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Lime light

Swedish foundation contractors are world leaders in the technology and application of lime cement columns. European Foundations looks at recent developements in the equipment.

Sweden has a long history of using lime modified soils for engineering purposes. In recent years attention has focused on lime cement columns, or lime piles, which are particularly cost effective for improving the stability of embankments, slopes, trenches and deep cuts. They can in addition be used to improve differential settlements and even reduce vibrations from traffic, or blasting or pile driving.

Sweden's leading foundation contractors have developed equipment and technical expertise which make them world leaders in this field.

For example Hercules Grundlaggning started manufacturing lime cement column rigs in 1994. Today, the company has three complete units and the fourth is being manufactured.

Its latest equipment comprises an installer and a batching plant. The installer is based on an Akerman rig and modified to fulfil the requirements for installing the lime cement columns. The batching plant is track mounted and houses two 14m3 silos for the lime and cement. When the silos are empty, the plant is tracked to an accessible part of the site and refilled directly from bulk tankers.

When in use, a specially designed mixing head attached to a hollow stemmed kelly is rotated into the soft ground to the required depth. The head is then lifted and rotated continuously as the dry lime/cement mix is injected to create a homogeneous column.

The installation process is fully governed by computers which secures that the mixing of soil and binding agents is properly performed. All installation parameters are automatically stored and presented in spread sheets or diagrams.

The method is mainly used to increase the stability and reduce settlements for embankments. However, there are a great number of additional applications which has proved to be cost effective and technically accurate.

For example In Uppsala, north of Stockholm, NCC was the main contractor for a wastewater purifying plant. For excavation of a sedimentation basin, the client accepted Hercules Grundlaggning's alternative bid to use lime cement columns.

The soil consisted of soft clay with a shear strength of 12kPa to 15kPa. The excavation depth was 4.2m. To withstand the earth pressures and increase the stability within the excavated area, the columns were installed in three concentric rings with an overlap of roughly 0.1m for each column (eg secant lime pile walls). To increase the total stability, additional columns were installed in a spoke pattern.

The diameter of the excavation was roughly 13m. Columns had a diameter of 600mm and an average effective length of 7.5m.

Vertical and horizontal displacements were monitored during the excavation. The average horizontal displacements did not exceed 2% of the excavation depth and the excavation proceeded as planned.

Part way through construction the design was modified with an access road, which was to have been piled, being taken into the excavation in open cut through the side of the cylindrical excavation. This would take away beneficial hoop stresses in the excavation walls and so a series of beams were formed, to strengthen the excavation in the area of open cut. However the stiffness of the lime-cement column improved ground meant that only moderate displacements occurred, and in fact no load was transferred to the beams. On this project the lime cement columns ended up being roughly 40% more cost effective compared with traditionally anchored steel sheet piles.

Hercules Grundlaggning is the largest foundation contractor of Sweden. The core business comprises driven piles, bored piles, underpinning, sheet piles, ground anchors, vibro compaction, soil nailing, lime/cement columns, rock bolts and shotcrete.

The company has a turnover of ECU40M, a staff of 280 and operates mainly in the Nordic countries.

Leading Swedish contractors are now promoting these advantages outside of their usual markets. Hercules Grundlaggning has recently concluded a joint venture project with Stent Foundations in the UK for the design and installation of dry lime/cement columns to stabilise a railway embankment. While in the US, Stabilator is carrying out the first major US project to use lime cement columns during reconstruction of a 50km stretch of Interstate highway at Salt Lake City in Utah.

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