Hi Matt,
This is a fantastic video and thanks for that.
I have designed waffle slabs for many years and I had only one slab failure due to plumbing leakage near the edge beam, there are couple points that I would like to hear your opinion:
- I believe plastic pits should not be used for drainage in reactive sites, it will get crushed/bend easily on site or due to soil movements, also there is no reference to plastic pits in AS 3500 to best of my knowledge
- Nearly all waffle slabs will have edge beams, this is the requirement of AS 2870 that edge beams need to be founded into controlled fill or natural soil
- I always design clay plugs where the pipe is entering slab and not exiting the slab, this will prevent water getting under the slab but allows it to flow out in the event of leakage under the slab, we always check and design that the pipe is graded min. 1:100 towards LPOD
- In regards to trees on reactive sites, we never allow trees to be planted closer than their mature height or will support the slab on deep piers, so a 10m distance for a gum tree might not be effective in my opinion
- What is more scary than using waffle slabs are these new products that specify 300 height waffle slabs with more internal ribs but essentially same volume of concrete, as you can imagine additional width will not compensate the structural capacity loss due to reduced height, yet there are engineers out there who persist that it works and do not understand the deference between Ig and Icr which was used to justify the moment capacity of these products
As you can imagine, I have lost business to competitors who either don't understand or don't want to understand these issues, unfortunately homeowners also think that engineers over design their house until it fails.
I have shared your video with my contacts.
Waffle slabs, slab heave and site drainage
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how often does this become a problem on reactive soils? and how much of the imbalance needs to happen in order for the problem to present itself?
we obviously read a lot of horror stories when they make the news/forums, but at the same time there are probably thousands more houses built on reactive soils that are doing fine, that we never hear about. I'm really curious (as a home owner) where is that fine line between being responsible, while still enjoying some greenery around your home, and being overly paranoid about trees, rains, hairline cracks etc.
we obviously read a lot of horror stories when they make the news/forums, but at the same time there are probably thousands more houses built on reactive soils that are doing fine, that we never hear about. I'm really curious (as a home owner) where is that fine line between being responsible, while still enjoying some greenery around your home, and being overly paranoid about trees, rains, hairline cracks etc.
Interesting video, I noticed you didn't talk about concrete aprons around the perimeter of the slab, I seem to see that crop up on quite a few articles on waffle slabs, is it strictly necessary?
I'm building with a raft slab, I assume some of the issues you raised with waffle slabs apply to raft slabs as well.
What should I be looking at to make sure I'm putting my raft slab in the best possible position?
I'm building with a raft slab, I assume some of the issues you raised with waffle slabs apply to raft slabs as well.
What should I be looking at to make sure I'm putting my raft slab in the best possible position?
It seems you are doing a great job of understanding the drainage issues that can cause problems with waffle slabs when you design them and that is half the battle. So well done for staying informed on the requirements of AS2870 and doing your bit.
However, I don't mean the engineers and designers when I say that the most important information on how to build waffle slabs well is not making it to through to the people who most need to know.
I don't even mean the builder's salespeople who say (when asked) "If it is on the engineer's drawings we *absolutely* will comply."
I am referring to the people who ACTUALLY need to do the work, the plumbers, landscape gardeners, the supervisors that need to watch and sign off, the certifiers who collect the paperwork at the end and say, "Yes, this building has been done exactly as required by the engineer's drawings and NCC requirements."
This is because so many of these elements are so hard to prove at the end of the job after the paint has dried and the driveway slab has cured.
In relation to your comments,
- I don't have a problem with plastic pits used in the yard for landscaping and surface drainage. If AS3500 is silent on these matters then it will come down to personal opinion. A plastic pit that has been installed well is going to perform (receive surface water) better than a poorly installed concrete pit. Careful installation is the key for getting the pits at the right height. That's the builder's responsibility. Maintenance and cleaning the pit - that's one for the homeowner. I think I might have stated that pits are not my preference because of the maintenance requirements. If another surface drainage solution is available I would lean that way.
- Nearly all waffle slab have edge beams? I have not found that to be true. Typically the 300 wide edge beams that I see are seated on the same surface as the internal beams. I know there is a note on some engineer's drawings that the beams are to be dug into the ground. Are you sure it is being done on site?
