Recent concept of mechanically stabilized granular layers is described. The penetration of grains into geogrid apertures result in .interlocking phenomenon. By this the grains are immobilized and such part of the layer is stiffened. The deformability of a geogrid plays important role in that mechanism. Results of laboratory experiments where deformation of geogrids with interlocked granular grains were measured are described and evaluated. The data are compared with other data both from laboratory and field observations