Machine trench excavation for utility connections

You need a trench whenever a service runs underground. Typical routes go from the public main to the house, from the house to a septic tank, cesspit or small treatment plant, to a soakaway for rainwater, or out to a garage, garden building or well. The trench is dug by machine wherever that is physically possible and safe. Hand digging is kept for short sections, the area around existing services and places a machine cannot reach.

In practice, "machine trench excavation" means digging the trench with an excavator and placing the spoil alongside it. Check the quote to see exactly what is included. These items are often priced separately:

• Hand trimming and cleaning of the trench bottom
• Trench support (shoring)
• Pumping out groundwater or rainwater
• Sand bedding and surround for the pipe
• Backfilling and compaction
• Loading and removal of surplus soil
• Cutting and reinstating tarmac, concrete or paving

When you compare quotes, compare the same scope. One contractor's metre may include backfilling, while another's covers only the digging.

Locating existing services

This step must never be skipped. Before digging, contact the network operators and ask for their records of underground services, and have the services located and marked on site. That means the gas network, the electricity network, the water company, telecoms operators and, depending on the area, the local council for street lighting and similar. In some countries a single locating service handles these requests, in others you contact each operator separately. Allow time, usually a few weeks, and note that the information is only valid for a limited period. Close to a marked service, the operator's conditions normally require hand digging, typically within roughly one metre on each side.

In practice an old cable or water pipe often turns out not to be on any plan. A good operator therefore digs carefully even on a "clean" plot and watches the colour and texture of the soil. A band of sand or a warning tape in an unexpected place is a signal to stop.

Design, consents and permissions

The route, depth and pipe material come from the design and from the network operator's requirements. Whether your connection needs permission, a notification or just the operator's approval depends on local building regulations, so check with your local building authority rather than assuming "nobody bothers". If the trench crosses a road, pavement or other public land, you will also need a street works or excavation permit from the council or highway authority, and often temporary traffic management. If the trench runs through a neighbour's land, get the owner's written consent first. In a block of flats, talk to the building management or owners' association before any digging in shared grounds.

Site and access

• Trench route marked on the ground with spray paint, pegs or string line, ideally set out by a surveyor on longer runs or drains that need a precise fall
• Access for the machine: gateway width, ground bearing capacity, low overhead lines, branches
• Space to stockpile soil next to the trench, set back from the edge
• A decision on whether topsoil is kept separate from subsoil
• Bedding and surround material delivered to site, so the trench is not left open longer than necessary
• Agreement on who lays the pipe, when, and who inspects it before backfilling

1. Check the service markings and route with the person in charge, and confirm depths and connection points.
2. Saw-cut hard surfaces such as tarmac and concrete so the edges do not break away in large pieces, or lift paving by hand.
3. Dig trial holes by hand where the trench crosses existing services.
4. Strip the topsoil and set it aside if it is going back on the garden.
5. Dig with a trenching bucket of the chosen width, checking depth continuously with a laser, level or staff.
6. For drains, check the fall of the trench bottom section by section, not only at the end.
7. Install trench support where depth or ground conditions require it.
8. Trim and level the bottom by hand, removing stones and roots.
9. Pump out the trench if water is coming in.
10. Hand the trench over for pipe or cable laying.

The excavator digs slightly short of the final level, and the last few centimetres are trimmed by hand. The reason is simple: the bucket teeth loosen the bottom, and a pipe laid on loosened soil would settle over time. In fine soils a toothless bucket or one with a smooth cutting edge is used.

The exact values are set by the design, the network operator and the relevant standards. The figures below are a rough guide so you know what to expect.

Water supply

A water pipe must sit below frost depth. Across much of Europe this usually means cover of roughly 0.8 to 1.2 m, and more in colder or higher areas. Your water supplier will state the required cover. The trench is usually about 40 to 60 cm wide so there is room to work at the bottom. A meter chamber needs a wider excavation.

Foul drain connection

For drains, a steady fall matters more than depth. Small-diameter house connections often run at a fall of roughly 1 in 50 (about 2 %), though the design may specify something different. The depth depends on the invert level at the public sewer or treatment plant and the level where the drain leaves the house. If the public sewer is shallow, the fall may not work and pumping may be needed. Find this out before digging starts.

Power cables and low-voltage services

Power cables in a garden or verge are usually laid at around 0.6 to 0.7 m, deeper and in a duct under a driveway or road. Cables for broadband, cameras or an electric gate go in ducts. Connection to the grid and any work on electrical installations must be done by a qualified electrician or the network operator. The groundworker only digs the trench.

Gas service

Gas pipes are usually laid with roughly 0.75 to 1.0 m of cover, depending on the gas network's conditions. Installation and connection are carried out only by a qualified, registered gas engineer or the gas network. If the excavator damages a gas pipe and gas escapes, switch off the machine at once. Do not smoke or use any naked flame or mobile phone near the leak. Move well away and call the gas emergency number or 112.

Shared trench

Several services in one trench save work, but the separation distances set by standards and network operators must be kept. Typically the drain sits lowest, the water pipe higher and to one side, and cables at a different level. Do not combine gas with other services without the gas network's approval. A shared trench is usually wider or stepped.

The price per metre depends on how many metres the machine can realistically dig in a day. These factors slow the work and push the cost up:

• Depth and width: more depth means more soil and stricter safety measures
• Soil type: estimators still use soil or excavation classes. Light sandy or loamy soil digs quickly, while heavy clay, cobbly gravel or rock is much slower and may need a hydraulic breaker
• Groundwater: the trench needs pumping, the sides collapse and the bottom turns soft
• Existing services and crossings that must be dug by hand
• Tree roots, old foundations, made ground full of rubble
• Restricted space: a narrow garden, nowhere to put the spoil, a smaller machine needed
• Hard surfaces that must be cut and later reinstated
• Short runs: getting the machine there and setting up is spread over only a few metres
• Working around traffic, for example beside a road or a driveway that must stay open

A level, dry, accessible plot with clearly marked services and room to leave the spoil beside the trench means steady work without waiting time.

