All buildings are subject to fractional but constant movement: bricks, timber and concrete will expand and contract based upon changing environmental conditions over monthly and seasonal cycles.
Normally, a building’s superstructure can comfortably accommodate this, but more acute movement may lead to evidential cracks in walls, floors and ceilings. These are normally categorised as either a failure in the initial design, building settlement or subsidence.
Settlement versus subsidence
Issues with the initial design would be channelled straight back to the architects and engineers, but what about settlement and subsidence? The former describes the compaction of a building’s individual material components under its own weight.
Most settlement occurs shortly after it has first been constructed, for example, as materials like timber joists lose a fraction of their moisture content and become squeezed under the weight of the roof structure. A typical building will settle into position over one to 10 years and cracks may manifest where stiffer elements are adjacent to others that are less so.
Subsidence, by comparison, is a change in the volume of the supporting soils under the building’s foundations and structural floors, causing downward movement. When this occurs, the key need is to identify the individual and specific cause for this volumetric change.
Ground, site & superstructure risks
The first consideration is the make-up of the soil. Granular earth, which normally contains larger particle sizes of sand or stone, is more porous and free-draining, whereas clay is much more likely to shrink or swell depending upon the prevailing moisture conditions.
With clay, ground moisture may bond to the surface of its fine particles, which will form a jelly-like consistency, rather than filling the spaces between the particles. This type of soil stay wet, and may potentially continue getting wetter than granular ones.
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Article reference Beginner’s Guide to Subsidence
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