Work on the foundations started on 28 January 1887.[13] Those for the east and south legs were straightforward, each leg resting on four 2 m (6.6 ft) concrete slabs, one for each of the principal girders of each leg but the other two, being closer to the river Seine, were more complicated: each slab needed two piles installed by using compressed-air caissons 15 m (49 ft) long and 6 m (20 ft) in diameter driven to a depth of 22 m (72 ft)[14] to support the concrete slabs, which were 6 m (20 ft) thick. Each of these slabs supported a block built of limestone each with an inclined top to bear a supporting shoe for the ironwork.