Trench at 3.6 m: Water, Spoil, and an Exposed Gas Main

Principal hazards:

• Collapse of vertical Type B walls at 3.6 m with seepage and sloughing - the most serious life-safety risk. Water reduces soil strength and can trigger rapid failure.
• Spoil surcharge at 0.8 m from the edge, adding load to the failure wedge.
• Plant loading from the excavator tracking within 2 m of the edge.
• Gas main exposed at shallow depth - strike, rupture or migration risk.
• Egress - a single ladder at one end of a 12 m trench.
• Inspection - no documented daily inspection for three days.

Protective system. For Type B soil, the standard approach is sloping at 1:1 (45°), benching, or a trench box/shoring designed for the depth and loading. Vertical cut faces for the full 3.6 m are not acceptable without an engineered system. The seepage and sloughing suggest the classification should be re-examined and may warrant treating portions as Type C (1.5:1 slope) or using a box. Spoil and equipment must be kept back at least 0.6 m from the edge - and further where surcharge is significant. The gas main needs support and protection, and excavation near it must be by hand until its position is confirmed.

Role. The competent person is designated by the employer, is capable of identifying hazards and has authority to take prompt corrective action. That includes classifying the soil, selecting and inspecting the protective system, checking for atmospheric hazards where required, and stopping work when conditions change. The absence of inspection records for three days is itself a failure: inspections are required at least daily and after events such as rain, seepage or ground movement.

Before work resumes:

1. Stop work and clear the trench. No one enters until the protective system is in place.
2. Re-inspect and re-classify the soil in light of the seepage and sloughing. Document the classification and the basis.
3. Install a system - slope to 1:1 or flatter, or set a trench box/shoring designed for 3.6 m and the actual loads. Account for the gas main.
4. Relocate spoil and plant to at least 0.6 m from the edge, and further for the excavator.
5. Provide adequate egress - ladders within 25 ft (about 7.6 m) of lateral travel for every worker, secured and extending above grade.
6. Protect and support the gas main, confirm its location with the utility, and use hand digging near it.
7. Reinstate daily documented inspections by the competent person, with records available on site.

The foreman's 'in and out by lunch' argument is not a control measure; the exposure is the same for one hour as for one day.

Gas main. Treat the exposed main as live until the utility confirms otherwise. Notify the gas utility before further excavation, confirm the line's location and depth, and ask whether a utility representative should be present. Hand-dig within the required clearance of the marked line, support the exposed span to prevent settlement or bending, and protect it from plant impact. Prohibit smoking, naked flames and running engines near the trench, and keep a gas detector available. If the main is damaged, stop work, evacuate the area, remove ignition sources and call the utility and emergency services. Do not attempt to repair a gas line.

Seepage. Water is a change in condition. The competent person must reassess the soil and the protective system. Options include diverting surface water, a sump and pump system, and a more conservative protective system such as a trench box or flatter slope. Watch for boiling or piping at the base, which signals potential instability. If water is entering from a utility trench or unknown source, investigate before pumping.

Who is involved: the competent person, the site supervisor, the gas utility, the designer/engineer if the protective system changes, and the utility owner for the sewer alignment. Record the decisions and the inspection in writing.