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Man of the moment

Reusing Victorian caissons to support a huge steel sculpture required careful assessment of the new loads

Millennium Man may come to acquire a particular meaning - think of the connotations of Renaissance Man or even Essex Man - but the original is a 28m high steel sculpture on the River Thames, a watchman for the Millennium Dome.

Sculptor Anthony Gormley set out to create a structure with no obvious signs of load transfer. Made up of a matrix of mild steel units, the image of a man is not immediately obvious. Client the New Millennium Experience Company says that only when viewed from a distance will its form become apparent. Close up, the sculpture is intended to resemble a random arrangement of interconnected elements.

Millennium Man stands on a new platform built on four Victorian caissons in the Thames. Safe reuse of these 'offshore' cast iron structures demanded careful assessment of the new loading conditions.

Tensile stresses in the mass concrete filled cast iron caissons were the main concern. Uncertainty over concrete quality ruled out anchorages solely within the caissons, so four high capacity deep ground anchors have been used. Principally these ensure the caisson iron is always maintained in compression and provide resistance to overturning from wind loading or impact from river vessels.

Geotechnical contractor Keller Ground Engineering proposed distributing the fixed length of the anchors both within the lower caisson concrete and to various remote depths in the underlying strata using its single bore multiple anchor system.

No specific ground information was available at the jetty location and time constraints prevented the ground conditions being fully investigated. Fortunately Keller had previously founded many of its single bore multiple anchors in the vicinity and was able to draw on information gained in its work on the nearby quay wall project and the Millennium Dome foundations.

Preliminary anchor design was developed on the basis of assumed values, but in the knowledge that the ground conditions and quality of concrete in the caisson were likely to vary from those anticipated. This meant final design would be determined as anchor drill logs were produced.

Each of the four anchors was drilled from a scaffold deck boarded out with sleepers using a Casegrande C6S rig, with Klemm rotary percussive drill head advance 178 drill casing through a 1.5m guide sleeve and on to the depth of the caisson. Water flush was provided by a high volume pump sited on the access barge.

Preliminary design was for six anchor units: one in the base of the caisson, one in the thin layer of Terrace Gravel, three units in the London Clay and one in the Lambeth Group. Each unit was to provide a 150 kN working load to provide a total prestress of 900kN.

But drilling of the first anchor revealed an unexpected soft to firm clay layer between the gravel and London Clay which meant redistribution of the unit anchors. Keller says this demonstrates the flexibility of the multiple anchor system which allowed the unit anchors to be located within the stronger beds of materials encountered during installation.

All other anchor systems are designed to found the entire anchor in a single stratum and thus normal systems could not have accommodated the variation in the encountered ground nor provided 900kN working load when founded in one of the, relatively thin soil materials, says Keller.

Installation of the four anchors, including penetration of the insitu caisson concrete took two weeks, following by three weeks of preparation of the caisson head concrete and anchor heads. All anchor tendons were fully protected against corrosion using the double plastic coating system approved in BS8081: 1989.

Anchor stressing used six hydraulically synchronised jacks (one on each unit anchor). This guarantees uniform and simultaneous loading of all six units. Each unit must satisfy three acceptance criteria, two related to load hold capacity and one related to extension behaviour. Hence, a six unit anchor has to satisfy 18 acceptance criteria.

Tony Barley, director of engineering, Keller Ground Engineering

Simon Dale, construction manager, McAlpine Laing

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