Beware Waffle Slabs - Poor drainage can wreck
ฝัง
- เผยแพร่เมื่อ 24 ก.ค. 2024
- Poor site drainage can wreck #waffleslab houses. But that doesn't have to happen to you. Watch this video and learn why poor drainage affects waffle slab houses so badly and what you can do to protect your own home.
This video is for homeowners, builders, plumbers, site supervisors and landscape gardeners.
The reason you that should beware of waffle pod slabs is that the most important information about how to build waffle footing and slab systems well is not making it through to the people who most need to know. This is leaving homeowners and home builders at risk.
⏱️Timestamps:
0:00 Introduction
1:10 Waffle pod slabs and slab heave problems
02:51 Site drainage problems on new developments
06:59 Site drainage problems caused by field inlet pits
09:44 Site drainage problems caused by roof water entering trenches
12:57 Site drainage and plumbing trenches
14:42 Why setdown patio slabs contribute to slab heave problems
16:58 How water gets under your patio slab
19:25 Why clay plugs are a great idea and why to use them in plumbing trenches
23:22 What you homeowners need to do to not contribute to slab heave
23:43 Why trees are the natural-born enemy of houses and what to do
25:04 Why you should fix leaks around your home
27:30 The impact of Lawn and garden irrigation on slab heave in waffle slabs
30:04 Why lifting the building pad height at development stage is a good idea
32:46 Why roof plumber need to run separate landscaping pipes
34:31 How engineers can document hybrid footings at setdown areas to reduce slab heave
36:38 Why engineers need to remember that deeper waffle slabs are stiffer
38:18 Why a sticker in the meter box discloses the site reactivity is a great idea
40:40 Why waffle pod slab construction should have a system of checking and verification
44:21 Conclusion. Thank you for watching!
This video is for homeowners to bring this video to the attention of their developers, builders, certifiers, plumbers, landscape gardeners, inspectors, and engineers. They each play a part in providing the framework for a performing footing and slab system.
The biggest challenge our project home construction industry is facing is getting drainage right so that waffle pod slabs can perform within expectations.
In our video, we talk about the challenges - super-flat blocks, plumbing trenches that are not free-draining, small allotments, yard drainage, field inlet pits, porous fill, and backfill.
Then we take those drainage challenges and provide some very clever ways to solve the issues.
Our solutions include clay plugs in drainage trenches, improving drainage on subdivision development sites, grading the base of trenches, separating downpipes and landscaping pipes, and stickers in power meters that tell homeowners and future homeowners about the risks of planting large trees and overwatering lawns.
Then we back it up with recommendations on how to ensure this work is getting done correctly.
This video is vital if you are building your home or you are a project home builder that builds homes for a living.
Show this video to your homeowners, your neighbours, your developers, your competitors, your builders, your plumbers, your inspectors, and your engineers.
Because while homeowners play a part too, the contribution of the homeowner is a lot less than some would suggest. So I’m also providing this information for homeowners so that they can limit their exposure and reduce the risk of being blamed for contributing to slab heave.
Don't forget that much of the requirements for good #drainage, clay plugs, and plumbing trenches have been on engineer’s drawings for a long time. Some of it comes from the 2011 version of the Australian standard for residential footings and slabs which is nearly ten years old.
So if your older home has some issues you might struggle to solve these issues alone, but you might also get some ideas on how to fix your drainage issues.
It is time these issues gain the recognition they deserve.
So I'm calling on #Metricon. GJ Gardner, #SimondsGroup, Burbank Group, #HenlyProperties, #PorterDavis, #CarlisleHomes, #CoralHomes to watch this video, get their plumbers to watch their videos, get their supervisors to watch this video, and get this video to their developers. Let's improve residential construction together and make Australia a better nation to build in.
cornellengineers.com.au/bewar...
Reference material:
AS2870 Australian standard for residential footings and slabs and Commentary to AS2870-2011.
CSIRO BTF 18 Foundation maintenance and footing performance: A home owner's guide.
C&CAA T49-2003 Guide to residential floors
QBCC Defective residential construction work causing subsidence - Contractor's responsibilities.
My next video is about drainage in sub-floor basement areas. After that, what would you like me to do a video about? Please comment!
Matt Cornell
Thanks so much Matt, your videos are educational to home owners looking for helpful advice to break down the jargons and understand the construction process. Would be very grateful if you could create a series of owners guide for inspection at each stages of the build? Sure that will be a hit! Thank you.
@@katiefan9867 What a great idea. Thank you for your suggestion!
Matt Cornell
How about something on drop edge beams? What are they, why they are needed, any particulars things to watch out for. I've read a number of posts from people who have been a given a quote for site works then later received the unwelcome advice of increased costs due to the need for drop edge beams.
@@markvanleeuwen1965 Thanks Mark. I would be keen to enter that discussion. Is there a way you could get more information to me on why the drop edge beams were specified after the original design was complete? Could we maybe do a case study on a specific site?
Matt Cornell
@@CornellEngineers These were posts I recall reading in the past so nothing current I can point you too. I don't think the designs had been completed but rather people being given a quote for the build which did not allow for drop edge beams, signing up for the build, then being hit with the cost increase.
Good informative video, thanks Matt.
Thank you Dewi! I'm glad you liked it.
Very helpful information Matt . Thanks
Thank you. Very glad we could help.
Great video thanks for sharing your knowledge. Glad I’ve watched it before buying.
Thanks for stopping by and leaving a comment. All the best with your purchase.
Matt Cornell
Good job easily understood
Thank you so much for your informative video. It was information that we needed to know as we commence the owner builder journey. We both feel now that waffle pods are not the best way for us to proceed.
Glad it was helpful!
I wish I watched this vdo before the slab pour but instead engaged a private inspector who said in his report 'Formwork is incomplete in some locations. Formwork is not the correct measurements as detailed in the approved plans. Location Multiple locations throughout' but then he was not able to check before the concrete pour and accepted fotos by the trades supposedly showing it has been rectified. Now the slab is in but not sure how that slab will last and if water is going to run under the waffle or if we need to put in clay plug etc after practical completion. Got about 12 bored piers on P sloping soil. I guess a lesson to prepare for the frame stage and not miss potential faults there.
Well, I hope everything works out well. Don't forget our other advice - take lots of photos every step of the way! Enjoy your new home when it's finished!
Matt Cornell
Very informative, now I am starting to worry all the waffle slab I approved. Drainage requirement will definitely on my checklist from tomorrow. Thanks for this video
The more you can do to inform your builders and concreters and clients WHY they need to manage site drainage, the less you will need to worry. Feel free to share mthis video.
Getting waffle slabs to place where they definitely will perform within expectation is a team effort that is not within the power of just the approver, (or the #structuralengineer, or the builder, or the plumber (although the plumber is critical), or just the landscaper.
Thanks for stopping by.
Matt Cornell
Cornell Engineers
Hi Matt
Thank you for a great video.
I have a problem which you are depicting here.
I have just completed my build (16 months ) with Porter Davis.
Even before handover we've had huge water pooling around the house. That water has at around the house for weeks as it is a clay soil very high to the surface.
