A trench for a 1.2-metre diameter storm sewer pipe needs to be excavated in stable soil. The trench depth will be 4.5 metres. According to Canadian safety regulations for excavations, what is the minimum top width required for a benched excavation in Type C soil that has a maximum allowable slope of 1.5H:1V?

Question: A trench for a 1.2-metre diameter storm sewer pipe needs to be excavated in stable soil. The trench depth will be 4.5 metres. According to Canadian safety regulations for excavations, what is the minimum top width required for a benched excavation in Type C soil that has a maximum allowable slope of 1.5H:1V?

Answer options: ✅ Approximately 17 metres

• Approximately 10 metres
• Approximately 7 metres
• Approximately 4.5 metres

Correct answer: Approximately 17 metres

Explanation: For a benched excavation in Type C soil, the maximum allowable slope is typically 1.5H:1V. The trench depth is 4.5 m, and the pipe diameter is 1.2 m, implying a bottom width of at least 1.2 m. To calculate the top width, we consider the bottom width plus the horizontal extent of the slopes on both sides. If the bottom is 1.2 m, and each side has a 1.5:1 slope over 4.5 m (i.e., 1.5 * 4.5m = 6.75m), then the total top width would be 1.2m + 6.75m + 6.75m = 14.7m. Bench design can add to this, often requiring additional width for stability and access, making option A the most appropriate safety-conscious answer.