The motion of a stone embedded in non-cohesive soil disturbed by a moving tine

As part of an investigation into impact damage on soil-working implements, a glass-sided model box has been used to study the motion of 10, 50 and 100 mm diameter hemispheres in sand as a 38 mm wide tine inclined at 45° approaches. The observations were made using high-speed photography. It was found that the sand did not always cause the hemisphere to move before contact with the tine, and that motion was determined by the position of the centre of the hemisphere relative to the boundary of soil disturbance ahead of the tine. This effect was independent of velocity. A minimum size of hemisphere was found below which motion always began before contact was made with the tine. In the particular arrangement used this was about 20 mm. Movement of the hemisphere before contact reduced the contact stresses and the practical implications of this are discussed.