As we now know, backfill is soil that's been removed as the foundation is dug. This soil can never be as hard-packed as the unexcavated virgin soil around it. It's looser, more porous, and much more absorbent of water. Additionally, some foundations intrude upon layers of natural bedrock that are guiding water to springs and other water sources. If your foundation stands in the way of the natural path of this water, it's going to build up against the foundation. To make matters worse, as the soil around the house settles, it begins to dip lower than the virgin soil. As the water runs downhill into this depression, it pools and absorbs into the dirt around the foundation.
It's no surprise that when it rains, water collects most densely in the excavated and backfilled area around your foundation. When you put a house in this hole, it's no different. In areas where the soil has clay in it, this is called the "Clay Bowl" effect.
This water is going to go anywhere it can, seeping through any opening and crack it can find its way it to. If pressure from the cracks formed on your basement walls during construction go all the way through, whether they're from the shock of the sudden weight on the walls before the concrete's completely cured or from rocks from the backfill striking the walls as it's backfilled, these cracks will certainly let water through.
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