Study on the immobile water of saturated compacted clay-rich materials by oil seepage

Immobile water is defined as a firmly bound thin film of water surrounding clay particles that somehow resembles solid features. The characteristics and amount of immobile water depend on pore water chemistry, the type of clay, and the contact area of water with particles. Seepage can occur only when the hydraulic gradient increases sufficiently. To date, no accurate method is available to determine the immobile water content. Therefore, an approach is proposed for direct measurement of the immobile water content, which applies vegetable oil to displace water in saturated compacted clay-rich materials. Under a high hydraulic gradient, the vegetable oil completely displaces the free porewater in the clay-rich materials, and the remaining residual water is assumed to be immobile water. By using carbon tetrachloride (CCl₄) as an extracting agent, the oil in the permeated sample was subjected to extraction and distillation, and the total liquid content was determined by the oven-drying method. As a final step, the immobile water content in the sample was calculated. A high-speed centrifugation method was used to determine the immobile water content from the corresponding soil water potential, which accounted for approximately 634.3 kPa. It was found that the immobile water of the clay-rich materials was essentially equal to the bound water. The effective porosity of clay was obtained based on the immobile water content. By comparing the relationship between porosity, effective porosity and hydraulic conductivity, the effective porosity revealed an improved linear relationship with log (k). The test results show the oil seepage method to be useful and reliable for the quantification of the immobile water content of saturated compacted clay-rich materials.