Grain size effect

Miniature cone penetration tests in centrifuge distinctively differs from field cone penetration tests, for comparatively small cone diameters with reference to the particle size of soils, and the existence of artificial boundaries imposed by the soil container, both of which may cause influences on the penetration resistance.

Cone penetrometer reduced in dimensions in proportion to the full size penetrometer used in field cone tests to match the modeling scale factor would be too small to use. However, a full size penetrometer would be too big to use in a model. Hence, miniature cone penetrometers are preferred over full size penetrometer but at the same time scale distortions of the miniature cone must be taken into account. To determine diameter of the base of the miniature cone (B), following points should be considered. For tests using a penetrometer of same prototype diameter NB with different scaling factor N, the stress in the models are expected to be similar. But penetrometer with smaller B tends to yield higher stress in the model than in the prototype because of B/D₅₀ ratio being not constant (where D₅₀ denotes the mean grain size). The figure shows the influence of so-called particle size effect which may be significant for smaller B/D₅₀. In addition, cone tip resistance versus depth curves observed in tests tend to be bumpy and indented for smaller B/D₅₀. To avoid the particle size effect and to obtain the smooth relationship between tip resistance and penetration depth, the diameter of cone larger than 20 times mean particle size D₅₀ is recommended.