- Clay plugs are shown on many of our local engineer's drawings. My point is that, like excavated edge beams, it is not being done as often as it should be. Are you sure the plumbers are doing clay plugs? Who signs off on their installation? Have you got written confirmation? How do the plumbers who do this work prove it to you?
- A design engineer may well think they have power over a homeowner or the next homeowner on the placement of trees but drawings sitting in someone's drawer are not looked at when the trees are being selected at the local nursery. Trees too close to a home ends up being a caveat that the builder uses to avoid blame. My solution is a sticker in the meter box to remind owners that trees can affect a home and that they need to be careful in selecting trees and their placement on an allotment.
- In relation to new waffle slab variants, various new solutions for "making it easier for a builder" are making their way into the market all the time. Engineer's and designers need to be wary in utilising these systems. The duty of care in a poorly designed/selected system rests with the specifying engineer because the suppliers will be covered by well-written disclaimers!
- Losing business to competitors that are willing to cut corners has become a way of life for responsible engineers. The up side is that these non-compliant designs are easy to spot and easy to prosecute. The downside is that the customer/consumer/homeowner goes through a traumatic experience of their young home moving, cracking, deflecting, lowered property values and fighting battles with the building company's lawyers. Anyone who has been through this experience will tell you they wished they had done things differently. Sadly, many of the fresh young people building homes here on homeone (and in the world) are looking for colour ideas more than what good design and construction looks like.
Thanks for sharing my video. Please also share it with the people who most need to know - the builders, plumbers, site supervisors etc.
Matt
However, I don't mean the engineers and designers when I say that the most important information on how to build waffle slabs well is not making it to through to the people who most need to know.
I don't even mean the builder's salespeople who say (when asked) "If it is on the engineer's drawings we *absolutely* will comply."
I am referring to the people who ACTUALLY need to do the work, the plumbers, landscape gardeners, the supervisors that need to watch and sign off, the certifiers who collect the paperwork at the end and say, "Yes, this building has been done exactly as required by the engineer's drawings and NCC requirements."
This is because so many of these elements are so hard to prove at the end of the job after the paint has dried and the driveway slab has cured.
In relation to your comments,
- I don't have a problem with plastic pits used in the yard for landscaping and surface drainage. If AS3500 is silent on these matters then it will come down to personal opinion. A plastic pit that has been installed well is going to perform (receive surface water) better than a poorly installed concrete pit. Careful installation is the key for getting the pits at the right height. That's the builder's responsibility. Maintenance and cleaning the pit - that's one for the homeowner. I think I might have stated that pits are not my preference because of the maintenance requirements. If another surface drainage solution is available I would lean that way.
- Nearly all waffle slab have edge beams? I have not found that to be true. Typically the 300 wide edge beams that I see are seated on the same surface as the internal beams. I know there is a note on some engineer's drawings that the beams are to be dug into the ground. Are you sure it is being done on site?
- Clay plugs are shown on many of our local engineer's drawings. My point is that, like excavated edge beams, it is not being done as often as it should be. Are you sure the plumbers are doing clay plugs? Who signs off on their installation? Have you got written confirmation? How do the plumbers who do this work prove it to you?
- A design engineer may well think they have power over a homeowner or the next homeowner on the placement of trees but drawings sitting in someone's drawer are not looked at when the trees are being selected at the local nursery. Trees too close to a home ends up being a caveat that the builder uses to avoid blame. My solution is a sticker in the meter box to remind owners that trees can affect a home and that they need to be careful in selecting trees and their placement on an allotment.
- In relation to new waffle slab variants, various new solutions for "making it easier for a builder" are making their way into the market all the time. Engineer's and designers need to be wary in utilising these systems. The duty of care in a poorly designed/selected system rests with the specifying engineer because the suppliers will be covered by well-written disclaimers!
- Losing business to competitors that are willing to cut corners has become a way of life for responsible engineers. The up side is that these non-compliant designs are easy to spot and easy to prosecute. The downside is that the customer/consumer/homeowner goes through a traumatic experience of their young home moving, cracking, deflecting, lowered property values and fighting battles with the building company's lawyers. Anyone who has been through this experience will tell you they wished they had done things differently. Sadly, many of the fresh young people building homes here on homeone (and in the world) are looking for colour ideas more than what good design and construction looks like.
