What determines foundation cost?

[kiwibloke]

Hi all,

How does building construction type ( brick, block work, timber frame etc) affect the foundation cost?

I assume that building construction types with higher mass (including walls, floors and roof) will require higher cost foundations. So for example a 100 square metre building area with brick construction would require a higher cost foundation than timber frame construction.

 

My second question is what dominates the foundation cost, is it the ground geology or the building construction type?

 

For the sake of argument If I am free to choose the geology and construction type what would be the best geology and construction type to minimise foundation cost?

 

Thanks

James

[nod]

First 

TF Or traditional The foundations will be the same 

Your SE will determine the depth Governed by ground condition and what’s nearby Trees hedges etc

[saveasteading]

If only we could change the geology of our sites.

 

It is a serious point though and the geology should affect the value of the site.

If the ground is fill or otherwise loose then the cost will be high.

At the other extreme is solid rock, and that too has a cost due to extensive breaking out.

Clay is not too bad but is affected by trees, so is sometimes a deep dig and lots of concrete.

 

The perfect site, which is what you asked, might  be dense sand and gravel...digs easily but is strong and free draining.

 

Agreed with nod, For  a normal house, the choice of construction will make little difference. Masonry is heavy but that is not the main concern.

[markc]

Flat level bedrock around 600mm  below the surface would be a be a foundation dream, but would then be a nightmare for drainage/inspection chambers etc.

[Mr Punter]

We have a lot of chalk around here which is normally easy to dig, drains well, not shrinkable.  We just need to be aware of are sink holes and other dissolution features.  @saveasteading's gravel sounds good.

[saveasteading]

Chalk is easy too. Rock for foundations but cheese for digging. But it is horrible across the site in winter as it turns to toothpaste.

Sink holes is a big issue as you say. Using swales rather than soakaways, and any soakaways spread over large areas can help.

[nod]

Our first build was on sand and clay We have a sandpit quarry across the road 

But we have hundreds of very tall trees to three sides 

All the trench foundations had to have 1800 mil of concrete and clay board Garages to 95 cube of concrete used 

 

Our next two are on perfect ground 

Flat Boulder clay with 100 mil of topsoil 

Perfect Well nearly Four small trees nearby 

SE has to assume that these will become big trees So rather than a cheap Ground baring slab I will have to have BB and the footings a little deeper at that side 

Still pretty good because the rest are very shallow 

Edited September 16, 2021 by nod

[ProDave]

Ours was "sandy clay" so no real problem for strip foundations. Our only issue was there was a LOT of organic top soil, which had to be removed over the whole house area, and then the strip foundations dug from there.

 

We think the excess top soil was a previous owner 40 years ago levelling what was a sloping site before getting planning.

 

Unless you have local knowledge, you are never going to know without digging test holes.

[Bitpipe]

We have clay on gravel on chalk - typical of thames valley.

 

When we excavated the basement (3.5m down, 11m square) it was interesting to see the strata - they were far from horizontal and undulated widely with one corner being almost all clay and the other exposing chalk.

 

Some kind of ground investigation is helpful ahead of excavation of footings, even if you're considering a slab style foundation. This often starts as desk investigation, the BGS maintains a free database of local boreholes.

[kiwibloke]

Thanks to everyone for sharing your experience and knowledge.

Several people have mentioned digging test bore holes. Who would analyse the results of the bore hole, is this a geotechnical engineer or structural engineer job?

What would be the cost of digging and analysing bore holes?

Thanks again.

James

[Redoctober]

It is my understanding that a Geo-technician would carry out the tests for you - they can provide details of the soil make up, which in turn will / could determine what type of foundations you require. They can also do percolation tests on the soil - [ although, this can be done by the individual ].

As for costs, well it can be dependent upon which area you are in - but to give you an idea I would say around the £1500 mark.

 

There are lots of useful Blogs and threads on this forum. Spend some time trawling through them as well as hammering the internet - I also found reading various magazines useful, especially as I was starting from such a low base knowledge wise.

[IanR]

I'll add a mention for an Insulated Raft foundation. Most "manufacturer's" of these will claim they are cost effective compared to traditional foundations due to the shallower dig, less concrete used and no need for a separate screed. I can't vouch for those claims as I never costed a traditional foundation, but went this route for my build due to the higher thermal performance.

 

They are suitable for most ground conditions, down to quite low bearing capacity.

An insulated raft for a timber structure is the simplest (cheapest) of the options, and gives the best opportunity for removing all floor to ground cold bridges.

