Trench fill acceptable tolerance in level and dip

[Stu84]

Greetings all

 

Trench fill happened yesterday for small side extension (3x4m footprint).  Footings are about 600mm thick and 600mm wide.   Checked levels today and there's up to 25mm deviation in height and if viewed in section there's a slight dish (up to a 10mm dip across the width of the foundations).  Also the pour is 10-35mm below the finished depth we agreed.

 

To be truthful I am upset as builders quoted to dig,rebar, box-in pipes and pour.  The digging was hard going so of course I helped dig. I boxed in the pipes and even resin-fixed all the rebar.  We've had some down pours so I've painstakingly kept the site as dry as possible with tarpaulins / pump etc.

 

I asked them to drive stakes in to ensure level all around which they argued against.  They said they'd use a laser and would get it to within 10mm and that I should trust them.  When the concrete arrived I thought best to leave them to it so they can focus.  4.5T of concrete was poured in about 10 min and I returned after about an hour to find they were packed up and about to leave.  I asked if they used the laser, they said they had.  There was a bit of floating water but not much and I covered with a tarp.

 

I'll get over the disappointment.  Just hoping it can be worked with.

 

Does the 'dip' across the width of the foundation matter?  Can this be leveled off with mortar before laying the first course of regular block?  It is definitely there (the water pools along the centre).  Should I be grinding down the sides a bit to better level?

 

Excuse long message.  Part of the healing process! 🥴

 

Thanks all.

[nod]

You can easily sort that out with your foundation blocks 

[Stu84]

On 13/09/2025 at 07:21, nod said:

You can easily sort that out with your foundation blocks 

Thanks nod, that does help.  On one stretch they managed to lose 15mm+ over 2m, impressive!  This is my first self-build so steep learning curve.  I know bed courses can be increased a bit but in the real world what is roughly deemed acceptable below ground for a standard 3:1 mix? 20mm?   Think if I had to go more I'd start looking to cut a few blocks.

 

There's an inverse camber across the surface so the mortar will be deeper on the inside than the outside otherwise blocks will want to tip in.  This is common??  At least the weak concrete mix that will go in the cavity will help blocks wanting to cave in.

 

Thanks!

 

 

[Redbeard]

Are the 'foundation-builders' taking on the rest of the build? If so it's only them  who will have the irritation of making the necessary adjustments because they have not met your stated tolerances. It'd be a different kettle of fish if they charged you extra because of their shortfall! I guess if it were me I'd be a little concerned about future works if, following assurances, the 1st bit was below par. OTOH if you'd done the footing yourself and it was a bit 'out' you'd just 'suck it up' on 'phase 2'.

[Stu84]

Thanks for thought Redbeard.  No, I'm doing the brick and block work and they knew that.  That's why I'm a bit irked.  Will be a challenge for me to sort.  I'll do it, but I put a lot of detail into the design to ensure I got off to the best.  That's been thrown away with their careless pour and I'm gong to have to play around with coursing/dolly blocks now to make it work, when before it would have been standard blocks all the way (that I'd already bought).

 

No point me ranting.  If it's normal for people to build off bad pours like this then that's some comfort.

 

Cheers

[Redbeard]

I wasn't saying it's normal, just that it's not catastrophic. If you paid for a tight job you should have got a tight job. I am not a groundworker but the last footing I set out for a client was less than 3mm out over a 6m diagonal, and was flat as a flat thing all over the top. Perhaps I got that so reasonable because I do not normally do it, and I hate it!

[Stu84]

On 13/09/2025 at 07:21, nod said:

You can easily sort that out with your foundation blocks 

 

22 hours ago, Redbeard said:

I wasn't saying it's normal, just that it's not catastrophic. If you paid for a tight job you should have got a tight job. I am not a groundworker but the last footing I set out for a client was less than 3mm out over a 6m diagonal, and was flat as a flat thing all over the top. Perhaps I got that so reasonable because I do not normally do it, and I hate it!

Thanks guys, am struggling with this one.  Dreaming of a footing with 3mm tolerance 😂  Here goes....

 

So I've taken more measurements and its one return (the rest is levelish) that slopes 20 or 15 mm (depending on whether you pick internal or external leaf) from the corner to the house over only 1.8m. 

 

On top of that it's all about 15mm below where is should be.  All considered to bring it back on track over 2 courses the beds are going to be massive (30mm).  Even using dolly blocks the first course is going to have to be about 30mm, dictated by the slope (granted its 30mm at worst tapering to about 10

 

Builder came around and couldn't really see the issue.  Said they'd level out the slope with some mortar, let it stiffen before laying another bed of mortar over that. 

