How should I fill in a trench left by replacement of a sewer pipe?

Question

I recently had my main sewer pipe that runs underground to the main pipe replaced, and I left it open for a few days to check everything is okay, and it seems to be. This is just a small run of pipe, about 2 meters in my back garden from our house to the main shared pipe.

What is the best way to fill the trench back in (I got rid of the original soil dug up as it was contaminated with faeces)?

I have done a few google searches, but found nothing conclusive. Is pea gravel (10mm) good for a base layer to just cover the pipe? After that should I use soil/ sand? Do I have to press it all down?

I am in the UK.

Answer 1

You don't say where you are located, but in the UK, to comply with Building Regulations, it would be typical to both bed the pipe on and backfill immediately around it with pea gravel or single-size stone.

Above that, it would depend what is on top of the trench as to how you'd backfill the rest of the trench.

Under a road, you'd likely use MOT type 1 stone, which is generally specified as being compacted in 150mm layers.

Under a grassed area, you could simply backfill with soil. You don't need to compact it, but be aware that it will settle over time. One way to counter this is to over-fill the trench, so it will (hopefully) settle to finish level with the surrounding area.

(Further assuming you are in the UK, if the work is subject to Building Regulations, the local authority Building Inspector will want to inspect and pass the drain before it is covered up).

Answer 2

I would bed it in sand (6" below, but too late for that by now, presumably, and 6" above and to each side.) After that "ordinary dirt" without large rocks should be fine, though if you are having material hauled in you might want to use sand most of the way just to make delivery worthwhile. You should compact it somewhat every 6" as you fill (though sand settles pretty well) and also mound it a bit more than level with the surface to allow for some settling. A complete job would involve stockpiling some material or calling for more next spring to correct the level after it's been through a winter.

If the depth seems at all dubious and you are in a cold climate, there's no time like the present to drop some extruded styrofoam insulation on top of it just after you have the sand bed level with top of the pipe - then continue with the sand fill on top of the insulation.