Sub-base and aggregates
Pipe bedding and surround (pea gravel) per run
- where
- bedding cross-section per metre (m³/m)
- trench width (mm)
- bed under the pipe (mm)
- pipe outside diameter (mm)
- surround above the crown (mm)
- bedding volume (m³)
- length of run (m)
- bedding stone to order (t)
- density of bedding stone (t/m³)
- margin (%)
Explain this equation
This works out the granular bedding and surround needed for a plastic drain laid in a trench, per metre and for the whole run.
B is the trench width, b the bed under the pipe, Do the pipe’s outside diameter and c the cover over the crown, all in millimetres. Ab is the stone cross-section per metre, V the total volume and T the tonnes to order.
The stone fills a rectangle B wide and (b + Do + c) deep, less the circle the pipe occupies (π Do²/4). Dividing by 1,000,000 or 4,000,000 converts mm² to m². Then it is volume times density plus a margin.
Sense check: a 600 mm trench with 100 mm under and over a 110 mm pipe needs about 0.18 m³ per metre, roughly 0.3 t per metre.
Watch out: use the bed and cover thicknesses from the pipe maker or drainage design, not just the defaults.
Granular bedding and surround for a flexible (plastic) drain: the stone under, beside and over the pipe in a trench.
Enter your values to see the result.
Good to know
Flexible pipes rely on the granular surround for support (Approved Document H para 2.43 and BS EN 1610). The bed and cover thicknesses here are inputs: take them from the pipe manufacturer’s installation guide or the drainage design (100 mm under and 100 mm over the crown are common minimums for 100–150 mm plastic pipe). 110 mm is the outside diameter of standard 100 mm (DN 110) PVC-U underground pipe; 160 mm for DN 160. Bedding is normally clean single-size or graded stone of the size the manufacturer specifies.
Common mistake
Forgetting that the pipe itself takes up room. On a narrow trench the pipe can be a quarter of the bedding cross-section.
Limits
Rectangular trench with vertical sides. Rigid pipes and concrete protection have different bedding classes: follow the design.
Sources
- Approved Document H: Drainage and waste disposal, 2015 edition (Table 6 minimum gradients for foul drains; Tables 11 and 12 access fittings, inspection chambers and manholes; para 3.15 rainwater drain gradients; paras 3.27–3.30 soakaways)
- BS EN 1610:2015 Construction and testing of drains and sewers (bedding, side fill and initial backfill)
- Caterpillar, Earthwork volumes reference tables (bank and loose densities; e.g. clay natural bed 2020/1660 kg/m³, topsoil 1370/950, sand damp 1900/1690, gravel dry 1690/1510)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.