Improvement of Collapsible Loess in Eastern Europe

Loess consists essentially of silt-sized (typically 20-30 microns) primary quartz particles that form as a result of high-energy earth-surface processes such as glacial grinding or cold climate weathering. These processes have resulted in the almost continuous deposit from the North China plain to south-east England, for example, although it is possible to isolate five major regions: North America, South America, and Europe including western Russia, Central Asia and China. These loess regions underlie highly populated areas and major infrastructure links, and are prone to collapse causing subsidence. The areas of most widespread concern are concentrated in Eastern Europe and Russia and to a growing extent in China although serious problems of potential collapse exist wherever loess is found. Natural loess typically has an open structure with a void ratio of 0.8-1.0 or more, and is found in three main forms: sandy, silty or clayey loess. The primary quartz particles are held in this condition by bonding. These bonds break down progressively as increased stresses are applied at the natural water content, but more importantly the structure undergoes immediate and considerable collapse (up to about 15%) if saturated. The structure is therefore metastable in its natural condition. Such collapse has resulted in disastrous failures that, in some cases, have caused con-siderable losses. More generally, they have proved enormously expensive in countries such as China, the USA, Bulgaria and other eastern European countries, and no less seriously, although less widespread, in countries such as the UK.
This chapter will first examine the nature and global distribution of loess, primarily focussing on the main hazards associated with this mate-rial and its consequences to the built environment. The main objective of this chapter is to present the state of the knowledge associated with the improvement of collapsible loess soils, drawing on the considerable ex-perience from eastern Europe, and in particular Bulgaria. This will be achieved by considering three questions:

1. What makes the loess problem still significant?
2. What is new in the principle of treatment of the loess base?
3. Which of the methods now being applied seem promising for the future?

Case histories, taken from Bulgaria will be used to examine design and analysis, measurement and post treatment evaluation of ground im-provement projects associated with loess soils. This will highlight the challenges and methods that can be used to overcome them.