Trenchfill or Not, is not a binary decision.

[epsilonGreedy]

I met my "wise owl" building adviser on site this week to discuss the dig and concrete pour. He recommended opting for something less than a full trench fill at a typical 150mm below ground level and instead advised pouring the concrete to 300m below ground level. This he said will provide more flexibility with fine tuning the finished level of the foundation blockwork and also simplify routing services through the foundations.

 

The building control chappie then joined us for a meet & greet and said he knew the ground conditions well enough at my plot to indicate a standard 250mm pour at 1m deep would suffice. In summary I had two wise gentlemen, with over 90 years in house building and building control between them, saying 850mm of trench fill is OTT for my plot.

 

If I follow their combined advice my concrete will be 700mm deep and 700mm wide.

 

Has any forum member ended up with foundation concrete somewhere between 250mm deep and full trenchfill (excluding passiv slabbers of-course)? 

Edited June 23, 2018 by epsilonGreedy

[Russell griffiths]

It’s all 6 of one half a dozen of the other

if you put a less amount of concrete in you will need more brickwork to get to ground level

if you fill to within 300 of finished ground level you will have more expenses in concrete but less in brick work. 

 

 

I would work out your damp course level, and measure down from that, making sure you have enough room for pipes to exit under finished ground level but above concrete level. 

 

Laying bricks in a skinny trench full of water is not fun, concrete in relative terms is a cheap product compared to a days wages. 

[Jeremy Harris]

FWIW, We've lived in three houses with no foundations at all.  Two just had slabs of stone buried around 200mm into the ground, with the stone walls built on that, one had brick walls with the foundations being two courses of corbelled bricks laid directly on to the ground.  All three had been standing for well over 100 years with no sign of any problems at all.  With one of them I needed to run a new water main inside the house (the old one ran up an outside wall and used to freeze) and only discovered the shallow stone foundation when I dug a hole outside to see how hard it would be to run a pipe in under the timber floor.  To say I was a bit surprised to find the foundation was less than a spade's depth down was a bit of an understatement.

 

There's a "London Effect" that I think has led to the belief that deep foundations are needed.  The UK is dominated by what happens in London, and London has both horrible clay and suffers badly from high water abstraction that has caused that clay to settle.  Consequently there were a lot of repairs to foundations with underpinning, that hit house insurers very hard, so as a consequence the rest of the country now has this view that deep foundations are absolutely necessary, even when it's obvious they are not from the surrounding houses.

[epsilonGreedy]

17 minutes ago, Russell griffiths said:

if you put a less amount of concrete in you will need more brickwork to get to ground level

if you fill to within 300 of finished ground level you will have more expenses in concrete but less in brick work. 

 

 

My original assumption was that 850mm of concrete finishing 150mm below ground level was right for me because I am willing to trade a larger concrete bill for lower brickie management costs. Although it is reassuring to hear the building control inspector indicate that 250mm of concrete will do, I am inclined to play safe and opt for something much thicker.

 

As you say it is a matter of juggling concrete cost v. blockwork labour costs. At this point the margin of debate comes down to 700mm or 850mm of concrete. I am erring towards 700mm as this provides more tactical flexibility up to DPC particularly with services. 

 

17 minutes ago, Russell griffiths said:

Laying bricks in a skinny trench full of water is not fun, concrete in relative terms is a cheap product compared to a days wages. 

 

 

At this time of year flooding on top of the set concrete 150 or 300mm below ground is not my current concern, should it be?

Edited June 23, 2018 by epsilonGreedy

[Construction Channel]

In our area it is a minimum of 650mm concrete irrelevant of the depth. 10” of concrete doesn’t sound like a lot to stand a house on to me, what is your ground like? 

 

Don’t get me wrong I have taken a lot of houses down with less footings than that that haven’t moved at all since they were built but regs are regs. 

[epsilonGreedy]

8 minutes ago, Construction Channel said:

In our area it is a minimum of 650mm concrete irrelevant of the depth. 10” of concrete doesn’t sound like a lot to stand a house on to me, what is your ground like? 

 

 

The building control inspector, who also acted as the inspector for the plot 40m away, said it is the sandy soil that looks like clay but does not behave like clay.

 

This is all subject to what we encounter on the day of the dig.

Edited June 23, 2018 by epsilonGreedy

[ProDave]

It does vary tremendously.  We had no depth stipulation other than we had to dig through the thick layer of top soil down into the sandy clay sub soil. But for some strange reason our SE specified reinforcing mesh in the concrete strip foundation pour.  No doubt it will be stronger but never seen that before.

