CSCS Excavation Safety 3 — Questions and Answers
Question 1: Who is legally defined as the 'competent person' required to inspect excavations under CDM Regulations 2015?
- Any labourer working on site
- A person with sufficient training, experience, and knowledge to identify hazards and specify the necessary precautions for excavation work (Correct answer)
- The client who commissioned the project
- The plant hire company representative
Correct answer: A person with sufficient training, experience, and knowledge to identify hazards and specify the necessary precautions for excavation work
Under CDM Regulations 2015, a competent person is someone with sufficient training, experience, knowledge, and other qualities to carry out the inspection properly. For excavation inspections, they must understand soil mechanics, support systems, and the hazards of excavation work. They must produce written inspection reports that are kept on site.
Question 2: What colour does a CAT scanner display or indicate when it detects a power cable in 'Power' mode?
- Blue for water
- The signal strength increases and an audible warning sounds — the operator must mark the position and determine the depth before any excavation (Correct answer)
- Green for safe to dig
- No indication is given in Power mode
Correct answer: The signal strength increases and an audible warning sounds — the operator must mark the position and determine the depth before any excavation
In Power mode, the CAT detects the electromagnetic field naturally emitted by live power cables. The signal strength meter shows increasing readings as the detector approaches the cable, and an audible tone becomes louder. The operator must mark the cable's position on the ground and use the Genny to establish its depth. HSE guidance HSG47 requires all modes (Power, Radio, Genny) to be used.
Question 3: What hazard does vibration from nearby plant or traffic pose to an open excavation?
- No hazard — vibration has no effect on excavations
- Vibration can destabilise the sides of the excavation, increasing the risk of collapse, and may dislodge shoring or trench box supports (Correct answer)
- Vibration only affects excavations deeper than 10 metres
- Vibration improves soil compaction and makes excavations safer
Correct answer: Vibration can destabilise the sides of the excavation, increasing the risk of collapse, and may dislodge shoring or trench box supports
Vibration from heavy plant, traffic, or piling operations can significantly destabilise excavation sides, particularly in granular soils. It can also dislodge shoring supports and cause previously stable soil to collapse. Under CDM Regulations 2015, the competent person must consider vibration sources in the risk assessment and may need to restrict plant movement near excavations.
Question 4: What must happen if an uncharted underground service is discovered during excavation?
- Remove it and continue digging
- Stop work immediately, make the area safe, and report the discovery to the site manager who must identify the service before work resumes (Correct answer)
- Cover it with soil and work around it
- Only stop if it appears to be an electrical cable
Correct answer: Stop work immediately, make the area safe, and report the discovery to the site manager who must identify the service before work resumes
If any unexpected underground service is encountered during excavation, all work must stop immediately. The area must be made safe and the discovery reported to the site manager. The service must be positively identified by contacting the relevant utility provider before any further work takes place. This is required under HSE guidance HSG47 and the CDM Regulations 2015.
Question 5: What is the purpose of monitoring equipment such as inclinometers used on large excavation projects?
- To measure the temperature of the soil
- To detect and measure ground movement around the excavation, providing early warning of potential instability or collapse (Correct answer)
- To test the quality of backfill material
- To track the position of underground services
Correct answer: To detect and measure ground movement around the excavation, providing early warning of potential instability or collapse
Inclinometers and other monitoring equipment measure lateral ground movement around excavations. They provide early warning of ground displacement that could indicate impending collapse, allowing preventive action before failure occurs. On large or deep excavations, CDM Regulations 2015 may require ongoing monitoring as part of the construction phase plan.
Question 6: Under HSE guidance, what weather conditions require additional excavation inspections?
- Only snow
- Heavy rainfall, flooding, frost/thaw cycles, and high winds — any conditions that could affect the stability of the excavation sides or support systems (Correct answer)
- Weather conditions do not affect excavation safety
- Only extreme heat above 30°C
Correct answer: Heavy rainfall, flooding, frost/thaw cycles, and high winds — any conditions that could affect the stability of the excavation sides or support systems
Heavy rainfall can saturate soil and reduce its strength; frost/thaw cycles can cause surface layers to crumble; flooding can undermine supports; and high winds can affect working conditions. Under CDM Regulations 2015 Regulation 22, the competent person must reinspect the excavation after any event likely to have affected its stability, including adverse weather.
CSCS Excavation Safety 3
Who is legally defined as the 'competent person' required to inspect excavations under CDM Regulations 2015?