A 12 m Trench Beside a Footing, a Gas Main and One Trench Box
A 0.6 m wide, 1.8 m deep, 12 m long trench for a sewer lateral is being cut with a 20-tonne excavator. The line runs 2.2 m from a two-storey brick building whose strip footing is at 1.2 m. The soil is sandy silt with cobbles, and the strip alongside the building has been excavated and backfilled before. The client wants the spoil piled on the building side to shorten the cycle. A 150 mm PE gas main crosses the line at about 1.0 m depth; paint marks are visible but the locator's accuracy is quoted as ±0.5 m. The water table is at 2.5 m and the base is dry so far. A worker will have to hand-trim the last 150 mm of the invert, and the crew has one 2.5 m trench box, one ladder and no gas monitor on site.
1. Where does the spoil go, and why is the building side the worst option?
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Spoil is a surcharge. A 1 m high stockpile of sandy silt adds roughly 18–20 kPa over its footprint. That load travels down and out at about 45°, so it reaches the trench wall and, at 2.2 m from the footing, the footing itself.
Why not the building side:
- The strip has been excavated and backfilled; its shear strength is lower and variable, so the trench wall there is already the weakest.
- Additional vertical load near a strip footing at 1.2 m causes settlement and tilts the footing; the building is two storeys of brick, which tolerates very little differential movement.
- The trench wall is loaded eccentrically, which is exactly the condition that produces a spalling or slab failure rather than a safe ravel.
- The toe of the pile can run into the trench if it is placed within the 1 m clear zone; sandy silt rests at about 35°.
Correct placement: haul it away, or stockpile on the opposite side at least 1 m back from the edge, keeping the ladder, the trench box handling area and the gas main crossing clear. Keep plant traffic off the building side as well.
2. How do you excavate around the gas main given the ±0.5 m locator tolerance?
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Treat the band, not the line. A ±0.5 m tolerance means the main could be anywhere in a 1 m wide strip. Add a margin and hand-dig or vacuum-expose from about 1.5 m either side of the paint marks.
Method:
- Machine only outside that band, with a banksman watching the bucket and the trench.
- Inside the band, use a blunt hand tool or vacuum excavation, no pickaxe, and expose the full circumference of the pipe before going deeper.
- Support or protect the exposed PE pipe; it is easily scored and a gouge is a leak path.
- If the pipe is not where the marks say, stop, re-scan, and treat the find as new information - do not chase it with the bucket.
- Keep a gas monitor and a fire extinguisher at the trench, and no smoking or hot work.
If it is struck: stop the engine, no switches or phones near the leak, evacuate upwind, keep people and vehicles out, call the gas emergency line and the site emergency number. Do not try to plug it with the bucket or the spoil, and do not drive over the escaping gas.
3. Select the trench protection and access arrangements, and decide whether entry is permitted today.
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Protection: 1.8 m is past the 1.5 m threshold, and the ground is sandy silt with cobbles next to a loaded footing and a backfilled strip - battering back to a safe angle is not possible in a 2.2 m gap. Use the trench box.
One 2.5 m box for 12 m means sectional working: set the box, work only inside it, move it with nobody in the trench, and never leave an unsupported length open for the trimmer. The box should sit with its top slightly above ground level to stop spoil and tools rolling in.
Access: a ladder within 7.5 m of every worker. A 12 m trench needs at least two ladders, so the single ladder on site is not enough.
Entry decision: not today. There is no gas monitor, and a sewer lateral crossing a gas main in a confined trench is exactly where you must test before entry and continuously while inside. Add a competent person's daily inspection (and after rain), keep the spoil and the machine back from the edge, and only then allow the trimmer in - inside the box, with a second person at the top.