Theory and Method
The permeability coefficient falls out of four measured quantities: the water volume passing through the specimen, the time it takes, the head difference driving it, and the thickness it crosses. Rearranging Darcy's relationship gives k = QL / (A · Δh · t), where Q is the collected volume, L the specimen thickness, A the flow area, Δh the head difference, and t the collection period. Each term carries straight into the answer, so a careless thickness reading damages the result as much as a mistimed collection.
Temperature deserves particular attention here. Hydraulic conductivity depends on the fluid moving through the pores as much as on the pore structure, and water viscosity drops roughly two percent per degree Celsius near room temperature. A laboratory that lets permeant temperature drift across a long run adds scatter unrelated to the liner under test. Holding the supply at a set point, then correcting readings to a reference temperature such as 20 °C, keeps results comparable between operators and laboratories.
The flow itself is minute because sodium bentonite seals as it wets. Montmorillonite layers pull water and hydrated sodium ions into the interlayer space, granules expand against their confinement, and the void network between them closes to narrow, tortuous paths. Confinement matters as much as hydration, since compressive stress decides how far that expansion goes. The clamping assembly must seal the perimeter tightly enough that every drop reaching the collection side traveled through the bentonite, not along the edge.
