Burying armoured cable in garden - what type of sand?

In most circumstances I would consider direct burial in the soil to be acceptable. Sand, or perhaps sifted earth is only required if the soil contains sharp stones or flints that would be liable to damage the cable. 

Above the cable should be buried some warning that a cable is below, plastic tape suitably printed is usual. 

And is there any more guidance these days as to how deep it should be buried? I seem to remember when I put mine in (long time ago now) the guidance was "sufficiently deep" which didn't help much! (Just asking out of interest.)

"sufficiently deep"

That famous NJUG diagram puts LV at least 450mm deep or 600mm deep under carriageways, which doesn't seem like a bad starting point. I'd try to have the trench at least 500mm deep under say a lawn so it's safe from being spiked by garden forks and the like used to aerate the turf. Under cultivated areas obviously much deeper, or something like a flagstone over the top to reduce the likelihood of the cable being hit. If under what's going to be a permanent path or such shallower might be OK.

If you do have issues with sharp stones and the like - I'd probably go for a twinwall duct rather than sand - much quicker and easier to install and probably more reliable as it's not going get displaced when backfilling or by future groundworks like sand might be. Just be careful that the duct's arranged so water can't collect inside it (i.e. no U shapes) - most cables don't like being immersed permanently.

   - Andy.

It's coming back to me that for mine "sufficiently deep" turned out to be dig down until you hit the (ceramic) sewer pipe, and then come back up a bit! But reassuring that (from memory) that was pretty much the same interpretation I made, including, being a cautious chap, running through duct.

Thanks, Andy

AJJewsbury said:

That famous NJUG diagram puts LV at least 450mm deep or 600mm deep under carriageways, which doesn't seem like a bad starting point. I'd try to have the trench at least 500mm deep

GN1 says 500 mm deep. Some of the Part 7 locations have requirements for 600 mm (705, 708, 730), but note also 705 requires 1 m deep under cultivated land.

Also worth remembering that there will be compacting when you back-fill (unless you compact as you go) ...

AJJewsbury said:

Just be careful that the duct's arranged so water can't collect inside it (i.e. no U shapes) - most cables don't like being immersed permanently.

That includes "normal" SWA. If SWA is buried in certain kinds of clay ground, with no ducting, best to try and provide some drainage or similar around the cable, by digging down at least another 200 mm, and back-fill with aggregate first, topping off with 1/4 down or similar, and then 50-100 mm of sand on which the cable is laid.

And don't forget if duct is not being used, cable tiles and/or marker tape above the cable.

if you are using sand then something free of sharps or ballast - 'building sand' would be fine, as would 'play' sand. Some types of sharp sand are surprisingly stony Nothing that would cut you if you crushed a handful of it - perhaps try that in the builders merchants yard before you buy.  +1 for some kind of duct or pipe though if you can afford it, so much easier, even if it is not the 'proper' duct that would be used in the street. I have even used scaff pole in the past as an embedded conduit in a funny place where things were shallow and being driven over.

At work we have a lot of cables in 4 inch 'soil pipe' with manhole covers at the changes of direction and we tend to leave a draw rope in,  pulling in a new one each time with the new cable.
Mike

For drainage we alway used pea gravel, small round pebbles rather than sand which can wash out creating an underground void.

Obviously I don't know anything about your work Mike so your use of 4" soil pipe as ducting is probably quite logical but out in the wider world I think the use of blue mpde water pipe, waste pipe and brown soil pipe as ducting is inviting trouble. Unless it's wrapped in warning tape of course, which it never is. I have a particular hissy fit over t&e in 20mm blue pipe for outside lighting.

That  is a reasonable point , perhaps I should be clarify - I fully agree the T and E in plastic pipe is not really adequate mechanical protection unless perhaps  cemented into a solid structure.  I'd not be that worried if it was deep in a solid floor in a hanger perhaps, as it will never be dug up until the building is demolished. Crossing a flower bed it is asking for trouble....

The sort of thing I was alluding to is  more organised, and involves armoured cables drawn into ducts that are well below the surface, that are indeed salt glaze pipes from the year dot to the older buildings,  or in a few places their modern plastic equivalent  The fact that it almost never needs to be dug up shows it is a good approach. It is however fully mapped.

From an electrical safety point of view in this thread the armour on the cable is the key thing, it could be direct buried or in sand or whatever, the fact it is in a tube just allows it to be pulled out and changed, rather than by digging up a pathway or something. If you have a choice of cable that means you are actually  relying on plastic  ducting as the sole  protection that is rather weaker. I'd not direct bury twin and earth either....
  I  suggest that on a site where one person is in control, so long  as the cable route is pretty clear from house to shed or whatever the risk of an SWA in the 'wrong' tube is very low - on a complex site with lots of other buried pipes and ducts and other folk rocking up to dig things in a weakly regulated way, then things like the marker tape and the correct colour of plastic duct become far more important.

I'm not advocating disregarding the guidance with gay abandon, more that there are low risk cases where doing something non-standard is still quite a lot better than nothing.

Mike.