A natural slope is the sloping side of an excavation, cut back far enough that the soil holds itself up — without shoring, trench boxes or sheet piling. Dig steeper than that angle and the ground will eventually find its own equilibrium: it slides. The underlying property is the angle of repose, or angle of internal friction.
How steep is safe?
A slope is written as a ratio of height to horizontal distance. 1:1 is 45 degrees; 1:2 is shallower, roughly 27 degrees. The safe angle depends on:
- Soil type — dry granular sand needs a shallow batter; stiff clay stands considerably steeper
- Water — groundwater, rain and seepage undermine any slope. A face that stood at knocking-off time can slump after a night of rain
- Surcharge beside the excavation — spoil heaps, materials, plant or an existing foundation all press on the crest
- Duration — a trench backfilled the same day may be steeper than an excavation left open for weeks
- Vibration — piling, traffic or a nearby compactor plate reduce stability
Safety rules
Collapse is lethal: a cubic metre of soil easily weighs 1,500 to 2,000 kg, and burial happens in a second.
- Depth triggers. Under US OSHA rules, excavations 5 ft (1.5 m) deep or more require a protective system — sloping, benching, shoring or shielding — unless cut entirely in stable rock; a competent person can require protection at shallower depths too. In the UK, the CDM regime sets no fixed depth: any excavation must be prevented from collapsing by battering, shoring or support. Elsewhere in Europe similar rules apply, often kicking in around 1 to 1.25 m
- Keep the crest clear of spoil, materials and people
- Inspect daily and after any break — especially after rain or frost — for cracks, washouts and slumping
- Provide two means of escape at opposite ends, and always use a ladder rather than climbing the face
- Deep excavations need design. Beyond roughly 6 m, OSHA requires a protective system designed by a registered engineer
Practical points
- A slope needs room: 2 m deep at 1:1.5 takes 3 m of extra width each side. If it will not fit on the plot, a retaining system is the only option
- Cover long-standing slopes with sheeting or geotextile against erosion
- Divert surface water away from the crest with a cut-off drain; water running over the edge carves gullies
- When digging beside an existing foundation, stay below its line of load spread or you will undermine it
Related terms
- Excavation pit
- Soil mechanics
- Soil compaction
- Sheet pile wall
- Dewatering
- Absolute settlement
- Shaft friction
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