Excavation Safety: Trench Support, Buried Services and CDM Notification Thresholds

Summary

Excavation collapse is a leading cause of death on UK construction sites. The statistics are sobering: the weight of soil is approximately 1.5–2.0 tonnes per cubic metre, and an unsupported trench collapse of just one linear metre can exert lethal forces on any worker in the trench. The time between a trench collapse beginning and the worker being completely buried can be as little as one second — there is rarely time to escape.

The legal framework governing excavation safety in the UK is the Construction (Design and Management) Regulations 2015 (CDM 2015), supported by the Health and Safety at Work Act 1974 and the Work at Height Regulations 2005 (which, despite the name, applies to excavation edges as a risk of falling into the excavation). The HSE provides detailed guidance in GS7 (Health and Safety in Excavations), which remains the primary operational reference for all excavation work.

Beyond the life safety risks, excavations also create significant risks from buried services — gas, electricity, water, telecommunications, and drainage — which can cause death if struck, and from CDM notification requirements that carry criminal liability if missed.

Key Facts

• 1.2m depth threshold — trenches over 1.2m deep require support or battered sides; below 1.2m, competent person judgement applies based on soil conditions
• 45° battered sides — the maximum safe angle for most soils (clay, gravelly clay, stiff silts) when battering rather than supporting; increase to wider angle (shallower slope) in loose sand, fill, or waterlogged ground
• 60° battered sides — may be possible in sound rock; competent person to assess
• Trench box (drag box) — the standard modular support system; steel panels and hydraulic spreader bars; inserted and extracted vertically; available for hire from all plant hire companies
• Hydraulic shores — adjustable hydraulic props set against the trench wall face; used for deeper excavations or unusual cross-sections
• Timber sheeting (close boarding) — traditional method; requires competent shuttering carpenter; still used for bespoke excavation shapes or where steel panels are impractical
• Competent person inspection — defined inspection requirement: before each shift, after any collapse, cracking, or event that may have affected stability, after any unexpected fall of material, and after heavy rain or frost
• Inspection record — must be in writing; recorded on Form F91 (or equivalent); entries must be made at the end of the working period during which the inspection was made
• CAT and Genny (C-Scan) — cable avoidance tool (CAT) and signal generator (Genny/Generator); detects buried services; should be used on every excavation before digging begins
• 500mm hand dig zone — HSE guidance requires hand excavation within 500mm of any marked service route; do not use mechanical excavation within 500mm of a positively located service
• 1m distance from trench edge — excavated spoil must be placed at least 1m from the trench edge to prevent surcharge collapse; further if the spoil heap is large or the ground is soft
• Atmospheric testing — in deep excavations (over 3–4m) and in confined spaces adjacent to sewers, gas main corridors, or fill, test for oxygen deficiency, carbon monoxide, and methane before entering
• CDM F10 notification — required where the construction phase will last more than 30 days with more than 20 workers simultaneously on site, or where the total person-days exceed 500
• Rescue plan — every excavation where a worker could become buried or entrapped requires a pre-planned rescue procedure; the rescue plan must be known to all workers and supervisors on site
• Safe toe distance — a minimum 1m clear at the bottom of the trench between the trench face and any worker or equipment is required; the toe of the trench is the most vulnerable point

Quick Reference Table

Detailed Guidance

The Collapse Risk

Soil is not a structural material in the engineering sense — it is a particulate medium with shear strength that varies with moisture content, composition, and loading. The angle at which a trench can stand without support (the angle of repose) depends entirely on soil conditions:

Stable soils (can typically stand at 45°):

• Stiff to very stiff clay
• Heavily over-consolidated clay
• Sound chalk
• Firm cohesive silts

Unstable soils (require near-vertical support or very shallow battering):

• Running sands and silts
• Loose or made-up ground (fill)
• Waterlogged or near-water-table soils
• Previously disturbed ground (backfilled trenches from previous services)

Factors that change stable soil to unstable:

• Rain — water reduces the cohesion of clay soils rapidly; a stable clay trench at 45° can collapse within hours of heavy rain
• Vibration — nearby traffic, piling, or compaction equipment can disturb soil structure
• Surcharge — excavated spoil placed too close to the trench edge adds to the destabilising force
• Temperature — frost and thaw cycles disrupt soil structure

The fundamental principle is: when in doubt, support or batch further back. The cost of a hire trench box is trivial compared to the cost of a fatality investigation, prosecution, civil claim, and — most importantly — the human cost.

Trench Support Systems

Trench box (drag box): The most widely used system in the UK. Consists of two steel panels (typically 2.4m × 1.2m to 2.4m × 2.4m) connected by hydraulic or fixed-length spreader bars. The box is lowered into the excavation after initial dig, then the trench sides are excavated down inside the protection of the box. As the work progresses, the box is jacked down or moved along ('dragged') to maintain protection.

