normal compression line is a boundary to all possible states for isotropic compression:
later we will see that this state boundary line forms part of a state boundary surface.
This is also a yield surface like that illustrated in Fig. 3.16 and the NCL is a yield curve
like that shown in Fig. 3.12.
At any state such as B inside the boundary surface the soil is overconsolidated and
its overconsolidation ratio is