Settlement of Shallow Footings

By Nancy Nichols, P.E. | March 24, 2020

A shallow foundation on soil can be made for an individual load (i.e., isolated), or to support multiple loads (e.g., strip, raft or mat).  The soil needs to be able to support the load evenly below the footing.  The load carried by the footing is transmitted to the soil below it.  The depth below the footing that is subjected to pressure from the footing load depends on the geometry of the footing and its dimensions.  For instance, for a square footing approximately 90% of the load is transmitted to soil below the footing for a depth of about 2x the width of the footing.  So, a wider square footing transmits the load to a deeper depth.  For a long/strip footing, the load is carried much deeper than for a square footing, i.e. approximately 7x the footing width.

The load is transmitted to soil below the footing and also to the side. As a rule of thumb, the footing load extends to soil within a 1:1 extending down and outwards from the edges of the footing.

A loaded footing may settle due to compression and/or consolidation of the soil below it. Consolidation is a term used when soil is compressed, and water squeezed out over time.  Consolidation of clayey or silty soil can occur for a decade or more.  Compression of granular soils also occurs, but largely during construction.

Soil is typically heterogeneous, or variable, due to differences in composition and compaction.  Natural soil can be in layers or pockets of differing composition.  For instance, in NH we have varved clays and silts that are due to historical glacial waters that flowed downhill from melting glaciers.  In the warmer months, there was more melting and higher flows, which deposited the silts.  In cooler months (not freezing), there was less melting and lower flows, which deposited clays.

To assess the engineering capacity of soil under a proposed footing, it is not enough to see the soil at the surface.  This is the reason that subsurface explorations are good practice prior to design and construction of a building.  For commercial properties, the 2015 International Building Codes require geotechnical explorations per Section 1803.  Are the footings of your building settling excessively or unevenly?  Call us to assess whether repair is needed and, if so, what is the appropriate repair.

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