Loose flooded and nearly saturated sand deposits from lignite mining in East Germany tend to collapse along slopes, leading to devastating avalanches. The local state of the soil is characterized by volume fractions and partial stresses as is usual in soil mechanics. In addition, an intergranular strain is introduced which is needed for the analysis of wave propagation. For the investigation of the field state in situ, frozen samples have sometimes been taken, and three novel sounding methods have been introduced: vibratory penetration; large vane with torsion shocks; and pressure shocks by air gun. The evolution of field states is described by a hypoplastic constitutive relation, the parameters of which can be estimated from granulometric properties. The influence of gas bubbles on the response to undrained loading cycles is explained. The loss of stability can be analysed by estimating the possible spontaneous release of kinetic energy, whereas conventional methods are insufficient. A deeper understanding of the loss of stability is obtained by analysing the propagation of transversal and longitudinal waves which are coupled by dilatancy or contractancy. Finally, a scenario for the onset of an avalanche is proposed which includes the formation of a gas cushion.