ABSTRACT
Subsidence cannot occur on solid, unmined rock - sandstone, granite, mudstone, slate - except by shear failure and rotational displacement to the surface under excessive load, or by landsliding where slope profiles permit (section 32). Hazard of potential subsidence can therefore be recognized by rock type on geological maps. All rocks do compact under load. Weak mudstone or sandstone can compact enough to cause settlement of structures, but normally well inside acceptable limits. COLLAPSIBLE SOIL HYDROCOMPACTION Some fine soils collapse due to restructuring when saturated for the first time; this hydrocompaction may cause subsidence by 15% of the soil thickness. The collapse is due to total loss of cohesion, after disruption of fragile clay bonds or solution of a soluble cement. Loess collapses most easily where it contains about 20% clay; with more or less clay, it is less unstable. Alluvial silts deposited by flood events in semi-arid basins, some tropical soils and some artificial fills may all exhibit collapse on saturation. Collapse potential is highest in soils with dry density < 1 -5 t/m 3, liquid limit < 30, and moisture content < 15% in dry climate zones. Potential can be recognized by consolidation test with saturation part of the way through the loading cycle.
