Dependency of the VFF on the compressive stress. The three curves on the left that cross at p = 10 Pa are artificial displays of Eq. (5), each with p_m = 10 Pa, φ_s = 0.15, and φ_d = 0.64 but with different values of ∆, namely 0.1,0.5, and 0.9, respectively. The two curves on the right are fit functions of Eq. (5) to the data of Meisner et al. (2012) and Omura & Nakamura (2021), respectively. Both datasets were fitted using Method B2, while the data of Omura & Nakamura (2021) were additionally fitted using Method B3 to account for the anomalous increase in VFF observed at pressures above ∼2 ∙ 10⁶ Pa. Solid lines indicate the measured pressure range, whereas dashed lines represent extrapolations of the fitted compression curves beyond the measurement range. All turnover pressures p_m are marked by filled circles.