Hydraulic properties of smectite rich clay controlled by hydraulic gradients and filter types

•The hydraulic conductivity evaluated from percolation tests depends on the gradient.•Low filter porosity clogged by clay gel gave too low conductivity at high gradients.•Gradient limitation in the ASTM can be raised to 100 m/m for dense smectitic clays.•A theoretical model can be used to predict the critical gradient.

The common practice in geotechnical laboratories is to apply high hydraulic gradients for getting results in reasonable time but this can jeopardize the accuracy. In this paper, the hydraulic conductivity of a smectite rich clay from Iraq was determined under different hydraulic gradients (5 to 10,000 m/m) using several densities and two permeants. Also, two types of filters were used, ordinary stainless steel sintered filter and sand/silt filter, in order to examine the possible effect of clogging by dragged clay gel particles. It was concluded that the outflow filter can affect the evaluated conductivity especially when applying high hydraulic gradients. For sintered filters the hydraulic conductivity was reduced as the gradient increased, while the conductivity increased as the hydraulic gradient increased when using sand/silt filters. For salt water the impact of the gradient was less obvious than for distilled water. A theoretical model was derived for selecting safe hydraulic gradients as a function of dry density, swelling pressure and permeant type. A major conclusion was that the gradient in laboratory testing should not exceed 100 m/m.