How to dig a trench for a cable for a garden office

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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 :)

Answer 9

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.

Answer 10

You need to rent a power trencher!!

Answer 11

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.

Answer 12

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:

Answer 13

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.

Answer 14

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.

Answer 15

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.

Answer 16

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 :)