NEBOSH CC Excavation and Demolition Safety — Questions and Answers
Question 1: Before any excavation work begins on a construction site, what is the essential first step to identify underground hazards?
- Obtain service plans from utility providers and carry out a CAT and Genny scan of the area (Correct answer)
- Start digging carefully with a mechanical excavator
- Ask local residents about any known services
- Check the site boundary for warning signs only
Correct answer: Obtain service plans from utility providers and carry out a CAT and Genny scan of the area
HSG47 (Avoiding Danger from Underground Services) requires obtaining up-to-date plans from all utility providers and then using a cable avoidance tool (CAT) and signal generator (Genny) to scan the excavation area. Plans alone are insufficient as services may have been diverted or added without updating records.
Question 2: What is the primary purpose of trench support (shoring) during excavation work?
- To prevent the collapse of trench sides, which could bury workers (Correct answer)
- To keep the trench dry
- To provide a working platform at the bottom
- To support equipment placed alongside the trench
Correct answer: To prevent the collapse of trench sides, which could bury workers
Trench support systems (shoring, trench boxes, sheet piling, battering) prevent the collapse of excavation sides, which is one of the most serious hazards in construction. Unsupported trenches can collapse without warning, burying workers under tonnes of soil. One cubic metre of soil weighs approximately 1.5-2 tonnes.
Question 3: Under what circumstances might battering (sloping the sides) be preferred over vertical shoring for an excavation?
- When there is sufficient space to slope the sides to a safe angle and the ground conditions are suitable (Correct answer)
- When the excavation is in rock only
- When the excavation is deeper than 5 metres
- Battering is never an acceptable method
Correct answer: When there is sufficient space to slope the sides to a safe angle and the ground conditions are suitable
Battering involves cutting the excavation sides to a safe angle (typically 45 degrees or less for most soils). It is preferred when there is adequate space, as it avoids the need for support equipment. However, the safe angle depends on soil type, moisture content, and surcharge loads. A competent person must determine the appropriate angle.
Question 4: What hazardous atmosphere might accumulate in a deep excavation, and what precautions are required?
- Carbon dioxide, methane, or hydrogen sulphide can accumulate; gas monitoring and ventilation are required (Correct answer)
- Only dust is a concern
- Deep excavations have no atmospheric hazards
- Only radon is a risk
Correct answer: Carbon dioxide, methane, or hydrogen sulphide can accumulate; gas monitoring and ventilation are required
Deep excavations can accumulate hazardous gases including CO2 (from decomposing organic matter), methane (from landfill or natural deposits), and H2S (from sewers or contaminated land). Oxygen depletion can also occur. Gas monitoring before entry, continuous monitoring during work, and provision of ventilation are essential precautions.
Question 5: A building is to be demolished. Under CDM 2015, what specific document must be prepared before demolition work begins?
- A detailed demolition method statement prepared by a competent person, included within the construction phase plan (Correct answer)
- A verbal agreement between the client and contractor
- A planning permission document only
- A structural warranty certificate
Correct answer: A detailed demolition method statement prepared by a competent person, included within the construction phase plan
CDM 2015 Regulation 20 requires that demolition or dismantling of a structure is planned and carried out in a safe manner under the supervision of a competent person. A detailed method statement must be prepared, describing the sequence of demolition, structural stability measures, hazardous material removal (especially asbestos), and exclusion zones.
Question 6: During demolition of a pre-2000 building, which hazardous material must be identified and removed before general demolition begins?
- Asbestos-containing materials (ACMs) (Correct answer)
- Lead paint
- Treated timber
- Old electrical wiring
Correct answer: Asbestos-containing materials (ACMs)
Asbestos is the single greatest cause of work-related death in the UK. The Control of Asbestos Regulations 2012 require a refurbishment and demolition (R&D) asbestos survey before any demolition work. All asbestos-containing materials must be identified and removed by licensed contractors (for licensable ACMs) before general demolition begins.
NEBOSH CC Excavation and Demolition Safety
Before any excavation work begins on a construction site, what is the essential first step to identify underground hazards?