Anyhow, the supervisor had connected (after weeks of sitting there) drain to the water pooled area. Water dispersed, etc etc.
Now that we've had some works around the house, we had to dig a hole to connect some pipes to drainage. Had a plumber excavate a hole next to foundation. As soon as he hit the area around the slab, water started pouring out from crushed rock around the slab and from under it.
I had estimated it to be several thousand litres over several days.. It took me days to pump out. Finally during these few hot days it drained completely...But as soon as the rain started again, the water started flowing out from the same location. I am very concerned about this....
Supervisor had witnesses this and said it is a concern to him as well....We've sent an email to the supervisor for records (who has now left PD), he had emailed his superiors but received no reply from them. We haven't received any reply either.... Do you have any advice?
Great Value Video!
Thank you, Matthew. I'm glad you liked it.
I have just inspected some cracking houses built on waffle slab footings in western Melbourne over the past two months. The builder is the most famous project home builder- Mxxx, they really have built too many rubbish houses on waffle slabs on reactive CLAY FILL! I wouldn't believe that they don't install either bored pier or screw pile to strengthen the foundation soil and increase its stiffness. The warranty manager from Mxxx claims that if they used bored pier or screw pile, the building contract price would have been increased a lot more, which most homeowners would not have agreed with. Thus, the rule of thumb is YOU JUST GET WHAT YOU PAY FOR.
Hi Xero
This is a contentious issue and I understand your passion on the subject.
Will you please help me share this information with as many people as possible - builders and homeowners?
We have a long way to go and I really think we all need to work together rather than getting bogged down naming names and trying to shame any particular player.
If some of these solutions became common practice right across the board, we might have a better chance (as a construction industry) of providing beautiful, well-built homes.
Matt
Hello Matt, Thank you for this video, so clearly explaining the construction of waffle slabs and the issues associated with them. We are in a house built on a waffle slab in 2004 and have 'slab heave', causing all the things you refer to - the ceiling rising from the wall in one room, doors at an angle to their frames etc. We don't have garden directly around the house, no leaking taps etc. What can we do to rectify this situation?
Hi Pam
You're welcome.
Without seeing the site, for an existing house that is 17 years old, your only real option is to identify sources of uneven soil moisture and eradicate them. It sounds like you have made a start by mentioning dripping taps and gardens. The other things to check is your plumbing (get a plumber to do a flood test of your pipes) and the presence of trees. Our video on slab levels (th-cam.com/video/zteewZSOZPs/w-d-xo.html) talks about using a water level to record floor contours - this tool is especially useful when you're not sure which part of your house is moving and which part s stable. it's not rocket science ad it might be something you can try yourself but otherwise, you might need help from a local engineer with a gas level or a water level.
Matt Cornell - Cornell Engineers
Hi Matt
Iam from Melbourne.I currently in a process to take my builder to court as my house has started breaking within 2 years of built.Builder has made all classic mistakes u have mentioned.
1.Clay reactive P site - Waffle slab with peripheral piers
2.Site was not drained during construction
3.Storm water pipes holding large volumes of water due to poor gradient.
4.Porous material used to back fill
5.Abscence of clay plugs
6.Lack of flexible joints in storm water/sewer
In addition builder has provided with
7. 3 Star picket breaks in storm water on north side of house
8.Blocked storm water pipe on the other side with brick mortar from builder.
All of this resulted in a heave of 140 mm.
I was looking to contact for your advice few months ago and contacted your office.Many thanks for very useful information
Regards
Ram
Hi Ram
Happy to help. Best of luck with your impending court case.
Matt Cornell
Hi Matt
Thank you for your response.We have engaged a building auditor to deal with our house.The Builder’s engineer has given plans for deeper edge beams of 600 mm .We have exposed the footings on 4 corners and found that the edge beam meets bored piers at 385 mm at set down alfresco area where the most heave of house is-140 mm.I am waiting for building auditor’s response .We have noticed water collecting on inner corners of our alfresco when ever it is raining.I have seen in your video of importance of deepened edge .The independent engineer who inspected our house last year has mentioned about recommendation of deep edge beam in drawings of builder’s engineer but couldn’t find any deep edge beams especially in the fill section on rear end of house near alfresco.Can I send the drawings for your response and 4-5 pictures of dug corners of house .Please send an invoice for your time
Thank you so much! This is the BEST info I could find about Waffle Pod Slab!
Our house is on P soil and builder put in a Waffle Pod Slab with 19 Bored Piers on a 25 squares single storey house. Builder didn't allow Private Inspection during Melbourne lockdown this year. So we couldn't do Pre-Slab Pour Inspection. However, no more Lockdown and we did a Frame Inspection and found Defects with the slab :
1) Uneven Slab which exceeds VBA standards and Tolerance
2) Very Poor Water Drainage on site during construction. Not enough temp downpipes aroudn house (French drainage?) or Temp downpipe is not installed properly. Hence rainwater falls directly on to slab or making large pools of water right up the footings!
My question is : Can we do any inspection if our slab's been damaged due to excessive of rain water around the footings? Our Slab has been in this state for over 2 months during this wild wet season of Melbourne.
Hi Annie
Thank you for your lovely feedback. We really appreciate your kind words.
Fixing drainage around a house is the first step. That should stabilise ongoing movement but long term flatness of the slab will depend on whether the slab was poured when the ground was wet or very dry. Stabilisation is the fist goal - fixing an overly uneven slab due to very wet or dry soil conditions when the slab was poured is a little bit harder.
Best of luck solving your problems.
Matt Cornell
Cornell Engineers
Hi Matt, thanks for these informative videos. We have a waffle pod slab home built in 2018, site classed H1 soil, and noticed cracks developing in a few areas especially at the joins between ceiling cornices and the ceilings over these few years - these seem to change with the weather, cracks open and close over the seasons, particularly the cornice cracks. Is this normal? We also have a few cracks forming in the corners of rooms and noticeably long cracks emerging from one particular window across plasterboard, and also one across the living room ceiling, none wider than a mm, but they worry us a lot. We worry considerably about the slab heave issue and these things getting worse. A concrete path has been laid around the entire perimeter of the house 3 years ago, the drains have been professionally checked with the flooding test and no issue was found - we are doing what we can to control the effect of moisture on this kind of soil and slab. Could this be considered within the range of "normal", like a seasonal movement issue? Thank you
Thanks for taking the time to make this video and share your insights.
One thing you recommended was to build up a 100-200mm compacted raised pad for the waffle pod slab to encourage drainage away from the slab. Could digging the pad lower than the surrounding hard ground also be an option - similar to how a raft slab edge is dug into the hard ground so that the slab edge forms a barrier to moisture entry? Or maybe dig a deeper perimeter footing around the entire slab like you described with the hybrid waffle slab.
Hi Mark
Great question. Thank you for asking.
A slab edge would have to be deeper than the depth of seasonal moisture changes - which is too deep to dig economically. For Brisbane for example, that would have to be at least 1500mm deep. Then the issue is ensuring that a heaving clay does not swell sideways, push against the footing and grip it and lift it.
