Collapsible soils will settle without any additional applied pressure when sufficient water becomes available to the soil. Water weakens or destroys bonding material between particles that can severely reduce the bearing capacity of the original soil. The collapse potential of these soils must be determined for consideration in the foundation design.
Many collapsible soils are mudflow or windblown silt deposits of loess often found in arid or semiarid climates such as deserts, but dry climates are not necessary for collapsible soil. Typical collapsible soils are lightly colored, low in plasticity with LL < 45, PI < 25 and with relatively low densities between 65 and 105 lbs/ft3 (60 to 40 percent porosity). Collapse rarely occurs in soil with porosity less than 40 percent.
Potential Collapse. The potential for collapse should be determined from results of a consolidometer test. The soil may then be modified as needed using soil improvement methods to reduce or eliminate the potential for collapse.