Drainage calculator
Stone, fabric and pipe for drainage trenches and drains behind walls, the bedding round a pipe, the size of a soakaway and the soil test that decides it.
Drainage calculator
How it is worked out
A trench: stone = width × depth less the pipe, per meter; fabric wraps it with an overlap. Clean stone laid loose weighs about 1.5 t/m³ (see 6.3 and 6.4). A pipe trench is the pipe diameter plus 400 mm wide, with a bed under and cover over the pipe (see 2.1).
A soakaway for a small roof stores the rain from about 16.7 mm of rain (area × 50 ÷ 3,000), divided by the share of voids: 40 % for washed stone, 30 % for rubble, 95 % for crates (see 6.5).
Infiltration rate = water between 75 % and 25 % of the depth ÷ (wetted area at half depth × the time). Do the test three times and use the slowest.
Sources
- FHWA-HRT-15-034, open-graded aggregates: No. 57 stone, loosest 95.4 lb/ft³ (1.53 t/m³), densest 108.7 lb/ft³ (1.74 t/m³)
- Washington State University Extension: improving drainage: “the trench should be sloped between 0.5% and 1%”
- Michigan DOT construction manual: underdrains: “The geotextile blanket should overlap the top of the trench a minimum of 1 foot”
- NCDOT Standard Specification 454: segmental gravity walls: “No. 57 stone for a horizontal distance of at least 12 inches”; geotextile between stone and soil, laps 18 in; 100 mm perforated pipe; 8–10 in lifts with hand equipment
- US Bureau of Reclamation, Pipe bedding and backfill (training manual 7): 100 mm bedding for pipes up to 250 mm, embedment stone up to 19 mm with 5 % or less fines, both sides raised together within 150 mm, compacted layers of at least 150 mm with tampers, 750 mm cover before heavy machines cross
- City of Hendersonville, sewer trench detail: bedding material to 150 mm above the top of the pipe; at least 150 mm of washed stone in rock or wet soil; sand around pressure pipes in 150 mm layers
- West Devon Borough Council: infiltration testing (BRE Digest 365 method): pit “0.3 to 1 m wide and 1 to 3 m long”, “Repeat the test 3 times”, rate from the fall between 75 % and 25 %
- CNC Building Control (council partnership): soakaway size for an extension: volume = area × 50 ÷ 3,000, voids about 30 %, pieces “no larger than 150mm”, “at least 5m from any building or road”
- Runnymede Borough Council: surface water drainage guidance: “n = 0.3 is usually assumed for granular filled infiltration systems”, crates 0.9–0.95, safety factor 2 for homes, 1-in-10-year storm, half empty within 24 h, water table 1 m below
- Philadelphia Water: drainage wells checklist: porosity 0.40 for stone storage; traps before the well; 10 ft from buildings
- Hastings Borough Council: soakaway advice note: “at least 5m away from any building”; water table at least 1 m below the base; specialist advice on clay and chalk
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