Browse Forums Paving & Concreting 1 Feb 10, 2023 9:36 am Hi All, The house is a ~1982 built, single storey, double brick exterior walls, single brick interior walls, hard wood used throughout the roof construction, large number of cement rendered interior walls, tiled & carpeted internal floors. Dimensions of dwelling ~9.2m x 22.2m with approx. 2.5m wide 'verandah' extension of slab on all sides under roof cover. Location is just inland of Coffs Harbour - very high rainfall area. Soil is clay and rocks just a few cms from the surface all over the property. Site of the building is on the top of a crest/small rise, falling away slightly on 3 of 4 sides. We purchased the property 7yrs ago - were aware of some of the cracks in the walls & foundation at the time. But didn't give it much consideration due to age of property & other factors. In hindsight something of a mistake but will not assist us now. There had always been cracks in the foundation of the house, and some cracking in areas of rendered brick walls. These were aesthetic issues only & remain so. However over the years they have in some areas gotten mildly worse, others much the same. I had asked a number of non-expert persons about what i should do & all including a neighbour felt it was only due to soil and rigid materials of house - so felt it was a non-issue. Clearly wrong, I knew this but ignorance is bliss sometimes. Not too long ago - in applying an under slab termite perimeter barrier (drilling 10mm holes around entire perimeter of the brick walls, 10mm from wall at 150mm intervals for injection of fipronil solution) - I found that in many areas there was significant voids under the concrete slab. Being a complete novice I thought this quite unusual - but focused on what was already a large job I didn't give it too much thought. However upon hindsight I realised this was almost certainly the cause of the foundation cracking and wall damage to the property. Thus here I am. I will note I am again near certain that these sub-foundation voids were caused almost exclusively by terrible drainage around the slabs edge after heavy rain. The large 14m x 27m roof had only a single downpipe on each corner and water would regularly exit the 'weep holes' from the quad style guttering as they overflowed during even mild rain. It was not unusual for significant pooling to occur on 3 sides after heavy rain and soil depression had occured in these spots making this even worse. I will note that I am certain that trees & animals are not a factor in the slab issues - the nearest instrusive tree (Liquidamber, is 35m from slab). To combat this I reinforced all the guttering and added an additional 4 downpipes, also adding extra soil to fill the depressions and tried to build up these areas into inclines away from the slab edges. But the voids under the slab remain. So how to address? I am unsure whether to go with 'mudjacking' or resin/polyurethane injection? Unsure if the latter is available in this non-metro area. As with many folks at this time, money is somewhat tight so I'd try and negate costs by doing as much of the work as I could but recognise that theres a limit to this in such matters. I should have taken care of this earlier - but would appreciate advice or thoughts to move forward with atleast ensuring no future damage occurs. Please let me know if additional info is required and thank you in advance for your assistance. PS. Tried to directly upload images - but failed twice so have posted linked to Google gallery of them - apologies. https://photos.app.goo.gl/Ve63FHgbpkBD3S8f7 Re: Voids under house foundation - how to address? 2Feb 13, 2023 7:43 am Bump... haha well if nothing else I feel a tiny bit better that I haven't completely snoozed on something incredibly important - if 1000+ learned folks view and I've not immediately been told WTF was I thinking. ![]() Re: Voids under house foundation - how to address? 3Feb 13, 2023 9:25 am ![]() Bump... haha well if nothing else I feel a tiny bit better that I haven't completely snoozed on something incredibly important - if 1000+ learned folks view and I've not immediately been told WTF was I thinking. ![]() Hey that's funny, Q. then seeing the cracks extend and get wider shouldn't worry you either? Here's a few things that you can also do while waiting for an geo/engineer/expert to visit your site 1. See if you can tract down the original certified geo report and engineering your local council may have a copy 2.Buy a laser level and check to see if the slab's sagging/ hogging, the tell tale cracking patterns should should match the movements 3.Hire a cheap auger drill a few holes, how would you classify the holes, polystyrene slabs flex, heavy rigid rafts less,etc 4. Talk to your neighbours and ask if they are having similar problems 5. Unfortunately you can't diagnose a problem using words alone.. more info required Cheers Chris Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Voids under house foundation - how to address? 