This study reformulates classical bearing-capacity theory into a depth-resolved framework for extracting apparent Mohr–Coulomb parameters from quasi-static flat-punch penetration tests. The measured force–depth response is converted to the mean contact stress \(\varvec{q(z)}\) and analysed as a function of depth, with cohesive, surcharge, and unit-weight contributions. Under homogeneous, axisymmetric conditions, the internal friction angle \(\varvec{\varphi }\) and cohesion \(\varvec{c}\) are obtained from the slope and intercept of the quasi-steady \(\varvec{q(z)}\) relation. Applied to an inert montmorillonite–glycerin reference material, the method yielded reproducible results across punch diameters \(\varvec{D=20}\) – \(\varvec{40~\textrm{mm}}\) , giving \(\varvec{\varphi _{pen}\approx 4.8^\circ }\) and \(\varvec{c_{pen}\approx 0.85~\textrm{kPa}}\) . Independent vane tests gave \(\varvec{c_{\textrm{vane}}\approx 0.70~\textrm{kPa}}\) , and direct shear box tests yielded \(\varvec{\varphi _{\textrm{dsb}}\approx 4.45^\circ }\) and \(\varvec{c_{\textrm{dsb}}\approx 1.16~\textrm{kPa}}\) . The friction angles from penetration and DSB testing were consistent, while the cohesion values differed in the ordering expected from their respective interface and deformation conditions. Within the quasi-static, low-stress regime examined here, the penetration-based approach provides an efficient method for quantifying apparent Mohr–Coulomb parameters of soft cohesive–frictional materials.
