This article reviews two types on concrete slab foundations, including monolithic and stem wall.
Placing footings on undisturbed soil without further testing works most of the time on residential structures because of the lower building weight. Footings depending on the load are being supported by the top layer of soil or several layers of soil several feet deep, see the Pressure Bulb or zone of influence the footing has on the soil below.
Commercial buildings depending on their size and items stored or used inside may be too heavy for the first layer of undisturbed soil you encounter. The undisturbed soil layer may be on top of a weaker layer of unsuitable soil. That’s why its common to have soil borings taken down several feet below the anticipated footing depth. Local knowledge from Geotech firms would be a good source, but its common to go 10 feet below the anticipated footing depth here in southern Missouri or to solid rock. On commercial jobs the borings are done at the preliminary stage and the footing type and depth in the plans have considered this information in the design process. Since borings may be limited to one or two bore holes on a small commercial site, it’s not uncommon to require the contractor once they excavate down to the designed footing elevation to drill down another 4 to 5 feet to ensure the subsoil closely matches the boring data. I worked on numerous bridge projects where the footings were design to be on solid rock, but specifications required to drill down a one inch hole 4 feet below the footing to check for compressible mud seams or open voids the limited number of borings may have missed. If either were found the footing was lowered, but only up to a point where column redesign was needed.
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Thank you Randy for your comments. The article is based on generalizations and not the specific engineering necessary to construct the building. In most cases we (Commercial Property Inspectors) will not be present to witness the construction of the building. This article and image was intended to be a reference of what to expect and not to substitute for engineering or specific regional or site specific issues. Your comments, relating to compaction and soils, are exactly right on the money and at no point should we (Commercial Property Inspectors) be in the position to comment on the stability of the soil. But, a commercial property inspector needs to understand what is known to ideal so that when something is not ideal we can understand a potential origin or cause for the condition.