Replacing clay drain with plastic - backfill question

4 Jan 2011
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United Kingdom

I'm replacing a leaky stretch of clay underground pipe with plastic. The run is around 5m in length and around 0.5m under the ground.

My question is around backfill. The clay pipe is currently just in the soil. It is sitting on firm soil/clay soil.

I understand I should lay the plastic on pea shingle and then backfill with "granular material" to the top of the pipe. Is this still relevant when just replacing damaged pipe that has sat on a solid bed already. What classes as "granular material".

Many thanks.
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I am not a drainage expert but I had always thought that a salt glazed drain pipe should ideally be laid on a bed of fairly weak mix cement, perhaps 200 mm wide and 75 mm deep.

As far as the plastic drain is concerned, I had also thought that they should be laid on a bed of pea shingle and then backfilled with either the same or sharp sand.

Although highly non technical, I actually find drainage quite interesting!

Shortly will need to relay about 7 m of mine!

Where is the pipe - in a garden, below a path or driveway etc?
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Where is the pipe - in a garden, below a path or driveway etc?


It goes down the side of my house. This was previously built over with a lean-to side passage, but I'm currently replacing that with an extension. So ultimately it will have a concrete floor above it. There will be 500mm of soil though below the concrete floor, before it gets to pipe level.

The type of backfill depends on the loading on the pipe but in your case that will be fairly minimal below a concrete slab. I'd fill it in with what you have available making sure there are no large stones or lumps of clay directly on the pipe. Granular material is generally 5-10mm pea gravel or other crushed stone.
The building regs don't specify what "granular fill" for flexible pipes is, but it's effectively pea shingle for rigid pipes. So I'd stick with that.

So fill with pea shingle to top of pipe. Get a bulk bag from builders merchants.

Be very careful with the fall. Support the pipe in shingle or concrete every metre or so and get fall consistent before filling and packing under. Falls on old pipes are generally rubbish, ours is 1/200 and took a lot of care not to be left with any flat spots.

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