Mechanical behaviour of an undisturbed soil subjected to loadings: Effects of load and contact area

This paper deals with the stresses and deformations induced by loading an undisturbed soil. Measurements of vertical stress and displacement were performed in two dimensions in large soil bins. The experiment included four loading treatments with two loads (F = 43 kN and 2F = 85 kN) and two contact areas (A = 0.45 m2 and 2A = 0.90 m2). The loads were applied by stiff plates to a very homogeneous and undisturbed loamy soil established in the soil bins 12 years prior to the tests. Twenty-one pressure transducers and nine displacement transducers were inserted horizontally from an inspection tunnel with minimal disturbance of soil. Vertical stress measurements included four depths (30, 50, 70 and 90 cm) and two to seven positions from the centre of load. Measured vertical stresses were compared to vertical stresses calculated with the Söhne model. Measured vertical displacements were used to evaluate the structural failure criterion. Doubling the contact area reduced stresses and strains in the topsoil, but load determined stresses and strains in the subsoil, which supports the elasticity theory. The Söhne model predicted the stresses in the soil profile with a maximum bias of 7 and 27 kPa for the low and high load, respectively. The best prediction was obtained with a concentration factor ν ≈ 5–6 and ν ≈ 8–9 for the low and high load, respectively. This indicates that the concentration factor depends on both soil mechanical characteristics and loading conditions. The model underestimated stresses when the soil deformation was large. No plastic strain was recorded if the calculated major principal stress was smaller than the precompression stress. For soil directly under the load axis, strain increased with increases in both the calculated major principal stress, if higher than precompression stress, and in the calculated minor principal stress. In contrast, for soil at the periphery of the loaded area plastic strain related better to the calculated minor principal stress and did not relate to the calculated major principal stress. This indicates that shear failure may be important, especially near the limits of the loaded area. More studies are urgently needed to evaluate the performance of the Söhne model in the field and to better understand the mechanical behaviour of pedologically mature soils.