Abstract: The shape of grains is one of the main factors influencing the compaction, mechanical and hydraulic properties of breakstone material. Two sets of limestone gravel with diameters of 2-5 mm and 5-10 mm were examined using the image measuring instrument and special fixture, from which the grain appearance images were obtained at different rotation angles. The geometric sizes of grains were measured with graphic software. The common grain shape parameters were calculated from each image at different rotation angles. The mean values of each parameter at different rotation angles for each grain were analyzed, so that the influences of human factors on measured value at just one section were avoided on grain shape evaluation. All the grain shape parameters of limestone gravel show significant differences compared to spherical grains, and the differences of the set with larger size are more obvious. The grain shape parameters of the two set samples can be characterized by skew distribution. The parameters of length-width ratio, flatness and sphericity can more sensitively reveal the differences. Compared to other parameters, it is more straightforward to determine the length-width ratio and sphericity.

Key words: grain shape, geometric size, shape parameter, measurement, statistical analysis