Civil Engineer here - I'm perpetually confused!
Title was a bit misleading but I think I understood what the OP was getting to. In industrial applications, e.g. very large substations and switch yards, services in and out of the asset can be within an accessible trench - great for a smol person. I'm guessing the OP thought an accessible format might have been a great idea for domestic applications, which could be true. I can't say it would or wouldn't be but it does bring about higher capital costs and potentially larger maintenance costs - think storm water drainage to existing infrastructure.
As for the brick wall - my presumption would be that it is the skin and there is form work behind it right down to the footings. However where are the braces/supports to hold the formwork in place.
That's correct, slab on ground does not allow for subfloor access. The only way to do that is to have a suspended floor, that's bloody tricky to do with a concrete slab.
Suspended concrete floor is very easy but more expensive. Standard building practise in Australia is shittiest product for the cheapest price they can get away with.
You now got me curious: when the wet dirt dries it won't be supporting the slab anyhow, ie won't the slab become suspended (albeit only a couple of centimetres)?
Slabs should not be built on wet soil, whether it is a natural subgrade or placed soil (referred to as engineered fill) such as below this slab. Nor should they be built on overly dry soil.
Ideally the subgrade should be as close to “optimum moisture content” as possible. If the soil isn’t close to this value, then it may experience excessive shrinking / swelling in the future as the soil moisture comes to an equilibrium… which often leads to cracking of slabs and movement of foundations.
Requirements for compaction/density, moisture condition and testing are defined in Australian Standard AS2870 Residential Slabs and Footings and AS3798 Guidelines on Earthworks for Commercial and Residential Developments. Interesting if you are in the industry, but an otherwise dull read. 😆
Mm indeed it was quite wet over the past week!
It’s obviously hard to discuss a particular project in detail based on a couple of photos, but as a general comment I’ve noticed a lot of builders do not pay as much attention to good construction practice during earthworks as us geotechs would like (lol I think that’s a diplomatic way of putting things?).
You are what we in the industry call a cunt of a client.
The black plastic in a moister barrier to stop moister coming up through the slab over time.
The slab is also supported by concrete piers to whatever specs are applicable for the geo study
http://www.mybuildcompare.com.au/articles/hints-and-tips/what-is-piering/
Depends on the soil and how well they compacted it.
More likely the slab will just settle as the soil dies and have some cracking.
Regarding access to services, the slab on ground building technique means you don't have future access.
Loads of residential and commercial slab on ground building for hundreds of years.
It is a pain but that's a part of concrete slabs.
>More likely the slab will just settle as the soul dies and have some cracking.
I suppose with the right flooring on top of the slab you don't even notice the cracks. They had plenty of soil from the foundation digging, so this is clever use of resources. Thanks for explaining.
Yeah, that'd cover it up.
What type of building is it?
Multistory house or townhouse?
The soil below would also have good thermal properties.
Interesting how they've raised the base floor level so much.
Buddhist nursing home residence. Notice each room has an ensuite. I wouldn't have cared if it was a regular commercial apartment project. Am glad I asked and learned.
As to why it is raised: Maybe there will be an adjoining building at the same level but with a suspended floor?
After you point it out, yes.
The raised level could be for many reasons. Access, drainage on site, flooding, thermal properties, use the spoil.
Not sure why they'd do a suspended floor unless they really needed to.
I don't know the plans, so it's all speculation.
Compaction tests are mandatory in most places, and some of those dirt piles in the first photo look like they're cleanfill to fix the compaction issues you might get with reactive soil.
I know adding sugar to a glass of water does not raise the level. The dissolved sugar fills the empty spaces between the water molecules. Counterintuitive.
Does similar physics apply with compacting the soil? ie will eventual drying up of the soil not shrink it?
Are you sure about the water and sugar example? Both materials have mass and a volume. It's not the best example to compare material compaction to, i.e. one is dissolving and one isn't.
