Previous strip foundation needs refilling in prep for a Slab on grade

[SHughesNI]

Any / all advice would be welcomed.
Ok a bit of background. Bought a small bit of land few years back in the hope of building an eco house. Unfortunately we had watched too many episodes of 'How to build a house for under £100k' and actually thought it could be done. Then Covid kicked in. I lived in a caravan for a year. Found electricity cable on the land (not in plans, drawings and NIE did not know it existed - they turned up one day cut the cable and knocked on the nearest neighbour 200 yards away asking 'Is your electricity on?) as digging and when NIE finally removed it 9 months later prices had skyrocketed. So we built a log cabin instead and decided to build the house as and when we have the money. There are 5 of us living in the log cabin. It's a bit tight to say the least but so far so good.
So we have the money we think for foundations but can't find a builder. Been let down 3 times already. All the best builders are booked up to at least 6 months.
Plans are for an ICF house. When we bought the land there was a strip foundation in place. Over the last few years of building the garage and cabin topsoil, car tyres (another story) and all manner of debris has fallen in so spent the last few days digging it all out hence the pile of earth in the background. See photos. 
Question - Going for a slab on grade foundation. What do I fill up the space with? Gravel? Better soil such as loam if I can even buy that? Sand and gravel? Clean stone? the soil excavated is clay and very porous in these parts hence the need for a raft foundation. I initially thought I'd be able to refill the excavated clay soil with a more stable soil such as loam, then 6 inch crushed stone/ gravel followed by rigid foam panels EPS, vapour barrier then the concrete. 
Apologies for sounding amateurish. 

 

[Russell griffiths]

Who designed the insulated slab ?

they will tell you what it needs to sit on, is the new house going to be the shape of those old footings ?

you will need to remove all that soil for the entire footprint of the house plus about a metre in all directions, then some form of crushed stone heavily compacted in layers until you are up to the level you need, no loam no clay. 

 

All down to the designer and any warranty company

[Thorfun]

I agree with @Russell griffiths. get the insulated slab designed by an SE and then proceed from there. depending on what the SE comes up with will determine what you do for the foundations.

 

I can recommend Tanner Structural Design for insulated slab design.

 

 

[SHughesNI]

Hi Russell & Thorfun
No design for the insulated slab. the house is ICF. The only builder with any experience in this field in the whole country has been away to England for the last 2 months. There is a forum on Facebuild selfbuilders NI and no one has stepped up to recommend anyone else. ICF is still seen as a mythical beast and even the banks here won't offer a mortgage as it's not been proven over here yet as a reliable building product. Mad but true. 
My SE was ill for 3 months and has a huge backlog. I did ask him the same question but hasn't responded to my last 2 emails hence why I came here. For the moment I just want to fill the void and thought it might be an easy answer to the experts on here. I have 3 young children and just don't want them falling into the void. As we have to walk 50 yards to the outside toilet this is a distinct possibilty 🤫
the attached was pretty much a summary of what I was hoping to do. 
Thanks for your advice though.

Edited October 20, 2022 by SHughesNI

[Carrerahill]

5 hours ago, SHughesNI said:

Any / all advice would be welcomed.
Ok a bit of background. Bought a small bit of land few years back in the hope of building an eco house. Unfortunately we had watched too many episodes of 'How to build a house for under £100k' and actually thought it could be done. Then Covid kicked in. I lived in a caravan for a year. Found electricity cable on the land (not in plans, drawings and NIE did not know it existed - they turned up one day cut the cable and knocked on the nearest neighbour 200 yards away asking 'Is your electricity on?) as digging and when NIE finally removed it 9 months later prices had skyrocketed. So we built a log cabin instead and decided to build the house as and when we have the money. There are 5 of us living in the log cabin. It's a bit tight to say the least but so far so good.
So we have the money we think for foundations but can't find a builder. Been let down 3 times already. All the best builders are booked up to at least 6 months.
Plans are for an ICF house. When we bought the land there was a strip foundation in place. Over the last few years of building the garage and cabin topsoil, car tyres (another story) and all manner of debris has fallen in so spent the last few days digging it all out hence the pile of earth in the background. See photos. 
Question - Going for a slab on grade foundation. What do I fill up the space with? Gravel? Better soil such as loam if I can even buy that? Sand and gravel? Clean stone? the soil excavated is clay and very porous in these parts hence the need for a raft foundation. I initially thought I'd be able to refill the excavated clay soil with a more stable soil such as loam, then 6 inch crushed stone/ gravel followed by rigid foam panels EPS, vapour barrier then the concrete. 
Apologies for sounding amateurish. 

