Relative stress σ₁/σ₀ for the adopted four basic systems of packing plotted against the relative density D = 1 − φ (after Rao & Chaklader 1972, their Fig. 6). The lines are truncated at the respective critical density D_c (see Table 1), above which the effective stress is computed using a linear relationship between the value at D_c and the lithostatic pressure.