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Sands of time

Letters

The Mystery of the Pharaoh's Obelisk on Channel 4 on 15 January speculated that the ancient Egyptians raised 500t granite obelisks by sand displacement.

This simple principle was used by modern Britons in West London during Laing's construction of Westway in the late 1960's. One continuous 1,160m long section of the elevated motorway is made up of 62m spans. Each span has 26 precast units, each weighing up to 132t.

During placement and before prestressing, the 3,040t dead load of each span was supported on temporary works consisting of girder sets, mounted on trestles. As the settlement of the temporary works was a variable, the 2m deep girders were set high and lowered to level by sandjacks, located on the trestle heads beneath the girder sets.

Laing designed sandjacks with a safe working load of up to 810t. Each had an outer cylinder, an inner ram and a dried sand in-fill. As sand naturally flowed out of two holes in the cylinder, the ram and its load gently descended.

Since the operation was irreversible it was imperative not to over-shoot, but this was never a problem, even under high pressure the sand only trickled out and the final level could be controlled to a millimetre. As sand built up outside the orifices, descent stopped and hundreds of tonnes were spectacularly supported by a few grains of sand!

Alan R Smith (M), 95 The Avenue, Sunbury-onThames, TW16 5HZ

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