What Is Land Drainage And How Does it Work?
A land drainage system uses perforated pipe to collect water from the surrounding ground. Water passes through the slots in the pipe before flowing towards an outlet or collector drain.
Unperforated pipe has a different job. It does not collect water along its length but carries water that has already entered the drainage system. In simple terms, perforated pipe collects water while unperforated pipe conveys it.
Land drains are widely used on building sites, agricultural land, sports grounds and landscaped areas where excess water could affect ground stability or the finished surface.
What Do You Need To Install Land Drainage?
The materials and equipment will depend on the drainage drawings and site conditions, but a typical installation may require:
- Perforated land drainage pipe
- Unperforated pipe where a carrier or collector drain is specified
- Land drain couplers, junctions and end caps
- Non-woven geotextile membrane where required
- Clean, free-draining aggregate or shingle
- An excavator or other suitable digging equipment
- A rotating laser, laser level or spirit level
- Measuring staffs and tapes
- A suitable pipe cutter or saw
- Marker paint, pegs and string lines
- Appropriate PPE
- Service detection and trench safety equipment
Check the specification before ordering. The pipe diameter, fittings, aggregate and geotextile requirements can vary considerably between projects.
Before Installing Land Drainage: Plan The System
On a commercial groundworks project, the drainage route will normally be shown on the drawings rather than chosen on site. Before any excavation starts, walk the proposed runs and check that the design works with the actual ground conditions.
Review:
- Drainage drawings and specifications
- Existing ground levels
- Required invert levels
- Underground services
- Soil and ground conditions
- The proposed outlet
- Existing drainage infrastructure
- Access for plant and materials
- Pipe diameters
- The arrangement of lateral and collector drains
- Junction positions
- Inspection and maintenance access
Pay particular attention to the outlet level. A perforated pipe may collect water effectively, but it will not solve the problem unless that water has somewhere suitable to go.
Where the levels or site conditions do not match the drawings, raise the issue before digging. It is far easier to adjust a drainage design at the setting-out stage than to correct a trench that cannot reach its intended outlet.
Where Should Land Drainage Discharge?
Depending on the approved design, land drainage may discharge into:
- An approved surface water drainage system
- A suitable watercourse
- A soakaway or infiltration system
- An unperforated collector drain
- Another approved surface water management system
The correct arrangement will depend on the project and any permissions that apply. Do not assume that the nearest ditch or drain is suitable simply because it is convenient.
What Fall Should A Land Drain Have?
A land drain should maintain a consistent fall towards its outlet. There is no universal gradient that suits every system, so the fall should come from the drainage design, project specification or manufacturer’s guidance.
Use the specified start and finish levels to establish the gradient. Check the pipe at regular intervals and watch for dips or backfalls, particularly around junctions and changes in direction.
A small low point can hold water and sediment, even when the rest of the run is correctly installed.
Choosing A Land Drainage Layout
The layout will depend on the size and shape of the area being drained, the ground levels and the position of the outlet.
Common arrangements include:
- Single runs: Used to intercept water along a boundary or defined section of ground
- Parallel lateral runs: Several drains laid across a larger area and connected to a main collector
- Herringbone systems: Branching lateral pipes that feed diagonally into a central drain
- Collector drains: Larger pipes that receive water from several upstream runs
Herringbone layouts are often used where water needs to be collected across a broad area. However, the spacing, pipe diameter and route should still follow the drainage design.
How To Install Land Drainage Step By Step
Step 1 - Set Out The Drainage Runs And Levels
Mark the route of each drain using paint, pegs or string lines. Include the outlet, junctions, changes in direction and any connections to other drainage systems.
Establish:
- The starting invert
- The outlet invert
- The required gradient
- Intermediate level points
- Junction positions
- A clear site datum
Confirm that the pipe can reach the outlet at the required fall before excavation begins. Do not rely on correcting the levels once the trench has already been dug.