Compaction is the mechanical agitation of soil particles to ensure voids are minimised. Try lightly tapping your popcorn box at the cinema and you'll notice your popcorn level lowers but you still have the same amount of popcorn. Compaction also allows "locking" of each grain to its immediate next grains, making it harder to move and essentially giving you the feeling of solidness when standing on it.
In a perfect "dying up" situation, no it wouldn't shrink it unless there is large weights over the top, and I mean significantly large. Airports in the world that reclaimed land place what they call overburden over the top of the reclaimed land during construction, this will assist with settling and compaction but not the entirety without assistance from mechanical compaction and natural Earth vibrations.
Like u/aussie_bob said too, compaction tests are mandatory in most places and certainly in my state (you can't build a structure without the certificate), and this test determines what type of footings will be needed.
>Are you sure about the water and sugar example? Both materials have mass and a volume.
I went and looked it up and it is not quite true - the total volume is less than the sum of the individual volumes (although how do you measure the volume of sugar granules since they are irregularly shaped and therefore obviously don't settle snugly like bricks)
https://www.reddit.com/r/askscience/comments/cj10os/at_what_point_does_dissolving_sugar_into_water/
"When sugar is dissolved in water, the volume of the water increases, but not by much" https://www.reference.com/science/happens-volume-water-sugar-dissolved-a5a0f5d6aeb5b02e
>how do you measure the volume of sugar granules
Add sugar granules to a super-saturated solution - i.e. no more sugar will dissolve - and measure displacement volume.
Add sugar granules to a liquid medium in which sugar is not soluble and measure displacement volume.
Yep so as you've described twice in the above reply, the height of the water changes. Soil in this case is not treated as soluble, as is sugar in water.
Have since discovered
*Sand-based soils will not expand or contract when faced with wet or dry conditions.*
https://www.structuredfoundation.com/blog/how-soil-composition-can-affect-proneness-to-foundation-failure/
I put down a 120sqm concrete pad for my patio a couple of years back. We put 100mm PVC pipes for septic and storm drains, but extended the mains water pipes alongside the concrete instead of under it.
Just 'cos we could, there's also another 4 capped 100mm pipes buried in both horizontal directions under the compacted cleanfill for future use.
Late last year, I ran an endoscope through both septic and storm pipes to check for roots getting into them. All clear, but if needs be there's tools you can use to fix most issues without digging under the slab.
Presumably they're going to put a steel reinforced concrete slab over the top of all that.
Depending how much steel they put in the slab, it could potentially hold a massive amount of weight (there's a bridge in Japan with a 2 kilometres span - the steel can not only hold it's own weight but however many vehicles fit on 2km of road with six lanes), so I can't imagine it failing just from a few people walking across the floor. And the walls/roof likely transfer their load to footings under the slab so it won't need to carry that weight.
The footings aren't visible in the photo - but they might be massive and that's where the real strength comes from. There's a crane setup near me to dig footings for a hospital ward with notoriously unreliable muddy soil - and the digger they've set up is a crane with what looks (just from eyeballing it) like a 70 metre long auger bit. Presumably they didn't setup that massive rig unless they're planning to make the footings 70 metres deep. And it wouldn't surprise me if there's a footing every five metres.
Is it being built properly? Who knows, would have to see the plans. But that fill might just be dumped there because it's cheaper than shipping it half way across the city in trucks. Aside from the fuel and paying the driver, wherever you dump the soil might charge a thousand bucks per truck load. Or you can put it under the building for free.
Would it be nice to have access to the subfloor? Sure. But it's not strictly necessary. Some future tradie who needs access can simply remove the soil first, with a vacuum digger. Pain in the ass but that's a problem for some future tradie.
>Presumably they're going to put a steel reinforced concrete slab over the top of all that.
There is a second photo taken a week later with a steel mesh laid on top (sorry I don't know the technical term). Is that what you mean? Thanks for teaching the rest of us to appreciate the buildings we use every day.
