Causes of Cave-ins
Soil properties often vary widely from the top to the bottom and along the length of a trench. Many factors such as cracks, water, vibration, weather, and previous excavation can affect trench stability. Time is also a critical factor. Some trenches will remain open for a long period, then suddenly collapse for no apparent reason.
The main factors
affecting trench stability are soil type, moisture, vibration, surcharge, previous
excavation, existing foundations, and weather.
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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.
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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 meter from the edge to the toe of the spoil pile is the minimum distance requirement. 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. 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.
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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. Soil in this situation is usually considered Type 3.
Weather
Rain, melting snow,
thawing earth, and overflow from adjacent streams, storm drains, and sewers all
produce changes in soil conditions. In fact, water from any source can reduce
soil cohesion. Frozen soil does not mean that you can have reduced shoring or
that a heavier load can be supported. Frost extends to a limited depth only.
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