Foundation depths and the reduced dig with beam & block floor

[sean1933]

Hi all,

 

I've posted a couple of times here and keep meaning to put up an introduction post with plans/photos etc and haven't got around to it yet as so busy - will sort soon I promise!

 

W'ere using one of the insulated B&B systems (probably jablite) as we're on a wealden clay site. I'm just starting to carry out the reduced dig and am trying to understand the best approach to the depth. Our SE plans (following site soil test) say a minimum depth of 1.2m from GL but to the bottom of B&B void is 450mm (best case as site is sloping), below that being trench fill. If I reduce dig on the entire site to bottom of the void then  the foundation dig will only be c.750mm which for clay doesnt look that deep. My question is will BC look to measure from original GL (which will be reinstated externally) or the reduced dig level? Or would I be wiser to just scrap topsoil off, trench fill to the required level then dig out the void afterwards (which would no doubt be an arse and very messy). I looked at the NHBC tech document for guidance but its a bit flaky and suggests either could be an option depending on how long soil is left to dry out, but still unclear in my eyes.

 

Appreciate a newbie question but I have heard stories of BC looking in the hole and telling people to go deeper despite firm grounding @ the base. Last thing I want is to have to dig another 450mm and trenchfill because I have got over zelous on the reduced dig!

 

thanks in advance!

[Oz07]

You can leave the perimeter at existing ground level and demonstrate your 1m depth off this. Providing you've scraped all the veg off. 

They don't expect you to take foundation level from reduced dig level

[Oz07]

[nod]

7 hours ago, sean1933 said:

Hi all,

 

I've posted a couple of times here and keep meaning to put up an introduction post with plans/photos etc and haven't got around to it yet as so busy - will sort soon I promise!

 

W'ere using one of the insulated B&B systems (probably jablite) as we're on a wealden clay site. I'm just starting to carry out the reduced dig and am trying to understand the best approach to the depth. Our SE plans (following site soil test) say a minimum depth of 1.2m from GL but to the bottom of B&B void is 450mm (best case as site is sloping), below that being trench fill. If I reduce dig on the entire site to bottom of the void then  the foundation dig will only be c.750mm which for clay doesnt look that deep. My question is will BC look to measure from original GL (which will be reinstated externally) or the reduced dig level? Or would I be wiser to just scrap topsoil off, trench fill to the required level then dig out the void afterwards (which would no doubt be an arse and very messy). I looked at the NHBC tech document for guidance but its a bit flaky and suggests either could be an option depending on how long soil is left to dry out, but still unclear in my eyes.

 

Appreciate a newbie question but I have heard stories of BC looking in the hole and telling people to go deeper despite firm grounding @ the base. Last thing I want is to have to dig another 450mm and trenchfill because I have got over zelous on the reduced dig!

 

thanks in advance!

Simalar ground conditions to ours 

We initially scraped the site Most trenches where full filled at 1.8 

By the time we had added drainage under the BB floor There wasn’t much of a void  

[Mr Punter]

Be very wary of the proximity of trees with the clay soil. Even if they are small, Building Control will expect you to calculate and design the foundation on the basis of their fully grown height.

[sean1933]

Thanks for all the replies everyone i appreciate it. Can i just check the measure from depth as looking at Oz07 picture has confused me a little. I have stripped the topsoil off c.1m around where the foundations which because of the site (overgrown wildflower/deep weeds)  has removed some 200mm (to be reinstated afterwards). Should i be measuring from this new 200mm depth or the original ground level - which will be used as reference for GL?

 

I wouldnt normally be too fussed but its a big house with lots of sleeper walls and wide (600 & 750's) foundations so at £125m3 theres a lot of money that could be wasted here!

 

Thanks in advance. As always the helpful responses are greatly appreciated.

[Russell griffiths]

What will happen @sean1933 if you dig to your stated depth and bc come around and want another 200mm how will you get a machine around it, I would get bc involved straight away and get a site visit the first day you start digging. 

 

Many years ago I had an extension passed at 1.8m, I decided to dig to 2m for safety sake, when bc came around 2 days later he wanted another 300mm down, I had to hand dig it. 

 

I would check they are happy before you start  splitting hairs over a few mm here and there. 