Thanks for sharing my video. Please also share it with the people who most need to know - the builders, plumbers, site supervisors etc.
Matt
Thanks for reply, agree with your points, unfortunately it is not only builders and certifiers, we show the edge beam approx. depth to avoid the builder leaving edge beams on the ground, most of the engineers will either have a hidden notes somewhere or don't care about it, I know a large engineering consultants which operates now in all states that allow upto 500mm rolled fill in some of their designs and then they have a hidden note that fill need to be controlled fill, do you really think the builder will read it? even their own engineers didn't know about this when I asked them to clarify.
I have lost significant business to this larger consulting firms as result of doing the right thing, so I decided to help home owners on this forum at free of charge to pick up engineering errors and designs generated by these larger firms, which is very enjoyable to watch as they can't justify their designs.
Interesting video, I noticed you didn't talk about concrete aprons around the perimeter of the slab, I seem to see that crop up on quite a few articles on waffle slabs, is it strictly necessary?
I'm building with a raft slab, I assume some of the issues you raised with waffle slabs apply to raft slabs as well.
What should I be looking at to make sure I'm putting my raft slab in the best possible position?
I'm building with a raft slab, I assume some of the issues you raised with waffle slabs apply to raft slabs as well.
What should I be looking at to make sure I'm putting my raft slab in the best possible position?
Concrete perimeter slabs, in my opinion, are worthless if the water running off the end of the perimeter slab soaks through the sandy/loamy fill (placed by the builder) down to the hard, clayey soil underneath and then drains back towards the slab.
If the clayey soil underneath has not been graded away from the perimeter of the dwelling at slab/site development stage you may as well not have a concrete slab.
Raft slabs are affected by uneven soil moisture conditions too so the points relating to site drainage, backfill of trenches, running of stormwater pipes separate to site drainage pipes are all still important.
The additional aspect for care with raft slabs is that pipes passing through the footings have to be done to AS2870 (and AS3500) and the articulation of pipes is maybe a little bit more important (in my opinion).
Hey Matt,
I came across your video while researching about waffle slabs. We will be knocking down and rebuilding, have a 2m drop (higher at the front) over 46m or so. We are at the bottom of a street that slopes towards us, and we like gardening and want to plant trees within reasonable distance of the building once we move back in. I am an engineer but unfortunately my area of practice is neither structural nor geotechnical, so while I can follow a bit, I am out of my depth.
The more I read about waffle slabs, which seems to be every builder's favourite, the more concerned about waffle slabs. I have a healthy mistrust towards builders and tradies (from my own site experience), and I am not sure a slab that needs perfect drainage, particularly in our specific situation, is the way to go for us. I have asked my builder whether a stiffened raft would be possible but I am yet to hear.
According to the geotech report, our site is a class P and the slab recommendation is M.
Am I being unduly paranoid about a waffle slab? It seems to me our situation would be best suited to something less sensitive to moisture changes and requiring perfectly installed drainage.
What do you think?
Sub Surface Soil Profile
An overview of the site subsurface profile as follows:
• FILL (Silty Sand mix) moderately compacted underlain by,
• Clayey SAND (SC) medium dense, dark-grey to light-grey mottled-orange in colour
underlain by,
• Silty CLAY (CL) stiff to very stiff, brown/grey to light-brown/grey mottled-orange in
colour extending to 1.8m below ground level.
It seems you are doing a great job of understanding the drainage issues that can cause problems with waffle slabs when you design them and that is half the battle. So well done for staying informed on the requirements of AS2870 and doing your bit.
However, I don't mean the engineers and designers when I say that the most important information on how to build waffle slabs well is not making it to through to the people who most need to know.
I don't even mean the builder's salespeople who say (when asked) "If it is on the engineer's drawings we *absolutely* will comply."
I am referring to the people who ACTUALLY need to do the work, the plumbers, landscape gardeners, the supervisors that need to watch and sign off, the certifiers who collect the paperwork at the end and say, "Yes, this building has been done exactly as required by the engineer's drawings and NCC requirements."
