Filling the trench

[Alex.]

Hello
I've 'sub-contracted' myself to do the labouring for the electrician putting mains out to my garage. I've done the digging, now I'm wondering what's best to put around the cable? Some suggest fine sand, others small gravel. The soil that came out the trench is mainly soft with a few pebbles, should I just put it back as was?
Next, should I put the warning tape just above the cable, or just below the ground level?
Cheers

 

[jimmyspeed]

the cable should be covered in shingle with the yellow and black striped wanting tape above it - this is how we did it recently on a site

gravel acts like warning for anyone digging /fences etc - it looks unnatural and gives you some leeway before you smash the pipe/cable with your graft !! supposedly

 

[Pete.]

Gravel around the cable to stop it getting cut by stones if theres any ground !movement topped with soft sand if you want as a visual warning, with the yellow warning strip on top of the sand a spades depth above the cable.

 

[Robotstar5]

and have you gone down 2' / 600mm?

 

[Welderpaul]

Nooo, not gravel - It can damage the sheath and acts like a drain making your cable permanently wet.
Excavate to 700mm deep. Lay 100mm of sand in the bottom of the trench, lay the cable, then backfill with 100mm of sand. Lay warning tape over the sand then backfill with arisings (Whatever came out of the trench). (We used to use bright orange sand - it comes from oop Cumbria way where the rock is orangey). We used to put a second layer of tape 300mm down from the surface as well.
That cheapy yellow warning tape has been replaced with real thick hard plastic these days - certainly stops a spade or pick...not an excavator bucket mind
Ducted cables are always the best way mind.

 

[ratherbewindsurfing]

The bedding in which an electricity cable is laid serves two main purposes, the first one is to protect the cable from damage from sharp stones etc.

The second purpose is to prevent the cable from overheating at the maximum rated current, This achieved by providing a bedding that has high thermal conductivity when dried out enabling. Generally having cable laid is a permanently wet environment is good is terms of keeping it cool.

Sand is not a good idea, it dries out and has low thermal conductivity, I have seen high voltage cables overheat and fail when laid in sand. Another good way of limiting the current rating or causing it to overheat is to put it in a duct.

The normal solution is to use limestone stone dust as it maintains a high thermal conductivity when dried out.

Having said all that if you are using SWA cable for the supply to your garage just use soil you have dug out taking out the obvious sharp stones and don't worry about the thermal stuff, the ratings used by electricians are very conservative.

 

[brightspark]

Hello
I've 'sub-contracted' myself to do the labouring for the electrician putting mains out to my garage. I've done the digging, now I'm wondering what's best to put around the cable? Some suggest fine sand, others small gravel. The soil that came out the trench is mainly soft with a few pebbles, should I just put it back as was?
Next, should I put the warning tape just above the cable, or just below the ground level?
Cheers

just put the soil back in as long as theres no sharp pieces of rubble likely to cut in the outer core as its only in your garden. its hardly likely to be dug in to unless your trench is less that a spade and a half depth .the tape doesnt matter as long as u know where it is / its not as if its on public land or highway or carpark and a 3d party could dig it up accidentaly . thats where u would use the tape

 

[Shox Dr]

Of coarse when you sell the house remember to dig the cable backup again, and cover with sand/pea gravel and then tape.

Or you could do it right the first time round.

 

[Alex.]

So no clear winner then - two votes for gravel,two for destoned soil and two for both! Guess I'll just take the cheaper option and put back the "arisings" (love that term) without the stones. Thanks everyone.

 

[Robotstar5]

Can we have a poll? I'd bed it in sand, warning tape on top then backfill.

 

[Pete.]

Why not sink a row of tall wooden poles...

 

[Robotstar5]

Why not sink a row of tall wooden poles...

To mark where the trench is?

 

[madkayaker]

My advice use duct the. You can part back fill with the material from the hole lay yellow cable warning tape then fill rest and level. Or sand base cable onto sand then more sand

The issue of depth is a difficult one to give proper answer. Utilities company's often quote the 700mm but if asked (trust me I have to very shove up people many times) you get the don't know we have always done that! The only reg I can find covers cultivated land. In a garden I would stay clear of borders etc and 300min would be as shallow as I would go unless I hit rock etc first

 

[brightspark]

To mark where the trench is?

yea and put orange cap[s on the top just like they do across fields with high pressure water and gas pipes ive just layed 300m of 4.omm swa down the side of a path for bollard lighting . borrowed a trencher and dropped it in . just shoveled the soil back in job done

 

[brightspark]

My advice use duct the. You can part back fill with the material from the hole lay yellow cable warning tape then fill rest and level. Or sand base cable onto sand then more sand

The issue of depth is a difficult one to give proper answer. Utilities company's often quote the 700mm but if asked (trust me I have to very shove up people many times) you get the don't know we have always done that! The only reg I can find covers cultivated land. In a garden I would stay clear of borders etc and 300min would be as shallow as I would go unless I hit rock etc first

so does that mean if u hit rock u would go shallower

 

[Robotstar5]

ive just layed 300m of 4.omm swa down the side of a path for bollard lighting

Why do you have to use expensive SWA when the DIY "sheds" sell much cheaper 1mm T&E for lighting circuits?

 

[the snooper]

My advice use duct the. You can part back fill with the material from the hole lay yellow cable warning tape then fill rest and level. Or sand base cable onto sand then more sand

The issue of depth is a difficult one to give proper answer. Utilities company's often quote the 700mm but if asked (trust me I have to very shove up people many times) you get the don't know we have always done that! The only reg I can find covers cultivated land. In a garden I would stay clear of borders etc and 300min would be as shallow as I would go unless I hit rock etc first

you need to be careful you could get arrested for that

 

[MrFluffy]

Do you people not have conduit for this job there? armoured cable or not, I'd still run it in flexy duct at least double the diameter of the cable outer to give it some airgap, preferred backfill is some slimey ultra fine grey sludge, but I use soil at a pinch and leave a plastic tracer about 80cm above it, so if I am the poor sod that has to dig in that area I always find a marker before finding the cable underneath. I find if Im using the jcb that depth lets me find the tracer before I find what is below too if Im being careful but if your only going to be using a mk1 spade you could probably reduce that to a spade spit depth or something. For the price of a roll of marker tracer, its worth doing and a roll will last you a lifetime unless your trenching a housing estate. I even use the proper colours for each one, being posh with 3 rolls of it.
When I knew I wouldnt be done with the trench (services out the workshop for eg, I plan to put a connection in for heating water to run the underfloor) rather than messing about re-opening it and re-backfilling it I run 100mm grey waste pipe in and pull some cords in so I can drag a cable through it easy in future then I just have to excavate the ends and space it off 20cm or so above whatever i dug the trench for the first time. Someone probably makes a really expensive modular ducting system with a fancy name, but I always have lengths of grey pipe knocking about.

 

[bump]

Seeing the cable and trench gang doing the 11KVa work round the corner, any old thing will do for backfill.

Personally I favour oversize ducting to run any cable in (i'll always end up with more than planned), with a sand bedding and top to take up 200 mm, with a barrier tape followed by 300mm pea gravel followed by more barrier tape with topsoil to cover.

 

[CompoSimmonite]

When I ran power to my garage (8' from house) I used blue 25mm water pipe as ducting. Seeing I had a roll of the warning tape that went over the top.

Paul H