[Oz07]

As above always get a visit morning of first day digging. If you strip the whole site have the conversation with bco early doors on phone see what he wants. I've left the top layer of soil in that pic as it's not going to be paved or anything. Always easier working if you strip the lot off. 

[Temp]

Did your SE say anything about needing to line the foundation trenches with foam board? We had to do that on clay.

[sean1933]

Thanks everyone I'll speak with the building inspector tomorrow about site strip and if he wants me to take photos and mark GL before I strip back - SE suggested just using a peg at lowest point (site slopes a bit) marked for GL and take a few photos.

 

On 20/08/2019 at 20:43, Oz07 said:

As above always get a visit morning of first day digging. If you strip the whole site have the conversation with bco early doors on phone see what he wants. I've left the top layer of soil in that pic as it's not going to be paved or anything. Always easier working if you strip the lot off. 

 

On 20/08/2019 at 20:29, Russell griffiths said:

What will happen @sean1933 if you dig to your stated depth and bc come around and want another 200mm how will you get a machine around it, I would get bc involved straight away and get a site visit the first day you start digging. 

 

Many years ago I had an extension passed at 1.8m, I decided to dig to 2m for safety sake, when bc came around 2 days later he wanted another 300mm down, I had to hand dig it. 

 

I would check they are happy before you start  splitting hairs over a few mm here and there. 

Great point thanks both, I'll also try and get BC to visit on the morning of the dig - its a big house with B&B floor (so sleeper walls too) so want to avoid double digging if I can avoid it. One saving grace is the site is big and open (2.2acres) so no problem getting around it. I dug a 1.6m pit the other day to bury some rubble and the ground clay was absolutely solid at 500mm (as per our 6 bore hole soil test) so I'm hopeful the 1.2m our SE set as minimum dig depth may be okay (subject to a few old removed trees which may well have routes in situ so require a deeper stepped section.

 

On 21/08/2019 at 17:18, Temp said:

Did your SE say anything about needing to line the foundation trenches with foam board? We had to do that on clay.

 

Yes our SE said we need clayboard for any trench fill foundation above the bottom 500mm. That being said he's confirmed its only required in virgin clay and not where there's any blockwork back filled foundation. Given we are building a 200mm cavity Brick/Block structure our trench fill is 750mm wide around the perimeter and 600mm on internal sleepers - annoyingly the SE has spec'd clayboard not only on the inside of the external walls but both sides of every internal sleeper wall so I'm tempted to just fill to 500mm and do away with the hassle/cost of claymaster and extra cost of trenchfill...but I need to do a quick cost/benefit analysis before hand. What I hate is the idea of either pre-buying a load of claymaster based on a 1.2m trenchfill only to be told by BC to dig deeper and needing to source more (it's notably cheaper direct from jablite), or overbuying from day 1 based on a 'worst case' depth. I'm still struggling with the concept of wasted materials as my profession is all about the detail, but I'll have to get over that or I'll drive myself nuts!

 

On 18/08/2019 at 09:32, nod said:

Simalar ground conditions to ours 

We initially scraped the site Most trenches where full filled at 1.8 

By the time we had added drainage under the BB floor There wasn’t much of a void  

 

Thanks Nod - looks very similar to what I envisage for our build. Am I right in thinking you had to put sump drains in or just a weak concrete top with a fall to deal with any potential under floor flooding... or did I make that up from a similar post? Reason I ask is our architect talked about putting in sump drains under the B&B but then helpfully put it in the spec to BC but left it as something to be discussed rather than pre-agreed at plan check stage...I'm not convinced its necessary as whilst site doesnt drain well due to heavy clay soil its not in a flood area and water table shouldnt be an issue. Also I was just going to put a DPM down with a bit of hardcore to stop any growth/smell but again architect has spec'd concrete (but without any real detail) which was a surprise to me when reading the approval! Another laborious expense I hadn't planned for!

[nod]

Hi

I put a very simple drain in (no sump) 

I simply laid a week mix concrete falling to one point

With a 90 degree bend flush with the concrete I added a anti flood valve one metre outside the property 

and a inspection chamber 

It did feel like overkill 

But something that couldn’t be done retrospectively