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I'm trying to dig a trench for the electrical cable for a shed/office I'm building in my back garden. This is about 7 m long and should be 0.5 m deep. My soil seems to be about 5 cm of soil/turf and then soil which is full of gravel, stones and building waste like bricks, etc. What is the best way to approach this?

I tried first just making cuts into the soil using a spade to make a narrow slot. That didn't work once I got to the stones.

I also tried using an SDS drill with a 600 mm long chisel bit, but that tends to jam in between the stones and gravel, so I spent a lot of time trying to dislodge it from the ground.

The third technique was with the pressure washer and wet vac, to blast away the soil and suck up the water and smaller stones. This seems to be doing something, but it is painstakingly slow. Since the slot is quite narrow it's pretty difficult to pull up the stones and gravel.

Is there some other piece of equipment I should consider using, or some other technique?

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    I dont see any easy way, I would use a mattock and crowbar which is hard work. Clever idea using pressure washer and vac! – Polypipe Wrangler Mar 30 at 21:23
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    I like that last photo of the crack! – Willk Mar 31 at 2:03
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    Is doing this sort of work without a license even legal where you live? I know it'd be illegal in Australia, for instance, unless you were a licensed electrician. Also, do you have any and all relevant approvals from your city council? – nick012000 Mar 31 at 5:46
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    @nick012000 AFAIK it's fine to do this sort of thing. I'm going to get an electrician to do the connections at the shed end and the house ends of the cable. – Matthew Dresser Mar 31 at 14:24
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    @nick012000 Matthew is in the UK (and not in a city). You don't need permission form the local authority to dig up your own garden. You do need to avoid damaging anything, but for modern houses you (the property owner) should have a (low-res) plan of the services run under the property from the solicitors' files when you bought the house. There's often nothing in the back garden, but I have a foul water drain under mine. It's fairly obvious from the access hatches where my drains connect (and the dip in the patio where the sand base wasn't rammed above the pipe). – Chris H Mar 31 at 14:43

16 Answers 16

46

You got a bad roll of the dice. Very good ideas and attempts that would work in more normal soil conditions. The problem is, if you continue then you will likely undermine the soil because that small rock will fall into the hole from the sides. That will be very difficult to back fill properly and over time it will settle and form a slight ditch / depression.

At this point I think I would call it on the attempt to minimize the work and / effect on the yard and just dig a proper ditch. It could still be narrow-ish, the width of a shovel. This will give you the space you need to backfill in layers compacting as you go and in the end (years from now) it will pay off in spades.

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    +1 for "It will pay off in spades". As well as the good advice. Obviously. – SiHa Mar 31 at 7:55
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    Agreed. Invest in a pick-axe and a strong back and tear out more width and depth and finish up with shovel work. – jdv Mar 31 at 14:37
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    s/role/roll/. And this is fairly typical for gardens in modern houses in the UK. I'm about 30 miles SW of this assuming the OP's profile is accurate and have grit sand, rubble, and a mixture of clay and stones below a thin layer of topsoil, most of which I put there myself. – Chris H Mar 31 at 14:47
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    I just dug a 25' (~8m) trench for conduit in my back yard ("garden") and did it the shovel-or-so width method. The worst part was hitting and cutting tree roots. It took a combination of branch cutters and a sawzall (in the non-™, generic sense) to get through them. Toward the bottom, the trench narrowed to just over the width of the conduit when I had to cut through obstacles. – FreeMan Jul 17 at 13:09
33

Change your wiring method

You started with non-metallic jacketed cable, requiring 500mm of cover. That's similar to the US where cover must be 24" for that type of cable.

However, the US has other wiring methods that only require 6" cover. (e.g. 150mm). This involves a tough metal conduit that you can't pierce with a shovel blade. The pipe is pricy but you can trench it with a garden trowel.

I would check your country's electrical code and see if they have a wiring method like that. Because if they call out 200mm for rigid metal conduit, well heck, you're already there. The conduit is no wider than your vacuum nozzle. You'd just need to straighten out the groove.

