110mm Underground Drainage
Complete guide to laying 110mm PVC-U underground drainage including trench preparation, bedding, gradient calculation, and connecting to inspection chambers.
Safety Warning — Excavation Work
Trench work carries serious risks including collapse, service strikes, falls, and water ingress. 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. No ground can be assumed to stand unsupported. 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.
What You'll Need
Tools
- Mini digger or hand-digging tools
- CAT scanner (cable avoidance tool)
- Laser level or spirit level and boning rods
- String line and line pins
- Hacksaw or pipe cutter
- File for deburring
- Tape measure and builder's line
- Whacker plate (for backfill compaction)
Materials
- 110mm PVC-U drainage pipe (3m / 6m lengths)
- Ring seal couplers
- Bends (15°, 30°, 45°, 87.5° as required)
- Inspection chambers / access points
- Selected granular bedding and surround specified for PVC-U pipe
- Kalsi soluble pipe lubricant
- Marker tape where required by the project specification
- Selected backfill material
Technical Specifications
Technical basis: the current Kalsi 110mm Underground Drainage data sheet, Approved Document H for England and HSE excavation guidance. Elsewhere in the UK, apply the relevant national building regulations. If a plain pipe has no factory insertion mark, obtain the current Kalsi socket insertion depth before jointing.
Step-by-Step Installation
1 Plan the Drainage Route
Confirm whether the run carries foul water or rainwater and establish an approved outfall before work starts. Keep pipes straight and at an even gradient wherever practicable, with large-radius bends close to access points. Approved Document H calls for access on or near the head of a run, at bends and changes of gradient or pipe size, and at junctions unless every run can be cleared from another access point. The permitted spacing depends on the type of access provided, so set the layout from Part H Table 13 rather than applying one distance to every run. Obtain the sewerage undertaker's approval before connecting to a public sewer.
2 Locate Existing Services
Obtain current plans from the relevant service owners and have a trained person scan the 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.
3 Excavate the Trench
Excavate to the design invert while maintaining the Kalsi minimum trench width of 410mm and allowing for the 100mm granular bed. Set the foul gradient from the approved hydraulic design: Kalsi's standard detail uses 1:40 (25mm/m), while Approved Document H permits 1:80 for a 100mm nominal foul drain where peak flow exceeds 1 litre/second and at least one WC is connected. Lay 100mm nominal rainwater drains no flatter than 1:100 (10mm/m). Keep plant and spoil far enough from the edge that they cannot overload the excavation.
4 Prepare the Bedding
Lay the Kalsi-specified 100mm 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 small socket recesses so the pipe barrel, rather than the sockets, is continuously supported. Bedding, pipe stiffness and cover work as a system; use a BS EN 1295 design where loading, depth or ground conditions fall outside the standard detail.
5 Lay the Pipe
Start at the downstream end and work uphill with sockets facing upstream. Cut square where necessary, deburr and chamfer the spigot, then clean the spigot, socket and seal. Check that the ring seal is present, correctly seated and undamaged. Apply Kalsi soluble lubricant as directed and push to the factory insertion mark. If a pipe has no clear insertion mark, obtain the required depth from Kalsi Technical before jointing. Check line and gradient after every length.
6 Install Inspection Chambers
Install the access points established at the design stage. Kalsi 320mm and 470mm chamber systems may be used where their base connections, permitted depth and cover loading suit the layout. Prepare and support each chamber exactly as required by its installation instructions; do not assume every plastic base requires the same concrete detail. Use compatible adaptors, preserve the channel direction and gradient, and fit a cover and frame with the correct load rating at finished level.
7 Surround and Initial Backfill
Place selected granular material evenly beneath the haunches and up both sides so the pipe is not displaced. Kalsi's standard detail provides 150mm side support and selected cover to 300mm above the crown. Compact the sidefill by hand in balanced layers and keep mechanical compaction away from the pipe until the specified 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 or the project specification, after the required bedding, surround and initial backfill are in place. For pipes up to 300mm, Approved Document H permits an air or water test. 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, not simply from pipe length. Secure test plugs and keep people out of their line of fire.
9 Complete Backfill
Complete the backfill with the specified material and compaction method, keeping large stones, debris and frozen material away from the pipe zone. For SN4 thermoplastic pipe in the standard Part 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 solution — Part H commonly shows a reinforced concrete cover slab separated from the pipe by flexible filler and at least 75mm of granular material. Do not substitute blanket concrete encasement without a drainage design.
Common Mistakes to Avoid
- Using one gradient for every drain — Select the fall from the flow, connected appliances, pipe size and approved design, then verify every invert before covering.
- Poor pipe-zone support — An uneven bed, voids beneath the barrel or unbalanced sidefill can deform PVC-U and alter the fall.
- Incorrect ring-seal jointing — A displaced seal, unchamfered cut end, unsuitable lubricant or wrong insertion depth can leak or prevent movement.
- Skipping the service scan — Striking gas, electric, or water mains can be fatal. Always scan before digging.
- Inadequate access — Check the siting and maximum spacing against the actual access-point types in Approved Document H Table 13.
- Guessing the protection detail — Traffic loading, cover, pipe stiffness and bedding must be assessed together; use the approved structural drainage detail.
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