Soil types
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. 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 metres 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.
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.
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. 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 in to 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. 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.
Many factors such as cracks, water, vibration, weather, and previous excavation can affect trench stability (Figure 2). Time is also a critical factor. Some trenches will remain open for a long period, then suddenly collapse for no apparent reason. Figure 3 shows the typical causes of cave-ins. The main factors affecting trench stability are soil type, moisture, vibration, surcharge, previous excavation, existing foundations, and weather. Moisture content The amount of moisture in the soil has a great effect on soil strength. Once a trench is dug, the sides of the open excavation are exposed to the air. Moisture content of the soil begins to change almost immediately and the strength of the walls may be affected. The longer an excavation is open to the air, the greater the risk of a cave-in.
Vibration Vibration from various sources can affect trench stability. Often trench walls are subject to vibration from vehicular traffic or from construction operations such as earth moving, compaction, pile driving, and blasting. These can all contribute to the collapse of trench walls. Surcharge A surcharge is an excessive load or weight that can affect trench stability. For instance, excavated soil piled next to the trench can exert pressure on the walls. Placement of spoil piles is therefore important. Spoil should be kept as far as is practical from the edge of the trench. Mobile equipment and other material stored close to the trench also add a surcharge that will affect trench stability. One metre from the edge to the toe of the spoil pile is the minimum distance requirement (Figure 4). The distance should be greater for deeper trenches.
Previous excavation Old utility trenches either crossing or running parallel to the new trench can affect the strength and stability (Figure 5). Soil around and between these old excavations can be very unstable. At best it is considered Type 3 soil — loose, soft, and low in internal
strength. In some unusual circumstances it may be Type 4 — wet, muddy, and unable to support itself. This kind of soil will not stand up unless it is sloped or shored. Existing foundations Around most trenches and excavations there is a failure zone where surcharges, changes in soil condition, or other disruptions can cause collapse. When the foundation of a building adjacent to the trench or excavation extends into this failure zone, the result can be a cave-in (Figure 6). Soil in this situation is usually considered Type 3.
http://www.king.ca/Government/Departments/Building/Documents/27_M026.pdf
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. 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 metres 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.
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.
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. 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 in to 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. 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.
Many factors such as cracks, water, vibration, weather, and previous excavation can affect trench stability (Figure 2). Time is also a critical factor. Some trenches will remain open for a long period, then suddenly collapse for no apparent reason. Figure 3 shows the typical causes of cave-ins. The main factors affecting trench stability are soil type, moisture, vibration, surcharge, previous excavation, existing foundations, and weather. Moisture content The amount of moisture in the soil has a great effect on soil strength. Once a trench is dug, the sides of the open excavation are exposed to the air. Moisture content of the soil begins to change almost immediately and the strength of the walls may be affected. The longer an excavation is open to the air, the greater the risk of a cave-in.
Vibration Vibration from various sources can affect trench stability. Often trench walls are subject to vibration from vehicular traffic or from construction operations such as earth moving, compaction, pile driving, and blasting. These can all contribute to the collapse of trench walls. Surcharge A surcharge is an excessive load or weight that can affect trench stability. For instance, excavated soil piled next to the trench can exert pressure on the walls. Placement of spoil piles is therefore important. Spoil should be kept as far as is practical from the edge of the trench. Mobile equipment and other material stored close to the trench also add a surcharge that will affect trench stability. One metre from the edge to the toe of the spoil pile is the minimum distance requirement (Figure 4). The distance should be greater for deeper trenches.
Previous excavation Old utility trenches either crossing or running parallel to the new trench can affect the strength and stability (Figure 5). Soil around and between these old excavations can be very unstable. At best it is considered Type 3 soil — loose, soft, and low in internal
strength. In some unusual circumstances it may be Type 4 — wet, muddy, and unable to support itself. This kind of soil will not stand up unless it is sloped or shored. Existing foundations Around most trenches and excavations there is a failure zone where surcharges, changes in soil condition, or other disruptions can cause collapse. When the foundation of a building adjacent to the trench or excavation extends into this failure zone, the result can be a cave-in (Figure 6). Soil in this situation is usually considered Type 3.
http://www.king.ca/Government/Departments/Building/Documents/27_M026.pdf
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