Trench Excavation Slope
To determine the maximum allowable slope for an excavation you need to know the soil type in the area. Remember that ALL of these sloped excavations have a maximum depth of 20 FEET, any deeper than that and you must use some form of shoring or layering.
|Soil Type||Max Allowable Slope (H:V)|
|Type A||3/4 : 1|
|Type B||1 : 1|
|Type C||1 1/2 : 1|
accordng to OSHA the slope can never exceed 1 : 2.
Solid mineral material that remains upright, stable, and intact when it is excavated.
Type A (compressive strength or greater)
These are cohesive soils that contain a good amount of clay. A soil cannot be Type A if:
1. it has cracks
2. there is vibration from traffic or some other source nearby (factory)
3. It has been disturbed recently (geologically recently, like a hundred years)
4. soil is part of a sloped + layered system
If a Type A soil has any of the listed defects it is downgraded.
Type B (compressive strength )
1. These are soils that have cohesion (clay)
2. some granular cohesionless soils are, gravel, silt, rocks etc)
3. previously disturbed soils (geologically recent)
4. Type A soil that has cracks
5. unstable dry rock
Type C (compressive strength less than )
1. cohesive soil with compressive strength
1. granular soils
2. submerged soil
4. submerged unstable rock
If the site you are working with cannot meet these conditions for a sloped excavation due to available space or some other obstruction, you need to look into using some kind of retaining structure to hold the soil back in the limited space.