This is because so many of these elements are so hard to prove at the end of the job after the paint has dried and the driveway slab has cured.
In relation to your comments,
- I don't have a problem with plastic pits used in the yard for landscaping and surface drainage. If AS3500 is silent on these matters then it will come down to personal opinion. A plastic pit that has been installed well is going to perform (receive surface water) better than a poorly installed concrete pit. Careful installation is the key for getting the pits at the right height. That's the builder's responsibility. Maintenance and cleaning the pit - that's one for the homeowner. I think I might have stated that pits are not my preference because of the maintenance requirements. If another surface drainage solution is available I would lean that way.
- Nearly all waffle slab have edge beams? I have not found that to be true. Typically the 300 wide edge beams that I see are seated on the same surface as the internal beams. I know there is a note on some engineer's drawings that the beams are to be dug into the ground. Are you sure it is being done on site?
- Clay plugs are shown on many of our local engineer's drawings. My point is that, like excavated edge beams, it is not being done as often as it should be. Are you sure the plumbers are doing clay plugs? Who signs off on their installation? Have you got written confirmation? How do the plumbers who do this work prove it to you?
- A design engineer may well think they have power over a homeowner or the next homeowner on the placement of trees but drawings sitting in someone's drawer are not looked at when the trees are being selected at the local nursery. Trees too close to a home ends up being a caveat that the builder uses to avoid blame. My solution is a sticker in the meter box to remind owners that trees can affect a home and that they need to be careful in selecting trees and their placement on an allotment.
- In relation to new waffle slab variants, various new solutions for "making it easier for a builder" are making their way into the market all the time. Engineer's and designers need to be wary in utilising these systems. The duty of care in a poorly designed/selected system rests with the specifying engineer because the suppliers will be covered by well-written disclaimers!
- Losing business to competitors that are willing to cut corners has become a way of life for responsible engineers. The up side is that these non-compliant designs are easy to spot and easy to prosecute. The downside is that the customer/consumer/homeowner goes through a traumatic experience of their young home moving, cracking, deflecting, lowered property values and fighting battles with the building company's lawyers. Anyone who has been through this experience will tell you they wished they had done things differently. Sadly, many of the fresh young people building homes here on homeone (and in the world) are looking for colour ideas more than what good design and construction looks like.
Thanks for sharing my video. Please also share it with the people who most need to know - the builders, plumbers, site supervisors etc.
Matt
However, I don't mean the engineers and designers when I say that the most important information on how to build waffle slabs well is not making it to through to the people who most need to know.
I don't even mean the builder's salespeople who say (when asked) "If it is on the engineer's drawings we *absolutely* will comply."
I am referring to the people who ACTUALLY need to do the work, the plumbers, landscape gardeners, the supervisors that need to watch and sign off, the certifiers who collect the paperwork at the end and say, "Yes, this building has been done exactly as required by the engineer's drawings and NCC requirements."
This is because so many of these elements are so hard to prove at the end of the job after the paint has dried and the driveway slab has cured.
In relation to your comments,
- I don't have a problem with plastic pits used in the yard for landscaping and surface drainage. If AS3500 is silent on these matters then it will come down to personal opinion. A plastic pit that has been installed well is going to perform (receive surface water) better than a poorly installed concrete pit. Careful installation is the key for getting the pits at the right height. That's the builder's responsibility. Maintenance and cleaning the pit - that's one for the homeowner. I think I might have stated that pits are not my preference because of the maintenance requirements. If another surface drainage solution is available I would lean that way.
- Nearly all waffle slab have edge beams? I have not found that to be true. Typically the 300 wide edge beams that I see are seated on the same surface as the internal beams. I know there is a note on some engineer's drawings that the beams are to be dug into the ground. Are you sure it is being done on site?
- Clay plugs are shown on many of our local engineer's drawings. My point is that, like excavated edge beams, it is not being done as often as it should be. Are you sure the plumbers are doing clay plugs? Who signs off on their installation? Have you got written confirmation? How do the plumbers who do this work prove it to you?