Choosing the machine

For house connections the usual choice is a mini excavator of roughly 1.5 to 3 tonnes. It fits into narrower gardens and does less damage to the lawn. On long, deep runs in open ground, a larger tracked excavator or a backhoe loader is faster and can also move and load the spoil. For short, shallow cable trenches there are chain trenchers, which cut a narrow slot with little disturbance but are unsuitable for stony ground.

Trench support and safety

A narrow trench with vertical sides is one of the most dangerous places on a building site. Soil can collapse without warning, even when the face looks firm. As a rule, nobody should enter a trench deeper than roughly 1.2 m without support or adequate battering of the sides, and local safety rules may be stricter. Trench boxes, trench sheets with props, or sloped sides where there is room are the usual solutions. Spoil and machines should not stand right at the edge, because the extra load can cause a collapse. Fence off the trench, especially if children or neighbours use the plot, and use a ladder to get in and out.

The trench on its own solves nothing. It is preparation for the plumber, the gas engineer or the electrician. Most problems arise in the gaps between trades.

• Plan dates so the installer arrives straight after the dig, because an open trench silts up in rain and the sides cave in
• Agree who provides the sand bedding and surround: the groundworker or the installer
• Before backfilling, an as-built survey is often required by the network operator and sometimes the building authority, and the trench should not be filled without it
• Water and gas pipes are pressure tested and drains are tested for watertightness, often while the trench is still open, depending on the operator's conditions
• Some operators want to inspect the laid pipe before backfilling, so contact them in good time

How long it takes

The digging itself is usually the quick part. On a typical house connection in good soil, a mini excavator can manage several dozen metres a day. In rock, in wet ground or with many crossings it may be a fraction of that. The whole process from dig to backfill takes longer, because it waits for installation, testing and survey. Allow time for the weather: after heavy rain, clay is often too wet to dig sensibly for a few days.

A backfilled trench settles. Even with good compaction the soil sinks a little over the first months, so final paving, tarmac or turf over the trench is usually laid after some time, or over a thoroughly compacted granular backfill.

Drain fall not maintained

If the trench bottom is not checked continuously, a dip forms. The pipe sags into it and sludge collects at the lowest point. The result is repeated blockages and smells, often years later.

Water pipe laid too shallow

A section that lies higher than the rest, for example where it joins a chamber or passes under a driveway, freezes in a hard winter. The burst then has to be found and repaired in frozen ground.

Pipe laid directly on stones

Without sand bedding and surround, a stone presses on the plastic pipe, which deforms or cracks over time. After laying, the pipe must be surrounded with fine material before the trench is backfilled with excavated soil.

Poorly compacted backfill

Soil tipped in all at once and only run over with the excavator's tracks will sink later. A hollow appears over the trench, or the paving drops. Backfill goes in layers, usually a few tens of centimetres at a time, compacted with a plate compactor or trench rammer, and carefully directly above the pipe.

Missing warning tape

A warning tape of the right colour is laid above each service, usually roughly 20 to 40 cm above the pipe. Without it, the next person digging has no warning and hits the service directly.

Digging without locating services

A damaged cable or gas pipe means danger, loss of supply for the neighbourhood and a claim for damages. If an electric cable is damaged, keep away from it, leave the machine where it is if that is safe, and call the electricity network and 112.

Subsoil mixed with topsoil

If the topsoil is not kept separate, the surface ends up as barren subsoil or clay after backfilling, and grass struggles over the trench for years.

The trench itself belongs to a contractor with a machine and experience. Some of the preparation, though, you can safely do yourself, and it often makes the day run much more smoothly.

1. Collect the service records from the network operators and keep them ready for the contractor.
2. Walk the planned route and mark it with pegs and string or spray paint.
3. Note manhole covers, stopcocks, meter boxes and inspection chambers, which show where existing services run.
4. Clear the route and the access path of garden furniture, plant pots, vehicles and loose items.
5. Lift turf along the route by hand if you want to relay it later, and store it rolled up in the shade.
6. Choose an area for the spoil heap and cover the lawn there with boards or a tarpaulin.
7. Protect nearby plants and trees, and tie back low branches over the access route.
8. Tell neighbours about the dates and keep children and pets away from the work area.
9. Photograph the route and the surroundings before work starts, as a record of the original state.

There are two checkpoints: before the service is laid and before backfilling. The second matters more, because afterwards you can no longer see into the ground. Before backfilling, photograph the whole route with a tape measure next to the pipe. The photos are useful for any later building work.

• The route matches the design and the markings and has not drifted towards the fence or trees
• Depth has been checked at several points, not only at the start and end
• For drains, the fall is even along the full length with no back-falls
• The bottom is clean, with no stones, roots or soft mud
• The sides are stable and deeper sections are supported or battered
• There is no standing water in the trench, or pumping is in place
• Existing services at crossings are undamaged and protected
• The pipe lies on sand bedding and is surrounded with fine material
• The pressure or watertightness test has been done and recorded
• The as-built survey was completed before backfilling
• Warning tape is in place above every service
• Backfill is compacted in layers, the surface is levelled and topsoil is back on top
• Hard surfaces and the surroundings are restored to the agreed condition
• You have the photos, records and documents the network operator and building authority need