Examining the effect of pore size distribution and shape on flow through unsaturated peat using computed tomography

摘要

The hydraulic conductivity of unsaturated peat soil is controlled by theair-filled porosity, pore size and geometric distribution as well as otherphysical properties of peat materials. This study investigates how the sizeand shape of pores affects the flow of water through peat soils. In thisstudy we used X-ray Computed Tomography (CT), at 45 μm resolution under5 specific soil-water pressure head levels to provide 3-D, high-resolutionimages that were used to detect the inner pore structure of peat samplesunder a changing water regime. Pore structure and configuration were foundto be irregular, which affected the rate of water transmission through peatsoils. The 3-D analysis suggested that pore distribution is dominated by asingle large pore-space. At low pressure head, this single large air-filledpore imparted a more effective flowpath compared to smaller pores. Smallerpores were disconnected and the flowpath was more tortuous than in thesingle large air-filled pore, and their contribution to flow was negligiblewhen the single large pore was active. We quantify the pore structure ofpeat soil that affects the hydraulic conductivity in the unsaturatedcondition, and demonstrate the validity of our estimation of peatunsaturated hydraulic conductivity by making a comparison with a standardpermeameter-based method. Estimates of unsaturated hydraulic conductivitieswere made for the purpose of testing the sensitivity of pore shape andgeometry parameters on the hydraulic properties of peats and how to evaluatethe structure of the peat and its affects on parameterization. We alsostudied the ability to quantify these factors for different soil moisturecontents in order to define how the factors controlling the shapecoefficient vary with changes in soil water pressure head. The relationbetween measured and estimated unsaturated hydraulic conductivity at variousheads shows that rapid initial drainage, that changes the air-filled poreproperties, creates a sharp decline in hydraulic conductivity. This isbecause the large pores readily lose water, the peat rapidly becomes lessconductive and the flow path among pores, more tortuous.