Summary

A 1m³ block of saturated clay weighs about a tonne. A trench wall collapse drops several tonnes of soil onto a worker faster than they can react and creates pressures that prevent the chest expanding — death by asphyxia, not crushing, is typical. HSE statistics put trench-related fatalities at 1–3 per year in UK construction, every one of them preventable.

The CDM Regulations 2015 require the contractor to plan and manage excavation work, and Regulation 22 specifically covers the safety of excavations. The Health & Safety at Work etc. Act 1974, the Construction Design and Management Regulations 2015 and HSE guidance HSG150 set the framework. PUWER (Provision and Use of Work Equipment Regulations 1998) covers trench boxes and shoring as work equipment.

The decision on shoring type is driven by depth, soil type, ground water, adjacent loading and how long the trench will be open. A 1.5m deep trench in a back garden for a drain run is a fundamentally different problem to a 3m service trench in a town centre street.

Key Facts

Quick Reference Table

Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
Depth Ground Conditions Typical Support
0–1.2m Stable, dry Optional — battering if loose
1.2–2.0m Stable, dry Trench box or hydraulic shores; or battered 45°
1.2–2.0m Soft / wet Close-boarded timber or trench box
2.0–3.0m Any Trench box (hydraulic) or piled support
3.0–6.0m Any Designed temporary works — engineer involvement
>6.0m Any Specialist piling / cofferdam

Detailed Guidance

Risk assessment before any dig

The Construction Phase Plan (CPP) under CDM 2015 should list excavation hazards specific to the site:

Excavation Risk Assessment Checklist
□ Depth and length of trench planned
□ Soil type (cohesive clay / granular / made ground / fill)
□ Ground water expected? (test bore / observation pits)
□ Buried services? (CAT and Genny scan; LSBUD plans)
□ Adjacent loading? (walls, plant, traffic, stockpiles)
□ Duration of works (longer = more risk)
□ Weather forecast (rain triples collapse risk)
□ Access for inspection and rescue
□ Confined space considerations if deep + narrow
□ Public exposure (footpath, schoolchildren)

A documented risk assessment, signed and dated, is required — verbal assessments are not compliant.

Battering and stepping

The simplest method of preventing collapse is to cut the trench wall back to a stable angle so that gravity holds the soil rather than vertical face resisting it.

Battering needs space — a 2m deep trench needs 2m of additional excavation each side at 45°. On confined sites this is often not possible and shoring is used instead.

Trench boxes

A trench box is a pair of vertical steel sheets (panels) connected by horizontal struts, lowered into the trench either by excavator or pre-set onto the bucket. The box protects workers inside the box from collapse outside it.

Installation:

  1. Excavate to depth 1m, then lower the box partially
  2. Continue excavating inside the box, pushing it down as ground is removed (push-in method) or
  3. Excavate the full trench length, then lift the box in (trench-and-shield method)
  4. Maintain at least 600mm freeboard above ground level
  5. Brace against differential ground pressure — boxes need ground contact both sides

Trench boxes do not stop ground above the box collapsing, only ground at the level of the box. If you stand a tall worker in a deep trench with a low box, their head and chest are unprotected.

Hydraulic shores

For narrower trenches (e.g. service trenches), hydraulic shoring uses vertical aluminium walings spanning the full depth, held apart by hydraulic struts. Quick to install, lightweight, but limited in groundwater and in long trench runs.

Close-boarded timber shoring

For soft, saturated or noisy (vibrating) ground, close-boarded timber shoring fills any voids and supports the trench face continuously. Construction uses:

Slower to install but the most adaptable system for difficult ground. Specified to BS 5975.

Inspection regime

HSG185 requires a competent person to inspect before each shift, after weather events, after any incident or sudden movement, and at intervals not exceeding 7 days. The inspection record covers:

The inspection is recorded in a written report (HSE Form F91 or equivalent), kept on site and accessible to HSE inspectors on request.

Adjacent structures

A trench next to a wall, road or other structure transfers load to the trench wall. The angle of influence is typically 1:1 from the bottom edge of the loaded surface — any trench inside this zone must have designed support, not nominal shoring.

For trenches deeper than 1.5m within 5m of any structure, treat the support as designed temporary works requiring engineer's input.

Ground water

Water in the trench is a stability hazard:

Never enter a trench where water level is rising faster than it can be pumped.

Manual handling and confined space

Public exclusion

Heras fencing or similar 1.8m mesh fencing around the work area; chapter 8 (DfT) signage on public highway works; pedestrian routes clearly diverted. The minimum offset for pedestrian barriers from a trench edge is generally 1m — but 2m or more on busy public routes.

Frequently Asked Questions

Do I need shoring at 1.2m exactly?

Below 1.2m, shoring is required unless the trench walls are battered or stepped to a safe angle. The 1.2m trigger is a rule of thumb; if the ground is unstable, shoring may be needed at lesser depths. HSG185 is clear that risk is assessed on the actual ground conditions, not depth alone.

Can I dig a 1.5m trench without a trench box if I work fast?

No. The work-fast-and-leg-it approach is a common excuse for trench fatalities. Even a few seconds inside an unprotected 1.5m trench is unacceptable risk. The CDM Reg 22 duty is absolute — the trench must be supported. Quick jobs use quick-install shoring (hydraulic shores, trench boxes).

What's the legal minimum trench inspection frequency?

CDM Reg 22(3) requires inspection by a competent person at the start of every shift, after any event likely to have affected its strength or stability (rain, frost, vibration), and after any unintentional fall of rock, earth or other material. Inspections must be recorded in writing.

What's a "competent person" under CDM?

Someone with sufficient training, skills, knowledge and experience to identify hazards and risks. For trench inspection this usually means a site manager or supervisor with CITB site management training and demonstrable experience of excavation work. SMSTS (Site Management Safety Training Scheme) is the typical baseline qualification.

Can a mini-digger collapse a trench wall?

Yes — vibration from tracking plant near the edge induces wall failure, particularly in loose granular soils. The HSE rule of thumb is to keep plant tracks at least the depth of the trench back from the edge (e.g. for a 2m trench, plant 2m clear). Heavier plant needs more clearance.

Regulations & Standards