Deeper edge beams (rat walls) have had their day but good site drainage #trumps it all.
Hybrid footings would suffer the same fate - so my recommendation is to get stormwater to flow away and stay away from your footings.
Matt Cornell
@@CornellEngineers Hi it's not only the storm water trenches but you have all the underground plumbing that is a problem with trade not back filling properly with soil taken out of the trench keep the same soil profile in the trenches but unfortunately trade back fill them with rubbish empty plastic bottles left over plastic reo tin cans. If you have even a 100mm fall from the rear to front the water will end up under your waffle pod slab. It's a discrase that big volume builders have become money hungry and grabbing cash from consumers not knowing what their rights are the salesman job is to sell you and promise you the world that it never comes the amount of promises that a salesman tells you is amazing that they can look at people in the face and tell them so much rubbish site supperviser or manager they should have knowledge and have a background in carpentry or dome sort of building background to be on site but yet they site supperviser or manager that are no where near the building industry actually 1 site supperviser or manager used to play football, ok where is his experience in building 1 guy I found out worked in IT, again no building experience, here in Victoria all site managers or supperviser must be registered with the VBA they must have knowledge of the building industry, but yet none of these guys have any experience in building a house. As I have proved it with our problem, oh my wife and myself last night going through all our photos and documents from our vcat case against the builder which they lost and agreed to underping parts of the house to stop the sinking or soil shrinkage, we just realized that the builder did not use a expert company in underpinning but used there own concreters to drill with a Auger attachment to his exscavator we asked our self only because I did some research on underpinning and we found out that a the engineer that designed the underpinning must also provide a certificate of compliance and a warranty on the underpinning that it compliance with the Australian Standard's and all building codes, but since the builder decided to cut cost and used his own driveway concreters who will gives a warranty and a certificate of compliance for underpinning. Now we have this problem that we cannot sell our house even after because we must disclose on our contract of sale that the house had issues and was underpinned on top of that no engineering compliance certificates issued, when our lawyer chased the engineer up he told our lawyer he cannot be part of any structural works due that its out of his expertise, but he designed the piers and drainage so who is responsible for all this work is it compliance is it with in the NCC, OR THE BCA, or the AS2870 there was no soil test done before the underpinning was done, the engineer disigned the piers at max of 3.5 metres deep but by reading his drawings it also stated that the underpinning should go to solid clay or rock, if that's not achieved a 50mm concrete base to be installed before the piers where to be pored none of that was ever done. It just goes to show the amount of short cuts builders take in the hope the home owners does not know or understand, lucky I am no pushover I do my research on everything and double check even with the VBA or other expert industry I get my facts before I say anything, last night myself and my wife where really shocked to find out the builder did this and thinking they were going to get away with it, I've told the builders state manager to his face if you have not learned by now the kind of people we are then you will never learn his reply was oh I am retiring end of 2021 I said and that does not change anything your name is plasted all over these documents so really makes no difference. This is the answer from a big volume builder which now have extended to NSW and Queensland and S.A . God help all these new home owners they have no idea what they are getting into. Stop the waffle pod slab get read of them.
@@CornellEngineers The footings dug into the ground that you suggested for patios and garages, wouldn't it have the same benefit to continue them around the entire perimeter? Also, if the waffle slab is supported on mass concrete piers, which of these issues does it negate? or do they all still apply since the soil swelling will still lift the slab?
If the piers are steel reinforced to tie the slab down, would that prevent it?
Hi Matt. Really love this video. Thank you so much for sharing your wonderful insight. One question I have for you. Does it make sense for a waffle slab to be poured less than two metres from a few mature neighbouring trees that are more than fifteen metres in height and clearly change the soil moisture levels in the ground surrounding them. What protections should a builder provide to deal with such a situation?
Hi
It's the design engineer's call and the answer depends on the site reactivity (clayiness of the soil).
The best two options I have seen are:
- a root barrier trench;
- supporting some or all of the waffle slab on piers that extend below Hs, the depth of influence of moisture changes;
AS2870 makes recommendations for designing near groups of trees and the soil report should have also noted the presence of the trees and provided an assessment of the effect of the trees (noted as yt in the report).
Matt Cornell
Hi Matt, thanks for posting this video it’s very informative and helpful. We have a waffle slab property on a partially flat, partially sloping block. Soil has eroded under a corner of the slab. From watching you video we can see poor site drainage is an issue we need to address but could you offer any advice on how to re-instate the soil under the slab before we address the drainage?
Hi Rachelle
Good question and thank you for your feedback.
First, it could be that your slab is supported on screw piers or bored piers - in this case your slab is supported on soil deeper down and you can generally place sand or gravel back under the void with no problems.
To see if your house is supported on piers, either check the plans or have a look under the slab where the void is to see if there are metal poles or concrete posts under the slab.
If there are not screw piers or bored piers, is there any damage to your house? Losing dirt sometimes causes the slab to drop (subsidence) and that often causes damage to the walls and ceilings . Getting the house resupported on firm ground with new deeper footings (underpins) is the solution in this case.
If you would like some specific advice for your house, head over to our website and drop me a line on our Contact Us page. cornellenigneers.com.au
Hi there. Great video. Im in NZ but guess the same applies. I am about to build on clay on a hill basically. 3 sides will fall away but one is towards the house. Anyway what I wanted to ask is, what about things like X-pod or Cupolex slabs. The idea being that because they are hollow there is some give when soil expands.
Hi. Thanks so much for your video and webpage. This has been very enlightening. We are looking at possibly purchasing a home that was built 5 years ago in central Victoria. The owner built the home (we don’t know yet whether he is a registered builder or not but will find out). They have not hidden the fact that it has been built on a waffle slab. As a result during our research we have found your video. We are yet to view the property and before we go any further (eg getting engineers report etc) we are wondering whether after 5 years there are no signs of issues does this necessarily mean it’s ok. There are no carpets so it’s easy to inspect the slab. The block does not appear to be completely flat (from the photos). The property ticks a lot of boxes but your video has frightened us (good thing!). Can you assist us?
Send me a message on my website with your phone number and I can call you for a chat.
Matt
It was one of the best video with great insights about Waffle pod slabs. Really appreciate your time and work Matt. Do you think having piers throughout the house slab vs only around perimeter of slab provides better strength against movements / heaves? I understand that height of footing is certainly is the key factor in strength as you mentioned but a number of times builders seem to do away with bare minimum (i.e. some builders would pour M class slab if they can, while doing H1 slab with higher footing may be a better idea for future stability). What are your thoughts?
Thanks for your question A Saini.
The use of piers with waffle slabs is a complex situation so my answer isn't a very concise response.
AS2870 allows waffle slabs to be built as suspended slabs on a full series of piers.
That tempts some engineers and builders to ignore the shrinkage portion of ground movement when designing waffle slabs. They say the slab is supported on the piers. When the ground shrinks the waffle pod slab is well supported and so they don't have to design/build such a stiff slab.
