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Expanding into clay Soilex expander body soil anchors have been used in stiff clay to support a sheet pile wall for a massive drydock in southern Sweden.

PILING AND FOUNDATIONS

The 0resundsvarvet shipyard in Landskrona on Sweden's west coast has one of the few drydocks in Europe able to repair passenger ferries and oil tankers. The dock was built in 1918 and, to make it more competitive for modern ship repair, it was decided to increase the 30m wide and 187m long facility to 40m wide and 195m long. This involved building a new quay with a sheet pile wall.

The extension was designed and built by Swedish contractor Peab for client 0resundsvarvet. Because of the tight construction programme, the firm opted for a scheme incorporating one level of high capacity ground anchors.

Before work began, field tests of a variety of anchors were carried out. Conventional anchors with between 5m and 8m bond lengths and grout pressures of between 10bar and 100bar reached failure loads of between 300kN and 450kN, well short of proof loads of 850kN required by the scheme.

Ground conditions are a stiff moraine clay containing up to boulder size rock fragments. Two expander body anchors from Swedish geotechnical contractor Soilex were also tested, despite the fact that there was little previous experience of using the system in such stiff clay conditions. These were loaded to 850kN without failing, giving around 6mm of deformation and 0.5mm creep, prompting Peab to select Soilex's EB530 type anchors for the project.

Introduced in the early 1980s, the expander body is a key component of both Soilex's anchors and piles. The system is typically used in soft clays and granular materials (Ground Engineering, October 1998) but this is the first time it has been used for a sheet pile wall in stiff clay.

The expander body is an up to 0.15m diameter package of folded sheet steel, which is inflated by injecting grout when in position. Expanded diameters range from 0.4m to 0.8m and bodies are up to 3m long. They are driven or vibrated into place or, as on this contract, inserted into predrilled holes.

The main advantage of the system is the way in which applied load is resisted. In conventional anchors, the majority of the load is taken by skin friction but with the expander body, capacity is derived by soil resisting the movement of the body. The expander body side resistance is mobilised at relatively small displacements and falls when the peak value is exceeded, as in conventional anchors. However, the base resistance increases gradually and requires relatively large deformations to reach the peak value.

During inflation, soil surrounding the expander body is displaced laterally along its length increasing the pressure against the sides. In clay soils, this increase results in a build-up of excess water pressure. After the injection phase the clay starts to reconsolidate, water content drops and the shear strength increases.

This effect is greatest close to the body side and is negligible at the base. This reconsolidation effect is similar to that of conventional anchors or driven piles in clay but affects a larger volume of soil, up to three times the inflated diameter, says Soilex.

In soft clay, grouting pressures of 3bar are typically needed to expand the bodies, but in the stiff clay on this contract, with an undrained shear strength of 160kPa, pressures were increased to around 15bar. Soilex's Erik Westerberg says there were no problems installing the anchors for the dock and boulders within the glacial clay did not restrict operations.

A heavy concrete beam was cast on top of the installed sheet piles with inclined holes for the anchors, so it was important that each anchor functioned properly due to the cost of drilling more holes in the beam.

Typically, in loose to medium dense soils, the expander body is driven directly using Soilex pipes - steel pipes spliced with a special coupler - but for this contract the expander bodies and pipes were installed in predrilled holes. A total of 152, 15m long anchors were installed by one rig over the month-long contract. Each anchor had seven, 12.7mm diameter strands bonded into the body and 550 litres of cement grout was used in each.

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