Use Postfix to encase underground drainage?

[dom]

I'm re-laying some underground (rainwater) drains. Of necessity these are laid to a shallower depth than BR recommendations. Also across location where I park my van.

Building regs recommend that such drains are encased in concrete to protect against collapse and tree roots.

Usually I would order a polybag of ballast and make up my own concrete. However it's not a demanding application, no structural loads, no exposed concrete - do you think Postfix would be up to the job?

[dennis]

Postfix isn't going to stop tree roots, you need dense concrete to stop tree roots. You also need strength to prevent collapse.

[sm_jamieson]

I've just concreted part of mine under where a new floor slab will be. Under where you park your van ? How shallow is the drainage ? If very shallow, put a reinforced concrete slab over. If you ensure 150mm around the pipe as I think the regs say, that should be OK. Put rebar over the top if you are worried (make sure its surrounded by

50mm concrete). Also, if its a long run, put in some movement joints (compessible board at a socket junction every 5 metres), since a long run of concrete can crack due to earth movement and take the pipe with it. Surely postfix would be a rather expensive way to do it. Also it may be a rather weak mix. Why not get some 25kg bags of ballast instead of a jumbo bag, and add cement to your own strength ? Having said that, I used a bag of postfix-type product in a plastic bag last year, and the set surface (streaky grey look) suggested a very strong mix. But if you mix your own at least you know what you've got ! Simon.

[Phil L]

If you are using the orange/brown underground drainage, then I would say don't bother with any concrete - I once used three12 inch sections of this to move a 1.5 tonne steel girder 200 metres, so unless your van has 18 or more wheels, they ain't gonna collapse, and therefore no tree roots will get in.

If OTOH you are using clay pipes, then you would be better off mixing your own concrete

[dom]

To answer additional questions, excavation depth (at worst) is about

350mm (120mm below pipe, 110mm pipe, 120mm above) - additionally I'll get about 70mm shingle coverage when I lay that.

Max run is about 4.5m before it couples into the original clay pipework.

Thanks for your replies - I've pretty much come round to the idea I need to make up and pour concrete. I was looking for a shortcut as I've got a contractor coming on site and need to get this job out the way.