However when the ground swells the waffle slab still lifts. That lifting is never even. Only some parts of the ground under the house are affected by an increase in soil moisture and so only some of the waffle slab lifts. The waffle pod slab tries to remain planar but an undersize slab will also sag deflect under the weight of a house where the house has lifted off the piers. That is when damage occurs in a house superstructure.
Uneven ground swelling doesn't allow an engineer to decrease the thickness/stiffness of a slab system even with perimeter and/or internal piers.
The latest idea is fully compressible EPS (expanded polystyrene) void formers. The whole system is a suspended slab supported on piers. The swelling forces are countered by compressible polystyrene. When the soil shrinks the system is supported on piers. The primary shortcoming is the plumbing running under the house needs enough flexibility to handle differential slab and ground movements and the long-term failure of the perimeter void formers when soil fills in under the edge beam (from seasonal movement, crushing and void creation which most homeowners feel the need to fill).
This incessant need to remove cost from footing and slab systems is a foolish game. The long-term pain is being felt by the very consumers being lured in believing that a 25 year guarantee actually means something. Well this video is all about helping those people hold their builders (and their subbies) to account. And the time to do that is during construction!
Have a great day
Hi Matt, i have an existing slab and we are renovating, moving the kitchen. we are relocating the sink drain pipe. so we are cutting the concrete slab 100mm wide approx 4000 mm long and its a waffle slab. do we need to fill in the waffle pods with concrete before filling the void with concrete? thank you for your time .
Hi Matt, very informative but I was wondering what to do for KDR site and the builder is specifying a waffle slab with 150mm and 350mm step downs for alfresco and garage?
Sounds like a sloping site. Overall drainage shouldn't be so hard on the sides and the downhill side. The uphill side will need some care to ensure the ground and the pipe trenches grade and are free-draining.
The detail at the step-down might also need some thought.
Finally the slope on the sides and how the ground grades where the slab steps need some extra thought for a building built on top of the ground.
Best of luck with your building project.
Matt Cornell
Hi Matt, it would be great if you could do a follow up vid on possible ways to rectify the water under waffle pod slab issue ? I have watched your other vids (drainage on slopes etc) and appreciate the info you are conveying on how this happens, but feel it would be helpful to see an engineers perspective on how to rectify these water under waffle (or raft slab) problems, specifically, is very wet clay under a waffle slab going to dry out eventually once the drainage is corrected (to prevent water intrusion under the slab) ? The problem I see here is that if there is no where for water vapour to go due to the clay soil and slab vapour barrier, then how can the underslab clay dry out, should deep subsoil drainage be implemented close to the walls, with their trenches draining away from the house ?
I came here to ask this same question. I have water under waffle slab and I would like to know Matt's approach to remediation of it.
My house has had slab heave and cracking. Its a great video. You dont mention a concrete apron around the house. Can you stop slab heave without concrete around house? If you need an apron can it be paver? What width? If you can get by with out a concrete or paver apron what do i do to stop slab heave?
In regard to the clay plug were pipes go under house if not done or not sure if done is concrete over the area sufficient?
Thank you for taking the time to make this video quick question I Have property off
8W X18L Meter Sitting on 10X27 meter Block the site is classified as P and M soil do you Recommend Concreting the easement side of the house To prevent any water going into the Slab?
Dear Lone Wolf (McQuade?)
Good question.
A well designed Class M/P slab with adequate natural site drainage should provide some protection from ground movement.
A couple of issues pricked my attention:
The first issue is the easement: The easement rules will dictate whether you are allowed to pour a concrete slab in the easement. Contact the easement owner (via your solicitor if necessary) to understand what you are allowed to build in the easement.
Second, it is better that the hard clayey ground around the building falls away from the building on all sides. This is irrespective of whether you build a slab or not.
A concrete slab and concrete spoon drain combination might help you improve drainage on your site and would probably be better than no slab.
Matt Cornell
Hey Matty, you really need a video on why timber frame suppliers should not be designing frames (they are not deemed competent) and why building designers/architects should not be doing bracing and frame designs either which are blindly signed off by a Certifier!
Hi Jared.
Some contentious issues there. Our industry needs ongoing training and we'll do our part - however there are a lot of competent designers/frames, architects and certifiers and I'm pretty sure they all try to practice within their expertise. The very same reason you and I don't try to be designers!
Matt
Hi matt
Thank you so much for such a informative video.
I need you opinion as my builder is going to start building my house soon. There is a small tree on nature strip at front, structural engineer recommended deepend edge beams. And builder is asking around $7000 for that even in contract I have fixed site cost.
Hi Umair
In terms of the structural engineering, a tree on a nature strip is owned by council which means you can't remove the tree if it affects your house. The footing and slab design therefore needs to accommodate the effect of the tree. Depending on your site classification and soil properties, a deepened edge beam might be an appropriate design response.
In terms of your contract, you should ask your solicitor if the builder is permitted under the terms of the contract to charge extra for a change in the design. You should also determine whether the effect of the tree on the footing design should have been foreseeable at quoting stage.
Best of luck resolving your issue.
Mat Cornell
Hi Matt great video's, learning heaps of good stuff. Researching on how to go about underpinning my parents house which was built on reactive clay soil and a 100mm slab with only an extra 100mm deeper footing, its a mess. Good info here on other factors in regards to site grading which i will need to address. Just thinking that the extra footing in the garage and patio step downs you mentioned in the waffle pod slabs might actually make the situation worse by damming the moisture under there. Maybe grading away from those step downs would be a better option as you mentioned in another comment that, Deeper edge beams (rat walls) have had their day but good site drainage #trumps it all.
Injected form seems to be the most cost effective way of underpinning these days, you can even raise the house bringing it back to level. Years ago when doing underpinning jobs it was dig a metre wide strip under the failed foundation and fill it with concrete, then skip a metre and dig another metre wide strip and fill with concrete and then repeat until you got to the end of the failed foundation. Once you had done this you would go back and dig up the skipped parts and fill them with concrete, thus giving a new continues foundation under the old failed one, but you would have day joints in the concrete every one metre due to the method of how the job was carried out.
@@nigelknight1435 thanks i looked at that but thinking of trying to pier down to more stable soil then jack up level. Then i think i might try and grade the block away from the house.
@@Billybob11345 I don't know your layout or what you are trying to achieve, but piles are normally dug with an auger (unless it's a driven pile) which can't be done with the house sitting on top, so you would dig pockets out under the existing foundation down to good bearing soil, then fill them with concrete. It sound like you want to remove the existing footing where you place the pile and use it as a jacking point, so do you have timber joists supporting the house that you want to jack up? because I don't think you will be able to jack up the existing foundation from piles positioned around the building unless it is a reinforced concrete ground beam type footing, if not reinforced you will just snap the existing concrete footing. The form mentioned in my first reply is pumped in under pressure and it also expands and lifts the existing foundation in a more uniformed way (it's a bit like point loads and line loads, your pile idea is the point load pushing up under the foundation and the form is the line load pushing up under the foundation.
@@nigelknight1435 yeah that was my idea to auger down, fill with concrete and jack up around the perimeter at 1200 centers.
