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Steel appeal

Major changes suggested by Swedish foundation contractor Stabilator, overcame difficult ground conditions and saved time and money, during work on a new office complex for Norwegian insurance company in Oslo.

Difficult ground conditions encountered during foundation work for Norwegian insurance company Uni Storebrand's US$120M office complex in Oslo, created both interesting problems and innovative solutions.

Depth to rock at the central Oslo site varied considerably. In some places rock had to be blasted away, in others the soft clay stratum was more than 30m thick. The shear strength of the soft clay varied between 15kPa and 30kPa. The rock consisted of clay shale with a compressive strength of approximately 100MPa.

In addition, the lowest excavation level was about 4m below the mean water level of the sea in the nearby Oslo Fjord, and so foundation design had to resist the resulting uplift forces.

For retaining at the site perimeter, some 5,000m2 of sheet piling was driven and anchored with 250 rock anchors.

In one section these sheet piles had to be driven very close to an existing road tunnel. To limit ground displacement in this area, lime/cement columns were installed on the passive side of the piling. Finite element calculated displacements corresponded well with those which actually occurred.

However the greatest challenge came from the load bearing foundations which were to be formed in conditions varying from soft clay to rock.

Under the original programme proposal precast concrete piles - or bored piles where depth to bedrock was less than 5m - were to be constructed from the already cast basement floor structure. To counteract the uplift force of the groundwater around the deep basement, a large number of both concrete piles and 800mm bored piles were to be anchored with permanent rock anchors.

However before work started it was realised that the proposed works involved a number of serious technical and logistical problems. First, limiting water and earth seepage, while rock blasting at the site, could prove difficult and give rise to substantial delays.

Also superstructure work would require drilling into the basement slab, and widespread use of formwork, concrete and reinforcement, all of which needed to be carried out during excavation and dewatering.

Equally collection and removal of excavated material from the bored piles would need to be carried out simultaneously with pile reinforcement and casting.

Furthermore drilling for the rock anchors could damage the already installed piles, while once placed prestressing forces in the anchors would place a load on adjacent piles. And should an anchor break during testing it would be difficult to find a new position for a replacement anchor, since the installation would have to be done from the surface of the finished basement floor slab.

All in all, given the short 22 month construction period and the fact that work on site and the design were to be done in parallel under so- called 'on-going planning', the foundation method introduced considerable uncertainty and risk.

After analysing the problem, Swedish foundation contractor Stabilator proposed an entirely different method to main contractor, Skanska. This consisted of replacing bored piles, most of the driven piles and the anchors with steel tube piles. These can act in both compression and tension, and were to be based 5m into the shale rock.

This essentially less complicated approach offered a much more manageable solution, and could be completed three months ahead of the original programme.

Stabilator was able to demonstrate to Skanska that the increased costs for steel tube piles would be more than compensated for by the considerable saving in time and the corresponding reduction in costs.

Boring for the steel tube piles was undertaken to depths of between 8m and 35m. The total length drilled was approximately 5,500m.

At the start of each pile, a guide hole was drilled in the concrete slab. As a result, the positional tolerance for piles were set at just 30mm, which presented the potential for savings in the fabrication of the superstructure.

Drilling was carried out with a down the hole hammer, which required special care in the relatively soft clay shale rock.

Piles were constructed in three sizes, between 115mm and 190mm in diameter, and loading capacity varied between 974kN and 2,052kN. The cores were jointed by welding or by means of API-type threaded joints.

In designing the piles, which were required to perform in both tension and compression, a number of factors had to be considered, including bond fracture, compressive fracture and fracture in the rock.

The pile caps and joints required special calculations. In order to take full advantage of the steel tube piles, the elements which transferred the force at the top were designed to function together with the beams, slabs and columns were connected in the superstructure.

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