Authors have developed an autonomous control system for wheel loaders to automate scooping and loading operation in quarries. In our previous research it is assumed that rock piles consist of homogeneous sized grains, and reaction forces from the piles during scooping are formulated and they are verified by fundamental experiments with our mechanical simulator. However, various sized rocks exist around the faces of quarries in practice. Therefore the reaction force model of such piles is necessary to automate the scooping in quarries. In this paper, we investigate the influence of the relationship between thickness of penetrating plate and sediment particle size, and tip shape of plate in penetration resistance.