Excavation work is routine for public works departments; installing culverts, replacing water lines, cleaning drainage structures, or repairing utilities. But once soil is cut, gravity becomes an unforgiving opponent. Trench collapses are one of the leading causes of fatalities in construction and municipal operations, and many occur because the trench walls were not properly sloped or benched.
When trench boxes cannot be used due to space, access, or equipment limitations, sloping or benching offers an effective, OSHA-approved method of protecting workers from cave-ins. Understanding these systems is critical for any municipal or highway department performing excavation tasks.
Sloping is the process of cutting back the trench walls at an angle, creating a “widened” excavation that reduces earth pressure and helps prevent the material from collapsing inward. Instead of vertical, near-vertical, or over-steep walls, the excavation is dug with stable, angled sides that naturally resist sliding.
Sloping increases the footprint of the excavation, giving the soil a more stable angle that resists shear failure. Different soils behave differently, so the required angle varies depending on the soil type.
Benching uses a series of horizontal steps cut into the trench wall. Each step reduces the height of unsupported vertical wall, decreasing collapse pressure. Benching is commonly used where space is available but soil conditions or job requirements prevent full sloping.
Simple Benching: One or more horizontal steps, but the bottom portion of the trench remains vertical.
Multiple Benching: A combination of vertical and horizontal surfaces creating multiple tiers, used in more stable soil.
Note: Benching is not allowed in Type C (the least stable) soil, per OSHA standards.
Before sloping or benching is designed, a competent person must classify the soil. OSHA categorizes soil into three main types:
Examples: Clay, hard packed soils
Maximum allowable slope: 3/4:1 (53° angle)
Benching allowed: Yes
Examples: Silt, previously disturbed soils, angular gravel
Maximum allowable slope: 1:1 (45° angle)
Benching allowed: Yes
Examples: Loose sand, submerged or saturated soils
Maximum allowable slope: 1½:1 (34° angle)
Benching allowed: No
If soil cannot be classified or varies along the trench, assume the least stable (Type C) and slope accordingly.
Sloping or benching is preferred when:
Working in wide or open areas (parks, fields, undeveloped land)
Installing long lengths of utility pipe where repeatedly moving a trench box is inefficient
The trench depth is significant but soil conditions are manageable
There are no nearby structures, roads, or equipment that increase wall pressure
Sloping or benching is not ideal when:
The road or right-of-way is narrow
Utilities limit how far the trench can be widened
Adjacent traffic or heavy equipment creates vibration
Soil is saturated, frozen–thawed, or previously disturbed
Required for trenches 5 feet or deeper unless protected by a trench box or shoring.
Designed by a competent person familiar with soil mechanics.
Daily inspections must occur, plus after any condition changes (rain, freeze–thaw, vibration).
Spoil piles must remain 2 feet back from the edge, regardless of slope design.
Safe entry and exit (ladders or steps) must be within 25 feet of all workers.
Standing water must be controlled before workers enter the trench.
Never assume “it looks safe.” Soil stability must be tested and documented.
If the soil is Type C and the trench is 10 feet deep:
Maximum slope is 1½:1
For every 1 foot of depth, the trench must extend 1.5 feet horizontally on both sides
10 ft depth × 1.5 = 15 feet of horizontal width per side
Total top width = trench bottom width + 30 feet
This is why sloping is often impractical in roadway settings—there simply isn’t enough room.
OSHA requires that every excavation be overseen by a competent person, trained to:
Identify potential hazards
Classify soil
Determine when sloping, benching, shoring, or shielding is required
Inspect and document trench conditions
Stop work if conditions become unsafe
This is especially important for municipal crews working in varied seasonal conditions.
Sloping and benching are proven ways to prevent trench fatalities, but they require space, soil knowledge, and daily inspection. When conditions allow, these systems can be more efficient than moving a trench box, especially for long, continuous excavations. But when in doubt, use the more protective system.
No excavation, no matter how routine, is worth risking a life. Applying correct sloping or benching techniques ensures that crews return home safely after every job.