 

I think I would be clearing your site of all top soil, ideally it will land about the height of the bottom of your old strip founds, then a base of hardcore compacted in 100mm rises to get heights then slab, however, SE will need to advise, it is just what I "think" might happen. I cannot see a slab would be going straight on top of that even if the trenches were not there. 

 

When we did our slab, we had to clear down to the clay - in our case about 24".

[Thorfun]

I presume you have planning permission? do you have building regs drawings? BCO will need (or at least did in my case) to see the SE calculations for the building. I suggest you spend the next 2 weeks trawling this forum on every possible subject and talk to the existing NI members who have done it all. if you can't get a mortgage on ICF then, rather than fighting the system, maybe pick a different build method?

 

I can empathise with your need to get this going but maybe just take a step back and get some solid plans in place before digging more stuff up!

[SHughesNI]

Thanks for reply. Yes planning was given over 2 years ago. SE calcs done though by you saying that  I am assuming you think the plans should include what foundation type?   I have added the plan for you to have a look. No way it is not going to be ICF. As I said we built a log cabin ourselves, no help from anyone except You Tube videos. Fairly competent DIYer, father was a builder and first job after leaving school was working on a building site (just 6 months though) so have some background in the trade. 
I know the easiest route is find a SE but like I said  the decent ones just like builders are unavailable. So far not tended to go the traditional route. Car tyres under the driveway. Car tyres, concrete piers as the foundation for the log cabin. Did the roofing, insulation, electrics, tiling, made the lights, everything myself and so far so good. Living off solar and propane- not connected to grid at all. 
As you can see from the screenshot of my bookmarks I have researched for the last 2 years - Just thought there might be some bright spark on here who would emphatically say YOU NEED THIS 🙃

[Gus Potter]

11 hours ago, SHughesNI said:

Apologies for sounding amateurish. 

Not at all.

 

I like your approach and attitude to life. I did something similar a "while ago".. bought a field, build the garage and fitted a shower and second hand kitchen in it, bought a 14"caravan that we slept in so the planners could not kick us out. Then worked away building the house.

 

Hope this helps.

 

My thoughts from an SE point of view and looking at your photo are:

 

Winter is coming so leave the soil dumplings between the strip founds where they are until you are ready. What we are trying to do here is not to let the frost into the ground and soften it. Let it flood as the water acts as an insulator. If a really hard winter is predicted (-18 C for two or more days) then chuck some stuff back over the strip founds say 200 -250 mm of soil.. this will give you a cover depth to the underside of the strip founds of about 450mm.. which is what BC recommend.

 

Design wise I would say.. what is the finished floor level of the house to be then work down from there and see what solutions are viable.

 

Ground bearing slabs are not that hard to design.. if you know what to look for and most folk don't so that is where the SE comes in. Why would they.. most folk have plenty other / better things to do.

 

From an SE point of view most low rise (domestic structures) slabs (could be ICF) are designed on the basis that we have a number of elastic layers; the concrete slab, the insulation say and the ground below. Think of this as layers of a cake. Each layer working progressivly deeper has different properties.

 

Now we also know that the ground is not uniform horizontally. In your case we have some strip founds (hard spots) intersperced with softer areas.. the dumplings. So now we have a 3D model!

