A microstructural investigation on hydraulic conductivity of calcareous clay

Calcareous clay is an important fine-grained component of the reclaimed island foundation. It is commonly incorporated with calcareous sand to form a mixture interlayer. Hydraulic conductivity is a crucial engineering property of foundation soil significantly influenced by fine particle composition. Thus, to investigate the role of calcareous clay in the seepage of sand-clay mixture, a series of consolidation tests and hydraulic conduction tests were conducted on calcareous clay and calcareous sand-clay mixture, and the effects of hydraulic gradient, void ratio, calcareous sand content and particle size on the hydraulic conduction were discussed. The test results indicated that the hydraulic conductivity of calcareous clay was maintained at the order of magnitude of 10⁻⁶ cm/s, and the implications of hydraulic gradient and void ratio were minimal. When mixed with the coarse calcareous sand, with the increase of sand content, the hydraulic conductivity of the sand-clay mixture was initially held at 10⁻⁶ cm/s, subsequently escalated rapidly to 10⁻³ cm/s when the sand content was between 30 % ∼ 80 %, and finally stabilized at this order of magnitude when the sand content exceeded 80 %. Based on this phenomenon, a three-stage model saparated by two critical ratios of calcareous sand-clay mixture seepage was proposed: no skeleton stage, partial skeleton stage and complete skeleton stage. The inference was verified by the stability of the hydraulic conduction results in the gradient reversal test. Furthermore, to analyze the seepage performance from the microstructure, several field emission scanning electron microscope tests were carried out after hydraulic conduction, and the pore size distribution was quantified. The incorporation of calcareous sand led to the transformation of the peak pore in the mixture from nanopores with the pore size less than 0.1 μm to macropores with the pore size exceeding 10 μm. Additionally, the correlation between the unimodal/bimodal model of pore distribution and the three-stage model of mixture seepage was established, and a microscopic explanation of the hydraulic conduction rate and stability of calcareous sand-caly mixture was provided.