Flooding Cause - Backfill Zone

When a new basement nears completion, soil or backfill is pushed into the gap between the basement wall and the clay walls of the open excavation. The illustration shows the basement wall, the backfill zone and undisturbed soil.

 Cut away diagram of a house and backfill zones adjacent to it

In most cases, backfill material is reused soil and is often the same soil removed during excavation. Upon removal, something new is added to the soil: air. This is not good. The air-soil combination becomes backfill that has empty spaces or voids. As a result, the soil will continue to settle for years.

Even after a few short years, you can see where the soil has settled, but may not be able to see depressions hidden by decks, stairs or patios. During rains, low areas become puddles next to the foundation.

To combat this problem, clay soil is recommended as a cap for the backfill zone (shown in the illustration above). Topsoils or sandy soil are not recommended. They allow water to percolate down into the backfill zone. But if you want a flower bed around the perimeter of the foundation, start with a clay base 15 - 25 cm thick. Slope the clay so that water will drain away from the house. Extend the clay 1.8 to 3 metres out from the house. Decorative landscaping materials or a flower bed may then be added.