A method to relate material's Young's modulus and the Kn( normal stiffness)

When analyzing sphere collision, the linear contact model in YADE calculate the Kn using Kn=EiDi X EjDj/(Ei*Di+EjDj),where E is the Young's modulus and D is the diameter of the particle. If it is not a sphere, one can use the equivalent sphere diameter with the same volume.
My simulation is about a rock impacting a slope where the slope is not made of a lot of bonded particles but just a rigid plate. The plate is assumed to have max D compared with the rock, which makes the equation goes like Kn=E(rock)D(rock). If we simulate a rock with the volume of 1 cubic meter, and the common value for a rock's Young's modulus is around 50 GPa, we will reach the value of Kn equaling 1e10. Does that make sense ? I did not simulate such huge dimension(size) [ I used to analyze microscopic issues], nor have I adopted such such big Kn before.
And I know a more popular way is to simulate the slope with a layer of particles bonded together. Any suggestions ?