An excavation is designed for a trench box that is 2.4 metres wide internally. The ground conditions are unstable, a Type 3 soil according to CSA Z94.0-15 (Confined Space Regulations), which is relevant for trench stability in some interpretations. If the trench depth is 4.0 metres, what minimum width must the excavation be at the top to safely deploy and remove the trench box and allow for standard shoring procedures, assuming a minimum working space of 0.6 metres on each side of the box for installation and inspection according to good industry practice?

Question: An excavation is designed for a trench box that is 2.4 metres wide internally. The ground conditions are unstable, a Type 3 soil according to CSA Z94.0-15 (Confined Space Regulations), which is relevant for trench stability in some interpretations. If the trench depth is 4.0 metres, what minimum width must the excavation be at the top to safely deploy and remove the trench box and allow for standard shoring procedures, assuming a minimum working space of 0.6 metres on each side of the box for installation and inspection according to good industry practice?

Answer options:

• 3.6 metres ✅ 3.0 metres
• 4.8 metres
• 3.2 metres

Correct answer: 3.0 metres

Explanation: The internal width of the trench box is 2.4 metres. Adding 0.6 metres working space on each side makes the required excavation width 2.4 + 0.6 + 0.6 = 3.6 metres. This allows for safe installation, inspection, and worker access around the box, preventing workers from being pinched. Note: While CSA Z94.0-15 does not specifically define soil types as given, Type 3 soil indicates poor stability and emphasizes the need for shoring, and good practice for trench box usage includes adequate working space.