Knitted-sock geotextile envelopes increase drain inflow in subsurface drainage systems

A knitted-sock geotextile envelope is commonly used in the USA, Canada, and parts of Europe to prevent sediment entry into agricultural subsurface drain pipes. Sand-slot pipes have a narrow width to prevent sediment from moving into the drain pipe, and they are used in the USA and Canada as a cheaper alternative to knitted-sock-wrapped pipes. The type of drain pipe (sand-slot or sock-wrapped) is important in subsurface drainage design because it affects the entrance head loss caused by the convergence of flowlines toward the drain perforations, thereby influencing the effective radius of the pipe, which in turn affects drain inflow. Although the theoretical effective radius of a knitted-sock-wrapped pipe can be estimated based on analytical equations, no study has experimentally measured it to verify the theoretical equations. This study had two objectives: (1) Experimentally measure the effective radius of sock-wrapped and sand-slot pipes to verify the theoretical method, and (2) comparatively evaluate the drain inflow performance of a sock-wrapped pipe and a sand-slot pipe. We tested the pipe materials in a sand-tank experiment equipped with manometers. The results showed that the measured and theoretical effective radii were 5.5 and 5.6 cm for the sock-wrapped pipe, 0.7 and 0.7 cm for the 4-row sand-slot pipe, and 1.6 and 1.6 cm for the 8-row sand-slot pipe, respectively. This agreement confirmed that the theoretical method reliably estimated the effective radius of the pipes. The sock-wrapped pipe had a considerably lower entrance head loss (0.4 cm) than the 8-row (6.1 cm) and 4-row (10.4 cm) sand-slot pipes, thereby increasing the effective radius of the sock-wrapped pipe. The estimated drain inflow of the sock-wrapped pipe was 12% and 20% higher than that of the 8-row and 4-row sand-slot pipes, respectively. In conclusion, we verified that the theoretical method accurately estimated the effective radius of the pipes and demonstrated that the sock-wrapped pipe had considerably higher drain inflow than the sand-slot pipe.