 

Starting to think best thing is to have the concrete breaker in to take 150mm of concrete out up to the corner.  Hand mix and re-pour this length.  😑

 

 

[mjc55]

I really think that you are overthinking this.  

 

Let the builder sort it out, he can get it level before it gets above ground level. 

[Alan Ambrose]

I think the reality is that groundworks is a often a rough and ready process - maybe on a fine day with the right team it isn’t, but on a bad day or with a less skilled team it just is. 1/2 an inch is a good tolerance in groundworks, anything better is probably uneconomic. It is upsetting to a certain kind of person and I’m one of those. I have internal drainage out by up to 200mm in horizontal position and the same in invert level. And I suspect I have not discovered all the faults yet. I wish I hadn’t trusted my groundworks guys and looked over their shoulders more.

 

Will it result in impossible-to-fix problems - probably not. Has it cost me much extra time and effort - definitely yes. I’ve resolved to buy a total station and do my own detail layout for the ‘next build’. My surveyor is good but a total station to use to check a position or level any time I want would be better.

[Nickfromwales]

It’s unfortunate, but the moment you turn your back it’s down to the people you chose; eg that they are able to perform admirably. This is when you find out who you chose, but as said above, often then too late to rectify and you’re left holding the baby.

 

On a job atm where the admirable but defo didn’t happen, quite spectacularly, but now there’s a new team and some (quality) control and it’s been recovered quite quickly.

 

When first there it was “down tools, pick up tape measures, and get the laser out BEFORE you do anything”. 
 

Found a few extra hidden faux pas, and then some, and made sure the guys got those deleted before ploughing on. 

 

You can spend more time panicking about what’s not right, vs just inputting the energy into getting it sorted and moving to the next item.

 

@Stu84, throw a 0-10/15mm bed down and leave it to go off, then offer the blocks down in another bed, tap it all into shape, and leave to set. Then you’ll have a (level and plumb) blank canvas to work from. By the time you’re on the 3rd course you’ll have all but forgotten about this.

 

Oh, and don’t even dream about taking a beaker to this!!!!!!!   

[nod]

Try not to overthink 

 

[saveasteading]

It's OK. I've seen very much worse.

Lasers aren't always accurate and are only used approximately in trench footings.

Successive processes get more accurate but

For perspective: the published tolerances in timber buildings are plus/minus 10mm.

[saveasteading]

23 minutes ago, saveasteading said:

only used approximately in trench footings.

For future reference, I tended to either bang pegs in the bottom and mark depth with a nail. In clay, knock nails or bars in the sides at finished level.

Very formally for precision ( big structures) we would have T profiles along the side at say 2m above finished level and a 'traveller' a T that we placed on the concrete and lined up with the profiles by eye. A laser can replace this if used skilfully. Big IF.  Hence the nails.

 

[Stu84]

2 hours ago, nod said:

Try not to overthink 

 

Thank you all for rallying and offering ideas and support.  Yes I am prone to overthinking.  I realise often much more happens below ground, but I wasn't quite happy with doubling Eurocode's 15mm max!

 

3 hours ago, Nickfromwales said:

It’s unfortunate, but the moment you turn your back it’s down to the people you chose; eg that they are able to perform admirably. This is when you find out who you chose, but as said above, often then too late to rectify and you’re left holding the baby.

 

On a job atm where the admirable but defo didn’t happen, quite spectacularly, but now there’s a new team and some (quality) control and it’s been recovered quite quickly.

 

When first there it was “down tools, pick up tape measures, and get the laser out BEFORE you do anything”. 
 

Found a few extra hidden faux pas, and then some, and made sure the guys got those deleted before ploughing on. 

 

You can spend more time panicking about what’s not right, vs just inputting the energy into getting it sorted and moving to the next item.

 

@Stu84, throw a 0-10/15mm bed down and leave it to go off, then offer the blocks down in another bed, tap it all into shape, and leave to set. Then you’ll have a (level and plumb) blank canvas to work from. By the time you’re on the 3rd course you’ll have all but forgotten about this.

 

Oh, and don’t even dream about taking a beaker to this!!!!!!!   

 

This is probably what we will do.  Throw down a 0-10/15 bed let it go off and treat that as the building surface.

 

One alternative idea is to trowel a bag of Structural, non-shrink grout over the area.  Could this be a forgotten panacea for low spots such as this?  It's cement and sand based, can be poured to as little as 10mm, can take high loads and can be used externally.  Just have to scabble the surface a few mm for a better key.  I suppose this is basically the same as smearing mortar but perhaps a bit more substantial?

