Beyond the Static Limit: Rate Effects on Apparent Mohr-Coulomb Parameters in Penetration Tests Using a Bearing-Capacity Framework

Penetration testing provides a simple, depth-resolved measure of the strength of soft cohesive–frictional materials, but interpretation at finite penetration velocity is not established for rate-sensitive systems. This study extends a previously introduced depth-resolved bearing-capacity inversion to controlled penetration velocities and quantifies how rate effects enter the identified Mohr–Coulomb parameters. Penetration tests were performed on a mechani- cally inert montmorillonite–glycerin mixture at constant velocities from 1 to 600 mm min−1 using a circular punch (D = 30 mm). For each velocity, the internal friction angle φ(v) was obtained from the steady-state slope and the cohesion-like parameter cpen(v) from the intercept of the surcharge-corrected stress. Independent vane-in-cup measurements with matched surface velocities provided a geometry-distinct reference trend. Across the investigated range, φ remained approximately invariant (φ ≈ 7.8°), whereas cpen(v) increased mono- tonically with velocity (from 0.49 to 0.76 kPa), mirroring the trend in vane shear strength. The results indicate that finite-velocity effects are partitioned primarily into the cohesion-like intercept, while the slope-based friction angle remains a stable descriptor of granular resistance. This supports penetration testing at practical velocities when representative static parameters are required, and motivates application to time-dependent materials where short test durations are necessary.