Retaining and trench safety
Trench battered back for safety: top width and extra dig
- where
- width at the top (m)
- width at the bottom (m)
- depth (m)
- batter angle of the sides from horizontal (°)
- volume dug (m³)
- trench length (m)
- extra dig compared with vertical sides (m³)
Explain this equation
This shows how wide a trench gets at the top, and how much more you dig, if the sides are battered back instead of supported.
B is the width at the bottom, D the depth, θ the angle of the sides from horizontal and L the length. Wt is the top width, V the volume dug and Ve the extra over a vertical-sided trench.
Each sloping side steps back D/tan θ, so the top is B plus two of those. The cross-section is a trapezium of average width B + D/tan θ; times depth and length gives the volume.
Sense check: at 45°, a 1.5 m deep trench 0.6 m wide at the bottom is 3.6 m wide at the top, with three and a half times the dig.
Watch out: this does not tell you a safe angle. A competent person must decide support or batter for the ground and conditions.
How wide a trench becomes at the top, and how much more you dig, when the sides are battered back instead of supported.
Enter your values to see the result.
Good to know
CDM 2015 regulation 22 requires steps to stop any excavation collapsing, by supports or battering. HSE guidance: decide the support before digging; batter slopes must be flatter than the natural angle of repose of the soil, much flatter in wet ground; protect edges where people could fall; keep vehicles away from the sides; a cubic metre of soil can weigh more than 1.5 tonnes. The old "1.2 m rule" is not in current law: shallow trenches can and do collapse. CITB toolbox guidance keeps spoil and materials at least 1 m back from a 1 m deep trench, further for deeper ones.
Common mistake
Assuming a trench less than about 1.2 m deep needs no support. People are seriously injured by collapses in shallow trenches, especially when kneeling or bending.
Limits
Geometry only: this does not choose a safe angle. Safe batters depend on soil, water, surcharge, vibration and time open, and must be set by a competent person.
Sources
- HSE, Excavations (support, battering, spoil, edge protection, inspection; a cubic metre of soil can weigh over 1.5 tonnes)
- Construction (Design and Management) Regulations 2015, regulation 22 Excavations
- HASpod, Excavations and the 1.2 m rule (HSG150 and CDM 2015 reg. 22: no safe unsupported depth)
- CITB, Excavations toolbox talk (spoil and materials at least 1 m from the edge of a 1 m deep trench)
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.