5. Excavation Safety

Question 1

What is excavation?

Answer 1

Any human-made opening in the earth that meets dimension requirements.

Question 2

What are the four key hazards associated with excavations?

Answer 2

Worker and machine contact

Contact with hazardous service (power lines, gas lines)

Falls into excavations (worker falling in, or materials/equipment falling in/on worker)

Trench collapse

Question 3

What is a trench?

Answer 3

Any excavation where the depth is greater than the width.

Question 4

What are the three general engineering controls used to protect workers in an excavation?

Answer 4

Sloping
Shoring
Shielding

Question 5

List the major components of an excavation shoring system.

Answer 5

Uprights
Walers
Cross braces/Struts

Question 6

What are the advantages and disadvantages of shoring systems?

Answer 6

Advantages:

• Can be constructed from readily available materials.
• Can be built around pipes, cables, etc. projecting from trench walls
• Can often be built on site by the workers that will benefit from them.

Disadvantages

• Materials need to be protected against deterioration (particularly in storage)
• Longer to install than a shield.
• Can be expensive in materials.

Question 7

Why must shoring extend 30 cm above excavation, and to within 60 cm of the bottom?

Answer 7

It ensures sufficient support for the top and bottom of the excavation wall.

It provides a barrier against material that might fall into the excavation.

It is high enough to be noticed by workers, reducing the tripping hazard.

Question 8

What is a spoil pile and why must it be more than 60 cm from the excavation edge?

Answer 8

A spoil pile is the material that was dug out from the excavation.

Maintaining 60 cm from the edge reduces the added pressure on the excavation wall, and reduces the risk that the material will fall into the excavation.

BONUS: if spoil pile height is greater than 60 cm, then it must be taken into account for the selection of shoring/shielding.

Question 9

What are the advantages and disadvantages of a trench shield of trench box?

Answer 9

Advantages:

• Ease of use, a single shield can be quickly placed into position.
• A single unit can be moved PRN and essentially cover multiple sections of an excavation.

Disadvantages:

• Not customized, they won’t fit around pipes/lines etc. in excavation wall.
• Does not prevent collapse. Material may fall against or around wall.
• Requires engineers approval, as such not easily built in-house.

Question 10

List 3 controls to reduce the risk of excavation workers being injured by contact with machines..

Answer 10

Skilled machine operators.

Skilled workers in area of machine operation.

Minimize number of workers near machines.

Question 11

Discuss vibration in relation to excavation safety.

Answer 11

Excavation work must have written instructions from a professional engineer if it is subject to vibration or hydrostatic pressure likely to result in ground movement hazardous to workers.
(example: near-by traffic vibrations)

Question 12

List environmental conditions that may present hazards in/around an excavation.

Answer 12

Surface water
Ground water
Leaks from damaged water/sewer lines
Rainfall, sleet, and snow
Freezing/thawing soil
Vibration
Soil contaminants
Air contaminants from leaking utility lines

Question 13

List 5 advantages of using a checklist in an excavation safety program.

Answer 13

Prevents critical steps being missed.
Promotes worker/supervisor responsibility
Encourages workers/supervisors to waTCH

Question 13

List 5 advantages of using a checklist in an excavation safety program.

Answer 13

Prevents critical steps being missed.
Promotes worker/supervisor responsibility
Encourages workers/supervisors to watch for changing conditions
Record of due diligence
Training aid