 

Having a masonry outer skin, or ICF build adds a little complication and cost to an insulated raft due to the likely need for a second, separated ring beam around the periphery of the raft to take the extra weight.

Edited September 17, 2021 by IanR

[saveasteading]

It depends on the site, and if the building is simple or complex.

If it a standard area with buildings already around, then it is likely that a local SE will be 90% sure of what to expect in the  ground. They may only want to witness a hole being dug  to a metre or so, to confirm their expectations.

For no cost you can chat to a local Engineer and see if they agree, and what the cost would be.

 

For an unknown or known problematic location, then deeper boreholes might be required. this is usually a primitive process with a tripod and dropping weight. once there, they can do several for the same sum. These can go quite deep if necessary and again the Engineer will advise what is wanted. 

 

Tests on the samples can be carried out in a lab, but that is often unnecessary.

[Conor]

1 hour ago, kiwibloke said:

Thanks to everyone for sharing your experience and knowledge.

Several people have mentioned digging test bore holes. Who would analyse the results of the bore hole, is this a geotechnical engineer or structural engineer job?

What would be the cost of digging and analysing bore holes?

Thanks again.

James

It's often as simple as a mini digger digging a couple small holes to about 2m deep and your structural engineer sticking their head in and grabbing a handful of soil. We did ours when we were laying our temporary electric supply, digger was already there so only had to pay the engineer her £250 site visit fee. We are fortunate to be on a high modulus sandstone based red clay. As we were digging down the basement, we came across a layer of river gravel... Fortunately it was quite thin and our target depth was just below. If it had been thicker, would have cost us thousands.

Edited September 17, 2021 by Conor

[saveasteading]

On 17/09/2021 at 12:46, Conor said:

we came across a layer of river gravel... Fortunately it was quite thin and our target depth was just below

 

Interesting, but I am wondering why was the gravel not a good place to stop, as is likely to be dense? If it was a thick layer it would have been a very good place to stop digging.

[epsilonGreedy]

On 16/09/2021 at 12:39, kiwibloke said:

Hi all,

How does building construction type ( brick, block work, timber frame etc) affect the foundation cost?

 

 

Beyond things you cannot control like geology, trees, heavy plant access, historic discoveries and gradients, then I reckon the greatest variable is your attitude to risk.

 

Many self builders try to insure their foundations by spending money on test digs, soil samples, engineer designed foundations, rebar and slip sheets. I had an easy site where I did the setting out, the trenches cost £500 to dig minus the garage and the concrete pour at 600mm x 600mm required about 38 m3 at £84 a cube. One semi retired builder recovering from a knee operation helped me on concrete pour day. Job done.

 

 

[ProDave]

Someone bought a wooden log cabin at the top of our road 15 years ago to knock down and build a new house.  He never questioned why it sat on a prominent high point, and never thought that it's name ".... mill" referred to the former saw mill that used to be on the site.

 

When he started digging he found the prominent mound it was on was a large pile of sawdust covered with a thin layer of soil.  I would have thought he would have piled it, but no he dug down and down and down until he hit solid ground to the point he could have installed a basement.

 

A test pit or 2 with a mini digger would soon have found that.

[Pocster]

Solid virgin rock . But what else would you want for an underground nuclear bunker 

[Conor]

2 hours ago, saveasteading said:

 

Interesting, but I am wondering why was the gravel not a good place to stop, as is likely to be dense? If it was a thick layer it would have been a very good place to stop digging.

Then our front door would have been nearly a metre above ground level, and our basement wouldn't have been very basementy.

[dpmiller]

good word that, basementy.  You should Copyright it @Conor

[Gus Potter]

On 16/09/2021 at 12:39, kiwibloke said:

Hi all,

How does building construction type ( brick, block work, timber frame etc) affect the foundation cost?

I assume that building construction types with higher mass (including walls, floors and roof) will require higher cost foundations. So for example a 100 square metre building area with brick construction would require a higher cost foundation than timber frame construction.

 

My second question is what dominates the foundation cost, is it the ground geology or the building construction type?

 

For the sake of argument If I am free to choose the geology and construction type what would be the best geology and construction type to minimise foundation cost?

 

Thanks

James

Hello kiwibloke.

 

"How does building construction type ( brick, block work, timber frame etc) affect the foundation cost?"

 

Not that much on standard strip foundations. To provide some context. A typical two storey house with well propotioned load bearing internal wall loads a strip found by about 40 - 50 kN/m, a bit less if TF. That is about 4 -5 tonnes per metre run of found. A common allowable soil bearing capacity, say a consolidated clay (not soft clays) is 100kN/m^2 .. 10 tonnes per square metre. Thus a 600mm wide strip found can carry 100 kN * 0.6 = 60 kN per metre run = 6 tonnes per meter run so gas in the tank there.. Practically you often need a 600mm wide found to fit the wall on anyway and to give the brickie / ground worker a bit of tolerance.

