An excavator is digging a deep foundation pit. The soil report indicates highly cohesive clay, identified as Type A soil. The pit is designed to be 6 metres deep. What is the maximum allowable slope angle for an unshored, temporary excavation in Type A soil, according to general OHS trenching/excavation requirements?

Question: An excavator is digging a deep foundation pit. The soil report indicates highly cohesive clay, identified as Type A soil. The pit is designed to be 6 metres deep. What is the maximum allowable slope angle for an unshored, temporary excavation in Type A soil, according to general OHS trenching/excavation requirements?

Answer options: ✅ 90 degrees (vertical)

• 53 degrees (a 3/4:1 slope)
• 45 degrees (a 1:1 slope)
• 34 degrees (a 1.5:1 slope)

Correct answer: 90 degrees (vertical)

Explanation: For temporary excavations in stable, highly cohesive Type A soil (e.g., hard compact clay), OHS regulations frequently allow vertical walls for depths up to 1.2 metres. However, for a 6-metre deep pit, even in Type A soil, a slope needs to be applied, typically at 3/4:1 (53 degrees) to ensure stability, or shoring is required. Vertical at 6 metres is not acceptable without engineered shoring.