Formation and collapse of metastable particle packings and open structures in loess deposits

Structure collapse and subsidence represent major geotechnical problems, particularly in areas containing loess, which is a widespread collapsing soil. The necessary open soil structure is formed by aeolian deposition of the constituent particles. Various aspects of the open structure can be modelled by simple Monte Carlo methods. These show how the typical vertical pores may be formed and allow certain basic structural types to be identified. By various methods, artificial airfall loesses can be recreated in the laboratory and the collapse behaviour studied by oedometer experiments. Large void ratio specimens (e > 1) can be produced, which have classic collapse behaviour. The artificial specimens mimic the behaviour of real loess samples remarkably well. The results suggest that the nature of the loess material may be as important with respect to structure collapse and related effects as the mode of sedimentation. The key controls are particle size and shape, the bond/weight ratio (R), the nature of the interparticle contact bonds, and the clay content (which modifies the latter two effects). In the geometrical models four pore types, six major substructural types and three collapse processes are identified.