Trench Soil Types: Stable Rock, Type A, B and C

Who Decides, and How0:00

The first step to safely entering a trench is to know what soil type you are working with.

When the soil type sets the slope or shoring, a competent person must classify it by at least one visual and one hand test. The visual check looks in and around the trench for old utilities, dug-before soil, and layers sloping toward it. A competent person can spot existing and predictable hazards, and has the authority to fix them right away. One hand test: roll a moist sample into a thread 1/8 inch thick; if 2 inches hang from one end without tearing, it's cohesive. For unclear soil or unusual conditions, it's good practice to have an engineer assess it on site.

The Four Soil Types0:40

Soil is sorted into four types, from most stable to least: Stable Rock, Type A, Type B and Type C. Type A is strong cohesive soil, like clay, or cemented soil, like hardpan. But soil is never Type A if it's cracked, shaken by vibration, or has been dug before. Type B is weaker cohesive soil, and granular soil like angular gravel, silt and sandy loam. Soil that has been dug before is Type B at best. That includes the backfill around old utility lines that cross or run beside your trench. Type C is the weakest. It's soft cohesive soil, and granular soil like gravel, sand and loamy sand. Soil that is underwater or freely seeping water is Type C. So are soil layers that tilt down into the trench, 1 foot down for every 4 feet across, or steeper. You can also read the soil by eye. Soil that is fine-grained, or stays in clumps as it's dug, is cohesive. Soil that is mostly coarse sand or gravel, or breaks up easily, is granular. Tension cracks, or chunks falling off a wall, can mean cracked soil, and small spalls are a sign the ground is moving.

When It's Not Clear-Cut1:52

Where the soil type sets the protection, a competent person classifies each deposit by the definitions of Stable Rock and Types A, B and C. With more than one layer, classify by the weakest one. Soil strong enough for Type A that is cracked or shaken by vibration is Type B. If unsure between two types, the safe habit is to treat it as the weaker one.

Keep It Current2:14

When protection is based on the soil type and anything affecting it changes, a competent person must check and reclassify the soil as needed. If rain or flooding wets the trench, the soil type must be changed to match.

Recap2:27

To recap. Where it sets the slope or shoring, a competent person classifies soil by look and by hand. From most stable to least, soil is classified as Stable Rock, Type A, Type B or Type C. Type A is strong cohesive soil like hard clay, or cemented soil like hardpan. Type B is in between: medium-strength cohesive soil, or granular soil like angular gravel, silt or sandy loam. Type C is the weakest: soft cohesive soil, gravel, sand, or soil that is underwater or freely seeping. If there is more than one layer, go with the weakest one. Cracked, vibrating or dug-before soil is never Type A. A good habit: when unsure, treat it as weaker. When protection depends on the soil type, a competent person must reclassify as needed when conditions change, including after rain.