Theory and Method
Discharge capacity describes the volume of water a drain carries along its own axis per unit time under a defined hydraulic gradient. Darcy's law relates that flow to the gradient, so the test holds the gradient constant and measures the volume collected over a timed interval. Dividing the flow rate by the applied gradient yields qw, the discharge capacity value reported to the designer. The relationship assumes laminar flow, so the method restricts gradient values to keep turbulence out of the result.
Confining pressure changes the answer. A drain buried under an embankment carries lateral earth pressure that compresses the plastic core, narrows the flow channels, and presses the filter jacket into the grooves. The tester reproduces that condition, loading the specimen anywhere from 10 to 350 kPa while flow continues through it. Running the full pressure range shows the laboratory how much capacity survives at working stress and where the drain loses the margin the consolidation design counts on.
Water conditioning protects the measurement itself. Gas released from solution inside a narrow drain channel forms bubbles that occupy flow area and cut the collected volume, which reads as a capacity loss the drain does not actually have. The conditioning tank de-aerates the supply water, and the dissolved oxygen analyzer confirms the condition before testing starts. Temperature control works on the same logic, since water viscosity falls as temperature climbs, and the ±0.5 °C hold keeps results comparable between specimens.