 

Was looking at something like this.

 

https://gbr.sika.com/en/construction/concrete-repair/grouts/cementitious-grouts/concrete-repair-andnewconstructiongrouts/hand-applied-grouts/sikagrout-111-gp.html

 

Thanks every one.  Starting to feel better about it.🤗

 

[saveasteading]

24 minutes ago, saveasteading said:

bars in the sides at finished level.

The point being that concrete is hard work, the workers are busy with what they do best,  and not esp skilled with lasers etc.

Ie do the precision at leisure.

 

10mm either way is absolutely fine though and better low than high. 15mm is probably typical of decent work.

[saveasteading]

1 hour ago, Stu84 said:

Starting to feel better about it.

Feel even better about it, as you are still overthinking. 

What comes next? Blockwork? As @nodsays, a bricklayer (you in this case) will use a string line and smooth that line out, and up. 

Relax, work out what level your first block goes at, and move on. 

If there is a high spot, you could regard that as your new datum and lift the building a few mm.

[Gus Potter]

On 12/09/2025 at 22:22, Stu84 said:

Greetings all

 

Trench fill happened yesterday for small side extension (3x4m footprint).  Footings are about 600mm thick and 600mm wide.   Checked levels today and there's up to 25mm deviation in height and if viewed in section there's a slight dish (up to a 10mm dip across the width of the foundations).  Also the pour is 10-35mm below the finished depth we agreed.

 

To be truthful I am upset as builders quoted to dig,rebar, box-in pipes and pour.  The digging was hard going so of course I helped dig. I boxed in the pipes and even resin-fixed all the rebar.  We've had some down pours so I've painstakingly kept the site as dry as possible with tarpaulins / pump etc.

 

I asked them to drive stakes in to ensure level all around which they argued against.  They said they'd use a laser and would get it to within 10mm and that I should trust them.  When the concrete arrived I thought best to leave them to it so they can focus.  4.5T of concrete was poured in about 10 min and I returned after about an hour to find they were packed up and about to leave.  I asked if they used the laser, they said they had.  There was a bit of floating water but not much and I covered with a tarp.

 

I'll get over the disappointment.  Just hoping it can be worked with.

 

Does the 'dip' across the width of the foundation matter?  Can this be leveled off with mortar before laying the first course of regular block?  It is definitely there (the water pools along the centre).  Should I be grinding down the sides a bit to better level?

 

Excuse long message.  Part of the healing process! 🥴

 

Thanks all.

Hiya. 

 

I get @Stu84's disappointment. But as others have said, and which I design for, is to be able to level things out with the masonry underbuilding.  You can often go for a 20mm thick mortar bed with no detrimental effect. Thicker mortar beds can be ok but you thicken the wall base on the first couple of courses.. it's still economic. 

 

A few SE / Geotek comments for all. 

 

Trench fill is a rough business.

 

Often we need to do it in sections, dig a hole and get the concrete in rapidly to stop the sides of the excavation falling in and, in sensitive clays, stop it from getting compromised once you expose it to air and let the pore water pressure change.  There is no time to work to 10mm tolerances.. it's like trying to polish a jobby (shite) , what is under the skin is the vital thing.  Pore water pressure? Some clay soils are quite happy to bear load  until you change the water pressure, by digging a hole and disturbing them. Sometimes digging a deeper hole is counter productive. 

 

Now there are lots of photos on BH where folk are doing trench fill. very badly..  They worry and sometimes try and shutter the sides of the founds, the ground water is rising and it all turns into a mess. Think.. if we need to excavate for a rear extension and doing trench fill.. how do we get rid of the extra muck in a hurry?

 

As an SE often what I want to see is you leaving the sides of the trench rough as we get a tacit beneficial key up the sides. Occasionally, if in made ground that may settle I'll say, drape some plasic down the sides so the settlement does not "drag down the founds", this is standard procedure in pile design. This sounds rough but it works and in the heat of battle on site when the weather is bad.. it's a pragmatic option. 

 

 

On 12/09/2025 at 22:22, Stu84 said:

I'll get over the disappointment.  Just hoping it can be worked with.

 

Now can I ask. Just say you had added the cost of a site visit to your SE design brief? Say that would top out at £500 quid.  Usually that will be about  £200 as for the SE what they charge as a headline rate  ( graduate Engineer cost at "£90.00 an hour)  you get back in recommendations.. it's good for business to provide "after sales service".