induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
Chassis ground is meant to protect people from shocks. It is a protective ground which is connected to the earth ground. Earth ground is a metal rod really dug deep in ground outside the building to sink current and provide ground zero. Signal ground need not be connected to this, Signal ground is a common return path for the signal.