Trench boxes must be designed for the depth and soil load — they are marked with a maximum permitted depth. Do not exceed the rated depth.

Hydraulic shores: Used for wider trenches, irregular shapes, or cross-bracing requirements. Hydraulic cylinders (Ackermann or similar) are set against both trench faces and inflated to the required pressure. They can be adjusted as conditions change. Particularly useful for excavations in traffic corridors where trench boxes cannot be lowered.

Timber sheeting (close boarding): Traditional method using 50mm × 225mm horizontal planks driven against the trench face, supported by vertical walings and horizontal struts. Requires a competent person (experienced in shuttering) to design and install. Can be adapted to any excavation shape. The timber must be structural quality — not pallet wood.

Battering (unsupported sloped sides): Where space permits, excavating at 45° to the horizontal rather than vertically removes the need for support. For a trench 2m deep, the surface must be 2m back from the trench edge on each side — this requires a total surface width of at least 5m for a 1m-wide trench at the bottom. Only practicable where sufficient space exists.

Buried Services: CAT, Genny and Safe Dig Procedure

Striking a buried electrical cable is one of the most frequent causes of serious injury in excavation work. In 2024, there were over 500 reported incidents of cable strikes in the UK construction industry. Gas line strikes cause explosions and fires. Water main strikes cause flooding. Telecom strikes can disrupt critical infrastructure.

Dial Before You Dig (now called LSBUD — Line Search Before U Dig): register at linesearchbeforeudig.co.uk (LSBUD) to obtain service maps from the major utility providers. This is mandatory before any excavation — it is free and takes less than 10 minutes. Utility owners provide maps of their assets; these maps are not guaranteed accurate but provide the starting point for service avoidance.

CAT and Genny:

• CAT (Cable Avoidance Tool) — an active scanner that detects electromagnetic emissions from live cables and pipe-borne radio signals; also detects passive signals from buried metallic pipes. Use in all three modes: power frequency (50Hz), radio frequency, and combined.
• Genny (Generator) — applies a low-frequency signal to a buried metallic service; the CAT detects the Genny's signal along the service route, allowing accurate location of specific services where the route is known.

CAT and Genny together provide reliable detection of most metallic services. Limitations: does not detect non-metallic pipes (plastic water, plastic gas, clay drainage, fibre optic cables without metallic tracer wire). Where plastic pipes are suspected, use a ground-penetrating radar (GPR) survey — a specialist service.

The 500mm hand dig rule: HSE guidance (HS(G) 47 and HSE information sheet IACL 71) requires that once a buried service route has been positively located (by CAT/Genny or through trial holes), mechanical excavation must stop 500mm from the located route. Final excavation within 500mm must be by hand. This is not a preference — it is an HSE requirement, and fatal incidents have occurred when this rule was ignored.

Discovering a service during excavation: If a buried service is encountered unexpectedly during excavation:

1. Stop all mechanical excavation immediately
2. Assess whether the service has been damaged (gas — smell and sound; electrical — no visible damage is reassuring but not conclusive; water — obvious)
3. If gas is detected, evacuate the trench and upwind area; call the Gas Emergency Service (0800 111 999) immediately
4. If an electrical cable may have been struck, treat it as live and do not approach; call the electricity network operator (116 123 for National Grid network)
5. Record the find and its location; update the service records for the project

CDM 2015 Notification Requirements

The Construction (Design and Management) Regulations 2015 require the client (or their appointed Principal Designer) to notify HSE of a project when the construction phase will involve:

F10 notification triggers:

• The construction phase will last longer than 30 working days AND at any time have more than 20 workers simultaneously working on site; OR
• The construction phase will exceed 500 person-days

Person-days = the sum of all working days contributed by all workers (not just the number of individuals). A project with 5 workers for 100 days = 500 person-days.

Who notifies: under CDM 2015, notification is the client's duty. In practice, the Principal Designer (PD) or Principal Contractor (PC) usually handles the paperwork. The F10 form is submitted online via the HSE F10 portal.

Non-notification: failure to notify when required is a criminal offence under CDM 2015 Regulation 6. The HSE can issue an improvement notice or prosecute. On most commercial sites, the Principal Contractor monitors project scope and ensures notification is submitted on time.

For excavation-specific projects (pipeline installation, utility infrastructure, drainage projects), person-days calculations often trigger F10 quickly because the work involves multiple operatives over extended periods.

