When to Backfill the Foundation Wall

Backfilling a Foundation

Introduction:

A house under construction down the road from where I live had one wall of its concrete foundation walls collapse during backfilling. About forty feet of wall just collapsed into the cellar area about the time it was backfilled to within two feet from the top of the wall.

This was an unfortunate and costly situation that should have never happened. The DIY homeowner builder that was performing the backfilling obviously was not aware that concrete takes a little longer than two days to harden.

For the DIY homeowner there is a lesson to be learned about this incident. Portland concrete begins the process of hardening immediately after water is combined with the mix but it takes a while before the chemical reaction is complete. The chemical reaction continues for several weeks before it is fully hardened. Wet concrete that is left to cure in the elements takes approximately three days to reach about 40% of its designed strength. After approximately three weeks, depending on the weather it will reach nearly 75% of its designed strength. It can take several months or a few years to get to 100% of its design strength. Keep in mind that a foundation wall is usually eight inches thick and eight feet high. It is a massive piece of concrete to cure.

As far a back filling goes I would not recommend completely back filling the foundation wall until the following three events take place.

• Three week after the foundation is poured

• The deck is installed to support the top of the foundation wall from lateral loads

• The basement floor is poured to support the bottom of the foundation walls

In the meantime it is alright to partially backfill the foundation. This really isn’t as much as a show stopper as one would think because after the forms have been removed there is still much that must be done around the exterior of the foundation wall. The foundation drain system must be installed and covered with a stone and a mesh and then straw. This takes a while. After these tasks are complete partial backfilling can be done to make life a little easier for the carpenters while they install the deck (the first floor platform). Partial backfilling should be limited to three feet until the deck is on and the foundation floor is poured if you don’t want risking a foundation wall collapsing.

Of course in cold weather this whole time line will slow down because the curing process of the concrete is slower or may stop completely. In addition, the vertical steel reinforcing bars could be installed to further strengthen the foundation wall much more quickly. This will certainly increase the cost of the house and that is why it isn’t or rarely done in residential construction.

After everything is said and done the best approach is to be patient and give the concrete time to cure before backfilling. Under most conditions you at least partially backfill it after ten days to a couple of weeks which will give you time to install the foundation drain system. These same considerations should be taken into account when backfilling the retaining wall too.

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