Complex sentence: The builders drove 5535 12-meter-long piles into the sea bed to reinforce the ground, which they then covered with a layer of sand, a layer of concrete blocks, and a layer of granite slabs.
Split into simpler sentences: The builders drove 5535 12-meter-long piles into the sea bed to reinforce the ground. They then covered the piles with a layer of sand, a layer of concrete blocks, and a layer of granite slabs.