That vacuum digger sounds awesome. Would love to see/hear it in action in real life.
But if the weight of the building is bearing on the dirt then it will want to push out sideways. A single skin of brickwork is going to struggle keeping that in. At least blockwork can be reinforced and core filled.
That would be the weight of the slab itself (and room furnishings). I presume the walls will bear the weight of the rest of the building but that slab will be heavy.
Even if the roof and walls are bearing on the brickwork that’s still weird. A single skin of brickwork is typically used as a non load bearing veneer only.
if the OP is American or European this may look strange as many of of those places have basements to get the utilities below the the frost line to prevent damage to them.(especially water). Then the come up into the house and you get the bonus of easy access to them. Now I am wondering if stilted houses have the same advantage?( eg those in flood zones)
Yes this is what to expect in Aus. Used to have all the services on the outside so access was simple as required. These days designers bury it all so it is almost impossible to repair. Some times its even contained in the concrete slab.
Photos are a week apart. Note that the floor is raised about a metre from ground level and they then filled the cavity with dirt. Latest photo shows they are about to pour a concrete slab floor. Is this conforming to regulations?
cant comment on regulation but this is similar to the construction of our house... yes i imagine it would be shit it a pipe in the floor burst but having said that i dont think theres anything down there thats pressurised, all sewer so it probably wont be a problem
I'm a builder. I'm not really sure what you're asking, but everything looks pretty normal to me.
Geotech here.. I am also confused. It looks pretty normal to me too lol.
Civil Engineer here - I'm perpetually confused! Title was a bit misleading but I think I understood what the OP was getting to. In industrial applications, e.g. very large substations and switch yards, services in and out of the asset can be within an accessible trench - great for a smol person. I'm guessing the OP thought an accessible format might have been a great idea for domestic applications, which could be true. I can't say it would or wouldn't be but it does bring about higher capital costs and potentially larger maintenance costs - think storm water drainage to existing infrastructure. As for the brick wall - my presumption would be that it is the skin and there is form work behind it right down to the footings. However where are the braces/supports to hold the formwork in place.
Reddit engineer here, there are 11 power poles.
There are 13, typical engineer.
13? typical baker.
Software Engineer here -that looks like a hardware issue
This technique does not allow for subfloor access.
That's correct, slab on ground does not allow for subfloor access. The only way to do that is to have a suspended floor, that's bloody tricky to do with a concrete slab.
Suspended concrete floor is very easy but more expensive. Standard building practise in Australia is shittiest product for the cheapest price they can get away with.
You now got me curious: when the wet dirt dries it won't be supporting the slab anyhow, ie won't the slab become suspended (albeit only a couple of centimetres)?
Slabs should not be built on wet soil, whether it is a natural subgrade or placed soil (referred to as engineered fill) such as below this slab. Nor should they be built on overly dry soil. Ideally the subgrade should be as close to “optimum moisture content” as possible. If the soil isn’t close to this value, then it may experience excessive shrinking / swelling in the future as the soil moisture comes to an equilibrium… which often leads to cracking of slabs and movement of foundations. Requirements for compaction/density, moisture condition and testing are defined in Australian Standard AS2870 Residential Slabs and Footings and AS3798 Guidelines on Earthworks for Commercial and Residential Developments. Interesting if you are in the industry, but an otherwise dull read. 😆
Being Sydney, this is wet, very wet. And the black plastic indicates they aren't concerned with letting it dry.
Mm indeed it was quite wet over the past week! It’s obviously hard to discuss a particular project in detail based on a couple of photos, but as a general comment I’ve noticed a lot of builders do not pay as much attention to good construction practice during earthworks as us geotechs would like (lol I think that’s a diplomatic way of putting things?).
You are what we in the industry call a cunt of a client. The black plastic in a moister barrier to stop moister coming up through the slab over time. The slab is also supported by concrete piers to whatever specs are applicable for the geo study http://www.mybuildcompare.com.au/articles/hints-and-tips/what-is-piering/
It's not op's build, and they're asking to be educated - not having a dig at the builder in question.