But yeah i was concerned about cracking. Its a single story trussed roof brick veneer home on a 100mm slab that i would hope has reinforcing mesh. But they have only dug an extra 100mm deeper perimeter footing that wouldn't have been reinforced by the looks of it. It hadn't moved a great deal more until recently.
We have just had a few very wet years and now its drying out its moving a lot. so much so that in one spot the brick work would be 40mm out that you can see all the house framing behind.
Its a real problem, the house needs repairing inside but cant do anything until i can sort out the foundations. I have looked at the injection before but will look again and get some prices but still think i need to address the lack of a footing.
Hi Matt, what a great video! Thank you!
Are engineers allowed to design waffle slabs without piers in the middle even though the site was classified as P - H2.?
Design Engineer recommended scrapping the site 150mm to obtain a better fundation to avoid piers and that’s what the builder did.
Are consumers protected by Australian Standars in that case?
More specifically, are Australian Standards allowing waffle slabs without piers in H2 sites or this is a basic design issue?
Obviously, house has slab heave issues now 🤦🏻♂️
Thanks
HI Adrian
A competent and experienced engineer is entitled to design the slab to suit the site conditions.
Class P sites come about due to one or more ''problem"' issues which can be managed by addressing those specific issues.
For example, a 200mm layer of soft topsoil might be the reason for the class P classification. If the topsoil is removed (and replaced with controlled fill) then the design can revert to a natural site classification (ie M/H1/H2) and piers become not required
Protection to consumers is a whole different ballgame . Designing to Australian standards gives justification to an engineer's design and a way of justifying 'doing a good job' in court. Section 4 of the standard provides room for the engineer to exercise discretion and design inputs. Protection to a consumer comes when everyone does their job - the soil tester, the engineer, the certifier, the builder, the concreter, the plumber. That's why we recommend plumbers and inspectors take photos of their work during construction - so they can defend their workmanship. The engineer's workmanship is on paper so that's an easy one to assess but a good engineering design is one very small part of the puzzle and can't be looked at in isolation.
Matt Cornell
An amazing video Matt!. From your experience, assuming no damage to the underground pipes, if a new house hasn't developed any problems for 5 years then is it safe to assume that there shouldn't be any major problems in the future?
Not exactly. Damage is caused by various factors and drainage is just one of them. Trees can have an influence at any age for a house. Pipes could move and leak. Slope stability issues could occur after many years. Your house could be affected by a really wet season or a drought.
Yes, there is some reassurance from a house that has been through a few years of seasonal moisture changes. It is the abnormal influences that you should be wary of.
This video is about getting the drainage right at the construction stage.
Life after that could still affect a home - even after many years.
Matt
Great video! Does that mean it will be necessary to install agi pipe system immediately outside of the building at least towards the higher sides of the site?
Great question Shu Guo
Agi drains are our least favourite way of solving drainage issues on sloping sites. Our website article and #TH-cam video on #drainageproblems for houses on sloping land explains why (in extreme detail!). cornellengineers.com.au/houses-on-sloping-blocks-with-drainage-problems/
Regards
Matt Cornell
I've got a house built on a waffle pod slab. Since reading up and becoming aware of all the short comings of it,I've since installed a French drain and profiled my backyard against water sitting.
Just can't believe that builders aren't made to put a large skirt around the slab to mitigate water getting under the slab.
It is rather unbelievable isn't it. But then waffle slabs wouldn't be a cheap, easy option if builders had to protect the slab against poor drainage.
@@CornellEngineers I reviewed my ascons for the land as profiled by the land sales / developers, all of it was absolutely flat. Then the pad and house was built, and filled to height to look pretty. However even still the neighbour's are still higher and that water flows to my backyard en mass...
Here's a video of my French drain that collects 35 meters of flow off neighbour's.
th-cam.com/video/o7TJDO9TZeA/w-d-xo.html
@@Slazmoservicing4209 Nice video. Love the "down the hole" shots! Flowing well.
@@CornellEngineers I've realised that it'll help, however there will be some water still going under the slab from backyard to the front yard.
Hello and thanks for a fantastic video. I have a question regarding the building of a pad for the house. We are demolishing our house and am wondering whether, if the demolition crew were to leave a pad approximately 100-200 mm higher than the surrounding surface, if that would suffice? Or does it need to be specifically built up with clay?
Thanks again
What maters most is that you're already thinking about site drainage. It doesn't need to be specifically built up with clay - but old sand and gravel filled trenches that used to service the old house need to be considered and perhaps the easiest way to eliminate these 'old' sources of uneven ground moisture is just to dig them up and and do a whole new pad.
Thanks for watching. Thanks even more for thinking about how to make your new build as good as possible. Glad I could be part of your journey!
Matt Cornell
@@CornellEngineers Thanks very much for the reply Matt, really appreciate you taking the time to share your expertise. Cheers. Chris
Thanks Matt for the wonderful video. Is it worth paying the builder to go one step above than what slab is recommended after the soil test. Eg. Can we ask to install a H2 slab when a H1 slab is sufficient per soil test?
Hi Augustin
Good question.
A stronger, stiffer waffle slab design will definitely provide "better" protection against uneven foundation movement than a less stiff design.
Yes, the easiest way to get a stiffer waffle pod slab is to ask for the slab to be designed for a more reactive site classification (ie H2 instead of H1). The trouble is the slab depth doesn't change for a H1 articulated masonry design upgraded to a H2 site - only the amount of reinforcement increases.
The better way to get a better performing slab design is to ask for higher waffle pods (and therefore deeper ribs).
Asking for higher pods and hence achieving deeper ribs (eg 460 high slab instead of 385) will give far superior performance than increasing the amount of reinforcement in the slab.
Be aware however that most builders aren't set up for 460 deep waffle slabs. They are going to charge you more for the concrete and more for the inconvenience!
Best of luck with your build.
Matt Cornell
Are waffle slabs even environmentally friendly? Construction site in the street is using this method. They are cutting the polystrene to size and the off cuts are being blown down the street, into people's garden and into the drains. Once it is covered in cement how in the future is this product going to be treated when they want to demolish the build?
Check out some articles about 'ball milling' but so far I think we're a little way away from working out what to do with polystyrene waste.
Good Video, very informative, Perhaps it would be best to avoid Waffle Pod slabs and stick to the old slab on the ground system with good internal beams, edge beams supported by 300 mm Dia Concrete Peers down to a meter below finish ground level, and stick to your excellent advises on keeping the area around the house, possibly to say 3 meters from the walls, as dry as possible and good Berms and Swales to take care of the water.
Houses should be kept separate from wet areas, linked internally but the kitchen, bathrooms, laundries and toilets should form part of a totally separate building. Toilets should never be anywhere near a bedroom or living areas, the kitchen is another area which should be kept separate, as if it were a restaurant.
keep the good work up.
Hi Franklin
I don't think we're ever going to get to a place where builders no longer use waffle slabs. There is a demand for quick and easy slabs and consumers are never going to pay more for what they see as the same outcome.
However, when done properly I think that waffle slabs probably come close in cost to traditional raft slabs. The trouble is many of the contributing factors to slab heave are being ignored, the issues are being outsourced and never checked or followed through.
