Trench shoring up to 2 m deep

The soil in a trench wall stays in place only because of its own cohesion and moisture. Rain, drying out, a lorry passing nearby or a heap of spoil piled on the edge can easily upset that balance. A typical collapse does not look dramatic: a slab breaks away from the top edge and drops into the trench. At a depth of around one and a half metres, that is enough to pin a person to the bottom and stop them breathing.

Site safety rules in many countries require support for vertical trench walls from a depth of roughly 1.2 to 1.3 m. Safety guidance also asks for the risk to be assessed at any depth. In loose soils, close to roads and buildings or where there is vibration, support may be needed in shallower trenches too. The alternative is battering, which means cutting the walls back to a safe slope. That takes far more space and means digging out much more soil. On a narrow plot, along a fence, next to the neighbour's house or beside a pavement there is usually no room to batter back, and that is where shoring comes in.

Shoring does not only protect the workers. It also protects nearby foundations, fences, paths and services already in the ground. When a trench wall moves, the surrounding soil shifts towards it, and voids can form under neighbouring structures. These often show up weeks later as sunken paving or a crack.

In the price list, trench shoring up to 2 m is measured in square metres. The figure is the area of the supported walls, not the area of the trench bottom and not the volume of the dig. For a trench 10 m long and 1.8 m deep with both walls supported, that works out at roughly 36 m². For a pit for a chamber, all four walls are added together.

When you compare quotes, check what the item actually covers. It normally includes:

• Delivery and preparation of the shoring materials or system
• Installing boards, walings, soldiers and struts in stages as the trench gets deeper
• Regular checks and tightening of the struts while work goes on

Removing the shoring is often a separate item in an estimate, because it happens later, together with backfilling. The excavation itself, carting away spoil, pumping water and backfilling with compaction are usually priced separately as well. If an estimate combines these, it should say so clearly. Otherwise it is easy to end up arguing about who pulls the shoring out at the end, and when.

The 2 m limit is not arbitrary. Up to this depth, lighter systems and timber usually do the job. Deeper excavations normally need a design by an engineer, heavier trench boxes or sheet piles, and that is a different category of work.

A handful of proven solutions are used for shallower trenches. The choice depends on the soil, the length of the trench, the space for machinery and how long the trench will stay open.

Horizontal timber sheeting

This is the classic method every groundworker knows. Timber boards, usually around 5 cm thick, are laid horizontally against the walls. Vertical timbers hold them in place, and struts are fixed between the opposite walls. Boards are added from the top down as the trench deepens. It suits cohesive soils where the wall stands for a while without support, such as clay or clayey loam. The advantage is flexibility: timber can be fitted around a pipe, a cable or a root. The drawbacks are slower installation and more manual work.

Driven vertical sheeting

In loose soils such as gravel or sand, the wall collapses before a board can be put in place. So the sheets, either timber or steel trench sheets, are set vertically and driven or pushed into the ground ahead of the excavation. The soil is dug out only behind them. This method takes more effort, but in poor ground it is often the only safe one.

Steel trench boxes

A trench box consists of two steel panels joined by adjustable struts. The excavator lowers it into the trench in one piece or pushes it down gradually as the dig goes on. For longer trenches for service connections and drainage, this is a quick and very safe solution, because most of the assembly happens above ground. The box is then dragged along the trench. It does need an excavator with enough lifting capacity and room to manoeuvre, so in a tight yard between houses it may not be an option.

Strut frames and hydraulic struts

For narrow trenches, light aluminium or steel frames with hydraulic or screw struts are also used to press panels or boards against the walls. They go in quickly, often from above without anyone entering the trench. They suit short jobs, such as repairing a burst water main.

For a family house, you will most often see timber sheeting for short trenches with bends and corners, and a trench box for longer straight runs. If the crew suggests something else, ask why. A good foreman can explain it in one sentence, for example "This is gravel, boards laid in afterwards would not hold".

Shoring goes hand in hand with digging, so preparation covers the whole job. Before the crew arrives, the following should be sorted out:

• Underground services located and marked on the ground by the utility owners (gas, water, electricity, telecoms, sewers), ideally checked again on the day with a cable locator
• Clarity on whether the trench is only on your own land or also on a public road or pavement, and if so, permission for street works from the local council or highway authority
• A check with the local building authority on whether the work needs approval or a notification under local building regulations
• In a block of flats, consent from the building management or owners' association if the dig is near shared services or on the building's land
• Access for an excavator and a lorry, plus space to store spoil and shoring materials
• An agreement with the neighbour if the trench runs along their fence or foundations

Do not assume that a new connection or drainage run "only needs a notice". Rules differ between countries and councils, and the decision on a specific case belongs to the building authority and the utility company. A short phone call or visit saves complications later.

Locating services is essential. When the trench is cleaned out by hand and sheets are driven in, it is easy to hit a cable or a gas pipe. If a gas pipe is damaged or you smell gas, leave the trench and the surrounding area immediately. Do not smoke, do not switch electrical equipment on or off, and from a safe distance call the gas network's emergency line or the local emergency number or 112. Repairs to gas pipes are for a qualified, registered gas engineer or the network operator only. If an electricity cable is damaged, do not touch it and call the network operator. Only a qualified electrician or the network operator may repair it.