 

Less concrete and more blocks will usually be cheaper. Also has the advantage of more room to correct things if the concrete pour is not dead level.

[PeterW]

Strip foundations -  which is what your BCO is talking about - have to be a minimum of 2w where w is the width of the finished wall.

 

The depth of the strip that most SE’s will specify is a minimum of w so either you have some skinny walls or your BCO is taking the regs and adding 100mm ... 

 

The biggest issue with strip fill is laying foundation blocks - you also need clearance and your brickies will hate you if there is no clear edges and they are having to hand ball foundation blocks from ground level to -750mm all day. 

 

A rough approximation from Spons shows Trenchblock at 305mm as £47 per square metre. The same linear metre with trenchfill is about £56 based on £80/cube for concrete. 

 

Trenchfill will take a couple of hours to complete - trench blocks are roughly an hour per square metre so 4-5 days depending on complexity. 

 

I can’t see why you would use strips and blocks based on that. 

[epsilonGreedy]

7 minutes ago, PeterW said:

The depth of the strip that most SE’s will specify is a minimum of w so either you have some skinny walls or your BCO is taking the regs and adding 100mm ... 

 

 

In my case we are talking about 100+100+100 brick and block walls.

 

9 minutes ago, PeterW said:

The biggest issue with strip fill is laying foundation blocks - you also need clearance and your brickies will hate you if there is no clear edges and they are having to hand ball foundation blocks from ground level to -750mm all day. 

 

 

As mentioned above the margin of debate is foundation blocks from either -300mm or -150mm.

 

10 minutes ago, PeterW said:

A rough approximation from Spons shows Trenchblock at 305mm as £47 per square metre. The same linear metre with trenchfill is about £56 based on £80/cube for concrete. 

 

Trenchfill will take a couple of hours to complete - trench blocks are roughly an hour per square metre so 4-5 days depending on complexity. 

 

 

Thanks for the Spons figures. My Gen-1 concrete is being supplied at £74/cube which sways things further from a financial aspect. Linear house trenches are 60m including a supporting walls. Using Spons I am trading £460 of concrete for blockwork at £423 + labour + time.

 

16 minutes ago, PeterW said:

I can’t see why you would use strips and blocks based on that. 

 

 

It comes down to having an extra 150mm in block height to level out any error in the finished height of the poured concrete and more flexibility for routing services through the foundations.

[PeterW]

So don’t pour a full load but bear in mind you’ve got to be a min 450mm below ground level with any foundation, and most services will be within the first 300mm at most below the surface, pouring a 600 x 600 into a 900 x 600 trench gives you lots to play with. One course of blocks and you’re into engineering bricks anyway so easy money. 

[PeterW]

And that spons block price includes labour but it takes much longer. 

 

Cash rich or time poor..?

 

[epsilonGreedy]

8 minutes ago, PeterW said:

Cash rich or time poor..?

 

 

It is more a case of foul drain gradient poor.

 

The wisdom of my wise owl adviser has become apparent to me today, he is always 10 steps ahead. The same chap is responsible for the foul drainage sewage connection at my plot and one other adjoining. He said he plans to be onsite the day the water company contractor digs the road to extend the mains sewer onto the plot otherwise he said they might come up at a high angle and not give us enough fall on the foul drainage run from my house to the sewage interconnect.

 

My most distant foul drain needs to exit the foundation at the right height for the drainage gradient arithmetic to hang together.

[epsilonGreedy]

30 minutes ago, PeterW said:

So don’t pour a full load but bear in mind you’ve got to be a min 450mm below ground level with any foundation, and most services will be within the first 300mm at most below the surface, pouring a 600 x 600 into a 900 x 600 trench gives you lots to play with. One course of blocks and you’re into engineering bricks anyway so easy money. 

 

In my case I reckon it will be 700 x 700 poured in a 1000 x 700 trench. Mr Wise Owl has twice tated his preference for 700mm wide trenches.

 

Would I be correct in thinking the mains water is the only deep service entry buried in the concrete? Hmmm must check what the underground LPG tank people expect.

[PeterW]

LPG can come through a wall - if you have a buried tank then the main regulator will be wall mounted outside so work out where you want that to be. 

 

Water needs to be planned but it can be as simple as a piece of 110mm underground pipe with a swept bend on it and you can shove the MDPE down it. 

 

What are your invert levels on the drains ..?? 

