Parametric study of the nonlinear failure envelope model parameters. Top row: the shear strength as a function of normal stress. Middle row: the apparent friction angle as a function of normal stress. Bottom row: the apparent cohesion as a function of normal stress. The left column shows the influence of changing φ_∞ within the range of 25° to 45°. The second column shows the influence of changing Δφ within the range of 5° to 25°. The third column shows the influence of changing σ_h within the range of 1 to 1000 kPa. The fourth column shows the influence of changing c within the range of 0 to 100 Pa. The black lines show the values taking into account the nonlinear model and the red lines show the linear Mohr–Coulomb model results (derived from a linear fit to data in the range [σ_h, 10⁴] kPa, where η ≥ 1). The line style (for red and black lines) indicates the parameter value as indicated in the legend. Only one variable is modified at a time. The reference values of the parameters when not being varied are (the same used in Figure 2) φ_∞ = 35°, Δφ = φ₀ − φ_∞ = 10°, σ_h = 10 kPa, and c = 0 Pa.