Trench Soil Types 1 to 4

Who Decides, and How0:00

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

In Ontario, the soil type is decided by looking at it and physically examining it. Check at the walls, and out from each wall as far as the trench is deep. Ontario's Ministry of Labour says the soil should be classified by a competent person: someone who knows the law for the work and the dangers on the site. Push a small sharp object in, look at the soil, and compare it with each type. For unclear soil or unusual conditions, it's good practice to have an engineer assess it on site.

The Four Soil Types0:34

Ontario law classifies soil as Type 1, 2, 3 or 4, using the regulation's descriptions. Type 1 is hard and very dense, with low moisture, high strength and no seepage. A small sharp object goes in only with difficulty, and only a machine can dig it. Type 2 is very stiff and dense, and a sharp object goes in with moderate difficulty. Type 3 is stiff to firm, or compact to loose, with surface cracks, seepage or low strength. Type 3 also includes dry soil that runs easily into a cone-shaped pile. Type 3 also includes soil that has been dug before, however hard it looks. That includes the backfill around old utility lines that cross or run beside your trench. Type 4 is soft, very loose and very sensitive, wet or muddy, and flows unless fully supported before digging. It also pays to learn the field signs. Type 1 is hard to drive a pick into, and its walls look smooth and shiny. Type 2 walls may stand for hours, then crack and fall in as they dry. Type 3, when wet, can drop chunks as it dries, especially with vibration. Type 4 flows, like muskeg, quicksand or leda clay.

When It's Not Clear-Cut and Keep It Current1:47

Soil is classified as the type it most closely resembles. With more than one type, go with the most conservative: the highest-numbered type present. If unsure between two types, the safe habit is to treat it as the weaker one.

Soil type can change with depth and along the trench. So it's wise to keep reassessing it, and change the protection when the soil changes.

Recap2:10

To recap. Classify by look and feel, at the walls and out as far as the trench is deep. If there is more than one type, go with the most conservative, the highest number. Dug-before soil is Type 3. A good habit: when unsure, treat it as weaker. When the soil changes, it's wise to reassess and change the protection to match.