ABSTRACT
13 Weathering and Soils GROUND CONDITIONS Top few metres of the ground profile generally consist of soi l , drift and weathered rock, with engineering properties very different from those of the underlying bedrock. Soil: mixture of weathered mineral debris and plant material, usually <1 m thick; may divide into plant-rich topsoil and clay-rich subsoil. Weathering: the natural decay and breakdown of rock or drift in contact with air and water; generally <10 m deep. Drift: transported, superficial sediment deposited on top of the bedrock; mostly unconsolidated clay, sand and coarser clastic debris; generally Quaternary age, hence too young to be consolidated; varies in thickness from 0 to > 50 m. Colluvium: slope debris, moved downslope largely by gravity alone; extent of sediment transport therefore drift > colluvium > soil ; includes debris from creep and sheetwash, also head and scree. Sheetwash by surface water increases greatly with loss of vegetation. Rockhead: the buried drift/rock interface; commonly a conspicuous boundary between weak soils and drift and strong rock; may be less well defined in deep profile of weathered rock; formed as erosion surface before drift deposition so its topography may be totally unrelated to modern surface. Engineering soil: weak material (UCS < 600 kPa) that can be excavated without ripping or blasting, therefore including soil, drift, weak rocks and weathered rocks.
