01-32_GENERALS_SUMMER25_PT - Flipbook - Page 16
HEALTH AND SAFETY
How soil conditions contribute to trench
collapses
Any time you are in a trench,
you are at risk of
being in a trench collapse.
The type of soil you dig in, however, is a
primary factor. It determines the overall
stability of your trench walls and must be
properly evaluated before any work occurs.
Section 226 of O. Reg. 213/91 describes
four types of soil based on factors such as
hardness, density, moisture content, and
internal strength. Type 1 is the hardest
soil—it’s almost like rock—while Type 4
is soft, loose, and 昀氀ows easily, like sand.
The higher the soil number, the more
likely it is to collapse if not properly excavated and supported.
For trenching operations in Ontario,
“The vast majority of work involves Type
3 soil, which has a low amount of internal
strength,” says IHSA Health and Safety
Consultant Ryan Smith. “In part that’s
because any soil that has ever been previously excavated or otherwise mechanically disturbed must be automatically
considered Type 3, if not Type 4 soil.”
A competent person must identify the
soil type found at a project and plan to
protect workers accordingly. (More on
that below.) It’s common for multiple soil
types to be present, even in a small area or
in soil that’s never been excavated before.
“Utilities are often buried in sand,
which is Type 4 soil, to protect the utility
from contact with rocks and other sharp
and abrasive objects,” Smith says, adding
that when more than one soil type is
identi昀椀ed, the law requires that the entire
excavation be protected based on the
highest number (i.e., the weakest) soil.
How changes to site conditions affect
trench safety
But a soil’s assessed “type” is not the
only determinant of its strength. And that
strength can change. It can be negatively
a昀昀ected by changing environmental and
jobsite conditions, such as:
EVAPORATION: Once a trench is dug, the sides
SHAREABLE RESOURCES
DOWNLOAD the Trenching chapter of IHSA’s
Construction Health and Safety Manual
(M029) to learn more about soil types and
protecting against cave-ins.
SIGN UP for our free, half-day Trenching
Safety Hazard Awareness training course to
help you understand the major hazards of
digging and working in trenches—and how to
control them.
16 the generals • SUMMER 2025
of the open excavation are exposed to
the air, which can decrease soil cohesion.
“Moisture is what binds the soil particles together,” Smith says. “In a hot, dry
summer, moisture will evaporate from
the trench walls over time, making the
soil looser and less consistent—and more
likely to fail.” The longer a trench is open
to the air, the greater the risk of a cave-in.
WATER: Excess moisture from rain, melting
snow, or thawing earth can weaken soil,
too. Heavy rain is particularly dangerous:
not only does it erode exposed trench
walls, it can create unseen changes to the
water table that further reduce soil cohesion. (Workers have also become trapped
in trenches due to 昀氀ash 昀氀oods caused by
torrential downpours.)
VIBRATION: Nearby vehicle tra昀케c or vibrations from construction operations
such as earth moving, compaction, pile
driving, and blasting can all contribute to
the collapse of trench walls.
EXCESSIVE LOAD: Also called surcharge, any
extra weight next to the trench puts pressure on its walls. This surcharge is most
often caused by excavated earth (a.k.a.
“spoil”) piled too close to the trench, but
nearby vehicles, equipment, tools, and
even workers can overburden a trench
wall. “It’s something that I see regularly,
even though the law says that you cannot
have anything within one metre of the
edge of an excavation,” Smith says.
How to protect against trench collapses
So how do you protect workers and prevent trench cave-ins? The hazard can be
eliminated for most people on a jobsite
by ensuring that only workers who are
directly involved in the trench operation
go near the excavated area.
For those who must be in the trench,
the law says you can never enter it alone.
Smith tells the story of a quality control
technician who entered an excavation to
get a soil sample without telling anyone:
“It would’ve been a couple minutes of
work for this person. But the excavation
hadn’t been 昀椀nished; the walls weren’t
properly supported. And the workers
were on break. So no one was aware that
the technician had even been on site
until they found him in the trench under
collapsed soil.”
Sloping the trench walls is the preferred way to reduce the risk of a cave-in.
This is done by cutting back the walls at
an angle—to stabilize the walls, prevent
spoil from sliding back, and to reduce the
pressure it puts on the walls. The slope
angle depends on the soil type. For example, for Type 3 soil, you slope the walls
at a gradient of 1 to 1 from the bottom of
trench: the walls of a three-metre-deep
trench would therefore be cut back by
three metres.
But tight jobsites do not always allow
for the proper amount of sloping. In that
case, you have two options:
Use a prefabricated, timber, or hydraulic
shoring system to support the trench walls.
Nowadays, hydraulic systems are often
preferred by contractors as the shoring can
be installed from above, without workers
having to enter the trench. However, it
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