NEBOSH CC Excavation and Demolition Safety 2 — Questions and Answers
Question 1: What is the minimum distance from the edge of an excavation that heavy plant and materials should be kept to prevent surcharge loading causing collapse?
- Materials and plant should be kept as far back as practicable; a common guideline is at least equal to the depth of the excavation (Correct answer)
- 30 cm from the edge
- There is no minimum distance
- 1 metre regardless of depth
Correct answer: Materials and plant should be kept as far back as practicable; a common guideline is at least equal to the depth of the excavation
Surcharge loading from heavy plant, vehicles, and stored materials near excavation edges can cause collapse. While the exact safe distance depends on soil conditions, a common guideline is to keep loads at least as far back from the edge as the excavation is deep. A competent person should assess the specific conditions and set exclusion zones.
Question 2: What protective measures should be in place around an open excavation to prevent people and vehicles from falling in?
- Substantial barriers or guardrails, clear warning signage, and edge protection (Correct answer)
- Cones placed along the edge
- Warning tape only
- Verbal warnings to workers at the start of each shift
Correct answer: Substantial barriers or guardrails, clear warning signage, and edge protection
Open excavations must be protected by substantial barriers (not tape alone) to prevent persons and vehicles from falling in. Guardrails should meet the same standards as for work at height (950mm top rail, toe board, intermediate rail). Clear warning signs, lighting for night-time, and vehicle stop blocks should also be provided.
Question 3: What is the correct sequence for demolishing a multi-storey building using the top-down method?
- Remove hazardous materials first, then demolish storey by storey from the top downwards, ensuring structural stability at each stage (Correct answer)
- Demolish the ground floor first to bring the building down
- Remove alternate floors to weaken the structure
- Use explosives on all floors simultaneously
Correct answer: Remove hazardous materials first, then demolish storey by storey from the top downwards, ensuring structural stability at each stage
Top-down demolition is the reverse of construction: remove hazardous materials (asbestos, chemicals), soft strip internal fittings, then demolish floor by floor from the top. Structural stability must be maintained at each stage. This method is common in urban areas where controlled collapse is not possible due to proximity of other buildings.
Question 4: An excavation is being dug near an existing building. What is the primary structural risk to the adjacent building?
- Undermining the building's foundations, causing settlement or collapse (Correct answer)
- Noise damage to the building's windows
- Dust damage to the building's paintwork
- Vibration damage to internal plumbing only
Correct answer: Undermining the building's foundations, causing settlement or collapse
Excavating near existing buildings can remove the soil supporting their foundations, leading to settlement, cracking, or structural collapse. A structural assessment must determine the depth and proximity of existing foundations. Underpinning or sheet piling may be required to protect the adjacent structure.
Question 5: How often must excavations be inspected under the CDM Regulations 2015?
- At the start of each shift, after any event likely to affect stability, and after any accidental fall of material (Correct answer)
- Weekly
- Monthly
- Only at the start of the project
Correct answer: At the start of each shift, after any event likely to affect stability, and after any accidental fall of material
CDM 2015 Schedule 3 requires excavation inspections at the start of every shift before work commences, after any event likely to have affected strength or stability (such as heavy rain, frost, or nearby vehicle movement), and after any accidental fall of rock or earth. A competent person must carry out and record inspections.
Question 6: During excavation work, a worker discovers an uncharted gas main. What is the correct immediate action?
- Stop work immediately, evacuate the area, prevent ignition sources, and contact the utility emergency service (Correct answer)
- Continue digging carefully around it
- Cover it over and continue work elsewhere
- Attempt to cap the pipe
Correct answer: Stop work immediately, evacuate the area, prevent ignition sources, and contact the utility emergency service
If an uncharted service is discovered, all work must stop immediately. For gas mains, the area must be evacuated, all ignition sources eliminated, and the National Gas Emergency Service (0800 111 999) contacted immediately. No one should attempt to repair or interfere with the service. The incident should be investigated to determine why the service was not identified.
NEBOSH CC Excavation and Demolition Safety 2
What is the minimum distance from the edge of an excavation that heavy plant and materials should be kept to prevent surcharge loading causing collapse?