Excavations and trenches

This talk is about excavations: trenches, pits, and anything we open in the ground that someone could be buried in, or fall into. HSE says every year people are killed or seriously injured by collapses and falling materials while working in excavations. I am not adding a count. The page does not give one. The risks it names are an excavation collapsing and burying or injuring people, material falling from the sides, and people or plant falling in.

The line I want remembered is HSE's. No ground can be relied upon to stand unsupported in all circumstances. Depending on conditions, a cubic metre of soil can weigh in excess of 1.5 tonnes. That is why a trench that 'looked fine yesterday' is not a method. Trenchless techniques should always be considered at the design stage, because they can remove the need for a major excavation. If we are digging because it is the habit, we have skipped that question.

Before any trench, pit, tunnel or other excavation, decide the temporary support and plan the precautions. Make sure the equipment is on site before work starts. HSE's examples are trench sheets, props and baulks. Deciding you need support after the sides have already been stood in is late. Battering the sides to a safe angle can also make it safer. In granular soils the slope should be less than the natural angle of repose of the material. In wet ground a considerably flatter slope is required. Rain overnight is a reason to look again, not a reason to hope.

Spoil and plant make collapses more likely. Loose material can fall from spoil heaps into the excavation. Edge protection should include toe boards or other means, such as projecting trench sheets or the sides of a box, to protect against falling materials. Head protection should be worn. Do not park plant and vehicles close to the sides. The extra load can make the sides collapse. People fall in as well as soil. Edges need substantial barriers where people are liable to fall. HSE's ways to do that are guard rails and toe boards inserted into the ground immediately next to the supported side, or fabricated guard rail assemblies that connect to the trench box, or the support itself, such as trench box extensions or trench sheets longer than the trench is deep.

What is next to the hole matters as much as the hole. Do not undermine scaffold footings, buried services, or the foundations of nearby buildings or walls. HSE says many garden or boundary walls have very shallow foundations, easily undermined by even a small trench, so the wall collapses onto the people in it. Before digging, decide if the structure needs extra support. A survey of the foundations and a structural engineer may be required. A wall that has stood for years is not evidence it will stand with its foundation cut away.

Services are a reason to stop, not a reason to be careful with the bucket and carry on. HSE says many serious accidents have happened when buried services were damaged. Contact with an electricity cable can cause explosion and burns to people nearby. Escaping gas that ignites can cause serious injury and property damage from fire and explosion. Excavation should not start until steps have been taken to identify and prevent injury from underground services. Overhead lines are the same family of harm. Burns and electrocution can result if a raised tipper body or an excavator touches a line, or comes close enough to arc. Everyone close to plant that becomes live is at risk, not only the operator. HSE says the need to excavate close to or under those lines should be considered very carefully and avoided where possible. The electrical talk has the overhead-line actions. This talk's rule is that we do not dig until we know what is in the ground and what is above the machine.

Water changes the support. Depending on how permeable the ground is, water may flow into an excavation below the groundwater level. The side supports should be designed to control that, and the design should take the extra water load into account. Pay particular attention near lakes, rivers and the sea. Water needs to be channelled to sumps and pumped out, and the effect of that pumping on stability has to be considered. Other ways of de-watering exist. Designers have to consider them. A pump thrown in because the bottom is wet is not that design.

People fall in as well as soil, and not only the people doing the digging. HSE's public-protection page says members of the public are seriously injured by falling into trenches, and that openings and excavations need barriers or covers. Getting in and out is part of the precaution, not an afterthought. Provide a safe means of access and egress. If a risk assessment says a ladder is a reasonable means, HSE says it must be suitable and strong enough, on a firm level base, secured so it cannot slip, and, unless there is a suitable alternative handhold, extend at least 1 metre above the landing place. Fumes are the quiet hazard. Do not use petrol or diesel engines in an excavation unless the fumes are ducted safely away or you have forced ventilation. Do not site a generator or a compressor in the excavation or near the edge. Exhaust can collect.

Inspection is every shift, not every time someone remembers. A competent person who fully understands the dangers and the precautions should inspect at the start of each shift. Also inspect after any event that may have affected strength or stability, or after a fall of rock or earth. A record is required. Faults are corrected immediately. HSE says a written report should be made following most inspections, and it should include who it was done for, the location and description, the date and time, anything that could give rise to a risk, action taken, further action needed, and the name and position of the person making the report. Today: what is supporting the sides, where are the services, how do we get out, and who inspected it at the start of this shift?