Let me say that again, "When done properly, waffle slabs are probably about the same cost as a traditional raft slab."
But the industry is forever looking for ways to outbid each other and increase profitability. Not so different to any other industry I guess.
Is resin Injection safe to use under waffle pad foundation?
I Have minor movement because one yard gully inlet was set too high.
I dug a open trench to drain away but obviously hasn't worked over time.
Soil is Clay sandy M waffle pad.
Hi Jason
I don't think resin injection is physically unsafe to use under a waffle slab. If everything else you have tried to fix the uneveness hasn't worked then resin injection might be your only choice.
great video. can you do a video on steel fixing please?
I could, Saiyd, but I haven't fixed steel since my work-experience days back in the 90s. I don't think it has changed much since then, but I am certain there are better professionals to comment on steel fixing than I.
Best Regards
Matt Cornell
Thanks Matt, do you think strip footing and raft slab have the same issue?
Some issues are similar and some issues the raft slab faces are specific to raft slabs.
A lot of the stormwater management issues are the same. Roof water and yard drainage should be kept separate for example.
Raft slabs have the benefit of an edge beam around the perimeter that stops water getting right in under the raft slab. However water can instead get into the perimeter trenches and run between the ground and the concrete footings.
The slab set down issue is less of an under-slab drainage nightmare in a raft slab than a waffle slab.
Plumbing running through raft slab edge beams is more complex and needs the concrete to be thickened locally.
So drainage around the outside of both types of slabs is paramount to reduce slab heave. The issues each faces are affected by how the edge of the slab is constructed.
Hi are waffle slabs good idea on a excavated site mostly on blue stone rock levelled
hi Matt, do you think paving an 1 metre wide concrete path surrounding a house can help? the water stays 1 metre away from the slab
Hi. A one metre wide path around a house is *absolutely useless* unless the hard, clayey soil that the house is built on (the hard soil under the path) also grades away from the house. A path cannot undo poor site drainage.
Hi Matt, Do the same problems apply to Polyvoid slab houses? How will you compare Polyvoid slab vs Waffle slab?
Hi Pulkita
I had a look at the sample design for a Class E site on the Polyvoid website. 0201.nccdn.net/4_2/000/000/087/64c/kelly-covey_polyvoid-design.pdf
The requirements for good site drainage, sloping trenches, elevated building pads and placement of trees are identical. I won't give my opinion on the design as this is not the place. As I see it, waffle slab and Polyvoid system will still come down to the builder, supervisor, plumbers, landscape gardeners and developers watching this video and doing a good job.
Hi,
Thanks for your video.
Built a wafflepod slab with bulk builder. Put a concrete path around it. 950mm down one side and 850mm the other. I have exposed soil about 250mm between the concrete and fence.
My neighbour has concrete butting up to the fence. So I will probably get their drain off. I should have fully concreted and am freaking out now I didn't.
Got any advice for me?
Hi Liam
Somehow you have to handle the runoff whether it is with a concrete slab, spoon drain, grated drain or some other system. Just remember water is logical. It runs downhill until something collects it or dams it up. You just don;t want your footings to be the thing that dams it up.
Matt
Hi Matt, great information. Thanks so much. Just wanted to get your thoughts. My off the plan house builder is going with M-class slab waffle pod for p class site 85mm thick 20mpa slab for a double storey. The concrete piers are set in. My question is whether the waffle pod slab just sits on it several piers or does have any mechanical connection? How does the piers help if it not connected to waffle slab?
Also I'm bit concern about having M-class slab design by structural engineer on a p class site. Read few horror stories about slab heave and our warranties is only 6 years. What all verification & Compliance certificates I can ask the builder during this slab foundation ? PS the house doesn't have concrete apron around either
Hi RSG
There's a couple of aspects to this question.
First, it is important (for the engineer) to understand why the site has been classified Class P. Check out our post of site classifications cornellengineers.com.au/soil-tests-what-it-means/
Second, a waffle slab design on piers is a legitimate solution so long as the piers and slab design comply with AS2870. Using piers to reduce the stiffness of the slab system is not a legitimate solution unless the engineering justification is clearly shown on the plans.
Third, the connection between the waffle slab and piers depends on the engineers intent. If the site is soft, non-reactive sandy soil or fill and never likely to heave up against the underside of the slab then no connection is required. It really depends on the site and the engineer's design.
As far as verification and compliance goes, being well-informed, keeping an eye on the builder, taking lots of photos during construction and taking independent advice when you need to all will help. However, at the end of the day, work with a builder you know and trust, that has good reviews, is proactive and trustworthy and doesn't want to cut corners.
Congratulations and best of luck with your building project. I hope it goes very well.
Matt Cornell
@@CornellEngineers thank you Matt for your time, very helpful.
Unfortunately this is the off the plan turn key where customers can only visit during PCI. Unlikely they will allow site inspection post slab foundation is done but i will try.
Interestingly noticed that the lot behind our block is the same house model (now 80% completed) and has used H1 slab with concrete piers, while in contrast it is proposed our house will use M slab. Moreover that block has A2 exposure classification & our shows none.
Understand that soil varies block to block but I thought I will flag this with the builder. Does retrofitting concrete apron around the house with proper drainage really helps to avoid future slab heave? We will avoid trees & minimise water use in the back & front yard.
@@rsg5850 A perimeter apron slab can't hurt but doesn't remove the need for the hard, clayey soil surface underneath the landscaping loam to be graded away from the dwelling. If water simply poured off the apron slab, through the sandy loamy fill and then ran back towards the house - then that concrete would be for nothing.
Matt
Our new home is having the slab poured this week. As we drove by the site yesterday, we wondered why we saw large stacks of styrene boxes. Hence, I ended up here.
I do remember the builder mentioned waffle pods but I had no clue what that was. While discussing where the shed and landscaping will go, he sternly urged not planting gardens near the house. Now I know why.
After watching this video, it seems the site we have is perfect for a waffle slab ( I hope). It's on a slope but has a plateau carved out which has been compacted. To me it seems the water will run right past the house through a drainage trench and down the hill.
My question is, if a waffle slab is at such risk of heave due to water underneath, why isn't a conventional slab at risk? Doesn't a conventional slab suffer potential heave?
Yes. It's true. A conventional (raft) footing and slab system is also at risk of slab heave if a site is poorly drained. Many raft slab foundations have heaved and caused damage to a house.
With your slab not yet poured, you are in an awesome position of being able to show this video to your builder so some of our recommendations can be passed on to his subcontractors.
Don't forget. Ask for verification that these requirements (which are more than likely shown on your engineering plans) are being carried out by the builder's subcontractors.
It is these loopholes that I'm trying to close with this video - and it is here for your benefit.