 

[Construction Channel]

39 minutes ago, epsilonGreedy said:

 

In my case I reckon it will be 700 x 700 poured in a 1000 x 700 trench. Mr Wise Owl has twice tated his preference for 700mm wide trenches.

 

Would I be correct in thinking the mains water is the only deep service entry buried in the concrete? Hmmm must check what the underground LPG tank people expect.

 

What exactly does mr wise owl intend to dig the 700mm wide trench with? 600mm is the standard and almost any groundworking firm will have a digger bucket to match that, 700mm sounds like a person that’s very good at drawing pictures but hasn’t had much real world experience of digging trenches. 

 

Just my 2 cents. 

[Russell griffiths]

I was thinking just the same. 

Never seen a 700mm digger bucket, not unless the bushes are shot and the bucket rattles around a bit. 

[dpmiller]

5 hours ago, epsilonGreedy said:

 

Has any forum member ended up with foundation concrete somewhere between 250mm deep and full trenchfill (excluding passiv slabbers of-course)? 

 

Ask me again later in the week.

[nod]

15 hours ago, epsilonGreedy said:

I met my "wise owl" building adviser on site this week to discuss the dig and concrete pour. He recommended opting for something less than a full trench fill at a typical 150mm below ground level and instead advised pouring the concrete to 300m below ground level. This he said will provide more flexibility with fine tuning the finished level of the foundation blockwork and also simplify routing services through the foundations.

 

The building control chappie then joined us for a meet & greet and said he knew the ground conditions well enough at my plot to indicate a standard 250mm pour at 1m deep would suffice. In summary I had two wise gentlemen, with over 90 years in house building and building control between them, saying 850mm of trench fill is OTT for my plot.

 

If I follow their combined advice my concrete will be 700mm deep and 700mm wide.

 

Has any forum member ended up with foundation concrete somewhere between 250mm deep and full trenchfill (excluding passiv slabbers of-course)? 

I had the same decision

But cane to the conclusion that it was cheaper to fill up with concrete than the exstra labour and block 

[Russell griffiths]

My personal opinion is that there really isn’t any other option I would do

trench fill, unless you dislike your bricklayers. 

[Russell griffiths]

19 hours ago, epsilonGreedy said:

 

 

 

 

At this time of year flooding on top of the set concrete 150 or 300mm below ground is not my current concern, should it be?

 

Flooding from ground water may not happen, but what about rain. 

A deep trench with 250mm of concrete in the bottom will fill up like a bath tub if you get a good down pour. 

Shallow trench can be swept out with a broom and bailed out at a low spot. 

Edited June 24, 2018 by Russell griffiths

[epsilonGreedy]

On 24/06/2018 at 12:15, Russell griffiths said:

 

Flooding from ground water may not happen, but what about rain. 

A deep trench with 250mm of concrete in the bottom will fill up like a bath tub if you get a good down pour. 

Shallow trench can be swept out with a broom and bailed out at a low spot. 

 

Sure, though as mentioned above I am not contemplating the complete opposite to trenchfill. My remaining choice falls within the range of the concrete finishing 300mm or 150mm below ground level.

 

A week ago I was not aware that, subject to engineering constraints, the concrete thickness could be 250mm or 1000mm (trenchfill) or any point between given a 1m deep trench. 

[Russell griffiths]

1 hour ago, epsilonGreedy said:

 

Sure, though as mentioned above I am not contemplating the complete opposite to trenchfill. My remaining choice falls within the range of the concrete finishing 300mm or 150mm below ground level.

 

A week ago I was not aware that, subject to engineering constraints, the concrete thickness could be 250mm or 1000mm (trenchfill) or any point between given a 1m deep trench. 

Ok 300mm it is then

sorted. ??

[Big Neil]

On 24/06/2018 at 12:12, Russell griffiths said:

My personal opinion is that there really isn’t any other option I would do

trench fill, unless you dislike your bricklayers. 

 I understand principally the different Materials used here, but what is the infill from poured concrete to ground level ?

[Temp]

Concrete. See.. .

http://www.nhbc.co.uk/Builders/Technicaladviceandsupport/TechnicalGuidance/61/filedownload,37205,en.pdf

 

the concrete in a cavity wall should stop at least 225mm below the base dpc

Edited August 22, 2018 by Temp

[epsilonGreedy]

5 hours ago, Temp said:

Concrete. See.. .

http://www.nhbc.co.uk/Builders/Technicaladviceandsupport/TechnicalGuidance/61/filedownload,37205,en.pdf

 

 

What is the recipe for such concrete fill? I have seen references to a lean mix and semi dry.