CITB Specialists Excavation and Groundwork Safety — Questions and Answers
Question 1: Before a specialist contractor begins excavation work, what must be done to identify underground services?
- Ask local residents if they know where pipes are
- Obtain service plans from utility companies, carry out a CAT and Genny survey, and use safe digging practices (Correct answer)
- Dig a trial hole with a mechanical excavator
- Only check for electricity cables as they are the most dangerous
Correct answer: Obtain service plans from utility companies, carry out a CAT and Genny survey, and use safe digging practices
Before any excavation, the contractor must obtain up-to-date plans from all utility companies, then verify locations using a Cable Avoidance Tool (CAT) and Signal Generator (Genny). Within 500mm of a detected service, only hand digging or vacuum excavation is permitted. HSG47 provides detailed guidance on safe digging practices.
Question 2: At what depth must an unsupported excavation in granular soil generally be considered as requiring engineered support under UK guidance?
- Any depth over 3 metres
- Any depth over 1.2 metres
- Only excavations deeper than 5 metres
- No specific depth; a risk assessment must determine the need for support in all cases (Correct answer)
Correct answer: No specific depth; a risk assessment must determine the need for support in all cases
There is no fixed depth at which support automatically becomes mandatory. The need for support depends on soil type, ground conditions, water table, surcharge loads, vibration, and weather. A competent person must assess each excavation individually. However, as a practical guide, most excavations over 1.2m in depth will need some form of support.
Question 3: How often must a specialist excavation be inspected by a competent person under the Construction (Design and Management) Regulations 2015?
- Once a week
- At the start of each shift before work commences, and after any event likely to have affected stability (Correct answer)
- Only after heavy rainfall
- Monthly by a structural engineer
Correct answer: At the start of each shift before work commences, and after any event likely to have affected stability
CDM 2015 requires that excavations are inspected by a competent person at the start of every shift before anyone enters, and after any event likely to have affected stability (heavy rain, adjacent loading, vibration from plant). The results must be recorded in a written report kept on site.
Question 4: What is the primary purpose of installing trench sheets and walers in a specialist deep excavation?
- To keep the trench dry
- To prevent collapse of the excavation sides and protect workers from being buried (Correct answer)
- To provide a working platform at the base
- To mark the boundaries of the excavation
Correct answer: To prevent collapse of the excavation sides and protect workers from being buried
Trench sheets (or trench sheeting) driven into the ground and held in place by walers (horizontal beams) and struts form a temporary support system that prevents the sides of the excavation from collapsing inward. This is critical — trench collapse is one of the leading causes of death in excavation work.
Question 5: A specialist groundwork supervisor notices that spoil from an excavation has been placed at the edge of the trench. What is the hazard and the correct action?
- The spoil may get wet; cover it with sheeting
- The weight of the spoil surcharges the trench sides, increasing collapse risk — it must be moved back at least 1 metre from the edge (Correct answer)
- The spoil is untidy; it should be spread evenly
- No action required if the trench has support
Correct answer: The weight of the spoil surcharges the trench sides, increasing collapse risk — it must be moved back at least 1 metre from the edge
Spoil heaps placed near the edge of an excavation impose additional surcharge loading on the ground, significantly increasing the risk of side collapse. As a minimum, spoil must be placed at least 1 metre from the edge of the excavation, though the exact distance may need to be greater depending on depth and soil conditions.
Question 6: What precaution must a specialist contractor take when excavating near a building foundation?
- Work quickly to minimise disruption
- Ensure the excavation does not undermine the foundation — use a design that maintains the bearing capacity of the soil supporting the building (Correct answer)
- Only excavate during business hours
- Place a fence around the excavation
Correct answer: Ensure the excavation does not undermine the foundation — use a design that maintains the bearing capacity of the soil supporting the building
Excavating near existing foundations can remove the lateral support to the soil beneath them, potentially causing subsidence or structural failure. A structural assessment must determine the safe angle of influence, and temporary works (underpinning, sheet piling) may be required to maintain the building's stability.
CITB Specialists Excavation and Groundwork Safety
Before a specialist contractor begins excavation work, what must be done to identify underground services?