Hi Matt , the sad part about the big volume bilders is the cost cutting they do at every turn they pay trade what they want if a trade person refuses they dont get the job, and the trade that do continue to work for big volume builders are the yes man cowboys. Recently well last 18 months we took our builder to vcat they lost and agreed to our terms and conditions on how to rectify a sinking slab by underpinning front and side of the house with civil and structural engineer to design a drainage system around our house with agricultural AG pipe 800mm deep into clay . Even by the builder agreed to do the works you'd think they do the right thing, no no no they used there own concreters that do driveways to drill 3.5 metres deep under the slab, at the time we assumed they where actual underpinning company but no was I wrong even after drilling into mains power burned out the transformer from across the road on the post pulled the cables out of the pit and fuse box 19 hours no power, then next day so called underpinners drilled into every storm water pipe, and water mains, no such thing of ring before you dig, it was a mess still till this very day builder continues to cut corners there is no policing these builders the VBA is absolutely toothless vcat they say is cheap actually it's not the DVBR have no idea when a builder is in the wrong to order the builder to fix all defects to be rectified by a specified time, mediation is a joke just another way to push people to walk way from complaining about the builder, if there is agreements the builder continues to cut corners. There is always been a hand shake between builders and building surveyors and soil testers. So how is the owners of past and future new homes have any chance in getting a quality built home when all these builders continue to build defects into new homes starting from the footings. I also heard about prefabricated concrete slabs and piers been manufactured in victoria and used in New homes but yet I see no documentation to say otherwise, it only come up in a forum when the owners posted that they where waiting on delivery of the slab and piers, i questioned this to that person she said yes it's true I asked where are they coming from she replied I don't know, is there any truth in this since you would have heard about if it was true or not, love to know your thoughts on this? Thanks in advance.
Hi Mario
I've not heard of prefabricated slabs or piers. I don't know how they could be built to comply with AS2870 unless they are up above the ground like a timber floor.
I applaud your resilience. You have been through a lot and now it is definitely your duty to help those bright-eyed young people build their dream home to make sure they do what they can to protect themselves.
It's a sad day when we realise that the building industry is flawed - sadder still if we do nothing about it.
Thanks for stopping by our TH-cam!
Matt
Idea: Keep ground under patio same level as rest of house and make the waffle pods 100mm shorter in height.
Thanks lasernerd1 for the idea. Don't forget that reducing the height of a waffle slab pod/slab decreases the strength and stiffness of the slab in that area. The idea works, but there is a height limit that you can't go under when you have reactive soils and you need a certain slab stiffness and strength.
Hi Matt,
I am a design engineer from South Africa and found this video to be extremely helpful, thank you very much. I have been asked by a developer to look at using a waffle slab design on their houses. The majority of the site has been classified as a H2 site (15-30mm movement expected).
We are also the civil engineers on the project so we can therefore make suggestions with regards to the height of the building pads. At the moment we have not specified any civil earthworks as the site has a nice gradual fall in one direction and we have therefore not deemed it necessary. What would your recommendation be regarding building platforms at civil construction phase and do you think that aprons around the house should be something that we should consider at all? Thank you in advance.
Hi Tremayne
Skip to this part in the video: th-cam.com/video/5Cu8PXN3Wl4/w-d-xo.html
The problem mainly arises when the developer and then builder create dead flat building pads for houses.
Yu have the advantage of a sloping site overall so use that to achieve falls in your trenches.
Apron slabs in my opinion are not really worth the effort. They cover up poor drainage issues especially when the concrete apron slab is poured on sand (which is porous).
Thanks for stopping by. I'm really pleased to have an engineer watching my videos!
Matt Cornell
@@CornellEngineers Hi Matt, thanks a lot for coming back to me, I really appreciate it. Your view on apron slabs makes 100% sense to me, I will definitely keep that in mind in the future. I will definitely keep a close eye on civil construction and make sure that the sites slope away from the house as far as possible.
With regards to patios and garages. What are your thoughts on designing patios and garages as separate waffle slabs to the house? My idea with this was to design them separately so that I can keep the bottom of all the slabs on the same level and so avoid the 'swimming pool' effect of water ponding in them due to them being lower than the surrounding area. Do you think that this is a good idea, or do you have any other suggestions?
Hi Matt, I've learnt so much from this video mate. Thankyou! Keep up the great work!
I'm planning to build on class H soil so I have a thousand questions but will keep it brief:
1. Aren't the voids in a waffle slab supposed to relieve pressure from slab heave? I've found collapsible void formers which advertise this feature but I'm not sure how polystyrene handles this,
2. Are suspended slabs or homes on pears/bearers/joists the only way to avoid slab heave? Isolation also makes plumbing easier to fix.
Hi. Thanks for your controversial set of questions! Waffle slabs do have voids that 'might' relieve soil pressure but the soil can still bear on the concrete ribs. Waffle slabs do move and crack so I'd say don't rely on the waffle slab voids to protect your house from movement.
Collapsible void formers are a new spin on an old solution. If the slab is designed by an experienced engineer as a suspended slab for downwards loads and also for the potential uplift loads from soil swelling, if the slab is well connected to tension piles and the tension piles have been load tested and well connected to the slab, if the void formers collapse at the load they are supposed to and if void formers are positioned under all of the perimeter footings then maybe!
2. Suspended floors give you better access to the ground under the building but they are still susceptible to foundation movement. Adjustable stumps, super-deep footings (deeper than the zone of influence), some screw piers, strip footing and pole systems are some solutions. This is a question for your design engineer. Don't cut corners. Use an experienced, local engineer when you can.
Hi Matt, mum’s 7 year old home has a great amount of slab heave at the centre of the property around the bathroom/toilet. An engineer recorded a level of +66 in that section. Builder has wiped his hands clean telling mum she created that problem with watering her veggies around the perimeter of her land.
What are your thoughts, should we take the builder to court?
Hi Peter
Uneven movement in a slab is caused by uneven changes in soil moisture after the slab is poured. So it depends on the moisture regime at the time the slab was poured and what changes occurred since then. This video might help explain th-cam.com/video/FTZXWrrp9VQ/w-d-xo.html
Higher levels in the middle of a slab mean either the outside has dropped or the inside has lifted. Without more knowledge of how the soil moisture conditions changed (and without a set of levels to confirm), I couldn't say who or what is the cause.
Hi Peter are you in melbourne , if so please contact me via Facebook, you can find me there and we an chat via messenger it's more private. I tell you how to go about it, its not as easy taking them to court if in Melbourne it's vcat there is bit more involved. I can tell you how to minimize your legal cost, till you need a lawyer. I don't know how to contact you privately on TH-cam. Hope I can help.
Matt, you haven't covered the
1) requirement to maintain constant soil moisture during construction (through the use of temporary down-pipes as soon as the roof is installed, etc).
2) recommendation for the home owner to engage an independent building consultant to ensure the building standards and best practice are adhered to during construction
3) recommendation for drain camera inspections prior to handover to detect problems including cracked pipes damaged during construction
Hi John
It's good practice to have temporary downpipe connections but I'm not sure that temporary downpipes are a "requirement". It isn't in the NCC and it isn't in AS2870 that I can see. That leaves erosion control provisions by local authorities and the plumbing code which I don't have. As a contribution to site drainage and foundation movement temporary downpipes are not on my hit list. Perhaps you can advise where the requirement comes from?