 

"My second question is what dominates the foundation cost, is it the ground geology or the building construction type?"

 

For most domestic stuff it's the "geology" that tends to govern. Type of generic soil type ; sands / clays/ rock, chalks filled ground. Next you drive down; water levels, proximity of trees, ground gas potential and so on.

 

"For the sake of argument If I am free to choose the geology and construction type what would be the best geology and construction type to minimise foundation cost?"

 

Kiwi.. is this the Xmas wish list?

 

Worked on a job a few years back. Top soil about 250/ 350mm thick from memory, great top soil all saved for the garden. A sub soil, clay / sand firm but easily dug and this was used to regrade the landscape. Then about three feet down (900mm) fractured weathered rock. This rock had not seem the light of day since the last mini ice age. Although fractured was great to support a house found. The site was slightly sloping and the rock was sufficiently fractured that we got good soakaway results for the septic tank and rain water run off. We excavated out the big hole for the septic tank with a JCB 3CX. We just carried on digging out the fractured rock here an there and used this as sub base for all the drive / hard standings.. saved a fortune. That was nearly perfect!

 

On 17/09/2021 at 12:46, Conor said:

It's often as simple as a mini digger digging a couple small holes to about 2m deep and your structural engineer sticking their head in and grabbing a handful of soil. We did ours when we were laying our temporary electric supply, digger was already there so only had to pay the engineer her £250 site visit fee. We are fortunate to be on a high modulus sandstone based red clay. As we were digging down the basement, we came across a layer of river gravel... Fortunately it was quite thin and our target depth was just below. If it had been thicker, would have cost us thousands.

Aye Conor you were lucky there!

 

For all. If you are new to building a house think twice before you do your investigation with a mini digger. Also, if you have say one house under your belt.. well pride comes before a fall if you think you are suddenly a ground expert.

 

It's often false economy. For a standard house set of trial pitting budget on something like a JCB 3CX for a day with a good driver. Get the SE to attend on the same day. Let the SE and the driver work together, stand back and just watch! you'll be amazed at what they can find and deduce! Make them tea and provide some buns and they will often include you in the process. It can be a day well spent and you can look back knowing that you have a good understanding of why your house "stays up".

 

Take a standard strip found at say 900mm deep. To do a proper job you want to dig down some 2.7m or until refusal to be sure. If you hit something solid with a mini digger at say 1.5 m how do you know it is not a boulder! a mini digger is like taking a knife to a gun fight!

 

A mini digger just does not have the "poke" and you end up disturbing the soil so much that the SE is left up with mince. What can often happen here is that you end up with an over designed found which will cost you a lot more than the extra expense of getting the right machine.

 

"grabbing a handful of soil"

 

Yes at times it may appear so. But there is a little more to it than that. I wonder.. you often pay your lawyer / solicitor a good fee without quibble, but balk when you need to pay say an Architect / Professional Contractor, Heating Engineer / Electrician for advice that will save you loads of money! Even though they are still all insured to the same if not a higher level!

 

 

 

[SteamyTea]

12 hours ago, Gus Potter said:

you often pay your lawyer / solicitor a good fee without quibble, but balk when you need to pay say an Architect / Professional Contractor, Heating Engineer / Electrician

That is because lawyers, accountants and medical doctors speak Latin, and we all know that means they went to a very posh private school.

Anything to do with making stuff was left to the school failures, they speak in grunts, never wash, drive a van, smoke and swear.

 

(I went to posh school, learnt Latin, and Greek, passed most of my exams, then downgraded to engineering. Made me look like the brightest in the class)

[kiwibloke]

21 hours ago, Gus Potter said:

 

Kiwi.. is this the Xmas wish list?

 

 

 

 

Thanks for your very detailed answer. No more questions on foundations for the time being ?

[Pocster]

9 hours ago, SteamyTea said:

I went to posh school, learnt Latin, and Greek

In Cornwall ? That’s not possible 

[WWilts]

On 16/09/2021 at 12:39, kiwibloke said:

If I am free to choose the geology and construction type what would be the best geology and construction type to minimise foundation cost?

Load bearing free draining brash under the topsoil, and free draining bedrock under that. Ideal for relatively shallow excavations. Shallower the required excavation, lower the foundation cost.

Minimising load bearing walls allows further economies in the foundations.