The procedure depends on the system. For a typical trench up to 2 m with timber or combined shoring, it looks roughly like this:

1. Mark out the trench line and check the position of the marked services.
2. Remove the surface (paving, tarmac, topsoil) and keep topsoil separate from the rest of the spoil.
3. Dig the first stage to a depth at which the wall still stands safely, usually only a few tens of centimetres.
4. Fit the first boards so they stand roughly 10 to 15 cm above ground level and stop stones or tools falling in from above.
5. Fit the vertical timbers and struts and tighten them so the boards press firmly against the wall.
6. Continue digging in stages, adding boards and struts at each stage.
7. Fill any gaps between board and wall with soil or wedges so the board has no play.
8. Place a ladder or other safe means of access that reaches above the trench edge.
9. Protect the trench edge with barriers or tape, and with lighting at night if the trench is on public land.

With a trench box the process is simpler. The excavator digs a section, lowers the box in or pushes it down gradually, and workers enter only once the box is standing. After the pipe is laid and partly backfilled, the box moves on.

One basic rule applies to every method: nobody stands in an unsupported part of the trench, not even "just for a moment while someone passes a board down". Those moments are exactly when accidents happen. Shoring a deep trench therefore needs experience, the right equipment and a person watching the walls from above, which is why this page has no DIY guide.

Spoil is placed at a safe distance from the edge, usually at least about half a metre, and further away for heavy machinery or larger heaps. A heavy pile pushes on the wall and on the shoring, and the struts may not have been sized for that load.

How demanding the shoring is, and therefore how much time and work it takes, depends mainly on:

• Soil type: cohesive clay is easier to support than loose gravel, sand or made ground full of rubble
• Groundwater: if water seeps through the wall, soil washes out, pumping is needed and the shoring may have to be closer
• Services crossing the trench: every pipe or cable means hand digging and adjusting the shoring around it
• Space: in a narrow yard a trench box may not fit and timber has to be carried in by hand
• Nearby buildings and roads: vibration and traffic loads call for more struts and more careful checks
• Trench shape: a long straight trench is simpler than one with bends, widenings and chambers
• Weather: rain softens the walls, frost firms them up, but they can loosen again after a thaw

As for time, an experienced crew can often dig and shore a short trench for a water connection in a day. A longer drainage trench with a trench box usually moves at about the same pace as the digging and pipelaying. With timber sheeting in poor ground the pace drops noticeably, because each stage has to be secured before the next one starts.

How long the trench stays open also matters. Shoring that holds for a day may not hold for two weeks of rain and changing temperatures. If work in the trench drags on, for example while waiting for the utility company to make the connection, expect more frequent checks and strut tightening.

Shoring is only a means of letting the main work in the trench happen. It pays to agree in advance who does what, and in what order:

• Before shoring: locating services and, where needed, lifting paving or tarmac
• During shoring: the plumber or pipelaying crew installing pipes, chambers or drainage
• After pipelaying: a surveyor to record the as-built position if the utility company requires it, and the site supervisor or the utility's inspector to check the work before backfilling
• At the end: backfilling in layers with compaction and gradual removal of the shoring

Removing the shoring is often underestimated. Boards are not all pulled out at once before backfilling. The work goes from the bottom up: a layer is backfilled and compacted, the matching part of the shoring is released, and so on. If the shoring comes out too early, the wall can slump into the backfill, damage the freshly laid pipe or leave the backfill poorly compacted. With a trench box, the box is usually lifted gradually as backfilling proceeds.

If the trench carries a drain, the gradient and pipe bedding are checked while the trench is still supported. Once it is backfilled, nothing can be corrected without digging again. That is why the shoring should stay in place until everything has been approved.

The same mistakes come up again and again. Some are dangerous immediately, others only show up over time:

• Entering an unsupported part of the trench, which can end in someone being buried, even at around 1.5 m
• Shoring that does not stand above ground level, so stones, lumps of spoil or tools fall into the trench
• Spoil piled right at the edge, loading the wall and bending the shoring
• Struts left loose or spaced too far apart, so boards bow and voids form behind them
• Gaps behind the boards left unfilled, leading to gradual soil movement and settlement of the ground around
• Damaged or rotten timber that cracks under pressure
• All the shoring pulled out at once before backfilling, after which the wall slumps onto the pipe and the backfill settles later
• No check after rain or over the weekend, when nobody looked to see whether anything had moved

The consequences often appear months later. Sunken paving above a service connection, a cracked kerb or a sagging pipe seen in a drain camera survey can often be traced back to a poorly shored or prematurely unshored trench. So it is worth checking the things that will no longer be visible after backfilling.

Shoring is not handed over as a finished product that lasts for years, but as a safe condition of the trench before the next work. Check it before people enter, after every rainfall, after any break in work and before backfilling. At handover, focus on these points:

• All vertical walls of the deeper trench are supported over their full height down to the bottom
• The shoring stands above the surrounding ground and protects the trench from falling objects
• Boards sit tight against the wall with no visible voids behind them
• Struts are firmly tightened, not bent, and nobody uses them as steps
• Timber and steel parts are not cracked, rotten or otherwise damaged
• Spoil and materials are kept at a safe distance from the edge
• There is safe access in the trench reaching above the edge
• The trench edge is fenced off and, on public land, signed and lit at night
• No water stands at the bottom undermining the walls, or pumping is arranged
• The order of backfilling, compaction and gradual shoring removal is agreed before backfilling starts
• The site records state the area of shored walls so it can be compared with the square metres invoiced