A 1.6 m trench in wet sandy silt, with concrete due in 40 minutes
A 22 m long, 1.6 m deep trench is being dug along the boundary of a school in sandy silt for a 150 mm SW pipe, in the last week of June. The sides are cut vertical. Spoil is heaped about 0.4 m from the edge and stands 0.8 m high. About 0.2 m of water stands at the bottom after overnight rain. A 2 m high boundary wall runs 2.5 m from the trench line. As you arrive, a helper is about to climb down a ladder to align a pipe; he says the mistry has gone for tea and that the concrete is arriving in 40 minutes. There are no barricades on the school side, no warning signs, and children use that path at 1 p.m. The helper is wearing canvas shoes and no helmet. The mistry's phone is switched off.
1. List the hazards in order of likely harm, and identify the two you would stop work for immediately.
Read the worked answer
Ranking by likelihood multiplied by severity:
- Trench collapse (highest). Vertical sides in sandy silt, saturated by rain, 1.6 m deep, with spoil piled at the edge adding a surcharge load. A wall of that soil can come down without warning and bury a person in seconds; 1.6 m of sandy silt is more than enough to suffocate an adult. Stopping work for this is not negotiable.
- Falls and unsafe access (second). A single ladder of unknown condition, no helmet, canvas shoes, wet and slippery bottom. If the sides do not fail, a slip or a falling lump of spoil will injure him.
- Children on the path (high severity, near-certain exposure). No barricade or sign on the school side at 1 p.m., with an open trench on the route.
- Flooding and drowning risk from the 0.2 m of standing water, which softens the base further and hides sharp debris.
- Damage to the boundary wall - at 2.5 m from a 1.6 m trench in sandy silt, the wall footing is within the likely zone of influence; a collapse here is a third-party property claim.
- Unsupervised work - the mistry absent and uncontactable.
I would stop work immediately on (1) the unshored vertical trench with a person about to enter, and (3) the unprotected open trench on a children's path. Everything else can be fixed while the trench is closed.
2. State the controls you would insist on before anyone enters, with the numbers you would hold to.
Read the worked answer
Controls, in the order they must be applied:
- No entry until shored or battered. In loose sandy silt, a trench deeper than about 1.5 m needs either shoring (sheeting and struts, or a trench box) or sides battered back to a safe slope - roughly 45 degrees in loose soil, which for 1.6 m depth means about 1.6 m of horizontal set-back on each side. On this site the batter would eat into the school path, so a trench box or close-sheeted shoring is the practical answer.
- Spoil and materials at least 0.6 m, preferably 1 m, clear of the trench edge, and never piled higher than 1 m. Move the existing heap back before anything else.
- Dewatering before entry: pump out the standing water and check the base, so the person is not working in slurry and the soil is not further weakened.
- Access: a ladder in every 7.5 m length of trench, secured and extending about 1 m above ground level.
- Edge protection: rigid barricades with warning signs on the school side and at both ends, plus reflective tape or lights; the trench must be closed or covered at the end of the shift.
- Personal protective equipment: helmet, safety shoes (not canvas), gloves, and a high-visibility vest; one person stationed at the top as a lookout while anyone is below.
- Supervision: a competent supervisor present before entry. Work does not proceed on a helper's judgement.
- Wall check: inspect the boundary wall for cracking before and after the work, and record its condition.
3. The mistry is uncontactable and the concrete truck is 40 minutes away. How do you handle the mistry and the client, and what do you record?
Read the worked answer
The concrete is a cost problem; a buried helper is not comparable, so the sequence is decided first and explained second.
Immediate actions
- Stop the helper from entering, in plain words: "Do not go down - the sides are wet sand and they will come in. Stand back from the edge."
- Move the spoil heap back, place whatever barricade material is on site across the school-side path, and post the helper as a lookout at the edge while you take over the supervision role.
- Call the mistry. If the phone is off, send a message in writing stating that work in the trench is stopped for safety and that he must report before entry - this creates a time-stamped record.
Handling the client and the concrete
- Tell the client's representative the facts and the options: the trench cannot be entered until it is shored or boxed; the pour can go ahead only into a trench that has been made safe, or it is rescheduled. Offer the rescheduled slot yourself rather than leaving the client to chase it.
- If a trench box or sheeting is available, use the 40 minutes to install it - that is the option that saves both the pour and the person. If it is not available, the pour waits. Do not accept "just five minutes to align the pipe".
Record to keep
- Time of the stop-work instruction, the condition found (depth 1.6 m, vertical sides, sandy silt, standing water, spoil at 0.4 m, no barricade, no helmet), the name of the person who was about to enter, the instruction given, and the mistry's absence.
- Photographs of the trench and the unprotected path, and the corrective actions taken with times.
- A short note to the client confirming the cause of any delay. This record protects the helper, the firm and the school, and it is the same record an inspector would ask for.