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20

Too narrow to be practical in soil with normal obstacles.

Some may advise renting machinery, though most typical "trenchers" are likely to have an expensive "oops" when they meet your buried debris. So if you rent equipment, you'll want a mini-excavator.

For a 7m (~23ft) long trench, I'd suggest (& probably have on several other questions) the good old fashioned pick and shovel. Trench needs to be at least a shovel wide, and may need wider spots depending what debris you hit that the pick won't break up. Pick is for breaking/loosening, shovel is for getting it out of the trench.

Trenches are expensive (in time and labor at least) so do put in conduit, and do put in at least one conduit other than power, even if it's empty right now. That way you only need to dig the trench once, rather than having to find out why you want conduit the hard way, and have to dig the trench again.

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  • 17
    I'm no fan of trenchers. Trenchers is how noobs slice gas lines. The mayhem you can cause with a trencher is off the scale for home DIY. – Harper - Reinstate Monica Mar 31 at 6:06
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    @nick012000 this answer is specifically calling out that renting a trencher is NOT a good idea; and I agree. Love the second conduit idea (what if WiFi doesn't work well? might want to run ethernet, or maybe coax for TV?). I also think it should be mentioned that the power conduit should be chosen a bit larger than what you need; if power needs change down the road and a heavier gauge cable needs to be run, it'll be easier if you have extra maneuvering room in the conduit. – Doktor J Mar 31 at 15:17
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    If you do end up running a second conduit, it can be useful to leave a piece of string in there to make it easier to pull any future cable. – Bob Apr 1 at 0:39
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    I recommend an intern, or other motivated and physically fit young person to do the groundwork. – Criggie Apr 1 at 2:25
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    One way to be a physically fit older person is to use the body you have, rather than leaving hard work to others (younger or not.) – Ecnerwal Apr 1 at 2:53
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I have dug hundreds of trench’s in my life and never seen one so narrow , wow. I use a narrow spade maybe 6” to 8” or . 2 meters wide to go down 18” max, and a full width of 10” or .25m wide to go down 24” or . 6 . I am amazed that the ground is holding. The deeper I have to dig the wider I make the trench . Even with a machine I usually dig at least 6” < .2 m wide Because dirt falls in. Depending on how far you have to go it may be best to continue , less soil to pack back in after you have the cable in place. I would suggest putting a conduit in so if there is ever a problem with the cable it will be easy to pull a new cable. That is really a clever way.

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    The conduit is a wise idea - I'd suggest one for power cables and one for data/phone cables in the future. Depends on the local regs for separation of low and medium voltage cables. Or OP could run fibre optic through the power conduit without issue. – Criggie Apr 1 at 2:26
  • I've seen cable - as in TV coaxial cable - buried in a trench as narrow as that or even narrower. It's common practice for my local cable company (Florida) - they even have a special tool designed for it that makes a trench so narrow it doesn't even disturb the grass. But I doubt it would be acceptable to bury electrical cable in a similar fashion. – Darrel Hoffman Apr 2 at 17:21
  • They usually use a plow and don’t dig a ditch the cable is channeled behind the bar into the ground. – Ed Beal Apr 2 at 17:53
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Having done this to pull 40A 220v back to my workshop, you're digging too narrow a trench. Get a shovel out and dig the trench at least the shovel-width. You're going to want to go down at least 18" (to get below the frost line). Wrap some tape around the shovel handle at your desired depth, and use that to measure progress. My house has been on the lot since 1875. I hit a number of strata of debris on the way to my target depth.

When you have your trench dug, plan to house your cable in PVC conduit to help make it more water resistant, and dig-safe for the person in 50 years that decides to excavate. Before you fill the trench, drop in an inch or two of soil all along, and lay some yellow police tape down on top of the conduit so as they dig, they hit tape first.

One of a number of images on Flickr of my doing just what you're planning

Good luck.

