How to Install Underground Drainage: A Step-by-Step Guide

Meta description: Complete guide to installing underground drainage - covers planning, excavation, pipe laying, inspection chambers, and testing for compliance.

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Introduction

Installing underground drainage correctly isn’t complicated, but it requires careful planning and methodical execution. Mistakes buried underground are expensive to fix, and failed inspections mean digging everything up again.

This guide walks through the complete process from initial survey to final testing. Whether you’re a builder installing drainage for an extension or a specialist contractor, following these steps ensures compliant, long-lasting installations that pass Building Control inspection first time.

Before You Start: Planning and Preparation

Survey Existing Services

Before any excavation, establish what’s already underground:

Request utility plans from electricity, gas, water, and telecoms companies
Use CAT scanner and signal generator to locate cables and pipes
Identify existing drainage routes (manholes indicate direction)
Check for private services (oil tanks, septic systems, soakaways)

Critical: Damaging services during excavation causes delays, costs, and potentially serious safety hazards.

Design the System

Plan your drainage layout considering:

Connection point – where does the new drainage join the existing system?
Fall required – typically 1:40 for 110mm pipes (minimum 1:80)
Depth at connection – this determines excavation depth throughout
Inspection chamber locations – at junctions, direction changes, and maximum 45m intervals
Pipe routing – shortest practical route avoiding obstacles

Calculate invert levels at each point working back from the connection. Allow for pipe diameter plus bedding material.

Obtain Permissions

Ensure you have:

Building Regulations application submitted (full plans or building notice)
Sewerage company approval for connection to public sewer
Build-over agreement if near existing public sewers
Party wall agreements if excavating near boundaries

Materials Checklist

Gather materials before starting excavation:

Underground drainage pipe (110mm PVC-U typically)
Fittings – bends, junctions, reducers as per design
Inspection chamber base and risers
Chamber covers and frames (appropriate for location)
Pea gravel for bedding and surround (10-20mm)
Marker tape
Concrete and granular backfill material

Step 1: Set Out the Route

Mark the drainage route on the ground surface:

Establish connection point location and invert level
Drive wooden pegs at chamber positions
Mark trench line with spray paint between pegs
Set up profile boards at each end showing invert level

Profile boards: Create simple frames with a horizontal crosspiece at a fixed height above invert level. String a line between profiles to guide excavation depth.

Step 2: Excavate the Trench

Dimensions

Minimum trench width:

110mm pipe: 450mm wide
160mm pipe: 500mm wide
Allows working space for jointing

Depth varies with gradient but typically:

Minimum 600mm cover under gardens
Minimum 900mm under driveways
Greater depth may be required to achieve falls

Excavation Technique

Remove topsoil separately for reuse
Excavate subsoil, storing away from trench edge
Keep trench sides vertical where possible
Shore trench sides if deeper than 1.2m or in unstable ground
Form bell holes at joint positions (extra depth for socket)
Check depth against string line regularly

Safety Considerations

Never enter unsupported deep trenches
Keep excavated material at least 1m from trench edge
Provide safe access (ladder) for trenches over 1m deep
Watch for groundwater ingress – pump if necessary

Step 3: Prepare the Bed

A good pipe bed ensures even support and prevents settlement.

Granular Bed

Spread 100mm of pea gravel along trench bottom
Level and compact lightly
Check level against profile line (allowing for pipe diameter)
Form socket holes where joints fall – prevent pipes bridging on sockets

Alternative: Concrete Bed

Required where:

Cover is insufficient for loads
Ground is unstable
Pipe passes through made ground

Pour 100mm concrete bed, allow to cure, then lay pipes on granular material above.

Step 4: Lay the Pipes

Inspection and Preparation

Check pipes for damage before installation
Remove any debris from inside
Clean socket and spigot ends
Have lubricant ready for ring seal joints

Laying Sequence

Start at the lowest point (connection end)
Lay pipes uphill so sockets face the flow direction
Insert spigot into socket with push-and-twist motion
Ensure ring seal isn’t displaced (mark insertion depth)
Check alignment with string line after each pipe
Verify gradient with spirit level or laser

Making Joints

For ring seal pipes:

Lubricate seal and spigot with silicone lubricant
Push home firmly – you’ll feel resistance from the seal
Ensure insertion mark aligns with socket edge

Cutting Pipes

When cutting is necessary:

Mark cut line accurately
Cut square with fine-tooth saw or pipe cutter
Deburr inside and outside edges
Chamfer the leading edge (essential for ring seal joints)

Step 5: Install Inspection Chambers

Chambers are required at:

All junctions and direction changes
Changes in pipe size or gradient
Maximum 45m intervals on straight runs

Chamber Installation

Excavate chamber position to required depth
Place 150mm concrete base, level and allow to firm up
Position chamber base on concrete, checking inlet/outlet alignment
Build up risers to finished level
Install rubber sealing adaptors where pipes enter
Connect pipes to chamber – ensure proper fall maintained
Fit cover and frame at correct finished level

Cover Selection

Choose appropriate covers for location:

Class A15: Pedestrian areas only (gardens, paths)
Class B125: Car traffic, driveways
Class D400: Heavy vehicles, commercial areas

Step 6: Surround and Backfill

Pipe Surround

After pipes are laid and checked:

Hand-fill pea gravel around pipes
Ensure full support under pipe haunches
Build up to 100mm above pipe crown
Never drop large stones or debris onto pipes

Backfill

Place warning tape 300mm above pipe level
Backfill in 300mm layers
Compact each layer (plate compactor or punner)
Continue to finished level
Replace topsoil last

Under Driveways

Where pipes pass under driveways:

Maintain minimum cover depths
Consider concrete protection slab if cover is marginal
Compact backfill thoroughly to prevent settlement

Step 7: Testing

Building Control requires drainage testing before sign-off.

Air Test (Preferred Method)

Plug all outlets except test inlet
Connect manometer and pump
Inflate to 100mm water gauge pressure
Stabilise for 5 minutes
Test for further 5 minutes – pressure drop must not exceed 25mm

Pass criteria: Maximum 12mm drop over 5 minutes for new installations

Water Test

Alternative where air test isn’t practical:

Plug lowest outlet
Fill system with water to top chamber
Allow to stand for 2 hours to saturate pipes
Top up to test level
Monitor water level for 30 minutes

Pass criteria: Less than 0.05 litres per metre per hour loss

Document Results

Record test results with photographs showing:

Test equipment in use
Manometer readings
Pipe runs before backfilling
Chamber connections

Provide copies to Building Control.

Common Problems and Solutions

Problem: Can’t achieve required gradient Solution: Start planning from the connection point. If impossible, consider a pumped system.

Problem: Groundwater flooding trench Solution: Pump continuously during work. Consider land drainage or concrete surround.

Problem: Test failure Solution: Check all joints systematically. Look for displaced seals or debris in pipes.

Problem: Misaligned chamber connection Solution: Use adjustable bend or short extension to correct alignment. Never force pipes.

Conclusion

Underground drainage installation follows a logical sequence: plan carefully, dig accurately, bed properly, lay methodically, and test thoroughly. Rushing any stage risks problems that are expensive to fix.

Building Control expects to see work at excavation stage – call for inspection before backfilling. This protects you as much as the client; if problems are spotted early, they’re still easy to correct.

Stock up on quality underground drainage materials before your next job. From pipes and fittings to chambers and accessories, we supply everything needed for compliant drainage installations.

Internal links:

Underground drainage pipes (110mm, 160mm)
Underground drainage fittings
Inspection chambers and accessories
Chamber covers and frames
Drainage bedding and surround materials

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