ABSTRACT

The loess of the semi-arid northern half of Gansu Province, China, is mantled by a variable thickness of Quaternary loess, the greatest recorded thickness being 400m. The region is subjected to frequent earthquakes and torrential summer monsoonal rains and this, taken together with the tendency of the younger loess units to collapse and hydroconsolidation, gives rise to recurrent and serious geological hazards. Landslides and debris flows threaten both urban and rural concentrations of population. Although the Gansu loess is rather uniform in its mineralogy, fabric, ambient moisture contents and chemistry, important diffferences exist because of variations in age and the extent of weathering and re-working. In addition, the sequence contains up to 37 buried palaeosols (ancient soils) which have significantly different properties. A classification for engineering purposes is proposed, based on both age (stratigraphical position) and pedogenesis (weathering). Water content is a vital variable in the loess. Ambient moisture contents are only 8–12 per cent. However, mechanical enlargement of the frequent joints, fissures and biogenic discontinuities gives rise to a widespread three-dimensional system of sinkholes and pipes (up to 2m in diameter) known as “loess karst”. This provides important by-pass drainage and so plays a role in slope failure. The Lanzhou loess is also rich in salts: carbonate may be a substantial source of strength, but washing out of sulphates and carbonates loosens the fabric and leads to collapse. There is a consistent relationship between loess age and weathering and its geotechnical behaviour, thus providing a simple but reliable guide to slope hazard assessment in this region.