Step 2 - Excavate The Land Drainage Trench
Excavate along the marked route to the depth and width shown on the drawings. The trench should provide enough room for the pipe, aggregate, geotextile and fittings without creating an unnecessarily wide excavation.
Clear loose material from the base and protect any exposed services. Maintain safe access and keep materials away from the trench edge where necessary.
There is no standard trench depth for every land drain. Ground conditions, loading, pipe diameter, invert levels and the outlet will all affect how deep the trench needs to be.
Excavation safety should be managed through the site risk assessment and method statement. Trenches must be stable before anyone enters them.
Step 3 - Prepare The Trench And Install Geotextile
Remove loose soil and debris from the trench base. Deal with any soft spots in accordance with the project specification, then check the levels again.
Where required, line the trench with geotextile membrane. Leave enough material on both sides to fold over the aggregate later.
Take care not to tear or contaminate the membrane during installation. Its purpose is to allow water through while helping to keep fine soil particles out of the drainage aggregate.
A separate geotextile is not needed on every project. Follow the drawings rather than adding one as a matter of course.
Step 4 - Place The Drainage Aggregate
Lay a bed of clean drainage aggregate across the base of the trench. Spread it evenly and check that it follows the required gradient.
The aggregate should be free-draining and suitable for the project specification. Material containing too many fines can fill the gaps between the stones, making it harder for water to move towards the pipe.
Do not prescribe an aggregate size based on another project. Use the grading stated in the drainage design or material specification.
Step 5 - Lay The Perforated Land Drainage Pipe
Carefully uncoil the perforated land drainage pipe and place it on the prepared bed. Follow the marked route and avoid tight bends or unnecessary deformation. The pipe should sit evenly on the aggregate without being forced into position.
Keep open ends clean while work continues. Soil and site debris entering the pipe during installation can restrict the system before it has even been completed. Check the alignment and levels as each section is laid. Flexible pipe is easy to handle, but it can move while fittings and aggregate are being installed.
Which Way Should Land Drainage Pipe Holes Face?
The correct orientation depends on the perforation pattern of the pipe being installed.
Some land drainage pipes have perforations distributed around most or all of the pipe, making orientation less significant. Half-perforated pipes have a defined perforated section and need to be positioned in line with the manufacturer’s instructions.
Do not rely on a blanket holes-up or holes-down rule. Check the product information before installation and ask the supplier where the perforation arrangement is unclear.
Step 6 - Connect Land Drainage Pipes And Junctions
Use compatible land drain fittings and couplers to join coils and connect lateral drains to the main collector.
The installation may include:
- Straight couplers
- Tees or Y-junctions
- Multi-junctions
- End caps
- Reducers
- Connections to collector drains
- Transitions into other drainage systems
Keep the pipework aligned through each fitting and maintain the required levels. Forcing a connection into place can pull the adjoining pipe off line or introduce a low point.
Where the land drain connects to rigid underground pipework or a different pipe material, use a suitable flexible fitting or adaptor. Avoid improvised joints that may move or allow the connection to separate during backfilling.
Step 7 - Surround And Backfill The Land Drain
Once the pipework and connections have been checked:
- Place the specified drainage aggregate around the pipe
- Hold the pipe in position while the material is added
- Bring the aggregate up to the level shown in the design
- Fold the geotextile over the aggregate where required
- Complete the remaining backfill and surface reinstatement
Place aggregate evenly on both sides of the pipe. This helps prevent the pipe from shifting or becoming distorted.
Keep excavated soil away from the clean drainage layer. Once fines become mixed into the aggregate, the movement of water towards the pipe can be restricted.
Step 8 - Check The System Before Reinstatement
Inspect the work before the drainage system disappears beneath the final backfill.
Check:
- Pipe levels and falls
- Alignment
- Couplers and junctions
- Collector connections
- The outlet
- Aggregate beneath and around the pipe
- Geotextile condition and overlap
- Visible damage or deformation
- Inspection points
- Compliance with the drawings
Where the project requires it, record photographs, levels and as-built information before reinstatement. These records can be useful if the drainage is extended or inspected later.
