160mm Underground Drainage
Installation guide for engineer-designed 160mm large-capacity drainage systems used in multi-dwelling, commercial, and infrastructure applications.
Safety Warning — Deep Excavation
Before digging, use current service plans and suitable locating equipment, and expose suspected services safely. A competent person must assess the ground, depth, groundwater, nearby structures, plant, and spoil loads and decide the required shoring, trench box, or safe batter; there is no depth at which unsupported ground is automatically safe. Provide barriers and safe access, inspect the excavation at the start of each shift and after anything that may affect stability, and do not enter until it is safe. Construction work must be planned and managed under CDM 2015.
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
- Excavation support selected by the temporary-works design
Materials
- 160mm PVC-U drainage pipe (3m / 6m lengths)
- 160mm ring seal couplers
- 160mm bends, junctions, and reducers
- Kalsi 470mm chamber system and adaptors where suitable for the design
- Selected granular bedding and surround specified for PVC-U pipe
- Kalsi soluble pipe lubricant
- Marker tape where required by the project specification
Technical Specifications
Technical basis: the current Kalsi 160mm Underground Drainage data sheet, Approved Document H for England and HSE excavation guidance. Elsewhere in the UK, apply the relevant national building regulations. Confirm the chamber arrangement, load rating and any missing socket insertion depth with Kalsi Technical and the project designer.
Step-by-Step Installation
1 Design and Regulatory Approval
Use 160mm only where the hydraulic and structural drainage design requires its capacity. Agree the design and Building Regulations route before excavation. In England and Wales, Section 106 of the Water Industry Act 1991 covers the right and process to communicate with a public sewer; a separate Section 104 agreement applies where new sewers are to be adopted by the sewerage undertaker. Obtain the relevant approval before connecting. Existing drainage may also need a survey and level check.
2 Service Detection and Setting Out
Obtain current plans from the relevant service owners and have a trained person scan the full route with suitable locating equipment. Mark detected services and confirm their positions with carefully dug trial holes, using the service owner's safe-digging requirements. Plans and scans are complementary and neither proves that the route is clear. Establish a verified datum and transfer the designed invert levels to secure profiles along the route.
3 Excavate the Trench
Excavate to the design invert while maintaining the Kalsi minimum trench width of 560mm (pipe OD + 400mm) and allowing for the 150mm bed. Keep plant and spoil far enough from the edge that they cannot overload the excavation. Install the temporary works selected by the competent excavation assessment before anyone enters. Control groundwater using the designed method; pumping can itself affect ground stability, so do not improvise dewatering.
4 Prepare the Bedding
Lay the Kalsi-specified 150mm minimum bed of selected granular material. It must be compatible with PVC-U, free from sharp or oversized material, and compacted evenly to the design gradient. Form socket recesses so the pipe barrel is continuously supported. Thermoplastic pipe depends on controlled sidefill and soil support: select the pipe stiffness, bedding, compaction and cover together using the project design and BS EN 1295. Road loading alone does not make full concrete encasement the default.
5 Lay the Pipe
Begin at the downstream end and work uphill with sockets facing upstream. Use a lifting and handling method suitable for the selected pipe length. Cut square where necessary, deburr and chamfer the spigot, and clean the spigot, socket and seal. Check the ring is correctly seated and undamaged, apply Kalsi soluble lubricant as directed, and push to the factory insertion mark. If no clear mark is present, obtain the required insertion depth from Kalsi Technical. Check line, gradient and continuous barrel support after each length.
6 Install Inspection Chambers and Junctions
Kalsi's 160mm TDS identifies its 470mm chamber system with the appropriate adaptors as compatible. Confirm the base connections, chamber depth, restricted-access requirements and cover load rating against the approved layout; a 470mm plastic chamber is not a universal substitute for every chamber or manhole. Install the chamber on the base specified by its manufacturer. Connect 110mm branches with the designed Kalsi level-invert junction or adaptor and keep channels aligned with the direction of flow.
7 Surround and Haunch the Pipe
Place selected granular material evenly beneath the haunches and up both sides so the pipe is not displaced. Kalsi's standard detail provides 200mm side support and selected cover to 300mm above the crown. Compact the sidefill in balanced layers to the project specification and keep mechanical compaction away from the pipe until the required protective cover is in place. Fit marker tape only where the project or adopting authority requires it.
8 Test the System
Test at the stage agreed with Building Control, the designer or adopting authority after the required bedding, surround and initial backfill are in place. Approved Document H permits an air or water test for pipes up to 300mm. The air test uses 110mm water-gauge pressure for about five minutes, followed by an initial 100mm pressure with no more than 25mm loss over seven minutes. A water test uses between 1m head at the highest invert and 5m at the lowest; condition the installation for one hour, then hold the test for 30 minutes. The permitted addition is calculated from wetted surface area under Part H paragraph 2.61. Secure test plugs and keep people out of their line of fire.
9 Backfill and Reinstate
Complete the backfill with the material and compaction method in the drainage design, keeping large stones, debris and frozen material away from the pipe zone. For SN4 thermoplastic pipe in the standard Approved Document H bedding condition, provide at least 600mm cover in non-trafficked ground and 900mm in light or main roads. If those depths cannot be achieved, use a designed protection detail; Part H commonly shows a reinforced concrete cover slab separated from the pipe by flexible filler and at least 75mm of granular material. Reinstate the sub-base and surface to the project specification.
Common Mistakes to Avoid
- Using an undesigned bedding detail — Loading, depth, ground, pipe stiffness, bedding and compaction must be assessed as one structural system.
- Inadequate cover under traffic — Standard SN4 thermoplastic pipe needs 900mm cover in light or main roads unless an alternative BS EN 1295 protection design is approved.
- Assuming components are interchangeable — Confirm socket, seal and adaptor compatibility before connecting another system or material.
- No gradient check before backfill — Once backfilled, gradient cannot be corrected without re-excavation. Check levels at every joint before covering.
- Wrong access product — Select the chamber or manhole from pipe size, depth, layout and loading; confirm the Kalsi 470mm base and adaptors suit the design.
- Voids or uneven sidefill — Flexible pipe relies on continuous barrel support and balanced, correctly compacted sidefill.
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