Atmospheric Testing in Deep Excavations

In deep excavations (typically over 3m depth) and in any excavation adjacent to known risks (sewers, gas infrastructure, chemical storage), test the atmosphere before entering:

Hazards to test for:

• Oxygen deficiency (safe range: 19.5–23.5% O₂) — can occur near sewers (methanogenic bacteria consume oxygen), in waterlogged soils, or adjacent to CO₂ leaks
• Carbon monoxide (CO) — from vehicle exhausts, nearby pumps, generators; WEL is 20 ppm (STEL 100 ppm)
• Methane (CH₄) — from sewers, landfill gas, natural gas leaks; explosive at 5–15% v/v; odourless
• Hydrogen sulphide (H₂S) — from sewers, organic putrefaction; highly toxic at >50 ppm; WEL 1 ppm

Use a calibrated multi-gas detector (four-gas or five-gas meter: O₂, CO, CH₄, H₂S minimum) before entry and continuously during occupancy in deep or enclosed excavations. If any alarm activates, evacuate immediately.

Rescue Plan

Every excavation site plan should include a rescue plan specifying:

1. Who is responsible for rescue (name and role)
2. What equipment is immediately available (hand tools for exposure, stretcher, first aid kit)
3. The procedure for a collapse or entrapment (call 999, specific actions for different scenarios)
4. The location of the nearest hospital with emergency trauma capacity
5. Assembly point and mustering procedure

The rescue plan must be communicated to all workers before they work near or in the excavation. A plan that exists only on paper but is unknown to the operatives is not compliant.

Frequently Asked Questions

Do I need a formal engineer's design for a standard 2m-deep trench box installation?

Not necessarily for standard depth within the trench box's rated capacity. Trench box manufacturers provide load ratings and maximum depth guidance for their products. For standard soils and depths within the rated capacity, the manufacturer's documentation is the engineering basis. For excavations over 4m deep, in unusual soils, with unusual surcharges, or with adjacent structures, a site-specific geotechnical or structural engineer's assessment is required.

What counts as a "competent person" for excavation inspections?

A competent person for excavation safety should have relevant technical knowledge and experience — typically a suitably trained site manager, engineer, or experienced foreman who can recognise the signs of ground instability, has knowledge of soil types, and understands the support systems in use. This is not a formal qualification requirement; it is a judgement of whether the person has sufficient knowledge. CITB and specialist training providers offer excavation safety and competent person training courses.

Can I backfill an excavation without LABC inspection if it's for drainage work?

Only if the drainage work doesn't require Building Regulations approval. Most drainage installations do require Building Regs approval (Part H), and the BCO inspection stage for drainage typically requires the BCO to inspect before backfilling. Backfilling without inspection may require the trench to be re-excavated. Check with your LABC before covering drainage runs.

What should be done with water-pumped excavations?

Any pumped excavation should be monitored for progressive destabilisation as water is removed — the soil can become less stable as pore pressure is reduced in the wrong conditions, or can become unstable as the rate of pumping affects artesian pressure. In suspect ground conditions near water (made ground, riverside, coastal), a geotechnical engineer should be consulted before dewatering deep excavations.

Is electrical cable detection mandatory before all excavation?

HSE guidance strongly recommends it, and it is standard practice across the industry. Technically, the legal duty is to carry out a risk assessment under CDM 2015 and to take reasonable precautions — and it is very hard to argue that not using a CAT scanner (which is inexpensive to hire) was reasonable when a cable is struck. CAT/Genny use before all excavation is industry standard and effectively mandatory for insurance and CDM compliance purposes.

Regulations & Standards

• CDM Regulations 2015 (SI 2015/51) — Construction (Design and Management) Regulations; F10 notification; Principal Designer/Contractor duties

• Health and Safety at Work Act 1974 — general duty to ensure safety of employees and others

• Work at Height Regulations 2005 — applies to edges of excavations as a risk of falling

• HSE GS7 — Health and Safety in Excavations; the primary operational guidance document

• HSE HS(G) 47 — Avoiding danger from underground services; CAT and Genny guidance

• BS EN 13331 — trench lining systems; requirements and verification

• COSHH Regulations 2002 — atmospheric testing requirements for confined spaces within excavations

• HSE: Excavations Safety — comprehensive guidance including inspection records, soil types, and trench support

• HSE: Avoiding Underground Services (HS(G)47) — definitive guide to CAT/Genny use and service avoidance

• LSBUD: Line Search Before U Dig — free service records search before excavation

• HSE: CDM 2015 — CDM regulations guidance and F10 notification portal

• HSE: Confined Spaces — atmospheric testing requirements

• hot works — permit systems and site safety management

• vibration havs — health surveillance and exposure monitoring

• part a structure — when engineers are needed for retaining walls and foundations