CSCS Excavation Safety 2 — Questions and Answers
Question 1: What is the purpose of a Genny (signal generator) used in conjunction with a CAT scanner before excavation?
- To generate electricity for site lighting
- To apply a detectable signal to specific underground services so the CAT can locate non-metallic and dead cables that would otherwise be undetectable (Correct answer)
- To test the soil composition
- To communicate with workers in the trench
Correct answer: To apply a detectable signal to specific underground services so the CAT can locate non-metallic and dead cables that would otherwise be undetectable
A Genny (signal generator) connects to or induces a signal onto specific underground services, allowing the CAT to detect cables and pipes that do not naturally emit a detectable signal. This includes dead electrical cables, fibre optic cables, and plastic pipes. HSE guidance HSG47 requires both CAT and Genny to be used together for a thorough survey.
Question 2: What is the recommended safe method for stockpiling excavated material (spoil) adjacent to a trench?
- Spoil can be placed right at the edge of the trench
- Spoil must be kept at least 1 metre from the edge of the excavation and not heaped to a height that could cause instability (Correct answer)
- Spoil should be pushed back into the trench as soon as possible
- There are no requirements for spoil placement
Correct answer: Spoil must be kept at least 1 metre from the edge of the excavation and not heaped to a height that could cause instability
HSE guidance requires excavated spoil to be placed at least 1 metre from the edge of the excavation. Heavy spoil near the edge increases the lateral pressure on the trench sides and significantly increases the risk of collapse. Additionally, loose spoil at the edge can fall into the excavation onto workers below.
Question 3: What action must be taken if water begins to accumulate in an excavation during work?
- Continue working in the water if it is below knee height
- Assess the risk, implement pumping to remove water, check the stability of the excavation sides, and ensure operatives are not at risk of drowning (Correct answer)
- Wait for the water to drain naturally
- Cover the excavation and move to another area
Correct answer: Assess the risk, implement pumping to remove water, check the stability of the excavation sides, and ensure operatives are not at risk of drowning
Water ingress is a serious risk in excavations — it destabilises the sides, increases the risk of collapse, and creates a drowning hazard. Under CDM Regulations 2015, the competent person must assess the situation, arrange pumping, reinspect the support systems for any deterioration, and ensure workers can safely egress. Work must stop if the risk cannot be controlled.
Question 4: What information must be included in a permit to dig before excavation work commences?
- Only the name of the machine operator
- The location and extent of the dig, known underground services, CAT/Genny survey results, support methods, emergency contacts, and authorisation signatures (Correct answer)
- Just the date and time of the work
- The cost estimate for the excavation
Correct answer: The location and extent of the dig, known underground services, CAT/Genny survey results, support methods, emergency contacts, and authorisation signatures
A permit to dig is a formal document that must include: the exact location and dimensions of the excavation; details of all known underground services from utility plans and CAT/Genny surveys; the proposed method of excavation; support systems to be used; emergency procedures; and authorisation signatures from the competent person and site manager. This is required under CDM Regulations 2015.
Question 5: What is the minimum distance from a known gas main at which mechanical excavation must stop and hand digging must commence?
- 1 metre
- 500mm (Correct answer)
- 2 metres
- No minimum distance — mechanical excavation can continue up to the pipe
Correct answer: 500mm
HSE guidance HSG47 states that when excavating near known gas mains, mechanical excavation must cease and hand digging must be used when within 500mm of the expected position of the service. This reduces the risk of striking and damaging the gas main. The same 500mm rule applies to other buried services.
Question 6: What type of shoring is most commonly used to support the sides of deep excavations in granular (sandy) soil?
- Open sheeting with widely spaced boards
- Close sheeting with continuous interlocking sheet piles or tightly spaced timber boards, as granular soil has no cohesion and collapses easily (Correct answer)
- No shoring is needed for sandy soil as it is naturally stable
- A single prop across the middle of the trench
Correct answer: Close sheeting with continuous interlocking sheet piles or tightly spaced timber boards, as granular soil has no cohesion and collapses easily
Granular (sandy) soil has very low cohesion and will collapse easily, requiring close sheeting where boards or sheet piles are placed continuously with no gaps between them. This contrasts with cohesive clay soils where open sheeting with spaced boards may be sufficient. The competent person must assess the soil type and specify appropriate support under CDM Regulations 2015.
CSCS Excavation Safety 2
What is the purpose of a Genny (signal generator) used in conjunction with a CAT scanner before excavation?