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  • Thanks for the answer, and welcome to Home Improvement! – IronEagle Mar 31 at 17:37
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    Good first answer, unfortunately conduit is listed as a wet location over many decades I have never pulled up any type of conduit that was dry. For this reason the NEC requires conductors or cable needs to be rated for wet locations in conduit outside. – Ed Beal Mar 31 at 21:12
  • All outside conduits are wet locations by definition, and almost always in reality. Anyone that thinks conduits being waterproof has any benefit whatsoever is ignorant, and at least one firm in the fiber optic business has lost a bid by flaunting that ignorance. You can assume that exterior conduit is full of water; If the cable in it belongs where it is, that's no problem at all. If an ignorant contractor has run non-wet-rated cable in there, they may have an expensive remediation some years later (seen that one as well, the cable manufacturer's engineer was on site and not amused.) – Ecnerwal Apr 1 at 2:49
  • Also the standard (and code required) tapes are red and state Buried electric line below - some have foil for easier detection, some don't. "Yellow police tape" is a strictly amateur move. – Ecnerwal Apr 1 at 3:23
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    @Ecnerwal OP appears to be in the UK, so US regs don't apply. This would appear to be relevant. – Will Crawford Apr 1 at 11:22
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My answer assumes you are certain there's nothing significant or structural buried under there and it's just backfilled with construction waste over the years. As a kid I dug my fair share of trenches on the family farm for various reasons, but I'm no expert here. I think the answer is just a pick mattock aka pick axe hoe (with a pointy side and a slightly wider blade) and elbow grease. A shovel or spade is the wrong tool, you'll just jar your hands when you hit stone. With a pick axe hoe, for this size trench it shouldn't take more than a couple of hours. Let gravity and your swing do the work, don't grip on the pick tight when it hits the ground or you may jar your hands. Your body won't ache so much the next morning!

If the law requires your cable be buried 500mm deep, I'd suggest digging it 700mm deep or so, filling the bottom 200mm with gravel to encourage good drainage before laying the conduit you feed your electrical cable through. For that depth I'd be digging it about 300mm wide. I'd also suggest using wider than the minimum conduit to allow additional cables in the future, perhaps for another circuit or fiber optic for network connectivity. Fiber and fiber equipment price isn't far above ethernet these day and removes any possibility of interference, as can occur running network cables parallel to mains power. Legally you may need to run fiber in a different physical conduit to mains though, but you'll have to check local regulation here if you want to play by the book. Leave a thin rope in the conduit hanging out each end to help you pull through cables in the future.

The depth and width I am suggesting may sound like overkill but if you have poor drainage the gravel will help with that, and you'd be surprised how much energy you burn trying to dig such a skinny trench.

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Long way around?

There used to be something in your back yard. Maybe a number of somethings over the years; I understand people have lived in the UK for some time. Maybe your direct line to the shed intersects where something was knocked down and is still where it fell?

Consider digging test holes at other sites representing possible paths to the shed - maybe along one or the other wall. Maybe the entirety of your yard is not full of these little rocks.

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    Yes, UK has many developments that were pre-WWII. It's kind of amazing really. – Harper - Reinstate Monica Mar 31 at 6:14
  • My house is a new build over a site with a previous house which was knocked down. I think what I am seeing is the building/demolition rubble. The back garden prior to the new development sloped down, so was in fact about 500 - 800mm lower than it is now – Matthew Dresser Mar 31 at 22:40
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Make the pit as narrow as the soil will handle given the angle that it wishes to adopt. Starting with one spade-width trench... make a test pit of 1m length, Chopping the sides away about 20cm wide and taking out cubes using the spade, keep the cubes and turf nicely on the side. Once you have a 20cm wide access pit, see the best way to get 50cm deep in a 1m test pit, and carry it forwards. You may even want to dig out the turf into a carpet of a certain width when you know the width that you need, then pack it and put the carpet back.

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You need to rent a power trencher!!

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    I don't think something like that can handle what he's describing. – Loren Pechtel Apr 3 at 3:13
  • I don't fully disagree, but need is a bit strong... – FreeMan Jul 17 at 13:15
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I see your question is tagged UK, and I recently had power run to an outbuilding in the UK too. In my case, the work was notifiable under Part P, as it was the installation of a new circuit. I hired a qualified electrician to do it.

