Curious Cracks

Beth is buying a 100-year-old house in Barre, and hired PERCH to investigate some cracks in a concrete foundation wall. The cracks were discovered by the current owners 10 years ago when they removed a wall covering in the basement. Beth needed a structural engineer to assess the cracks’ severity before proceeding with the sale.

The terrain outside the wall slopes at a steep angle, up to 45 degrees, with a window above the high point of the grade. Several roof lines direct water into the corner; gutters were added at some point to divert the water. The outside face of the wall jogs inward at two locations: a stair-step jog near the southwest corner, and a vertical jog near the bottom corner of the window.

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Exterior view of the foundation wall.

On the interior side, there are several vertical and diagonal cracks in the wall between the floor slab and the window. The widest cracks extend from the top east to the bottom west, and vary from ¾ inch to 2 inches wide. (Other cracks are smaller.) There’s also a crack in the floor slab itself, which has a slight downward slope toward the corner. All the cracks were filled with spray foam by the current owners and have remained stable over the last 10 years.

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Wall crack – filled with spray foam. Floor slab crack – slopes down into corner.

PERCH determined that the culprit for the cracks was a familiar one: differential settlement. Soils under a heavy load may compress at different rates, causing one location to sink lower than another. This is especially true of old houses built on whatever soil happened to be on site, with no fill or compaction. Here it’s clear that the corner of the floor slab has settled lower than the middle of the slab. The stress concentration in that corner, along with the lack of damage elsewhere, makes differential settlement a better explanation than frost heave.

As for the wall, concrete is most likely to crack at stress points, where the forces in the concrete change rapidly. The bottom corner of a window, in which the wall next to the window supports two floors and a roof while the wall under the window supports only the window itself, is a textbook example of a stress point. Here one can imagine the wall sliding along the diagonal crack, down to the lower point as a result of differential settlement.

So should Beth be concerned? In this case, no. The cracks have not widened in the last 10 years, and gutters divert water to prevent further impact on the soil. Also, the wall is plumb and the cracks don’t extend all the way through. (There’s a simple explanation for the oddities on the outside of the wall: imperfect formwork when the concrete was originally placed.) PERCH delivered a report to Beth with some long-term suggestions for keeping the cracks insulated and the soil dry. The sale proceeded on schedule.

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