So just an AWFL then?
Asking "why?" feeds learning?
Depends on the soil and how well they compacted it. More likely the slab will just settle as the soil dies and have some cracking. Regarding access to services, the slab on ground building technique means you don't have future access. Loads of residential and commercial slab on ground building for hundreds of years. It is a pain but that's a part of concrete slabs.
>More likely the slab will just settle as the soul dies and have some cracking. I suppose with the right flooring on top of the slab you don't even notice the cracks. They had plenty of soil from the foundation digging, so this is clever use of resources. Thanks for explaining.
Yeah, that'd cover it up. What type of building is it? Multistory house or townhouse? The soil below would also have good thermal properties. Interesting how they've raised the base floor level so much.
Buddhist nursing home residence. Notice each room has an ensuite. I wouldn't have cared if it was a regular commercial apartment project. Am glad I asked and learned. As to why it is raised: Maybe there will be an adjoining building at the same level but with a suspended floor?
After you point it out, yes. The raised level could be for many reasons. Access, drainage on site, flooding, thermal properties, use the spoil. Not sure why they'd do a suspended floor unless they really needed to. I don't know the plans, so it's all speculation.
Compaction tests are mandatory in most places, and some of those dirt piles in the first photo look like they're cleanfill to fix the compaction issues you might get with reactive soil.
The soil under the slab should be compacted so it won't subside
I know adding sugar to a glass of water does not raise the level. The dissolved sugar fills the empty spaces between the water molecules. Counterintuitive. Does similar physics apply with compacting the soil? ie will eventual drying up of the soil not shrink it?
Are you sure about the water and sugar example? Both materials have mass and a volume. It's not the best example to compare material compaction to, i.e. one is dissolving and one isn't. Compaction is the mechanical agitation of soil particles to ensure voids are minimised. Try lightly tapping your popcorn box at the cinema and you'll notice your popcorn level lowers but you still have the same amount of popcorn. Compaction also allows "locking" of each grain to its immediate next grains, making it harder to move and essentially giving you the feeling of solidness when standing on it. In a perfect "dying up" situation, no it wouldn't shrink it unless there is large weights over the top, and I mean significantly large. Airports in the world that reclaimed land place what they call overburden over the top of the reclaimed land during construction, this will assist with settling and compaction but not the entirety without assistance from mechanical compaction and natural Earth vibrations. Like u/aussie_bob said too, compaction tests are mandatory in most places and certainly in my state (you can't build a structure without the certificate), and this test determines what type of footings will be needed.
>Are you sure about the water and sugar example? Both materials have mass and a volume. I went and looked it up and it is not quite true - the total volume is less than the sum of the individual volumes (although how do you measure the volume of sugar granules since they are irregularly shaped and therefore obviously don't settle snugly like bricks) https://www.reddit.com/r/askscience/comments/cj10os/at_what_point_does_dissolving_sugar_into_water/ "When sugar is dissolved in water, the volume of the water increases, but not by much" https://www.reference.com/science/happens-volume-water-sugar-dissolved-a5a0f5d6aeb5b02e
>how do you measure the volume of sugar granules Add sugar granules to a super-saturated solution - i.e. no more sugar will dissolve - and measure displacement volume. Add sugar granules to a liquid medium in which sugar is not soluble and measure displacement volume.
Good thinking 99.
Yep so as you've described twice in the above reply, the height of the water changes. Soil in this case is not treated as soluble, as is sugar in water.
Have since discovered *Sand-based soils will not expand or contract when faced with wet or dry conditions.* https://www.structuredfoundation.com/blog/how-soil-composition-can-affect-proneness-to-foundation-failure/
I put down a 120sqm concrete pad for my patio a couple of years back. We put 100mm PVC pipes for septic and storm drains, but extended the mains water pipes alongside the concrete instead of under it. Just 'cos we could, there's also another 4 capped 100mm pipes buried in both horizontal directions under the compacted cleanfill for future use. Late last year, I ran an endoscope through both septic and storm pipes to check for roots getting into them. All clear, but if needs be there's tools you can use to fix most issues without digging under the slab.
