures had only minimal changes during piling. Also displacements and vibrations of surrounding buildings were minimal.

Sauli Maanpää, geotechnical designer at SM Maanpää Oy promotes high strength steel piles: “Piling work didn’t cause any damages to surroundings, thanks to slender piles with maximized resistances. Higher resistances enable reduction of the needed amount of piles, which is significant advantage in dense urban areas where soil displacements can be a problem”. “Usually driving of relatively slender steel piles also causes less vibrations than driving of concrete piles”, he adds.

Very straight long piles

Ground conditions at site consisted of approximately 40…45 meters thick quite stiff clay layer, 2…3 meters thick moraine / gravel layer and the bedrock. Pile lengths varied from 45 to 50 meters and all piles reached the surface of the bedrock.

Piling and earthwork contractor Turun Siirtomurske Oy made a fill layer over the heads of the old piles. Due to this uniform working platform the piling rig was able to move freely without hindrances. Piles were driven next to old piles. Top part of the clay layer was penetrated just by pushing, but lower parts of the clay were so stiff that impact driving was needed.

Piles were equipped with mechanically attached rock shoes and the element length was 12 meters. Piles were driven with 5 ton Junttan hydraulic hammer. Installation speed of piles was over 10 piles per day. “Piling work itself was standard work without specialties. Pile elements with mechanical splices are the most important thing when piles are this long”, says Ari Virtanen, project manager at Turun Siirtomurske Oy

 All piles were very straight. A torch inspection was made for all piles after installation. Even though the piles were very long the light of the torch was visible from the bottom of every pile.

Exactly scheduled pile deliveries

The location of the site at the city center and just next to another big construction site set own rules for deliveries. According to Ari Virtanen, the main issue in this project was the logistics “All pile deliveries were exactly scheduled. Some of them having time frame of only 15 minutes. And due to lack of space on site, all deliveries needed to be done with trucks having cranes in them.” Piling contractor and transportation company agreed together the exact delivery times “Everything went extremely well and all deliveries were on time and with self-unloading equipment” Ari Virtanen praises.

Piling work begun January 14th and the last piles were installed March 3rd. SSAB delivered all together 9,4 kilometers of pile elements. All pile elements were delivered in 12 meter lengths. In addition to 615 pile elements of RRs220/12,5 also 170 pile elements of RR140/10 were delivered to the project.

Project organization

  • Project owner: LähiTapiola Keskinäinen Henkivakuutusyhtiö
  • Main contractor: YIT Rakennus Oy
  • Contractor for piling and earthwork: Turun Siirtomurske Oy
  • Geotechnical designer: SM Maanpää Oy

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