Unbalanced Backfill & A Garage Foundation Wall

Posted by in Construction & Shortcuts, Foundations, Masonry, Soil Issues | June 16, 2014

It is not uncommon for a garage floor slab to be raised above the original grade, especially where the grade around the garage slopes down from front to back or side to side.  This hillside or sloped grade setting will usually result in a garage floor that is elevated above the grade against one or more of the perimeter walls.  See Figure UB-1.

 

Typical garage foundation wall with unbalanced backfill.

 

The most common problem we have seen with this setting is the outward leaning of the short block or concrete foundation walls that retain the backfill under the floor slab.  See Figure UB-2.  The outward lean is due to the lateral pressures generated by the backfill under the floor slab and the surcharge pressures generated by the weight of the floor slab and the weight of the contents on the floor.  Lateral pressures are generated by the outward movement of the soil under the floor slab similar to the outward movement of the soft ice cream of a partially melted ice cream sandwich when the sandwich cookies are squeezed together.

 

Garage foundation wall leans outward.

 

A garage foundation wall is usually not capable of adequately restraining the movement because the top of the wall has not been restrained, or held back, by attachment to the garage floor slab or the wall has not been designed or built as a cantilever retaining wall.  Rather, most short height garage foundation walls resist the outward movement by the combination of their own weight and the limited bending strength of unreinforced masonry or unreinforced concrete.

Analysis of the restraint provided by the weight of the wall and the limited bending strength of unreinforced masonry show that most foundation walls are no match for the lateral earth pressures of the back fill material.  Analysis further shows that an unbalanced fill greater than 16 inches will probably cause a block wall to lean outward over time.  Unbalanced fill is the height of the garage floor slab above the outside grade.  However, we have observed outward movement of garage foundation walls where there had been unbalanced fills as little as eight inches.  Due to its greater weight, a solid concrete wall is capable of retaining a little higher unbalanced height of backfill.

To reduce the risk of outward leaning of a garage foundation wall with an unbalanced back fill, the conditions need to be evaluated by a structural engineer and the top of the wall tied back to the floor slab or the wall reinforced with vertical steel reinforcing rods which extend into a footing or base similar to that for a retaining wall.

If the soil and surcharge pressures are not considered in the design and construction, a BIG separation may develop over time.  See Figure UB-3.

 

Separation between floor slab and leaning garage foundation wall.

 

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