Soil Types – Trench Excavation

 

Soil Types – Trench Excavation



The type of soil determines the strength and stability of trench walls.

Identifying soil types requires knowledge, skill, and experience. Even hard soil may contain faults in seams or layers that make it unstable when excavated.

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The foreman or supervisor must be knowledgeable about soil types found on a project and plan protection accordingly. This knowledge must include an awareness that soil types and conditions can change over very short distances. It is not unusual for soil to change completely within 50 meters or for soil to become saturated with moisture over even smaller distances.

The Construction Regulation sets out four soil types. If you are unsure about the soil type, have the soil tested to confirm the type.

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Type 1 — It is hard to drive a pick into Type 1 soil. Hence, it is often described as “hard ground to dig”. In fact, the material is so hard, it is close to rock.

When excavated, the sides of the excavation appear smooth and shiny. The sides will remain vertical with no water released from the trench wall.

If exposed to sunlight for several days, the walls of Type 1 soil will lose their shiny appearance but remain intact without cracking and crumbling.

If exposed to rain or wet weather, Type 1 soil may break down along the edges of the excavation. Typical Type 1 soils include “hardpan,” consolidated clay, and some glacial tills.

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Type 2 — A pick can be driven into Type 2 soil relatively easily. It can easily be excavated by a backhoe or hand-excavated with some difficulty. In Type 2 soil, the sides of a trench will remain vertical for a short period of time (perhaps several hours) with no apparent tension cracks. However, if the walls are left exposed to air and sunlight, tension cracks will appear as the soil starts to dry. The soil will begin cracking and splaying into the trench. Typical Type 2 soils are silty clay and less dense tills. 

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Type 3 — Much of the Type 3 soil encountered in construction is previously excavated material. Type 3 soil can be excavated without difficulty using a hydraulic backhoe. When dry, Type 3 soil will flow through fingers and form a conical pile on the ground. Dry Type 3 soil will not stand vertically and the sides of the excavation will cave into a natural slope of about 1 to 1 depending on moisture. Wet Type 3 soil will yield water when vibrated by hand. When wet, this soil will stand vertically for a short period. It dries quickly, however, with the vibration during excavation causing chunks or solid slabs to slide into the trench. All backfilled or previously disturbed material should be treated as Type 3. Other typical Type 3 soil includes sand, granular materials, and silty or wet clays.

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Type 4 — Type 4 soil can be excavated with no difficulty using a hydraulic backhoe. The material will flow very easily and must be supported and contained to be excavated to any significant depth. With its high moisture content, Type 4 soil is very sensitive to vibration and other disturbances which cause the material to flow. Typical Type 4 material includes muskeg or other organic deposits with high moisture content, quicksand, silty clays with high moisture content, and Leta clays. Leta clays are very sensitive to disturbance of any kind.

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