Collapsing soils are those that rely on a water soluble mineral cement to maintain their strength. When wet, the mineral cement dissolves and these soils collapse, leaving a bumpy terrain. Loess and some other soils of a loose, open structure are subject to this type of subsidence.
Figure 254-9 Block diagram of permafrost morphology.
A similar effect is observed in hydrocom-pactive soils. In arid regions, alluvial fan deposits and soils which were originally deposited as a result of mudflows or mudslides and have a loose structure containing a relatively large quantity of air can form hydrocompactive soils. Wetting causes a reorientation of the soil particles and can result in significant subsidence [4.5 m (15 ft) in California after the introduction of irrigation water].
In regions where hydrocompactive soils or collapsing soils are common, site-specific geologic reports that assess potential subsidence should be obtained
These soils should be avoided for rnosl types of development. When avoidance is impossible, a common engineering approach is to compact the area by the addition of water prior to construction.
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