 

One design approach in your case is to ask..  what do we have under the strip found .. is there a uniform layer of say clay 3 -4 m down or do we have a number of different layers. We then look at for example how "elastic" and thick these layers under the founds you have are and how much they are likely to settle when we put a building on top. Next we look at what you are putting on top of the soil. Say some compacted hard core, then say EPS and a concrete slab.

 

We then look at the most elastic layer.. which usually is the EPS and design for that. On a large project we may develop a more comprehensive model.. but that cost thousands as you need to use expensive software, have folk that know how to use it properly and a good comprehensive compatibale ground investigation that also cost a lot.. no point in trying to race an F1 car if the fuel is made by me in my garden shed.

 

But we also look at the number of the soil layers and recognise that horizontally they will not be uniform so depending on the soil and number of layers we also design for the fact that there may be some soft spots under the EPS and we reinforce the concrete slab to span over these soft spots.

 

That's a bit of one design approach.

 

Your best bet here may to try and get your hands on some recycled aggregate.. some 6F2 lay that and roll it in with a 8 tonne roller. What you are looking to do here is pinch stuff from road design. Before you do that try and find a local SE that will come by the site and expand on what I have written. I'll not go into any great depth here but you can use a loaded concrete lorry to do a rolling proof test! Yes it's old school stuff but it works! Leave the strip found where they are as although they are a  potential "hard spot" the EPS ect will easily mitigate.

 

I would love to work with you but you need someone local who will be on tap you all the way through your journey.

 

Hope this helps.

 

Keep us posted as interested in your project.

 

All the best.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SHughesNI]

On 20/10/2022 at 22:57, Gus Potter said:

 

Nuts - Had whole reply did a backspace and poof it's gone 😲

Short version - Many many thanks Gus for replying and with a real effort to guide me in the right direction. Apologies for not replying sooner but life got in the way.

yes you are right hardcore, EPS and then raft. There are some great articles by ecohome which talk about Insulated Raft Foundations and even frost protected versions. One that I was following here https://www.ecohome.net/guides/2264/stage-one-of-the-ecohome-demo-house-slab-on-grade-construction/
This of course is on standard soil (whatever that is?) and not clay else they would have been more specific. This is the route we intend to go except we are going to use car tyres with sidewalls cut out as we did in our driveway. A process called mechanical concrete article here https://www.buildwithrise.com/stories/mechanical-concrete-for-home-foundations
You might think we are nuts but have had some very positive feedback from trucks on our driveway some saying they felt like the truck was bouncing up th drive - possible something to do with the physics of resultant forces. Have photos if interested. 

I will take your advice and source out another SE and if you are still interested will update you of any progress 

Regards

Sean Hughes

 

On 20/10/2022 at 22:57, Gus Potter said:

 

 

 

 

 

 

 

On 20/10/2022 at 22:57, Gus Potter said:

 

 

 

[Gus Potter]

On 25/10/2022 at 11:15, SHughesNI said:

I will take your advice and source out another SE and if you are still interested will update you of any progress 

Yes keep us posted.

 

On 25/10/2022 at 11:15, SHughesNI said:

and even frost protected versions.

The frost protection is an interesting one.

 

For all. You'll often see BC /  NHBC saying you need 450mm frost cover. But why?

 

From memory if you go back to say 1936 the housing stock was pretty bad so the government set up the NHBC as a gov body. Their remit was to carry out research and develop better housing for us all. Some time later the BRE were formed as an off shoot, much later the NHBC became the insurance comapny that we all love.

 

To deliver better housing we needed some standards that "worked in most cases for mass housing". We know that in a bad winter the frost will get into the ground some 9" -12", in a really bad one maybe a bit deeper, especially if you get a cold east wind blowing at the same time.

