Distribution of subsoil stress (σ _s) and geogrid stress (σ _r) and variation of geogrid stiffness (s = 2·5 m, h _e = 6 m): (a) σ _s from B to C (edge); (b) σ _s from B to A (diagonal); (c) σ _r from B to C (edge); and (d) σ _r from B to A (diagonal)