Subdrainage

Subdrainage, or underdrainage, is an underground drainage system designed to remove excess groundwater and moisture from soil layers beneath the surface. The objective is to lower pore-water pressure, improve soil bearing capacity, and reduce problems such as waterlogging, frost heave, and foundation settlement. Subdrainage supports structural stability in buildings, roads, railways, and agricultural lands by keeping the root zone or foundation zone drier than the surrounding soil.

A typical installation comprises perforated or slotted pipes laid in a trench at a depth determined by the structure and soil conditions. The pipes are surrounded by clean gravel or crushed rock to facilitate water flow and are wrapped with a geotextile filter to prevent fine soils from clogging the system. The trench is backfilled and the drained water is discharged to a designated outlet, such as a ditch, storm sewer, stream, or a sump pump where necessary. In some designs the subdrainage is connected to a pumped drainage system to overcome flat or elevated outlets.

Design considerations include soil type (cohesive versus granular), groundwater level, drainage area, desired outflow rate, and the gradient to the outlet. Pipe diameter, spacing, trench depth, and filter materials are chosen to meet the required drainage performance. Subdrainage is commonly used around building foundations, basements, highway embankments, and agricultural fields (where it is often referred to as tile drainage).

Maintenance focuses on preventing clogging and ensuring outlet flow. Periodic inspection of outlets, cleaning of sediment buildup, and replacement of degraded geotextile or filter aggregate may be required. Poorly designed or maintained subdrainage can fail to relieve moisture, leading to persistent wet conditions or structural distress.