Built by Roma Agrawal

Author:Roma Agrawal
Language: eng
Format: epub
Publisher: Bloomsbury Publishing

Extraction holes radiating from the large shaft.

The team then bored 32 cylindrical access shafts, 3.4m in diameter and between 14m and 25m deep, through the original masonry raft of the cathedral and into the ground. These were dug laboriously by hand (accessing this confined space with diggers would have been both difficult and dangerous). At each stage of the descent, concrete was cast in a ring around the edge of the hole, creating a tube to keep the soil in place. When the shaft was finished, a second layer of concrete was cast inside the tube to stop the hole from collapsing in on itself. At the base of each shaft the engineers sank four mini-wells from which they could pump out the excess groundwater that would otherwise rise and flood the shafts.

These, though, were not the shafts that were going to save the cathedral. They just provided the means for drilling about 1,500 holes, slightly inclined from the horizontal, with a diameter the size of a fist and between 6m and 22m long, through which soil could be extracted. The plan was that, after the soil had been removed, these holes would naturally close up over time, causing the foundation of the cathedral to settle.

Since the north side of the cathedral was the highest and needed to come down the most, the largest amount of soil was extracted in that area, while much smaller amounts were taken from the south-west corner. More than 300 cubic metres were removed from one north-eastern shaft, whereas only 11 cubic metres were taken from another in the south-west corner. In total, through this vast warren of shafts and tunnels burrowed deep beneath the historical cathedral, and with nearly 1.5 million extraction operations, 4,220 cubic metres of soil were removed from underneath the structure – enough to fill about one and a half Olympic-sized swimming pools.

As you might expect, this soil removal was done carefully and cautiously, in stages, over a long period (four and a half years). All that time the levels in the cathedral were strictly monitored to make sure that any movement stayed within the limits of what the engineers wanted. The arches and columns inside the cathedral were supported with steel beams and props to prevent any damage from sudden, unexpected or large movements. Meanwhile, soil samples were continually taken out of the ground to be tested for stiffness and water content, and were compared with the computer model to make sure reality matched prediction.

The difference in floor level between the north-east and the south-west had been more than 2m, but in 1998, once the north end had settled down by just over a metre, the process was suspended. Even though this left the foundation slightly tilted, the engineers had become concerned about damaging the structure. The lean of the towers had been brought back to an amount that was deemed safe – and so, for the time being, work has stopped.

The large cylindrical access shafts have been left open.

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