What are collapsible soils?
Collapsible soils are soils which have a high ratio of voids, low bulk density and water content, high percentages of fine-grained particles with slight to no plasticity. These soils gain density as water is introduced, resulting in the compaction or 'collapse' of the spacious voids between fine particles.
How are soils determined to be 'collapsible'?
Collapsible soils occur naturally and are generally wind-blown deposits. Typically silts and fine-grained sands with a small amount of clay. Collapsible soils will display a high apparent strength in their dry state but have a low density, porous structure making them susceptible to large amounts of settling when water is introduced.
How can damage from expansive soils be prevented or mitigated?
The absolute best way to prevent damage from swelling soils is to extend the foundation of a structure beyond the depth at which water content in the soils will regularly be fluctuating. More recent studies have shown that these depths often extend further than was previously thought. Most areas have a regular wet season and dry season which is easily accounted for, but in the case of a drought followed by an extended wet season, these zones may extend in depth and experience greater differences between their dry volume and wet volume.
What are potential issues caused by collapsible soils?
The most apparent and most damaging effect of collapsible soils is the 'settlement' of a foundation/structure. Pictures below show just how damaging these soils can be to foundations and walls of homes, as well as roads and sidewalks. Piping (especially plastics/PVC) not buried beneath collapsible soils can be subjected to stress and will be likely to break as collapsing occurs - no longer offering support to these lines. It is worth noting that if water/sewage pipes break they will leak into the soil and exacerbate the collapsibility of these soils, possibly causing further issues. Along with utility line issues, landscape irrigation, poor surface drainage, and intentional ponding can have profound effects on potentially collapsible soils. Concrete drainage piping and drainage ditches are also subject to very similar issues. Cracking will leak water run-off into expansive soils and the vicious cycle of heaving, cracking, and leaking continues.