Abstract: This study employed a composite organics-amended bentonite in creating soil-bentonite backfills for the construction of a vertical barrier. The backfill consisted of in-situ clayey soil and bentonite amended with tetramethylammonium chloride (TMA) and sodium carboxymethyl cellulose (CMC), hereinafter referred to as STCMB. The vertical barrier was constructed using the excavation-backfill method, with dimensions of 10 m in length, 0.6 m in width, and 5 m in depth. For comparison, a conventional (unamended) soil-bentonite (SCB) vertical barrier with the same dimensions was also constructed. Field slug tests and laboratory flexible-wall hydraulic conductivity tests were conducted to evaluate and compare the hydraulic conductivity of the two types of vertical barrier materials under field and laboratory conditions. The results of field slug tests revealed that the hydraulic conductivity of STCMB was approximately 38% lower than that of SCB, with measured values of 8.1 × 10 -11 m/s and 1.3 × 10 -10 m/s, respectively. Based on previous study, this could be attributed to the synergistic interactions between TMA and CMC, which facilitated the formation of a three-dimensional hydrogel network structure among bentonite particles. This structural configuration enhanced the tortuosity of fluid transport pathway, thereby reducing the hydraulic conductivity. Flexible wall hydraulic conductivity test results showed that the hydraulic conductivities of STCMB were 8.0 × 10 -11 m/s in tap water and 6.9 × 10 -11 m/s in 1000 mg/L phenol solution. In contrast, the SCB exhibited higher hydraulic conductivities of 1.4 × 10 -10 m/s in tap water and 1.6 × 10 -10 m/s in 1000 mg/L phenol solution. The ratios of hydraulic conductivity in tap water to that in the phenol solution for STCMB and SCB were 1.159 and 0.875, respectively, indicating minimal impact of the phenol solution on hydraulic performance and demonstrating favorable chemical compatibility. Furthermore, the hydraulic conductivities obtained from the field slug tests closely aligned with those obtained from the flexible wall hydraulic conductivity tests. With respective advantages in test scale and parameter control, these two methods collectively facilitate a comprehensive evaluation of the hydraulic conductivity of soil-bentonite vertical barriers.