He told me the cable could safely be run only two inches below my lawn, as (a) it was steel wire armoured cable which a spade wouldn't get through; (b) it was not a location that would obviously see a lot of digging, like a vegetable patch or flower bed would; and (c) I would know where the cable was anyway.

I was a bit surprised by that, and a cynic might say his decision was biased by the fact he'd be the one digging the ditch - but he had a good reputation and all the relevant qualifications.

You should ask the person who's going to sign off the installation certificate as he/she might say your ditch is already deep enough. And indeed, they might be happier signing things off knowing they told you to do it that way :)

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1

I would recommend investing in (or renting?) two tools : a trenching shovel and a digging bar.

A trenching shovel is narrow but attaches to the handle it's easier to scoop into the trench to remove soil. If you don't have access to a trenching shovel, you can try a transplanting shovel as they're narrow but deep.

The digging bar is good for loosening up the soil. It works like a pickaxe or mattock, but the long bar means that you can just lift it up and drop it (which I find easier on my back) and the longer length means that you can use it to lever out larger rocks rather than needing brute strength.

It's possible that you already have a digging bar, as there's a spear looking thing to the lower right of your trench ... but a digging bar is all metal, so it can be used in places that would snap wooden or fiberglass handles.

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I have dug many a trench using a Pick Mattocks in the US, in Iraq, etc. Anything from sand to asphalt. It makes just about a wide enough hole, you can go back and forth to make it a little wider if you want. I prefer about one and a half blade thickness. It's hard work in rocky soil, but you need enough space to backfill properly so you don't get a dip in your yard. You could even use a little concrete for stability if you're worried about the ground moving.

Also, use outdoor cable rated to be "buried" but I would ALSO use a strong conduit. You don't want to cut into that power in the future.

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    You want to use individual wires in a conduit, not a cable, lest you need to call a sparky in to bail you out of a chicken-choke pulling job! – ThreePhaseEel Apr 3 at 1:15
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Perhaps an AirSpade or even Hydro Excavation!

A Case for AirSpade:

This tool uses a high-velocity air stream to move and aerate soil so that it can be vacuumed away.

Better yet, find a contractor who has one of these and will come excavate your trench for you using this technology. Here is a video blurb showing its efficacy: Video

Disclaimer: I have no affiliation with this company and no first-hand experience with the product I suggested. I ran across it while searching for something else and happened to think of it while reading the OP's question.

It is possible that the rocks under your soil would prove too much for the AirSpade, however. If that is the case, it's time to talk about hydro excavation

A Case for Hydro Excavation:

This process works much like the AirSpade above but uses WATER instead! It is simply amazing what the technology can do, but is likely to be far more expensive to get a crew out to do your job. It was used by Google while running fiber here in Austin, TX. It created the most beautifully square holes in the ground, with perfectly straight sides. These went down over 15 feet in places. This would really do exactly what you need, as evidenced by this picture:
this picture

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    Smells spammy, without quite causing me to reach for the flag button – Ecnerwal Apr 1 at 20:26
  • I saw that product before in another video, but I don't think it will cut it in this case. Too much rock and stone to get through. – Matthew Dresser Apr 2 at 21:14
  • I would have loved something like that 40 years ago replacing the water main of my parent's house, but I doubt it would do anything in the mess of rock we have and what he's describing is even worse. – Loren Pechtel Apr 3 at 3:16
  • and still has the same issues, only works in more ideal soil conditions. Also, need the truck needs access which might not be workable for a back yard (I don't know in this case if fenced, if a septic tank, etc) – Ack Apr 3 at 22:36
  • @Ack - Actually, the dig that was done for Google here in Austin, went straight through our caliche soil like a laser. It was pretty amazing. Also, the linked article mentioned the dig can be as far as 500 feet from the truck by using a 'remote kit' which is basically longer hoses... – MrWonderful Apr 3 at 22:45
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Depending on what equipment you will have in your office, you could connect it with just a 12V DC cable. That's what used for garden decorative lightning and you won't need to bury so deeply.