How Deep Should Land Drainage Be?
There is no single installation depth that applies to every land drain. The trench needs to be deep enough for the pipe to meet the designed invert and reach the outlet.
Factors affecting depth include:
- The purpose of the drainage system
- Existing and proposed ground levels
- Ground and soil conditions
- The outlet invert
- Surface loading
- Required cover over the pipe
- Existing services
- The drainage layout
- Inspection requirements
- The project specification
A depth quoted for a domestic garden should not automatically be applied to a construction or commercial drainage scheme. The conditions and loading may be entirely different.
What Size Land Drainage Pipe Do You Need?
Land drainage pipe size should be based on the catchment area, expected flow, run length and the role of the pipe within the system.
Common flexible coil diameters include 60mm, 80mm, 100mm and 160mm. Larger perforated twinwall pipes may also be specified where greater capacity or structural strength is needed.
Application | Main Considerations | Pipe Selection |
Small or localised drainage run | Catchment, run length and outlet | As specified |
Lateral drain | Spacing and expected flow | As specified |
Main or collector drain | Combined flow from connected laterals | Based on the drainage calculation |
Larger commercial or agricultural scheme | Catchment, flow, ground conditions and outfall capacity | Engineer or specifier requirement |
Do not choose the diameter simply because it is familiar or readily available. A collector pipe receiving several lateral runs may need considerably more capacity than the individual drains feeding into it.
Changes to the layout can also affect pipe sizing, so alterations made on site should be checked against the design.
Common Land Drainage Installation Mistakes
Installing A Drain Without A Suitable Outlet
Perforated pipe collects water but does not remove the need for a proper discharge point. Without a suitable outlet, water may remain within the trench or be transferred to another part of the site.
Confirm the outlet and its invert before excavation starts.
Losing The Required Fall
Uneven bedding, inconsistent aggregate depth and movement during backfilling can all introduce dips into the pipe.
Check the levels throughout the installation, not just at the beginning and end of the run. Correcting the aggregate bed is straightforward while the pipe remains exposed.
Using The Wrong Backfill Material
Unsuitable material can restrict the movement of water towards the pipe. Soil or aggregate containing excessive fines may gradually clog the drainage layer.
Use the material stated in the specification and keep it separate from excavated spoil.
Allowing Soil To Contaminate The Drainage Aggregate
Loose soil can fall from the sides of the trench or become mixed into the aggregate during backfilling.
Where geotextile is specified, keep it intact and close it properly before placing the remaining backfill. Heavily contaminated aggregate should be replaced rather than covered over.
Damaging Or Kinking Flexible Land Drainage Pipe
Land drainage coils are flexible, but they can still be crushed, kinked or punctured during handling.
Uncoil the pipe carefully, avoid tight changes of direction and do not drop heavy aggregate directly onto an unsupported section. Check for damage before completing the backfill.
Incorrect Connections Between Drainage Runs
Improvised joints can move apart or leave adjoining pipes out of alignment.
Use the correct couplers, junctions and adaptors for the pipe sizes being connected. Support each junction properly so it stays in position while the aggregate is placed.
Treating Land Drainage As Foul Drainage
Land drainage is intended for groundwater and surface water. It should not receive foul water, sewage or wastewater from buildings.
Keep the two systems separate unless a specific connection has been designed and approved as part of the project.
Get In Touch With Drainfast
Drainfast supplies perforated and unperforated land drainage pipe, couplers, junctions, end caps, geotextiles and flexible drainage fittings for commercial construction, civils, groundworks, landscaping and agricultural projects.
Our technical team can help you check product compatibility and identify the materials required for the installation. You can also send us your project drawings for a free material take-off, reducing the risk of ordering too much or running short on site.
Call 01420 555600 or contact Drainfast online to discuss your land drainage requirements.
