Direct and Indirect Permeability of Fissured Tills

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Courtesy of ASTM International

Comparison of direct and indirect measurements of water permeability on relatively large undisturbed and compacted Scottish till specimens in rigid-wall (Rowe cells) and flexible-wall (triaxial cells) permeameters shows the interaction of macro-fabric, test method, evaluation basis, effective stress, and possibly sidewall leakage. At low stresses, the sidewall leakage in a Rowe cell is the most likely cause of the directly measured permeability, kd, and pore-pressure-dissipation-based permeability, kp, becoming higher than settlement-based permeability, ks. Similarly, fissures raise permeability measured in both permeameters. Generally, kp is higher than ks (or volume-change-based kv from triaxial cells) with ka in between but closer to ks (or kv). The differences decrease with increasing stresses; the reduction rate for fissured specimens is dependent on fissure coating thickness and permeability.

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