Excavation Safety Flashcards

What colour coding is used on UK utility plans to identify gas mains?

Answer: Yellow

On UK utility plans, gas mains are shown in yellow. The standard colour coding is: red for electricity, blue for water, yellow for gas, green for telecommunications, and black or grey for sewers/drainage. Knowing this colour coding is essential when reviewing utility plans as part of the permit to dig process required by HSE guidance HSG47.

What is the maximum safe period that a trench can remain open without being inspected, according to CDM Regulations 2015?

Answer: Until the start of the next working shift — inspections must occur at the start of every shift during which the excavation is occupied

CDM Regulations 2015 Regulation 22 requires excavation inspections at the start of every shift before work begins. A trench cannot be left open between shifts without reinspection before re-entry. Additionally, inspections are required after any event that may have affected stability (rain, vibration, etc.). This ensures conditions are checked before anyone enters.

What method should be used for backfilling an excavation that runs adjacent to an existing building foundation?

Answer: Controlled backfilling in compacted layers using approved granular material to avoid settlement or damage to the adjacent foundation

Backfilling near existing foundations must be carried out carefully in compacted layers (typically 150-300mm) using suitable granular material to prevent future settlement. Uncontrolled backfilling can cause ground movement affecting adjacent structures. The method must be specified in the method statement under CDM Regulations 2015, and a structural engineer may need to approve the approach.

What is the legal requirement for fencing open excavations that are left unattended, particularly near public areas?

Answer: Under CDM Regulations 2015 and the Highways Act 1980, open excavations must be securely fenced with rigid barriers, adequately signed, and lit during darkness to prevent unauthorised access

Open excavations left unattended must be securely fenced with rigid barriers (not just tape), clearly signed, and provided with adequate lighting during darkness. If near a highway, the Highways Act 1980 and New Roads and Street Works Act 1991 impose additional requirements for signing, guarding, and lighting. CDM Regulations 2015 require these measures as part of the site management plan.

What are the three modes of operation on a CAT scanner and what does each detect?

Answer: Power mode (detects live electrical cables), Radio mode (detects metallic services receiving long-wave radio signals), and Genny mode (detects the signal applied by a connected Genny unit)

The CAT operates in three modes: Power mode detects the 50Hz electromagnetic field from live power cables; Radio mode detects long-wave radio signals re-radiated by metallic pipes and cables; and Genny mode detects the specific signal applied to a service by the connected Genny unit. HSE guidance HSG47 requires all three modes to be used for a comprehensive survey before excavation.

What training and competency requirements apply to operatives using a CAT and Genny for underground service detection?

Answer: Operatives must be trained and assessed as competent in the use of CAT and Genny equipment, with refresher training typically every 3 years, as recommended by PAS 128 and HSE guidance

HSE guidance HSG47 and the PAS 128 specification require that operatives using CAT and Genny equipment receive formal training covering the principles of operation, correct use of all modes, interpretation of results, and limitations of the equipment. Competency must be assessed and refresher training provided typically every 3 years. Records of training must be maintained.