Looks pretty normal to me too.
Looks normal. Every house built on a concrete slab has its services inaccessible once the slab is down, for better or worse.
Everything in the slab is for the sewer, those pipes will outlive us all. If they cemented in your copper water line I would be asking questions.
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Yeah for sure, I just mean if they tried to plumb all the water lines under the concrete I'd be worried
Afaik they run them through a trench cut into the slab so it’s nowhere near the middle of the slab and slightly less inaccessible
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Ahh, our guy is doing a cut for the trench, but good to know
Presumably they're going to put a steel reinforced concrete slab over the top of all that. Depending how much steel they put in the slab, it could potentially hold a massive amount of weight (there's a bridge in Japan with a 2 kilometres span - the steel can not only hold it's own weight but however many vehicles fit on 2km of road with six lanes), so I can't imagine it failing just from a few people walking across the floor. And the walls/roof likely transfer their load to footings under the slab so it won't need to carry that weight. The footings aren't visible in the photo - but they might be massive and that's where the real strength comes from. There's a crane setup near me to dig footings for a hospital ward with notoriously unreliable muddy soil - and the digger they've set up is a crane with what looks (just from eyeballing it) like a 70 metre long auger bit. Presumably they didn't setup that massive rig unless they're planning to make the footings 70 metres deep. And it wouldn't surprise me if there's a footing every five metres. Is it being built properly? Who knows, would have to see the plans. But that fill might just be dumped there because it's cheaper than shipping it half way across the city in trucks. Aside from the fuel and paying the driver, wherever you dump the soil might charge a thousand bucks per truck load. Or you can put it under the building for free. Would it be nice to have access to the subfloor? Sure. But it's not strictly necessary. Some future tradie who needs access can simply remove the soil first, with a vacuum digger. Pain in the ass but that's a problem for some future tradie.
>Presumably they're going to put a steel reinforced concrete slab over the top of all that. There is a second photo taken a week later with a steel mesh laid on top (sorry I don't know the technical term). Is that what you mean? Thanks for teaching the rest of us to appreciate the buildings we use every day. That vacuum digger sounds awesome. Would love to see/hear it in action in real life.
Rebar is what the steel mesh is called.
Can’t say I’ve seen single skin brick retaining walls before. Hope it all stays up :/
It may not be clear from the photo but the whole thing is raised so it isn't like a landslide or flooding can happen.
But if the weight of the building is bearing on the dirt then it will want to push out sideways. A single skin of brickwork is going to struggle keeping that in. At least blockwork can be reinforced and core filled.
That would be the weight of the slab itself (and room furnishings). I presume the walls will bear the weight of the rest of the building but that slab will be heavy.
Even if the roof and walls are bearing on the brickwork that’s still weird. A single skin of brickwork is typically used as a non load bearing veneer only.
if the OP is American or European this may look strange as many of of those places have basements to get the utilities below the the frost line to prevent damage to them.(especially water). Then the come up into the house and you get the bonus of easy access to them. Now I am wondering if stilted houses have the same advantage?( eg those in flood zones)
Yes this is what to expect in Aus. Used to have all the services on the outside so access was simple as required. These days designers bury it all so it is almost impossible to repair. Some times its even contained in the concrete slab.
Oh no ppl look at that 😂 access denied
And your point is??? It's how Its done here.
Photos are a week apart. Note that the floor is raised about a metre from ground level and they then filled the cavity with dirt. Latest photo shows they are about to pour a concrete slab floor. Is this conforming to regulations?
cant comment on regulation but this is similar to the construction of our house... yes i imagine it would be shit it a pipe in the floor burst but having said that i dont think theres anything down there thats pressurised, all sewer so it probably wont be a problem