RECOGNITION OF COLLAPSIBLE SOILS BASED ON GEOLOGY, CLIMATE, LABORATORY TESTS, AND STRUCTURAL CRACK PATTERNS

As with many geologic hazards, some of the problems caused by collapsible soils stem from a failure to recognize them ahead of time, or from a failure to recognize them as the cause of observed damage. Several tools are available to identify collapsible soils. First, they should be distinguished from other types of subsidence in a general classification scheme, where collapsible soils would include subsidence by hydrocompaction, piping, and gypsum dissolution. Geologically, collapsible soils have long been associated with alluvial fans and gypsiferous soils and rocks in arid climates, but they also can occur in windblown materials and colluvium. Laboratory identification of collapsible materials uses swell-consolidation tests, soil density, liquid limit, gradation analysis, porosity and void ratio. Field testing can include plate load testing and measurement of in situ moisture. In all cases, gysiferous soils may require more time to react than typical silty hydrocompactive soils because they require dissolution rather than dispersion to remove material. Finally, crack patterns in structures can be used to identify the direction of foundation movement and the principal stresses involved in the damage. Cracks associated with settlement can be distinguished from those caused by heave, landslides, or earthquakes based on position of the cracks relative to slabs, doors and windows; orientation of the cracks; and variation of crack separation along their lengths.