Foundation

Published on November 14th, 2012 | by Henri

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Replacing backfill next to concrete-block foundation

Q.    I have a problem with the basement wall on the higher-grade side of my house. The wall is built of 11 courses of cement blocks and was backfilled with the native red clay and other soil when the house was built 48 years ago. I suspect I was in a hurry when I built the house so now I have to redo the backfill. The basement wall has not moved much but the hairline crack in the mortar joint indicates that the wall is moving.

I need to know the proper fill material to use and the proper way to grade the soil to prevent a future problem. The wall is 36 feet long and the house is a one-story brick ranch. I don’t see any cracks in the mortar joints of the bricks yet. I would appreciate your views on this situation.

A.    If you only see a hairline crack in a 48-year-old concrete block foundation, I wonder why you would want to go to the trouble and expense of excavating the affected wall and redoing the backfill. Keep in mind that there are some risks involved in removing the existing soil next to the foundation; it will take an operator who is very experienced with the use of a backhoe, as well as some hand shoveling. At this point, I would suggest that you simply make sure that the grade slopes away from the wall in question at the rate of about 2 inches per horizontal foot and plant a healthy stand of grass on it to control the travel of water.

However, if you want to replace the existing backfill, have the soil excavated to the base of the footings. On the side of the excavation away from the house, pin a sheet of geotextile fabric wide enough to start at the bottom of the trench and eventually cover the stone bed you will install. Spread a couple of inches of egg-sized crushed stones at the bottom of the excavation.

Lay a 4-inch perforated drainpipe on top of the stone bed and, at the lowest point in the grade around your house, connect it to a solid drainpipe leading to daylight, since your land slopes. Add a minimum of 1 foot of the same crushed stone (more if you can) over the perforated drain pipe. Fold the fabric to cover the stones. Continue the backfilling with coarse sand or bank-run gravel to within one foot of the top of the excavation and complete the backfilling with native soil sloping about 2 inches per foot. Plant grass on it; avoid shrubs or trees.

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