Excavations result in injuries or deaths annually, due to a lack of knowledge and site preparation. Soils combine clay, sand and roc, resulting in different performance and characteristics.
Understanding Soil Characteristics
Recognizing the type of soil present is possible understanding its physical characteristics:
- Clay: Ranging from very soft to hard clay, this material can be easily penetrated with a fist at its softest or indented with difficulty by thumbnail at its hardest.
- Sand: Ranging from loose clean sand to dense clean sand, this soil can take a footprint from as much as 10mm deep to less than 3 mm deep. Gravel is sometimes considered a type of sand.
- Rock: Broken or decomposed rock can be dug with a pick, while sound rock is not usually diggable.
Excavation preparation varies according to soil composition.
Understanding the Angle of Repose
The angle of repose is a function of the natural slope of a particular soil, impacting excavation safety:
- Granular soils have a slope ratio of 1.5:1 and a slope angle of 34 degrees.
- Weak cohesive soils have a slope ratio of 1:1 and a slope angle of 45 degrees.
- Cohesive soils have a slope ratio of 0.75:1 and a slope angle of 53 degrees.
The slope ratio corresponds to the minimum distance that equipment should be from the edge of an excavation pit to minimize the chance of a cave-in. For example, a 3-foot-deep pit in a weak cohesive soil should have no equipment within 3 feet of its edge. A higher slope angle is more favorable, but moisture reduces this value.
There are three popular methods for preventing soil collapse:
- Battering: Sloping the sides to the angle of repose
- Benching: Cutting steps into the sides of a pit
- Shoring: Using mechanical devices to hold up sides
It’s incumbent upon you to recognize signs of changing conditions that could lead to collapse. These include wall tension cracks, loose soil sliding, soil chunks toppling, wall bulging, pit floor heaving or collection of water in the bottom.