- A design engineer may well think they have power over a homeowner or the next homeowner on the placement of trees but drawings sitting in someone's drawer are not looked at when the trees are being selected at the local nursery. Trees too close to a home ends up being a caveat that the builder uses to avoid blame. My solution is a sticker in the meter box to remind owners that trees can affect a home and that they need to be careful in selecting trees and their placement on an allotment.
- In relation to new waffle slab variants, various new solutions for "making it easier for a builder" are making their way into the market all the time. Engineer's and designers need to be wary in utilising these systems. The duty of care in a poorly designed/selected system rests with the specifying engineer because the suppliers will be covered by well-written disclaimers!
- Losing business to competitors that are willing to cut corners has become a way of life for responsible engineers. The up side is that these non-compliant designs are easy to spot and easy to prosecute. The downside is that the customer/consumer/homeowner goes through a traumatic experience of their young home moving, cracking, deflecting, lowered property values and fighting battles with the building company's lawyers. Anyone who has been through this experience will tell you they wished they had done things differently. Sadly, many of the fresh young people building homes here on homeone (and in the world) are looking for colour ideas more than what good design and construction looks like.
Thanks for sharing my video. Please also share it with the people who most need to know - the builders, plumbers, site supervisors etc.
Matt
Thanks for reply, agree with your points, unfortunately it is not only builders and certifiers, we show the edge beam approx. depth to avoid the builder leaving edge beams on the ground, most of the engineers will either have a hidden notes somewhere or don't care about it, I know a large engineering consultants which operates now in all states that allow upto 500mm rolled fill in some of their designs and then they have a hidden note that fill need to be controlled fill, do you really think the builder will read it? even their own engineers didn't know about this when I asked them to clarify.
I have lost significant business to this larger consulting firms as result of doing the right thing, so I decided to help home owners on this forum at free of charge to pick up engineering errors and designs generated by these larger firms, which is very enjoyable to watch as they can't justify their designs.
Thanks Matt for your video. I will definitely passing this on when my drainage plans are designed.
@Structural.Review regarding your comment '500mm rolled fill and hidden note the fill needs to be controlled fill'. I found this on my engineering plan. I should get this clarified by the Engineer?
Hi Matt, Great video.
With regards to clay plugs and back filling trenches - Is it more common for the plumbers to back fill with a material that isn't found on site (i.e. gravel etc.) or just to back fil with the material they've dug out for the services?
Im on H2 and I have already put the builder on notice that i expect photos from the trenching and services work and expect to see the clay plugs being installed. Though after thinking about this a little, I assume they just pack the earth back in over the top. This seams the most logical and cheapest route than back filling with another material and disposing of the soil from the site. If it is the same h2 soil just packed back over the services, is this in effect just one large clay plug along the entire service trench(s)?
With regards to clay plugs and back filling trenches - Is it more common for the plumbers to back fill with a material that isn't found on site (i.e. gravel etc.) or just to back fil with the material they've dug out for the services?
Im on H2 and I have already put the builder on notice that i expect photos from the trenching and services work and expect to see the clay plugs being installed. Though after thinking about this a little, I assume they just pack the earth back in over the top. This seams the most logical and cheapest route than back filling with another material and disposing of the soil from the site. If it is the same h2 soil just packed back over the services, is this in effect just one large clay plug along the entire service trench(s)?
actually in hindsight i think i know the answer to this. The services will be bedded in appropriate material regardless of the trench back fill and its the "ends" of those trenchecs at the slab edge if you will, that need the clay plug
On highly reactive site they should back fill with clay. one issue that does come up in slab heave cases is the pipe trench is dug flat or even sloping towards the house and then gravels are used to get the correct slope away from the house.This doesn't work and can cause slab heave especially for the sewer trench at the rear of the house as watering the backyard lawn can be directed water under the slab.
Another issue apart from not using clay plugs is flexible plumbing joints which are really done although required by AS2870.
Another issue apart from not using clay plugs is flexible plumbing joints which are really done although required by AS2870.
Another issue apart from not using clay plugs is flexible plumbing joints which are really done although required by AS2870.
