When coarse particles are transported in pipes, mechanical friction occurs near the pipe invert, and hence large pressure gradients are required. This behaviour is described by the sliding bed model, from which an approximate expression for the solids effect has been derived. Pilot-plant tests were carried out with coarse gravel in a large recirculating pipe loop. Initially, all particles were larger than 10 mm in diameter, and the solids effect was in good accord with the equation derived from the sliding bed model. However, as the testing continued the particles degraded, and the pressure gradient diminished significantly. It was seen how the solids effect changes with exposure time or, equivalently, with the number of passes through the pump.