A trench for a new water main is 2.5 metres deep. According to Ontario Reg. 213/91, what is the minimum height a trench box must extend above the bottom of the trench excavation?

Question: A trench for a new water main is 2.5 metres deep. According to Ontario Reg. 213/91, what is the minimum height a trench box must extend above the bottom of the trench excavation?

Answer options:

• 1.2 metres
• 1.5 metres ✅ It must extend at least 300 mm above the top of the trench when soil is sloped back at 1:1.
• It must extend at least 300 mm above the top of the trench if the trench is less than 1.2 metres wide.

Correct answer: It must extend at least 300 mm above the top of the trench when soil is sloped back at 1:1.

Explanation: Ontario Regulation 213/91 (Construction Projects) section 236(1) states that if a trench box is used, it shall extend above the top of the trench at least 300 millimetres where the trench is sloped back at an angle of not more than one horizontal to one vertical. This prevents material from falling into the trench.