Problems with collapsible soils: Particle types and inter-particle bonding

A collapsible soil is composed essentially of a packing of mineral particles and a set of interparticle bonds holding the system together. Failure requires the bond system to fail and the soil structure to collapse. A natural hazard is presented. The soil structure may collapse inwards (consolidate), as in loess failure, or it may collapse outwards (disperse, disintegrate), as in the failure of quick-clays, some collapsing sands, some silty estuarine deposits, and in wind erosion of silty soils by saltating sand grains. Generalising about bonding systems allows two types of interparticle bond to be recognized: long range bonds and short range bonds. Long range bonds are found in clay mineral systems and allow the occurrence of plasticity. They are represented by c in the standard Coulomb equation. Short range bonds are found in inactive particle systems. These are soil systems where the constituent particles do not have a significant electrical charge. A slight deformation of a short-range bonded system causes much loss of strength. It is short range bonds which tend to dominate in collapsing soil systems, although in the complex case of loess the bond failure is initially mediated by long range bonds at the interparticle contact regions. A collapse failure involves a large scale remaking of the soil structure, and thus total failure of the bonding system. Generalising again- it can be claimed that five types of particle make up engineering soils: A active clay mineral particles (the smectites), B inactive clay mineral particles (e.g. kaolinite, illite), C very small inactive primary mineral particles (close to the comminution limit in size- mostly in the quick-clays), D silt (usually quartz silt), and E sand (usually quartz sand). The nature of type D particles contributes to the collapse of loess soils, the most widespread of the collapsing soil phenomena. The nature of type C particles controls the behaviour of quick-clays. C and D systems are essentially dominated by short-range bonds.