Building foundations sit on piles which were driven into the ground until it set (hit something hard)
Piles are joined in long lengths as long as they can still be pushed in.
Ground beams and columns do not sit on top or subsoil. It has nothing to do with reclaimed land.
You are right, high rise buildings are supported by piles driven into the ground until it hits the solid stratum.
But when the reclaimed land begin to settle and consolidate slowly, which may take up to 20 years, the whole reclaimed area will be extremely undulating and the roads and drains, including some low rise building will all be damaged.
The Marina South and Marina Bay area was left to settle for about 20 years before construction began.
If you have been to the Jalan Eunos Industrial area where the low rise HDB terrace factories and the auto workshops are, you'll noticed some pavement around the buildings had sunk by one foot and 2 steps up became 3 steps up.
It was because that area was reclaimed from a big swamp and build over almost immediately with low rise buildings.
Anyway, those terrace factories are all due to be demolished very soon.