Trench digging technique

[Stroppymonkey]

Yep. Anglian Water have actually been really helpful, and I'm getting them to sign off my approach, including all pipe/connection/adapters etc. before I buy or dig anything. They're happy with the pipe being in a twinwall duct, as long as it's not super stoney, which I'm hoping it won't be......otherwise I'm digging deeper and spending a shedload on sand.

Are you sure that using a duct negates the need for protective back fill?
I have a 'dig deep' 760mm bucket for putting in water lines. Its only 3" wide at tip so the pea gravel bill isnt as painful. You have to be VERY confident that there are no other services present though as it takes no prisoners.

 

[blackhorse]

Are you sure that using a duct negates the need for protective back fill?

Yep, as long as the engineer inspecting is satisfied that stones/flint won't be an issue, then I'm told it'll be fine. The duct is really for my peace of mind, which I suggested to AW and they were happy with. Of course, I may well find out that stones/flint are an issue once I get digging, in which case I'll definitely need sand or pea shingle surrounding the pipe. I'm hoping that won't be the case though, as I'm already having to run barrier pipe for part of the run, which is hideously expensive.....having to surround the pipe with pea single on top of that is going to make me cry a little inside.

 

[6feetdown]

Going 300mm wide would allow me to run some more conduit for armoured electrical cable and fibre optic too (hoping to put an outbuilding up part way along the trench at some point in the future - money allowing).

This would be handy but somehow I've cropped the photo.
Worth googling as water company may want something similar if you've multiple services going in

 

[blackhorse]

This would be handy but somehow I've cropped the photo.
Worth googling as water company may want something similar if you've multiple services going in

Thanks for that - I haven't mentioned other conduit/cables to AW, so will definitely check with them before doing anything.

 

[Smiffy]

Yep. Anglian Water have actually been really helpful, and I'm getting them to sign off my approach, including all pipe/connection/adapters etc. before I buy or dig anything. They're happy with the pipe being in a twinwall duct, as long as it's not super stoney, which I'm hoping it won't be......otherwise I'm digging deeper and spending a shedload on sand.

I would expect twin wall duct to be more expensive than sand. Don't buy sand from a builders merchant. Buy it from An aggregate supplier. Ask for prices on reject sand. It's all we use for this and usually very chap.
I personally would want shingle around a duct regardless. Especially the ones you run for elec and fibre. If you don't intend to run cables straight away a pull them in later, had to many problems with poorly laid ducting collapsing.

 

[blackhorse]

....had to many problems with poorly laid ducting collapsing.

Can you elaborate on this, Smiffy? That's something I'm keen to avoid so if shingle/sand is more foolproof then I'd be inclined to go down that route.

If using shingle, would you also use a duct? If using a duct as well as the shingle then that's going to bump on the volume of the shingle somewhat. Is sand surrounding a bare mdpe pipe possibly the best cost/robustness trade-off option?

 

[Smiffy]

Can you elaborate on this, Smiffy? That's something I'm keen to avoid so if shingle/sand is more foolproof then I'd be inclined to go down that route.

If using shingle, would you also use a duct? If using a duct as well as the shingle then that's going to bump on the volume of the shingle somewhat. Is sand surrounding a bare mdpe pipe possibly the best cost/robustness trade-off option?

Cost wise using reject sand and perhaps not quite doing 100mm all around is the cheapest over proper twin wall duct. Flexi duct is cheaper but not as robust.

Ducting and pipe aren't actually very strong against compressive force. They rely on equally distributing pressure around the circumference to an immovable surround for there strength. Any void around the pipe doesn't allow this to happen and the pipe to collapse. This is why pea shingle is used and it flows freely into all voids but when covered over and surrounded then bonds and becomes incompressible.

 

[Furniss]

yep i imagine uk quarries do same but our local quarry does a declassified sand, cheap and compacts well

 

[Alan caddy]

If your using detectable tape put it below your pipes so doesn't get broken if your ever digging down again, also I wouldn't put the pipe in duct if your having to use barrier pipe on some of it as any problem with contamination later on will go down your duct and cause more issues later on

 

[hiluxman]

Also if you've a wash plant near by you can get recycled 10mm pipe bedding alot cheaper.

They will also do recycled sand too which could also be used for backfilling.

 

[craig]

yep i imagine uk quarries do same but our local quarry does a declassified sand, cheap and compacts well

Different areas use different names, round here and would guess the most common is just fill sand.

 

[Vinpetrol]

As doobin says , dig it a foot wide , if it's in a field you can use your bucket to give it a bit of a compact half way then put your marker tape in and pull in the rest of the clay . Give it a good track in and then pull the soil in and track it in again . Ideally you would use sand around the pipe but if your on a budget and its your own pad you could use a bit of the topsoil to protect the pipe . So long as it's not Stoney and you spread it with a shovel .

 

[blackhorse]

Thanks so much for the info. So with a 300mm wide trench, 200mm fill height surrounding the pipe, and say a 70m trench run (less than I originally thought), I get the following costs using Longwater Gravel based on 1.8 tonnes per cubic metre (8 tonnes in total):

• Fill sand - £286.18
• Pea shingle (crushed) - £445.82
• Pea shingle (natural) - £551.52

The cost of the twinwall duct from JDPipes would be about £200.

