Excavation bowtie example — map the activity step by step

Step 1 — Name the activity

Activity: Working in an excavation.

Keep it that tight. Not “civil engineering”. Not “streetworks”. The people on the board should recognise the job they are about to do.

Step 2 — Name the top event

Top event: Excavation collapse (loss of support to the sides / face).

Other top events exist for excavation work — struck by plant, striking a live service, engulfment by water. Start with one board and one top event. Collapse is the classic knot for this activity. You can build a second board for services or plant later.

If you cannot say the top event in one line, stop and fix that before adding threats.

Step 3 — List realistic threats

Threats are the ways you get to collapse. Examples commonly worth debating on a real board:

1. Sides left unsupported — no trench box / shoring / batter where the ground and depth need it.
2. Spoil stacked at the edge — surcharge loading the face.
3. Plant working too close — vibration and surcharge from excavators or wagons.
4. Groundwater / water inflow — face weakened, or materials washed out.
5. Undermining / over-dig — toe removed, vertical face overcut.
6. Disturbed or unknown ground — previous excavations, made ground, voids (site-specific — only include if true).

You do not need twenty threats. You need the ones your crew believes can actually happen this week.

Step 4 — Place preventive controls on each threat

Controls only earn their place if they interrupt that threat.

Threat: sides left unsupported

Possible preventive controls:

• Design / temporary works decision for support or batter
• Trench box or shoring installed before entry
• Daily inspection by a competent person before work in the excavation
• Permit or authority to enter linked to support being in place
• Depth / entry rules in the method statement that match what is on the ground

Threat: spoil at the edge

• Spoil distance rule (set and marked)
• Spotter / banksman when tipping near the excavation
• Planned spoil location on the briefing sketch

Threat: plant too close

• Exclusion zone sized for the machine and ground
• Banksman for plant movement near the open excavation
• Plant selection and positioning agreed before the dig

Threat: water

• Dewatering plan where needed
• Stop-work trigger when water or slumping appears
• Weather / groundwater check in the daily inspection

Threat: over-dig / undermining

• Dig plan and dig level control
• Competent excavator operator briefed on toe protection
• Inspection after machine dig before people enter

Be honest about control type and strength. A toolbox talk is not the same as a trench box. If your tool records strength and type, use that — colouring thin administrative controls next to one physical control teaches the room fast.

Step 5 — Add what defeats a control (optional but useful)

A control can exist on paper and fail in the field. Examples:

• Inspection deferred “because we were behind”
• Box left out because “it’s only a quick job”
• Spoil pushed closer as the day runs out of space
• Agency operative not told the exclusion distance

Write the defeater next to the control, then write what you do about it (stop authority, supervisor check, no-entry until fixed). This is often the most valuable part of the session.

Step 6 — Mitigative controls (after collapse)

These do not stop the collapse. They limit harm if it happens:

• Nobody in the excavation unless needed (reduce exposure)
• Emergency plan for excavation collapse / buried person
• Rescue equipment suitable for the depth and ground (and people practised — not just a photo in a van)
• Alarm / shout / stop-plant protocol when the face moves
• First aid / trauma response and emergency services access kept clear
• Standby person who knows the plan and is not buried in other tasks

If mitigative controls are weak, do not pretend the left side makes you safe enough to skip them. Collapse still happens on sites that “usually” shore.

Step 7 — Outcomes

Say what happens to people if the right side fails too:

• Worker buried / crushed in collapse
• Multiple casualties if several people were in the excavation
• Rescuer injured in an improvised rescue

Severity belongs here. The point of the picture is not to frighten; it is to keep the real end-state visible while you argue about controls.

Step 8 — Use the board on the job

1. Brief from the left. Today’s threats and the controls that must be in place before entry.
2. Walk the physical controls. Box, spoil distance, exclusion — eyes on, not “signed the sheet”.
3. Confirm the right side. Who is standby, where is rescue kit, what is the stop trigger.
4. Update when ground changes. Rain, deeper dig, different machine, night shift — same top event, fresh look at threats and defeaters.

Link the board to your written risk assessment. Actions from thin controls go on the assessment and get owners. The bowtie is not a parallel universe.

Critical controls for excavation (SIF lens)

If you mark critical controls, candidates often include: temporary works / support decision, physical support before entry, competent inspection, and exclusion of people until support is verified. See SIF and critical controls on a bowtie.

Do not mark twelve controls as critical. Mark the few you would never allow to be “skipped for speed”.

Build this in Bowtie One

Open Bowtie One, start from Map an activity, and put this structure on the canvas. No account needed to build. Add threats, controls, defeaters, outcomes; set strength and risk if you use those fields. Upgrade to Pro only when you want cloud save, sharing, PDF or review dates.

The homepage demo board is this territory for a reason: if the excavation picture makes sense to your crew, the method will make sense for height, confined space and hot work next.

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