Theory and Method
In-plane flow describes water moving parallel to the surface of a geosynthetic instead of passing through it. The behavior follows Darcy's law for laminar flow, where the volumetric flow rate stays proportional to the hydraulic gradient driving it. Hydraulic transmissivity, written as theta, equals the flow rate per unit specimen width divided by that gradient, and carries units of square meters per second. Transmissivity also equals the in-plane permeability of the material multiplied by its thickness under load, which explains why thickness and drainage capacity move together.
Normal stress is the second variable that governs the result. Compressing a drainage core closes pore space, intrudes the adjacent geotextile into the flow channels, and reduces the cross section available to water. The loss is non-linear, so a product that performs well at 20 kPa can fall short at 200 kPa. Testing across the 10 to 250 kPa range produces the flow capacity curve that designers interpolate against actual overburden pressure at the project site.
The QualiGTX™ IPF C reproduces these conditions mechanically. A servo system applies and holds the target normal load on the 100 × 200 mm specimen while the motorized reservoir sets the upstream head for a hydraulic gradient of 0.1 or 1.0. Water enters one edge of the clamped specimen, travels the length of the flow path, and discharges at the downstream edge. The operator collects the outflow volume over a timed interval once flow reaches steady state, then calculates flow rate per unit width and transmissivity from the measured discharge, specimen width, and applied gradient.
