Effect of Perforation Arrangement on Entrance Head Loss of Horizontal Drains and Selection Criterion for Permeability of Geotextile Envelope

Horizontal drains (HDs) are used widely for effective groundwater management in agricultural lands and geotechnical applications. These drainage systems facilitate water flow through perforations in the HD pipe wall, causing a head loss known as entrance head loss (he) across the pipe wall. This he typically is added to the groundwater table, impacting the achievement of the anticipated groundwater level. The extent of he is influenced by both the perforation arrangement of the pipe and the material used for the pipe envelope. In this study, a numerical model, which was validated using a physical sand tank model, was utilized to simulate HDs and assess the impact of perforation arrangement and envelope material permeability on he. The analysis revealed a significant influence of the perforation arrangement on he, with closely spaced smaller perforations exhibiting lower he values than widely spaced larger perforations. Under the specified model conditions, he accounted for 13%–43% of the head loss. Envelope wrappings, such as geotextiles, commonly are employed in HDs to prevent particle loss from the surrounding soil through the perforations. The study demonstrated that geotextiles can substantially reduce he, thereby enhancing the overall performance of HDs. Notably, the effect of he becomes negligible when the permeability of the geotextile exceeds 500 times that of the surrounding soil. An investigation of 25 geotextiles available on the market revealed significant anisotropy in permeability; the in-plane permeability could be as low as 1/500th the cross-plane permeability. This discrepancy can lead to a substantial increase in he. Consequently, the study suggests a selection criterion for geotextile envelope permeability to minimize he, emphasizing the need for careful consideration when choosing materials for HD applications.