Subsurface Drainage

The concept of subsurface drainage can vary from simple to complex depending on the predominant form of moisture ingress.

The intent is to capture and remove and free-water entering the pavement materials, particularly in areas where moisture ingress is of concern.

Moisture conditions naturally fluctuate within a pavement structure from the point of construction until equilibrium moisture contents are achieved and can vary from years to decades, depending on the surrounding soil profile, rainfall and drainage conditions.

Where free-water is at risk of entering the pavement, it is generally attributed to:

1. Distress in the surfacing – where the surface is heavily distressed and has lost its waterproofing characteristics, the pavement is exposed to moisture ingress, particularly if the surface shape or the waterproofing seal is compromised
2. Grassed medians/nature strips – a common area for moisture ingress which typically interface directly to the pavement box
3. Poor pavement drainage – where culverts are backed up, table drains have reached capacity or stormwater pits are clogged, sustained periods of inundation can result in capillary rise through adjacent materials, allowing moisture to enter the pavement structure
4. Subsurface Geological Springs and/or moisture paths through rock faces – whilst not so common, subsurface geological features can act as a natural path for transferring moisture into the pavement structure

The use of subsoil drainage is typically characterized by an arrangement of shapes and forms but ultimately takes the form of a trench running adjacent to or beneath the pavement structure. Depending on the source and severity of the moisture ingress, the location and configuration of the subsurface drainage can vary, including:

1. Parallel Subsoil drains running adjacent to the pavement box aimed at intercepting moisture before entering the pavement
2. Perpendicular Subsoil drains that run beneath the pavement, typically used to drain high-side longitudinal drains
3. Rock-blankets placed underneath the pavement, allowing the water to freely flow beneath the pavement box and discharge at nominal locations

Conventional subsurface drainage infrastructure is constructed using a permeable material which commonly comprises a nominal size stone to ensure a high void content and free-draining characteristics. Where the drains are to be adopted in areas of structural significance, including underneath pavement shoulders, the use of no-fines concrete is also adopted.

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For more information on Global Road Technology or subsurface drainage please contact GRT.