 

Here is one theory.. we had concrete founds so we made them 6" thick ( 150mm = two courses of brick) then we wanted say a ventilated solum space and we needed to get some drains round the house but did not want the drains to be below the founds, or at least not below the underside of the founds. Folk also planted things round the outside of the house so they needed some soil depth here.

 

In summary 18" (450mm) to the underside (formation level) of the found seems like a good number.

 

Frost heave happens when the water in the soil forms ice crystals they expand with great force  and can lift a found.

 

For significant frost heave to occur you need the following to all happen / soil conditions.

 

1/ Sustained temperatures below zero deg C so the freezing gets deep enough... that is the obvious one.

2/ A continuous supply of unfrozen water to allow the ice crystals to keep growing... the water table usually.

3/ A medium that has small voids so that when the ice crystals expand they have something to push against to cause heave.

 

One way of explaining this it to look at say a sandy / silty / CLAY. Here the clay is in captial letters indicating that the dominant component is clay. For all.. you'll see this capitalisation cropping up in your ground investigation reports. Excuse the spelling / grammer (I'm left handed)

 

While a true clay is pretty water proof (we build dam cores with it) the above type of soil will probably have sand and silt lenses in it that provides the water path for the ice crystals to grow = frost heave.

 

On the other hand you could have what is called a "well graded" sand or gravel. Well graded means that the soil particles are more single size.. open textured... like a French drain aggregate. If the water table is well below (say 800- 900mm) the underside of the founds then the water can't get anywhere near the underside of the founds to freeze. If so then no frost heave other than a tiny bit where the surface water on the soil particles freezes.. and that is not a lot. You'll get much more movement than this between the house heating and cooling between winter and summer. A big fall of snow ion the roof and so on.

 

You could also technically have a house on large sized stones with big voids, if the water table is down a bit then any ice crystals will just grow into the voids and not cause significant heave.. point 3 in some ways.

 

You could also have a found on fractured weathered rock. Here if the water table is say 600mm below the ground level all tends to be fine as the rock would normally have been there since the last ice age.. your house is just a "moment" in geological time so it ain't going to heave.. unless you are doing some big rescaping of the land and you have really soft rock.. chalks require some careful thought. Yes you might get the " rock flower on the very top" swelling a bit but just scape that off.

 

I mention clay soils in terms of frost heave. Technically you could have say a really good pure clay.. say a cracking London Clay that can be used for dam cores. While it would swell a bit on freezing it would then stop as it is so waterproof that no new water can get in to make big ice crystals. Now that all sound great in terms of frost heave.. but it also shrinks like a "..." in the summer when it dries out and when there are trees and so on.. which is why we don't waste too much time on the technical argument about frost heave in most clay soils.

 

However when it comes to other types of soil, insulated rafts , ICF it is worth having a look at all the design parameters. If you find that the frost cover is driving the design then you need to look at why and the above is a starting point on where to look.

 

The last step is to gather enough site investigation info to make your case to BC and others.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

17 hours ago, Gus Potter said:

NHBC saying you need 450mm frost cover. But why?

Just a bit more to add to Gus' excellent explanation.

There is nothing magic about 450mm depth, probably originally foot and a half, but it is a good guide to how deep frost will go in the uk. Frost and freezing causes changes to the ground as explained.

If you were in an area that gets especially cold or for long periods then the frost can creep deeper. 

My own house was built from timber in the 1920s. It has 2 bricks above dpc, then 2 below on 150 concrete. So not remotely 450mm. The house moves up and down seasonally, mostly through clay shrinkage, causing doors to stick. But it is still standing, even the brick chimneys. 

I have often designed buildings with 450 or 500 deep edge strips where they are not structural. It works nicely with 2 blocks, and keeps the strip and internal slab safe from frost.

 

Hence also 450 or so depth for water pipes.

[SHughesNI]

Thank you all for responding to my query. Speial thanks to Gus Potter who went above and beyond what I asked for. Plenty of food for thought and has helped me tremendously in steering me in the right direction.