Below-Ground Drainage (Leaving Cert Construction Studies): Revision Notes

Below-ground drainage

Below-ground drainage systems transport wastewater from where it enters the underground pipework to either a public sewer or a private septic tank. This critical infrastructure requires careful design, installation and maintenance to function effectively.

Understanding siphonage problems

Siphonage occurs when pressure differences create suction that can empty water traps, allowing sewer gases to enter buildings. Three main types affect drainage systems:

Self-siphonage

This happens when the waste pipe beneath an appliance runs completely full across its diameter into the stack. The flowing water reduces pressure inside the waste pipe, creating suction that pulls water from the appliance trap.

Induced siphonage

When water flows down through the stack past a connecting waste pipe, the main flow creates suction within the waste pipe. This pulls water from the connected appliance trap.

Back pressure

This develops at the bottom of the stack, typically where it changes direction. Water flow slowing through the stack increases pressure, which builds up in the waste pipe and can force the appliance seal to fail.

Function and design principles

The primary role of below-ground drainage is moving wastewater from underground entry points to sewerage systems or septic tanks. In urban areas, homes typically connect to local authority sewers, whilst rural properties often use septic tanks with percolation areas.

Since underground pipework requires minimal maintenance once installed, the design must prioritise reliability:

Materials and installation components

Modern installations commonly use uPVC (unplasticised polyvinyl chloride) piping due to its strength, flexibility, chemical resistance and ease of installation. However, concrete, cast iron and clay pipes are also suitable for underground drainage.

Access points for maintenance

Underground drainage systems require various access points for inspection and maintenance:

Manholes

Used when drain depth exceeds 1 metre. These substantial structures need cast iron, pressed steel or precast concrete covers due to loading requirements. Internal dimensions range from 1200 x 750mm to 1200 x 840mm, built on minimum 150mm concrete bases.

Inspection chambers

Installed for drain depths up to 1 metre. Modern construction often uses uPVC products rather than traditional concrete and brick. These provide access at key locations: drain heads, level changes, direction changes, and new connections to main drains.

Gully grates

Surface drainage points that connect to the underground system, typically handling rainwater and some waste discharge.

Access junctions and rodding points

Additional maintenance access points positioned strategically throughout the system to enable clearing potential blockages.

Backdrop manholes

Specialised manholes that handle significant level differences in the drainage system.

Ventilation requirements

Drainage systems connecting multiple properties need proper ventilation. Install vents at each house connection or wherever branch lengths exceed 6 metres. When pipework changes direction, use the largest possible radius to maintain smooth flow and reduce blockage risk.

Ensuring watertightness

Preventing leakage requires installing rubber seals at pre-formed pipe ends. Connect pipes squarely to avoid damaging these seals and maintain system integrity.

Preventing blockages

Several design considerations help prevent blockage formation:

• Internal pipe surfaces must remain smooth
• Use adequate pipe diameter (typically 100mm minimum) for expected flow volumes
• Install pipes at correct gradients
• Use large radius bends for direction changes
• Prevent any objects projecting into the pipework

Installation specifications

Gradients and depths

Drainage pipes connecting to sewerage systems require proper gradients and installation depths. Standard pipe diameter is 100mm, with gradient requirements varying based on the number of connected dwellings and system span.

Trenching and backfill

Excavate trenches and backfill with aggregate to embed drainage pipes properly. The pipe bottom must sit 550mm below ground level, with 300mm topsoil over 500mm aggregate depth.

Inspection chambers

These provide drain access for regular inspection and maintenance. Position them at drain heads, level changes, direction changes, and where new connections join the main drain.

Chambers can use traditional concrete and brick construction or modern uPVC products that offer easier, cheaper and faster installation. The chamber sides slope or 'bench' to direct overflow back into the drainage channel.

For depths up to 1 metre, standard inspection chambers suffice. Beyond 1 metre depth, the structure becomes a manhole requiring stronger construction.

Testing the system

Underground drainage must be tested before backfilling to ensure system integrity. Three testing methods are available:

Air test

Water test

Smoke test