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Under existing conditions at depths on the order of hundreds of meters gravitational force is comparable with the strength of rocks, and with an increase in depth it becomes controlling.Therefore, in models which pretend to model the processes and structure of the actual earth's crust it is necessary to consider the action of gravitational force.Consideration was given in [1][2][3] to the behavior of the upper layer of a two-layer model represented by a thin layer of wax or paraffin in which gravitational force was small compared with forces of materi~l strength, and it was possible to disregard it. With tension for the lower layer of this model breakdown of the upper layer was observed as a system of tensional cracks.The initial cracks played an important role by directing the sequence of formation and position of subsequent cracks.In this work a model is considered in which the upper layer is almost dry sand, i.e., a material where gravitational force is controlling. With tension for the lower layer, formation of tensional cracks over the whole layer thickness becomes impossible in this model. With initiation at the lower boundary of the layer, only a layer of thinning arises. Above it without interruption as the crack edges separate there is collapse of sand along certain slip planes.In addition, it is evident that under actual conditions, crack formation should precede uniform deformation of the whole layer (shell). Only at a specific stage of deformation is the uniform deformation field disrupted and conditions created for forming a zone of increased deformation followed by development of discontinuities in the form of zones of thinning. Above them collapse commences.A study of mechanisms for deformation and breakdown of the upper layer of a two-layer model in which gravitational force is controlling with uniform tension or compression for the lower layer is presented in this article. UNIAXIAL DEFORMATIONA diagram of the experimental device is shown in Fig. i. On a sheet of rubber i00 • 70 cm in size a layer of sand 2-20 cm thick is poured.To remove the effect of boundary walls they were discarded after pouring the sand layer. As a result of this, natural slopes formed with angles 33-36 ~ along the edges of the layer (block) of sand.The underlying layer of rubber was stretched along the greater sheet dimension by two horizontal jacks.In order to avoid rubber compression in the perpendicular direction, the long sides of the rubber layers were secured in fixing grips, which, however, did not interfere with stretching the rubber. A prior study of the deformation field for the rubber surface (without the sand layer) showed that to a first approximation it may be considered uniaxial.With rubber deformation up to 5% no visible breakdown of the sand surface is observed. (In order to observe changes better, the surface of the white sand was powdered by a similar dark colored sand). The first breakdown at the surface was seen in the form of a system of subparallel wavy lines which gradually developed as fault fissure...