Sloping and benching

Sloping is the simplest protective system: cut the walls back at an angle flat enough that the soil cannot slide. It needs no special equipment, no installation crew and no rental. It also needs a lot of room, moves a lot of soil and only works where the soil type is known and stays the same. This lesson covers the required slopes, how to calculate the space and volume a sloped excavation needs, benching, and the conditions that make sloping the wrong choice.

How sloping works

A vertical trench wall fails because the weight of soil behind it exceeds the soil's ability to resist sliding along a diagonal plane. Cutting the wall back to a flatter angle removes the wedge of soil that would otherwise slide, and leaves a face whose angle is within what the soil can support. The weaker the soil, the flatter the slope must be. Slopes are written as a ratio of horizontal distance to vertical rise, or as an angle from the horizontal or the vertical. Because different documents use different conventions, always check which way a ratio is written.

• A slope of 1 horizontal to 1 vertical (1H:1V) is 45 degrees from horizontal.
• 3/4H:1V is about 53 degrees from horizontal, steeper than 1:1.
• 1.5H:1V is about 34 degrees from horizontal, flatter than 1:1.
• 3H:1V is about 18 degrees from horizontal, very flat.

Required slopes

OSHA Appendix B to Subpart P gives the maximum allowable slopes for excavations less than 20 ft deep, based on soil type:

Excavations 20 ft or deeper, or slopes steeper than the table allows, require a design by a registered professional engineer. If the competent person does not classify the soil, Appendix B requires the excavation to be sloped no steeper than 1 1/2:1, the Type C slope. Spoil must still be kept at least 2 ft from the edge of the slope.

Calculating width and volume

The top width of a sloped trench with a flat bottom is the bottom width plus the horizontal run of each slope. For a slope ratio \( k \) (horizontal per unit vertical) on both sides over a sloped height \( h \):

The cross-sectional area of a trapezoidal trench is the average of the top and bottom widths multiplied by the depth, and the volume is that area multiplied by the length. These numbers matter because they decide whether the slope fits on the site, how much spoil must be stored and how much backfill and compaction will be needed afterwards.

Benching

Benching cuts the wall into a series of horizontal steps, so that the overall profile follows the required slope. Benches give workers places to stand and make it easier to remove soil in lifts, but they add risk if done badly: each vertical riser is a small unsupported wall, and a bench that is too tall or too narrow defeats the purpose.

OSHA Appendix B permits benching in Type A soil and in cohesive Type B soil, but not in Type C soil or granular Type B soil. In Type A, benches may be up to 4 ft high, and the overall profile must stay within the 3/4:1 slope. In cohesive Type B, benches may be up to 4 ft high within the 1:1 slope. Appendix B also allows Type A excavations 8 ft deep or less to have an unsupported vertical lower portion up to 3 1/2 ft high, with the walls above sloped at 3/4:1; between 8 and 12 ft deep, the vertical portion is still limited to 3 1/2 ft and the walls above must be sloped at 1:1.

When sloping is the wrong choice

Sloping is cheap only when there is room for it and the soil is reasonably strong. It becomes the wrong choice when:

• There is not enough room: urban streets, trenches beside buildings, property lines, live traffic lanes and existing utilities rarely leave space for wide cuts.
• The soil is weak or wet: Type 4 or Type C soils need such flat slopes that sloping becomes impractical, and wet slopes can still slump.
• The excavation will be open for a long time: slopes erode, dry and crack, and must be maintained.
• Nearby structures could be undermined: a sloped cut beside a foundation may remove its lateral support.
• The volume of soil to move, store and replace makes the job uneconomic, or the restoration cost of a wide cut in a paved area is greater than renting a trench box.

Combining methods is common: a slope at the top of an excavation with a trench box or shoring below, or a sloped section where space allows and shielding where it does not. The combination must be planned so that the transitions do not create unprotected vertical walls.

Maintaining slopes

A slope is only protective while it keeps its shape. The competent person should watch for slumping, erosion gullies after rain, tension cracks behind the crest, water seeping from the face and loose material rolling down. Slopes should be scaled of loose material, kept free of surcharge at the crest, protected from surface water with diversion berms or ditches, and covered with tarps if rain is expected. A slope that has slumped no longer matches the angle it was cut to and must be re-cut or supported before work continues below it.

Combining slopes with other systems

Many excavations use a combination: the upper part of the wall is sloped and a trench shield or shoring protects the lower part. This is efficient where the top few feet are in weaker fill and a shield would otherwise need to be very tall. OSHA Appendix B allows supported or shielded vertically sided lower portions in Type A and B soils, provided the support or shield extends at least 18 in above the top of the vertical side and the slope above meets the maximum allowable angle for the soil. In Type C soil, the support or shield must likewise extend at least 18 in above the vertical side, with the slope above at 1 1/2:1. The transition is where errors happen: a slope that ends at the top of the shield with no freeboard, or a shield set so low that the slope above it effectively becomes a vertical wall, gives no protection at all.