Whether you’re building a highway, a house, or a hotel, subsurface drainage cannot be overlooked.

We’ve all seen it happen. Construction projects that undergo meticulous planning are completed, and then lo and behold, begin experiencing moisture problems in the walls and floors only weeks/months after construction wraps up. 

The culprit? Too many times is subsurface drainage overlooked when designing new houses, public spaces, and roads. Not even agricultural facilities are safe from subsurface drainage issues, which require these systems to prevent crop flooding. 

In this article, we examine the importance of subsurface drainage in road, building and agricultural designs and provide examples of instances where subsurface drainage can be of the greatest benefit.

Subsurface drainage in road design

On the surface, highways and pavements appear to dry quickly after rain. But this is not always true! Moisture can easily be absorbed by these road materials, which can lead to large cracks or potholes in the road.

While pavement sealants are effective in preventing absorption from the surface, they won’t stop moisture from seeping into pavements through the wet soil underneath the road. This is where subsurface drainage can be used as an effective tool in road design.

Typically, there are only a few layers under asphalt or pavement that form a road’s subsurface drainage system. These include:

• A permeable base to provide for rapid removal of water which enters the pavement structure. Asphalt treated permeable base layers may strip and become clogged with fine particles, thus weakening the overall pavement structure.
• A method of conveying the removed water away from the pavement structure. This can consist of anything from a base sloped towards a drainage ditch or a pipe collecting system.
• A filter layer (such as a geotextile – read more about geotextiles in our recent blog post) to prevent the migration of fine particles from moving into the permeable base from the subgrade, subbase or shoulder base material. Excess fine particles in the permeable base will clog its drainage routes and render it ineffective. Depending upon the subgrade and pavement structure a filter layer may not be used.

Subsurface drainage in building design

Specifying adequate and reliable subsurface drainage to complement the waterproofing as a complete system is critical for any building design. This is especially true for those designs that include balconies, green roofs, decks, flat roofs, and terraced buildings.

Saturation of tiling and topping screeds can result in water finding its way into the substrate. The water then becomes locked in place until it either evaporates back out through the path of ingress or leaks through the substrate to the level below. Constant water saturation underneath tiles can also lead to the debonding of tiles and the deterioration of the waterproofing membrane. Salts and minerals that are present in the concrete can be drawn up from the tile bed through the grout. This becomes more of a problem as the moisture evaporates, leaving the salts and minerals to crystallise and build-up efflorescence (lime/calcium deposits) that appear as white coloured staining on tiles.

Subsurface drainage can help to remove water from the surface under buildings. The purpose of subsurface drainage in buildings is to provide a permeable base layer that serves as an escape route for excessive rain and stormwater that may find its way into the substrate. 

Stormwater can pass through a property in three forms:

• surface run-off (overland flow)
• piped flow
• subsurface flow (groundwater)

While not all properties will require a formal subsurface drainage system, most retaining walls and recessed buildings will require additional drainage. A subsurface drainage system is usually needed behind retaining walls to reduce constant water seepage, as well as adjacent to any walls that are recessed into the earth.

Subsurface drainage in agricultural design

Many soils throughout the world remain wet for several days after a rainfall without adequate drainage, preventing timely fieldwork and stressing growing crops. Saturated soils don’t provide sufficient aeration for crop root development and can be a significant source of plant stress.

That’s why artificial drainage of poorly draining soils has become integral to maintaining profitable crops. Some of the world’s most productive soils are drained, including 25 percent of the farmland in the United States and Canada.

Drainage helps the soil dry sooner, which provides timely field operations (planting, harvesting and other processes). Earlier drying of the soil provides earlier seed germination because the soil warms faster in spring. Drainage helps reduce surface runoffs by allowing water to infiltrate the soil and flow into the perforated drainpipes rather than flowing over the land. This also helps to make crop yields become less variable. The overall impact of subsurface drainage is a healthier, more productive soil with more stable crop yields.

It’s important to note that in areas where the aquifer is in decline, drainage provides fewer opportunities for groundwater recharge due to less deep percolation.

Several options are available for draining excess subsurface water to a specific depth from the soil profile. The most suited drainage system is influenced by:

• soil type
• topography
• rainfall
• outfall type and location.

How to solve subsurface drainage problems?

While drainage systems can be expensive to install, if they’re well maintained, their benefits outweigh the initial cost.

Subsurface drainage problems usually call for the knowledge of geology and the application of soil mechanics. Project engineers should request assistance from geotechnical engineering professionals for projects involving cuts, sections depressed below the original ground surface, or whenever the presence of groundwater is likely. 

As with any remedial system, there are potential risks involved with having an unskilled person attempt this type of work.

• Ineffective positioning of weep holes
• Damaging other finishes
• Blocking drains
• Damaging pipework or puddle flange
• Drilling through the membrane system
• Insurance coverage

It’s important that project designers consider the potential for large fluctuations in groundwater levels. Wet periods after several years of drought, or changes to recharge practices can lead to considerable rises in groundwater levels. For tunnel, structure abutments, or other structure projects which might require relief of hydrostatic pressures, it’s best to contact a geotechnical professional to help with the hydrostatic design, which will usually be based on a geotechnical design report. 

Ultimately, when it comes to the ground, you only get one chance to get the subsoil drainage system right!