Analysis of Hydraulic Gradient Effects for Laboratory Hydraulic Conductivity Testing

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Courtesy of ASTM International

A theoretical model is presented that characterizes the effect of hydraulic gradient on measured hydraulic conductivity in the laboratory. A closed-form equation is derived for the distribution of total head within a hydraulic conductivity test specimen as a function of specimen height, boundary stress conditions, compression index, and change of hydraulic conductivity index. From this expression, corresponding equations for the distributions of local pore pressure, effective stress, void ratio, hydraulic gradient, and hydraulic conductivity are presented. Two laboratory experiments were performed using a clay slurry to assess the validity of the theory: (1) an end-of-primary incremental loading consolidation test with direct hydraulic conductivity measurements, and (2) a rigid-wall hydraulic conductivity test with local pore pressure measurements. Using material properties obtained from the consolidation test, the theory predicted correctly the behavior of the hydraulic conductivity test specimen for two values of equivalent hydraulic gradient. It is concluded from this research that excessive hydraulic gradients applied during hydraulic conductivity testing can cause reductions in measured hydraulic conductivity. The magnitude of the effect is expected to be more important for normally consolidated soils with high compressibility, such as soft clays and soil-bentonite slurries.

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