Safe Work Practice 

Trenching/Excavation

General

Soil conditions and stability can vary greatly from one area to another. Factors that can affect soil stability include, but are not limited to: previously disturbed soil, drying of walls and sub-surface weeping.

Possible Hazards

Cave in

Water

Underground / overhead electrical lines

Underground gas mains

Underground water mains

Falling loads

Slips/trips/falls

Hazardous atmosphere

Equipment malfunction

PPE

Hard hat

Safety vest

Steel toed footwear

Eye protection

Protective Systems

There are different types of protective systems. Sloping involves cutting back the trench wall at an angle inclined away from the excavation. Shoring requires installing aluminum hydraulic or other types of supports to prevent soil movement and caveins. Shielding protects workers by using trench boxes or other types of supports to prevent soil cave-ins. Designing a protective system can be complex because you must consider many factors: soil classification, depth of cut, water content of soil, changes due to weather or climate, surcharge loads (eg., spoil, other materials to be used in the trench) and other operations in the vicinity.

Hard Compact

Hard compact is defined as: 

•	Hard to hand excavate, 

•	an excavating bucket can leave well defined teeth marks in the soil, 

•	the soil has been verified as hard compact by a Professional Engineer, 

•	a soil testing kit indicates that it is hard compact.

Soils Other than Hard Compact

1.	Require a 45 degree or greater cutback from the vertical side wall in all soil conditions above 1.2 meters in height.

2.	Require the use of shoring, or

3.	Require the use of an engineered trench box. 

Frozen Soils

Frozen soils cannot be considered hard compact due to the risk of wall failure below the frost line.

Set-Backs

Trenches in or near roadways and construction sites are subject to wall movement from vibration. Vehicles and equipment must be kept back from the trench a distance equal to the depth of the trench.

Spoil Piles

1.	They must be set back a minimum of 1.0 m from the trench/excavation edge.

2.	If placed too close to the trench or excavation edge, they can exert excess downward pressure causing wall failure.

3.	Excavated loose material should be scaled back away from the edge of the trench.

Inspection

Trenches to be inspected daily and as conditions change by a competent person prior to employee entry to ensure elimination of excavation hazards

Access and Egress

Safe access and egress to all excavations, including ladders, steps, ramps, or other safe means of exit for employees working in trench excavations 4 feet (1.22 meters) or deeper. These devices must be located within 15 feet (4.5 meters) of all employees.

The Occupational Health and Safety Act, Occupational Safety General Regulations -Section 14- Trenches always takes precedence over all company rules.