High-concentration bentonite grouts require chemical modifications to increase its penetration through soils; however, this modification complicates the evaluation of the maximum penetration distance due to the changes in the physicochemical and rheological properties of the bentonite grout. In this study, the yield stresses of 7.5%, 10%, and 12% bentonite grouts modified by 1–4% sodium pyrophosphate (SPP) (by dry weight of bentonite) were determined by matching a constitutive model (Herschel–Bulkley model) to the shear stress–shear rate curves obtained through rheological tests. The bentonite grouts were injected into sand columns with an injection pressure of 35 or 140 kPa. The maximum penetration distances of the bentonite grouts through the sand columns were calculated using analytical equations (based on the yield stress of the grouts), and then compared to the measured maximum penetration distances. The results showed that the analytical equations could capture the maximum penetration distance at high yield stress (greater than 28 Pa), but the equations significantly overestimated the maximum penetration distance at low yield stress due to filtration. A new filtration model is presented in this study to estimate the maximum penetration distance of the SPP modified bentonite grouts that can provide better prediction, particularly for grouts having low yield stresses. The model was calibrated based on the 1-D column tests and then independently validated with an additional set of column tests using a bentonite with different chemical composition.