Abstract Computer simulation is used to study the effect of particle size distribution on the structure of sediments of hard spheres in three dimensions. The sediments are obtained by ballistic random deposition according to the rain model, followed by a compression process by means of Monte Carlo simulation. It appears that the Monte Carlo technique is an effective method to obtain loose random packings with densities up to 0.60, but that the method is less effective in producing dense random packings with densities of about 0.64. The polydisperse packings clearly show a size segregation effect. The structure of the sediments changes from medium-range ordered to short-range ordered if the width of the Gaussian particle size distribution is increased from 0 to 0.5 mean particle radius units. The pore structure of the sediments is investigated on a 100 3 grid by means of the Hoshen—Kopelman algorithm. It is shown that the pore structure of the loose random packings is such that particles with radii up to 0.15 units can percolate through the sediments.