It sounds like it's probably worth paying the £86 extra and going for the fill-sand option. Unfortunately our topsoil is very stoney and so I don't think we'd be able to use that around the pipe.

 

[Smiffy]

Thanks so much for the info. So with a 300mm wide trench, 200mm fill height surrounding the pipe, and say a 70m trench run (less than I originally thought), I get the following costs using Longwater Gravel based on 1.8 tonnes per cubic metre (8 tonnes in total):

• Fill sand - £286.18
• Pea shingle (crushed) - £445.82
• Pea shingle (natural) - £551.52

The cost of the twinwall duct from JDPipes would be about £200.

It sounds like it's probably worth paying the £86 extra and going for the fill-sand option. Unfortunately our topsoil is very stoney and so I don't think we'd be able to use that around the pipe.

I would think if you got on the phone Monday morning you should be able to get that down a bit. I don't know what your area is like for aggregates but that seems a little expensive for essentially a waste product. We always collect but I think the last lot was £12 a tonne.

 

[bobthebuilder]

Mole it

 

[hiluxman]

I would think if you got on the phone Monday morning you should be able to get that down a bit. I don't know what your area is like for aggregates but that seems a little expensive for essentially a waste product. We always collect but I think the last lot was £12 a tonne.

I was going to say that didn't sound too outlandish. Especially for a one off delivery to a none account customer (assuming that's a delivered price). And I'd dare say thts also from such as an aggregates yard where it'll of come in and been rehandled

Then factor in 20% of it will be vat

 

[Smiffy]

I was going to say that didn't sound too outlandish. Especially for a one off delivery to a none account customer (assuming that's a delivered price). And I'd dare say thts also from such as an aggregates yard where it'll of come in and been rehandled

Then factor in 20% of it will be vat

I know it depends on the area. most on this forum pay a fraction of the price we do in the south east for quarried stone. Sand and brick clay are the only things quarried around here so reject sand is cheap.
Plus I don't think any company I have ever dealt with puts their best price on the website.

 

[Canal Navvy]

If you actually need barrier pipe due to contaminated land then you really need the whole run in barrier. If you only do a section then your lovely granular pipe bedding will allow ground water to track up to the unprotected part and migrate through the pipe wall. Things like fuel oil in the ground can penetrate the walls of a live water pipe and the sort of analysis the water providers use can identify the product and the likely timescale it was refined.
As an aside, cross-country pipelines that are bedded in self compacting pea gravel have clay stanks at intervals to stop them acting as drains.
If at dig depth you are in moist, fine clay then it is pretty good bedding material itself.

 

[blackhorse]

If you actually need barrier pipe due to contaminated land then you really need the whole run in barrier. If you only do a section then your lovely granular pipe bedding will allow ground water to track up to the unprotected part and migrate through the pipe wall. Things like fuel oil in the ground can penetrate the walls of a live water pipe and the sort of analysis the water providers use can identify the product and the likely timescale it was refined.
As an aside, cross-country pipelines that are bedded in self compacting pea gravel have clay stanks at intervals to stop them acting as drains.
If at dig depth you are in moist, fine clay then it is pretty good bedding material itself.

To elaborate on my situation a little more; I have an existing water pipe to the house which is a short run of around 6 metres, which I'm replacing in order for Anglian Water to then replace the lead communication pipe. Because the existing meter point is just a couple of metres from my oil tank, Anglian Water are insisting (reasonably I suppose) that we use barrier pipe in case that tank ever ruptures. That 6 metre run from the existing meter point to the house will be entirely in barrier pipe. Since Anglian Water will be closing the road off while they do the work, I'm taking the opportunity to get another meter point added for a field supply. I'll then use the remaining 40+ metres of barrier pipe on the 70m run to the field. There's no pre-existing contamination of the land at all.

Your point about the shingle acting as a drain is really interesting. I imagine it would be even worse with a duct. I might dig a test hole this weekend and see what the ground is like down at 800mm. Even if I do end up using sand/shingle, it would presumably be worthwhile adding in a few carefully constructed clay stanks.

 

[Smiffy]

To elaborate on my situation a little more; I have an existing water pipe to the house which is a short run of around 6 metres, which I'm replacing in order for Anglian Water to then replace the lead communication pipe. Because the existing meter point is just a couple of metres from my oil tank, Anglian Water are insisting (reasonably I suppose) that we use barrier pipe in case that tank ever ruptures. That 6 metre run from the existing meter point to the house will be entirely in barrier pipe. Since Anglian Water will be closing the road off while they do the work, I'm taking the opportunity to get another meter point added for a field supply. I'll then use the remaining 40+ metres of barrier pipe on the 70m run to the field. There's no pre-existing contamination of the land at all.

Your point about the shingle acting as a drain is really interesting. I imagine it would be even worse with a duct. I might dig a test hole this weekend and see what the ground is like down at 800mm. Even if I do end up using sand/shingle, it would presumably be worthwhile adding in a few carefully constructed clay stanks.

The duct shouldn't work as a drain as each end has to be sealed against rodent ingress to get signed off, at least it does with ses water and Thames water. So water ingress is only via joints, so minimal traveling.
I have never added anything in trenches to stop them acting as drains. It's only a problem if there is somewhere for the water to go. If it is a solid uninterrupted trench, in clay, it doesn't matter to much. Different if you have voids in the soil as a trickle of water can cause erosion and sink holes.