160mm Underground Drainage
Installation guide for 160mm large-bore drainage systems used in commercial, multi-unit residential, and highway drainage applications.
Safety Warning — Deep Excavation
160mm drainage installations typically involve deeper trenches than domestic 110mm systems. Trenches deeper than 1.2m must be shored, battered, or benched. Always use a CAT scanner and consult utility plans before digging. Wear full PPE including hard hat, steel-toe boots, and high-vis. Never enter an unsupported trench. Follow CDM Regulations 2015 for all groundwork operations.
What You'll Need
Tools
- Mini digger or excavator
- CAT scanner and signal generator
- Laser level or dumpy level
- Boning rods and string line
- Pipe cutter or hacksaw
- Deburring tool and file
- Whacker plate compactor
- Trench shoring equipment (for deep trenches)
Materials
- 160mm PVC-U drainage pipe (3m / 6m lengths)
- 160mm ring seal couplers
- 160mm bends, junctions, and reducers
- Inspection chambers (460mm or larger)
- Granular bedding material (10mm pea gravel)
- Silicone pipe lubricant
- Drainage marker tape
- Concrete (for chamber bases and surrounds as needed)
Technical Specifications
Step-by-Step Installation
1 Design and Regulatory Approval
160mm drainage is typically specified for larger installations — multi-unit residential blocks, commercial premises, or public sewer connections. The drainage layout should be designed or approved by an engineer. Submit a Building Regulations application under Part H and, if connecting to a public sewer, obtain a Section 106 (Water Industry Act 1991) adoption agreement or connection approval from the water authority. A CCTV survey of existing drainage may be required.
2 Service Detection and Setting Out
Scan the full route using a CAT scanner and Genny. Obtain utility plans through LSBUD or direct requests to utility providers. Mark all detected services and set out the trench route with spray paint. Establish datum levels and transfer the pipe invert levels to timber profiles at each end of the trench and at every change of direction. Set a string line between profiles at the correct gradient.
3 Excavate the Trench
Excavate to the required depth, maintaining a minimum trench width of 460mm (pipe OD + 300mm). For 160mm pipe, trenches are typically deeper than for domestic 110mm work. Allow for 150mm of bedding below the pipe invert. Shore trenches exceeding 1.2m depth. Keep spoil at least 1m from the trench edge and use barriers and signage where appropriate. Pump groundwater if present — pipe should not be laid in standing water.
4 Prepare Bedding — Bedding Class Selection
The bedding class depends on the ground conditions and loading. Class N (granular bedding, 150mm below pipe) is suitable for most applications with stable ground. Class B (180° granular surround) provides additional support in poor ground. Class S (full concrete surround) is required under roads or where cover depth is insufficient. Lay and compact the bedding material to the calculated gradient, checking levels frequently with a laser level.
5 Lay the Pipe
Begin at the lowest discharge point and work uphill. Clean all sockets, spigots, and ring seals. Apply silicone lubricant generously to the spigot end. For 160mm pipe, two people are needed to align and push joints together — the larger diameter requires more force than 110mm. Push to the insertion mark only, leaving the expansion gap. Check alignment and level after each length. 160mm pipe must sit firmly on its bedding with no voids beneath — tap bedding into any gaps.
6 Install Inspection Chambers and Junctions
Use 460mm or larger inspection chambers for 160mm drainage. Set chamber bases on a minimum 150mm C20 concrete bed. Where 110mm branches connect to the 160mm main run, use level-invert reducing junctions. Ensure all benching in chambers is smooth, self-cleansing, and provides a minimum half-pipe channel. Adjust chamber risers so that the cover sits flush with the finished surface level.
7 Surround and Haunch the Pipe
Carefully hand-place granular material around the pipe up to 150mm above the pipe crown. Compact in layers, working evenly on both sides to prevent pipe displacement. For Class B bedding, ensure the granular surround extends to at least the pipe centreline on each side. Place drainage marker tape 300mm above the pipe crown. This surround is the pipe's primary protection against backfill loads.
8 Test the System
Perform a water test or air test before completing the backfill. For a water test: plug the downstream end, fill to 1.5m head above the upstream invert, allow 2 hours for absorption, then top up and monitor for 30 minutes. Maximum permitted loss for 160mm pipe is 0.08 litres per linear metre over 30 minutes. Building Control will typically want to witness this test. Document results with photographs and test certificates.
9 Backfill and Reinstate
Backfill in 150–200mm layers, compacting each layer with a whacker plate (but not directly above the pipe until there is at least 300mm of cover). Use clean, selected fill free from large stones, frozen lumps, or organic material. Where the trench crosses vehicle-trafficked areas, reinstate with sub-base and surface material to match the existing construction. Reinstate soft landscaping areas to existing levels.
Common Mistakes to Avoid
- Wrong bedding class for ground conditions — Class N bedding is not suitable in waterlogged, made-up, or unstable ground. Consult an engineer if ground conditions are poor.
- Inadequate cover under trafficked areas — 160mm pipe under roads needs minimum 600mm cover with concrete surround (Class S) or SN8 pipe. Standard SN4 without protection will fail.
- Mixing pipe standards — Never mix pipe from different manufacturers in the same run. Socket dimensions and seal profiles vary between manufacturers.
- No gradient check before backfill — Once backfilled, gradient cannot be corrected without re-excavation. Check levels at every joint before covering.
- Undersized inspection chambers — 160mm drainage requires 460mm minimum diameter chambers. Standard 315mm mini chambers are too small.
- Failing to compact bedding properly — Voids beneath the pipe create point loads that crack the pipe over time. Bedding must be uniform and fully supporting.
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