4Feb 13, 2023 9:51 am ![]() ![]() Bump... haha well if nothing else I feel a tiny bit better that I haven't completely snoozed on something incredibly important - if 1000+ learned folks view and I've not immediately been told WTF was I thinking. ![]() Hey that's funny, Q. then seeing the cracks extend and get wider shouldn't worry you either? Here's a few things that you can also do while waiting for an geo/engineer/expert to visit your site 1. See if you can tract down the original certified geo report and engineering your local council may have a copy 2.Buy a laser level and check to see if the slab's sagging/ hogging, the tell tale cracking patterns should should match the movements 3.Hire a cheap auger drill a few holes, how would you classify the holes, polystyrene slabs flex, heavy rigid rafts less,etc 4. Talk to your neighbours and ask if they are having similar problems 5. Unfortunately you can't diagnose a problem using words alone.. more info required Cheers Chris Chris, Appreciate the reply. Umm as I said I was 'bumping' the thread. The progression of the crack have concerned me - hence I'm here, my comment was in rerference to the normal online practive which is folks telling someone they're a moron is they'd done something incorrectly. But as i said, just tryng to bump a thread for a response and appreciate yours. Just curious - as mentioned I know there are decent sized voids under the slab - as I've drilled around the entire perimeter of it and found it consistantly, largely in the areas where the worst pooling of water after heavy rains occurred. So unsure what the laser level will confirm that I cannot already see the results of? RE: 1. I will check but I highly doubt they will have them - these would have been submitted to council in ~1980. I have original hand drafted plans for house etc but no Geo report etc. As mentioned it's all incredibly reactive soil here, clay pan just a few cm down. RE: 3. Could you please clarify what you mean by this? I do not believe the slab is one incorporating polystyrene. RE 4. The neighbours would not be relevant. We're rural and many hundreds of metres between dwellings, also dwellings are very different construction from vastly different construction dates. Also alas there's not a lot of neighbourly spirit - if you know what I mean. RE 5. I did attach an image gallery of some of the cracks, but as stated I am near certain that the cause of this is the voiding under the slab. The cause of the voids I am near certain is the improper guttering and drainage, soil erosion etc. I believe I have corrected these - so would have thought it logical to address the voids. Thank you for your reponse it is appreciated. Re: Voids under house foundation - how to address? 5Feb 13, 2023 10:17 am Quickly Re1. You will need to determine the footing type, depth width, reinforcement Size.. might wish to consider a scan? Re3. The newer slabs are Polystyrene/voids designed to float and flex? Older rafts have grid thickenings, they should try to line them up with masonry walls Re.4 Contour/slope causes water to flow down. do you have drain channels, swales diverting the runoff stormwater away? Re5 the crack show the consequences not the cause, you'll need to address the cause first, search underpinning, just a thought the perimeter footings may need to go deeper, remedy might take a while Also start mapping the data, otherwise you'll never know if things are improving? Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Voids under house foundation - how to address? 6Feb 13, 2023 10:28 am ![]() Quickly Re1. You will need to determine the footing type, depth width, reinforcement Size.. might wish to consider a scan? Re3. The newer slabs are Polystyrene/voids designed to float and flex? Older rafts have grid thickenings, they should try to line them up with masonry walls Re.4 Contour/slope causes water to flow down. do you have drain channels, swales diverting the runoff stormwater away? Re5 the crack show the consequences not the cause, you'll need to adress them first, search underpinning, just a thought the perimeter footings may need to go deeper Appreciate your effort as i'm sure you're busy. RE 1. Yes, this is getting beyond the info I can rattle off with accuracy As mentioned it's ~1982 built. I know it has steel reo grids in it, can see a very small part of this in sections - and also hit this a bunch of times when drilling the holes for the sub-slab termite treatment. Width and length is easy enough, thickness is harder to say. RE3. Understood - highly doubt it's the former, defintely the older style - which I suspect was a poor choice given the highly reactive clay soil here but assume thats just how things were done then. RE4. On atleast 2 sides the water runs away naturally to some degree - my focus had been limiting the water getting there in the first place - hence focus on the gutters which were major issue. Re5. Understood and agree.Am aware of underpinning and will look into again but think this is a tad beyond the anticipated scale of the issue. Atleast I'd certainly hopeso as limited info on that indicates its very expensive - that aside what is needed, is needed. Apreciate your replies and info. Re: Voids under house foundation - how to address? 7Feb 13, 2023 11:17 am RE1 You can't keep drilling the slab unless you have engineering drawings. That's why scanning has become popular get an engineer not an inspector with a new tool? (humour) RE3 deep edge raft beam with grid thickenings is far superior. I've inspected many houses 50year and older hardly any cracking. RE4 It could be gutters, below ground cracks, etc a case of elimination and monitoring RE5 You'll need to check as how far down you need to go.. unfortunately no silver bullet fix If you need to go deeper, maybe beam , offset piles. Make sure the advice giver is a qualified engineer Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Thanks 🙏 I was worried if the roof went on before it’s fixed! 2 14231 Building Standards; Getting It Right! Make sure you don’t dig below footing base level, so you don’t undermine your foundation. Also the backfilling of trench need to allow for stormwater to drain away… 1 12231 Hi all, It appears they have cut and jackhammered the house footing for a service they were planning to run? Will there be any potential structural issues? Noting that… 0 11463 |