Beyond void ratio, what is the most significant intrinsic soil property that controls its coefficient of permeability?

The most significant intrinsic soil property that controls its coefficient of permeability, beyond void ratio, is the particle size distribution. This property fundamentally dictates the dimensions and connectivity of the internal pore network, which are the channels through which water moves. The particle size distribution describes the range of particle diameters present in a soil sample and the percentage of each size. A crucial representative measure derived from the particle size distribution is the effective particle size (D10), defined as the particle diameter at which 10% of the soil particles are finer by weight. The D10 value is highly influential because it provides a good indication of the average size of the smallest passages, known as pore throats, that water must navigate. Larger soil particles, and consequently a larger D10, create larger average pore throats. These larger, less constricted pathways offer less resistance to water flow, resulting in a higher coefficient of permeability. Conversely, smaller soil particles lead to smaller average pore throats, which significantly impede water movement and reduce the coefficient of permeability. Beyond the effective size, the gradation of the soil, which describes the uniformity of its particle sizes, also plays a role. For instance, a well-graded soil containing a wide range of particle sizes can have smaller particles occupying the voids between larger ones. This packing arrangement effectively reduces the overall pore throat sizes and connectivity, leading to lower permeability compared to a uniformly graded soil with similar average particle size and void ratio. While particle shape and mineralogy, particularly for fine-grained soils where specific surface area and surface charge become critical, also influence permeability by affecting water adsorption and pore geometry, the particle size distribution remains the primary intrinsic property controlling the physical dimensions of the pathways available for fluid flow.