How to count a drainage trench: gravel, pipe and geotextile

1. The fall and the excavation

The total fall is the length times the slope: 1 % over 20 m is 0.20 m. The depth entered is at the high end and the trench is deeper by the fall at the outlet, so the volume uses the average depth: the high-end depth plus half the fall.

2. Gravel, backfill and geotextile

In a French drain the gravel fills the whole trench; with a pipe it fills the bed, the pipe and the cover, and the rest is backfilled with soil. From the gravel volume subtract the pipe volume, then add the compaction allowance. The geotextile wraps the gravel: the width of the trench, both sides and an overlap on top.

Worked example: French drain, 20 m × 40 × 60 cm, 1 %

Pipe Ø100 in 6 m lengths, overlap 20 cm, 10 % compaction:

• Fall 0.20 m; average depth 0.70 m; excavation 20 × 0.40 × 0.70 = 5.600 m³.
• Pipe volume π × 0.05² × 20 = 0.157 m³; gravel (5.600 − 0.157) × 1.10 = 5.987 m³.
• Geotextile 20 × (0.40 + 2 × 0.70 + 0.20) = 40.00 m².
• Pipes: 20 ÷ 6 = 3.33, so 4.

With a pipe in a gravel surround

Same trench, 10 cm of gravel under the pipe and 20 cm above it: the gravel reaches 0.40 m, so gravel 3.347 m³, soil backfill 20 × 0.40 × (0.70 − 0.40) = 2.400 m³, geotextile 28.00 m².

What this does not cover

The count does not size the pipe, choose the fall for a soil or check the outfall. Widths, surround, overlap and compaction are starting values to replace with the drawings.