NEBOSH CC Excavation & Ground Work Hazards 3 — Questions and Answers
Question 1: What type of ground movement hazard involves soil suddenly losing its strength and behaving like a liquid?
- Liquefaction (Correct answer)
- Subsidence
- Heave
- Sinkholes
Correct answer: Liquefaction
Liquefaction occurs when saturated, loose, granular soils lose their shear strength due to vibration or rapid loading, causing them to behave like a liquid. This can result in sudden and catastrophic excavation collapse.
Question 2: Which permit-to-work system is used for excavation work near live underground services?
- Excavation permit (Correct answer)
- Hot work permit
- Confined space entry permit
- Electrical isolation permit
Correct answer: Excavation permit
An excavation permit is used when working near live underground services. It records the checks carried out, identifies the services present, specifies the methods to be used and identifies who has authorised the work.
Question 3: What personal protective equipment is essential for all workers entering an excavation?
- Hard hat, high-visibility clothing, and safety footwear (Correct answer)
- Only a hard hat
- Only high-visibility clothing
- Only safety footwear
Correct answer: Hard hat, high-visibility clothing, and safety footwear
As a minimum, workers entering an excavation must wear a hard hat to protect against falling objects, high-visibility clothing so they can be seen by plant operators, and safety footwear to protect feet from sharp objects.
Question 4: What is the purpose of a trench box (also called a drag box) in excavation work?
- To provide a pre-made support system that can be dragged along as the trench progresses (Correct answer)
- To store tools safely in a trench
- To filter groundwater from an excavation
- To measure the depth of an excavation
Correct answer: To provide a pre-made support system that can be dragged along as the trench progresses
A trench box or drag box is a steel or aluminium pre-fabricated support system that is placed in the excavation and dragged forward as work progresses. It protects workers from collapse without the need for traditional timber shoring.
Question 5: What is 'heave' in the context of excavation hazards?
- Upward movement of the excavation base due to stress relief or groundwater pressure (Correct answer)
- Lateral movement of excavation sides
- Settlement of soil above an excavation
- Collapse of the excavation sides inward
Correct answer: Upward movement of the excavation base due to stress relief or groundwater pressure
Heave is the upward movement of soil at the base of an excavation caused by stress relief when overburden is removed, or by groundwater pressure. It can destabilise the excavation and supporting structures.
Question 6: When should a competent person inspect an excavation after heavy rainfall?
- Before any workers are allowed to enter the excavation (Correct answer)
- During rainfall so the rate of water ingress can be observed
- Only if water is visible in the excavation
- At the end of the shift following rainfall
Correct answer: Before any workers are allowed to enter the excavation
After heavy rainfall, a competent person must inspect the excavation before any workers are allowed to re-enter. Rainfall can significantly weaken soil, increase water pressure and raise the risk of collapse.
NEBOSH CC Excavation & Ground Work Hazards 3
What type of ground movement hazard involves soil suddenly losing its strength and behaving like a liquid?