If homeowners find peace of mind in an independent building consultant inspecting the building then I think that is fine. However my dream is that builders and inspectors do their job and that thewre isn't a need for indepedent verification. Pie in the sky but I am sure there are builders that take pride in their work.
@@CornellEngineers I would start with AS2870 6.6(c)
@@johnpeters4214 Temporary downpipe connections are a very nice and easy (and very visible) way for a builder to show that they are doing their best to comply with these drainage requirements.
However AS2870 doesn't specifically ask for temporary connection of downpipes. In my opinion, the channelling of water away from the building under construction could be handled better, and more permanently, by achieving grade in the hard soil surface away from the building.
From the moment the builder steps on site, and preferably from the moment the developer finishes grading the site, good drainage should be deeply inherent in the site/subdivision setup so that it is nearly impossible for anyone to stuff it up.
Sadly, the hardest, most important and influential aspects of site drainage on development sites (like clay plugs, separate pipes and pipe trenches with grade away from the building) are the hardest to police.
Does anyone have any good ideas for how a plumber could prove (to an independent building consultant if you like) that the base of the plumbing trenches has been graded away from the building?
We currently were meant to have our slab down all the waffle foam has blown away and been damaged. If the foam is damaged would that effect the slab
Hi Andrew
The polystyrene is just there to make voids in the slab. If the foam is damaged it isn't the end of the world - the concreters will have to be careful not to fall through when walking on the polystyrene and they might use a little more concrete.
Otherwise I don't see any disadvantage for a home owner.
For anyone else reading - when polystyrene is stored on site waiting to be installed in a slab, it has to be bagged and tied down. It will blow away and broken pieces of polystyrene are an environmental hazard (and ugly-looking).
Matt Cornell
I can see a problem, you mentioned thinking and plumber in the same sentence... how can you tell if a house is built on a waffle slab years after it has been built?
I think my point is that solving issues with slab heave and waffle slabs is a collaborative issue rather than blaming any one trade. But everyone has to play their part well for this to work and the information has to get through to all the players.
To see if a slab is a waffle slab after it has been poured look you have a few options: original plans, ground-penetrating radar, dig under the edge beam and feel up the inside edge for polystyrene or ask the builder. I recently saw a set of plans that had both options - raft slab and waffle slab in the set - so even original plans don't always help so if the slab design was important to us we'd engage a GPR. Finally, if you're really keen you can drill the slab and see if you hit polystyrene.
If it is a waffle slab you can tell if their is styrofoam by drilling into it. Waffle slab if it is built right shouldn’t have those problem. All based on the builder. Also the builder should have soil report provided by a geotechnical engineer who goes out and does borehole logs classifies the soil . I have built with builders who utilise waffle and raft if built correctly their shouldn’t be any of the problems he mentioned. Than again I am more for raft slabs due to less issues and a peace of mind. Would I built raft slabs over waffle yes but then again builders want to save money and most houses are designed for 40 years or so then they are tore down and rebuild. New technology new styles ect. Most houses from 1980s are being tore down as we speak built again. Also I would ask about the builder and ask him to provide at least 5 reference for projects he has completed and visit those houses he has build and ask the occupiers if they have had any issues. E.g leakage in roof cracks in walls due to slab movement ect. Always do your research. I have been working in construction for the past 4 years and a lot of builders don’t even conduct site visits they let the site supervisor run it. Most doesn’t care they just want to finish as fast as possible and get paid. Coming from a civil engineering background and working on houses and multi units I can tell you most builder don’t care. That’s why I want to get my builders licence because I think honesty and quality is the best policy. Most builder I have worked with don’t have a clue I always thought how they got their licence. Remember to do your research before purchasing a house. I remember a house that was build next to my project was literally sinking due to land being land filled and built on. Imagine you bought that you would be cursing yourself. It was in box hill Sydney new development area.
simple solution that every Italian did when he built his house in the 60s: lay concrete over the entire block. No grass, no weeds, no water damage.......no worries mate! 😂
remove the clay? or how about mixing the soil with a blend of different sizes of gravel? clay with the stone allows an adhesion of with the stone. say 5 feet or so, with compacting. this also allows for increase volume of soil so house pads can be much higher, and drainage should be great.
Thank you for your thoughts. We need more thinkers like you.
Interesting concept. Sounds like a lot of work but might save some cost compared to removal of the soil and replacing it. However, it is very difficult to blend anything with clay. It is sticky. It is hard to dig. By trying to mix gravel with it your results are likely to be haphazard at best. We are aware that some engineers specify tyning heavy clays to loosen them and this pracice is just as difficult.
So the mixing idea has merit but it would be hard to ensure evenb mixing and then the soil needs to be compacted back into place in layers.
Matt Cornell
@@CornellEngineers ah ok. I have a question so really the only way to have the soil type one wants is to place it? and if you have clay soil it should be removed all the way down to a different layer of soil? and when you remove soil can it be replaced with say 2 inch clean stone, will that allow for compressive strength to be meet and water drainage ? I know I should go to college and study civil engineering, 🙂. I'm old now. I'm in US. thanks. my thinking before was that a mix of stone and clay might allow the clay to expand in the voids of the stone to stone but I see your point. and of course that swelling does not limit the direction. ok. yeah not a good idea. also thought that clay might act as a binder to hold the stone. oh well. man! this stuff is so interesting.
@@gdotone1 I love your enthusiasm for knowledge. Keep up the good work. Yes, you can place the soil you want but the reactivity of soil (the depth of soil that is affected by seasonal moisture changes) varies depending on the climate. So to achieve a fully stable site you might have to replace 2m of soil. In reality this isn't cost-effective compared to making the slab stronger (we have suggested this to builders previously). We ended up just making the slab and inground beams nice and strong.
Matt
@@CornellEngineers ah I see. you know I just may see if I can take a class or two. maybe the brain cells will hold together long enough for me to enjoy the knowledge. thanks.
Climate wise, from what I have read. Waffle slabs are not very good for the climate here. They are suited to cold climates. Also not being able to plant trees near the house is a big downside. May as well live in an apartment.What is the advantage? Price I guess.
Hi vin van
The advantage is more consistency and less risk for the builder when pricing the concrete footing and slab. The cost savings might get passed on to the client, but the benefit is nearly all in the builder's favour.
Incidentally, the risk on these slabs is now starting to get passed back to the engineers who specify them. forums.whirlpool.net.au/archive/3p8n8lx3
@@CornellEngineers Cheers.
They shouldn't be building slab or waffle housing on reactive clay soils . Should all be timber flooring 600mm minimum above the ground with 4 metre deep piers .
Timber floor framing has its own issues with clayey soils. Adjustable stumps are not everyone's favourite idea.
@@CornellEngineers I said with 4 metre deep piers . The reactive zone is 2.2 metres deep in SE QLD . Virtually every house underpinned by the QBCC is a waffle pod .
Hi there, Do you think deeper pier passing the reactive clay soil is a good solution to prevent the slab heave, or we can change the reactive clay soil on developing block before waffle slab ? Thx so much
:)
;o)