According to the Victorian Building Authority and other industry groups, they are not required as the slab comes under building regs but flexible joints come under plumbing. To wit, a representative from Storm Plastics was shot down when he said they were mandatory during a meeting attended by the VBA and other building bodies a couple of years ago.
Unfortunately, rather than advocate correction to regulatory shortcomings and ambiguities, many regulatory bodies adopt laissez-faire doctrines.
AS 2870—2011
- (b) Drains attached to or emerging from underneath the building shall incorporate flexible joints immediately outside the footing and commencing within 1 m of the building perimeter to accommodate a total range of differential movement in any direction equal to the estimated characteristic surface movement of the site (ys). In the absence of specific design guidance, the fittings or other devices that are provided to allow for the movement shall be set at the mid-position of their range of possible movement at the time of installation, so as to allow for movement equal to 0.5ys in any direction from the initial setting. This requirement applies to all stormwater and sanitary plumbing drains and discharge pipes.
According to the VBA, it depends on what is referenced. If the slab is referenced, then external plumbing does not apply.
I'll contact Storm Plastics in the new year and ask what their recollection of reasoning is.
I'll contact Storm Plastics in the new year and ask what their recollection of reasoning is.
What do they mean by referenced ? When your at VCAT it doesn't matter what the VBA say if the builder hasn't complied with AS2870 then they haven't fulfilled their contract obligations. It is one of many things the VBA let happen in the building industry that don't comply with the Australian standards.
What do they mean by referenced ? When your at VCAT it doesn't matter what the VBA say if the builder hasn't complied with AS2870 then they haven't fulfilled their contract obligations.
Australian Standards are only law if they have been legislated or written in a contract.
The VBA person was adamant when he said that when the slab is referenced, only the buildingcode applies, not the plumbing code.
I'm pretty sure the VBA person I spoke to referenced an ABCB meeting he attended as the VBA representative when the Storm Plastics representative was deemed wrong after claiming flexible joints were compulsory use when building on reactive soils. I had asked for a written ruling on my Email query but the VBA don't like supplying them. I'll search my Email sent folder later to see if I still have the query I sent.
Don't rely on V-CAT historic rulings, they have used unlegislated and outdated Guides as references in the past and only got away with it because industry 'experts' were none the wiser. The same happened in NSW but it was falsely denied when the question re the use of an unlegislated handbook was put to the NSW parliament several years ago.
I'll investigate again in the new year.
Australian Standards are only law if they have been legislated or written in a contract.
That is also my understanding, specific codes, alternate solution's, etc must be referenced in the contract documents.
If it's non compliant then the builder is to guarantee the work for the specified warranty time ie structural 6 years.
Its bad luck if it falls apart after then, but best to bring the non compliance and subsequent effects to the builders attention ASAP.
Build a solid case based on facts, data/proofs and put the builder on notice.
Start with non compliant -> Not fit for purpose -> Followed by negligence.
But get legal advice
Australian Standards are only law if they have been legislated or written in a contract.
That is also my understanding, specific codes, alternate solution's, etc must be referenced in the contract documents.
My NCC Part 2 (which hasn't been amended as far as I know) only states that a footing or slab must be constructed in accordance with AS 2870.
As per the above and that stated by StructuralBIMGuy, no plumbing provisions external to the slab that are contained within AS 2870 apply as they are not referenced in either Parts 2 (BCA) or 3 (PCA) of the NCC but would apply if they were referenced in the contract. This is my understanding as explained to me by the VBA and my interpretation of the text from the NCC Part 2 3.2.0 (a) pasted below .
"Acceptable Construction Manuals
3.2.0 Application
Performance Requirements P2.1.1 and P2.2.3 are satisfied for footings and slabs if they are installed in accordance with one of the following:
(a)The footing or slab is constructed in accordance with AS 2870.
(b)Piled footings are designed in accordance with AS 2159.
Explanatory information:
Composite construction — design requirements for other materials
State and Territory Variations
In New South Wales delete 3.2.0(a) and insert NSW 3.2.0(a) as follows:
(a)The footing or slab is constructed in accordance with AS 2870 except that for the purposes of Clause 5.3.3.1 of AS 2870 a damp-proofing membrane is required to be provided."
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