We come across two types of point loads, loads from above pushing down on the concrete and sub-grade supports that cause point loads when we attempt to raise the concrete. With this study we will be discussing the latter, sub-grade supports.
Last year we came across another porch that had settled even though the builder had installed Sonotubes or footers under the concrete to support the posts going to the overhead roofline.
This happens way to frequently and why we prefer the “big foot” style footers that flair out at the bottom as opposed to the 6″ 8″ or even 12″ Sonotubes with no flare at the base.
This porch had four Sonotubes and all had settled. Because of previous experience we knew not to attempt a lift unless we could disconnect the under slab supports. Fortunately the Sonotubes were partially exposed which enabled us to use a concrete saw on them which allowed us to raise the porch without damaging the slab.
Sub grade supports are a great idea but like most things need to be thought out and done properly to function as designed. Simply pouring a tube of concrete into the ground may not provide any support, in fact if not to load bearing strata it will exacerbate the problem. A small 6″ diameter Sonotube that is six feet long weighs roughly 1724lbs! Now imagine all that weight standing on its end and just six inches in diameter, it’s understandable that it settles.
Without being able to disconnect the support from the structure, whether it be a porch or patio, damage to the concrete is almost a certainty.
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