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It's a longer way round, but I'd be inclined (and have been several times) to go around the edge, close to the fence. With underground 4" sewer pipe. Sturdy enough to withstand shovels in the future. It wouldn't need to be too deep, but lay some plastic mesh over it, so anyone in future will find that first. Put a rope in as you go, at the same time as cable - armoured is best, but not cheap, so you could pull something else through at a later date if needed.

This doesn't strictly answer the question posed, but does offer a solution to the problem. Given that you will have it protected by an RCD - either separate or integral with the house, and the ground there may well be more amenable.

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    No. Sewer pipe and conduit are similar, but different products. Pulling wire around sewer pipe elbows is miserable, cable worse (or impossible), armored cable is utterly pointless (and even harder to pull, and a waste of money) if using conduit which is the "Armor" if done properly. Minimum burial depths are matter of what the local code sets (and as Harper notes, choice of particular types of Electrical Conduit can reduce those in most jurisdictions.) "Buried electric line below" tape is both inexpensive and the correct/required item to warn of a buried electric line below, not "mesh." – Ecnerwal Apr 1 at 11:17
  • @Ecnerwal - why pull round bends? Put the wire in as you go. Simple! – Tim Apr 1 at 12:04
  • @Tim and what happens if you ever need to replace a wire? – Dan Is Fiddling By Firelight Apr 1 at 17:31
  • @DanisFiddlingbyFirelight - with a 4" tube, complete with an extra rope anyway, it really isn't that difficult. Yes it might need replacing in 20 or 30 yrs time, but isn't that going to be the same wherever and whatever? Hardly worth discussing. – Tim Apr 1 at 18:13
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    @Tim Putting wire in as you go happens to be illegal, and plumbing bends are a total bear to pull around. – Ecnerwal Apr 1 at 20:25
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Why on Earth are you going to bury this cable? There are some disadvantages:

  • Do you think it will be safe? Not in, let's say, next 30 years. And even after 3 years you won't be able to find it unless you make a thorough photosession of your work

  • You won't be able to examine the cable condition in case when something will go wrong. To do this you'll just have to "unbury" the whole cable.

Why not just go by air? The cable can follow the perimiter of your area. For example near the top of your fence? Or just go high over the area, connecting the spire on the shed/office with the main line.

Personally I'd say the 0.5 meter deep is not quite enough to walk barefooted on it in the next 30 years :)

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    Hello, and welcome to Home Improvement. This is really a comment, not an answer. With a bit more rep, you will be able to post comments; in the meantime, please take our tour so you'll know how best to contribute here. – Daniel Griscom Mar 31 at 11:15
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    @DanielGriscom Just a perspective. I'd see that as an answer. He provides reasons why he considers it unwise to do the job as it is being done and then an alternative. There is enough merit in the argument for it to remain as an answer rather than getting buried [ :-) ] in comments. – Russell McMahon Mar 31 at 11:52
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    It's an answer. It happens to be an answer I, as a practitioner of the electrical arts (Low voltage flavor professionally, both sorts at home, as my LAHJ permits) throughly disagree with, but it's an answer. As already mentioned I prefer conduit in trenches, not direct burial, but for longevity I will take a buried cable 10 times out of 10, or 9,999 times out of 10,000 (if it crosses a known active fault line, sure, in air is better.) Per the applicable code in most places, finding it again is as easy as digging a few inches for the "buried electric line below" tape in the top of the trench. – Ecnerwal Mar 31 at 12:55
  • Attaching wiring to the fence most likely won't meet code in any jurisdiction with codes and in which they're actually enforced. Also, 0.5m (~18") is just fine for wires in conduit and 24" (~.6m) is perfectly acceptable for direct burial cable. There's nothing about either situation that would preclude "walking barefoot" above it in 15 minutes or 30 years. – FreeMan Jul 17 at 13:22

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