Browse Forums Building A New House 1 Jul 14, 2015 10:52 pm Does anyone know why we'd have no rubble/hardcore/blinding layer under our slab? We told our Site Supervisor but he didn't believe us. But we clearly don't ... So why not? We have E-D P extremely reactive soil with uncontrolled (clay) fill. Our slab is 125mm thick with double mesh, 1 metre drop edge beams + 1 metre piers ... But no rubble Plastic was laid straight on the earth Thought you had to have rubble ?? Re: Do you have rubble under your slab? 2Jul 15, 2015 5:22 am Hey TomCat I can't comment on the rubble / crusher dust but, We have a similar footing design and situation. We will be having up to 1.8 mt deep continuous piers. (All the beams will be 1.8mt deep) I was wondering the same, But basically the fill doesn't support the slab the trenched piers support it from the stabile soil deep down. The only problem as I see it is if dry clay is used as fill and becomes wet and expands after the build, causing heave. I chased this up with the engineer and he confirmed it. Clay fill should be avoided although difficult in Adelaide. Given the wet weather this may not be a problem as the clay will be wet and expanded.... After the build the fill will dry out and settle on the piers? Anyone else got any thoughts? Our Build Here: viewtopic.php?f=31&t=74037&p=1255246#p1255246 Re: Do you have rubble under your slab? 3Jul 19, 2015 3:48 am Thanks Claypot. Yeah the extra piers are (hopefully) anchoring/supporting the slab and the way it looks right now (swimming in a pool of soupy wet clay ) keeping it from sliding down the street! I'm really curious why we had no crusher dust/blinding layer/rubble. I thought that was standard for all builds. Perhaps I need to trawl through everyone's threads looking at their site prep/trenching pics to see if they have it. Thanks for responding Re: Do you have rubble under your slab? 4Jul 19, 2015 6:55 am Hi TC, Was going to respond when you first put this up but had nothing useful/helpful to say. Every build I have seen has had a layer of rubble/sandy crusher dust, we did on our last build. This build we did too but it was a darker coloured sandy rubble and definitely set hard when it was compacted. They laid the same stuff around the whole house and in the driveway so there really isn't much clay exposed where they are working and when it is compacted trucks, boots etc it sets hard. I'll look through our engineering document (we got a 20 plus page document!) and see if it is actually specified and get back to you. Re: Do you have rubble under your slab? 6Jul 19, 2015 8:31 am On my preliminary engineering of my waffle pod slab the Engineers recommend a layer of not more than 50mm of crushed rock compacted and under the moisture barrier. (My summary) The paragraph is a lot longer but I take it is recommended and not essential. Maybe one of our engineers on the forum could comment. I couldn't copy and paste from the PDF from Simonds but if required I could write it all out. Do you have rubble under your slab? 7Jul 19, 2015 9:10 am Hi TC. It's Mr Tribe. Without seeing your soil report, bore logs and slab design I say this with a caveat of general advice. I wouldn't be too worried about not having a rubble barrier under your slab. From memory your slab is a traditional slab with deeper rafts and no waffles. Your soil rating has been given because of the amount of uncontrolled fill on your lot which hasn't got a certified compaction rate attributed to it. If it was certified you would probably sit around the H1 level like us. Having a E-P rating will actually work in your favour as the engineer will bulk up your rio grades, increase piering numbers and depth and sometimes diameter. Also your slab (depth and volume of concrete) has been beefed up due to this soil result. The piers will act to buffer your clay heave and should work like a suspended slab. Also your rafts (the trenches between your lamingtons) will also act to buffer this heave (shrinking and swelling of the clays as they dry and expand under moisture). So my thoughts. You're ok. Crusher dust or gravels won't do much against heave. Moisture will still penetrate to the clays and start the swelling. It's the slab design that will combat this. Also you will find your fill will naturally compact overtime especially under the new house loads and the heave in your underlying soils will actually reduce. Soil testing for slab design is really basic for houses. An engineer has to go off two boreholes only. What if the rig went through a rock? Or hits a pocket of unconsolidated fill or hits clays in one spot and silty soft clays in another. All this leads to an engineer accounting for uncertainties and they will all be conservative and add this into their slab designs. Hope this eases your worries. Feel free to flick through your slab design. Re: Do you have rubble under your slab? 8Jul 19, 2015 9:56 am TomCat Thanks Claypot. Yeah the extra piers are (hopefully) anchoring/supporting the slab and the way it looks right now (swimming in a pool of soupy wet clay ) keeping it from sliding down the street! The slab would be treated as a suspended slab supported on piers and designed accordingly. The question is how many piers, where are they, what was the soil conditions like when they were put in,. etc,etc The piers would need to be reinforced if there are horizontal loads applied. Generally, they may not necessarily be attached to the slab? Your engineer would have designed the slab accordingly based on the loads and site conditions expected.Here in WA engineers inspect slabs before the pour and I am on site on difficult pours as a rule. TomCat I'm really curious why we had no crusher dust/blinding layer/rubble. There are factors that influence as to whether a crushed layer is put down..you will need to ask the engineer and not the builder TomCat I thought that was standard for all builds. Perhaps I need to trawl through everyone's threads looking at their site prep/trenching pics to see if they have it. Thanks for responding If people dont put photos up you are wasting time reading the posts/threads as they are difficult to understand without Just curious did you take decent photos? Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Do you have rubble under your slab? 9Jul 19, 2015 10:27 am Tribe7 Hi TC. It's Mr Tribe. Without seeing your soil report, bore logs and slab design I say this with a caveat of general advice. I wouldn't be too worried about not having a rubble barrier under your slab. From memory your slab is a traditional slab with deeper rafts and no waffles. Your soil rating has been given because of the amount of uncontrolled fill on your lot which hasn't got a certified compaction rate attributed to it. If it was certified you would probably sit around the H1 level like us. Having a E-P rating will actually work in your favour as the engineer will bulk up your rio grades, increase piering numbers and depth and sometimes diameter. Also your slab (depth and volume of concrete) has been beefed up due to this soil result. The piers will act to buffer your clay heave and should work like a suspended slab. Also your rafts (the trenches between your lamingtons) will also act to buffer this heave (shrinking and swelling of the clays as they dry and expand under moisture). So my thoughts. You're ok. Crusher dust or gravels won't do much against heave. Moisture will still penetrate to the clays and start the swelling. It's the slab design that will combat this. Also you will find your fill will naturally compact overtime especially under the new house loads and the heave in your underlying soils will actually reduce. Soil testing for slab design is really basic for houses. An engineer has to go off two boreholes only. What if the rig went through a rock? Or hits a pocket of unconsolidated fill or hits clays in one spot and silty soft clays in another. All this leads to an engineer accounting for uncertainties and they will all be conservative and add this into their slab designs. Hope this eases your worries. Feel free to flick through your slab design. That is a great explanation Mr Tribe! Thanks Re: Do you have rubble under your slab? 10Jul 19, 2015 10:40 am Thank you Mr T (Eng) and SBIMG (eng) It was raining when they dug trenches. http://i1292.photobucket.com/albums/b572/MandyBarlow/Avalon/Now%20building/6MayTrenching_zpsznr8paoo.jpg ^the Adelaide rains had just started. Was wet but as you can see from the dust, not completely wet through. And it poured (rain) when slab was poured. Best pic I have pre-pour. Plus the video (shows it more clearly). I cant see any mention of rubble on my footing design Though only an expert would know how to read! https://mmmanderine.files.wordpress.com ... report.pdf Re: Do you have rubble under your slab? 11Jul 19, 2015 10:54 am And thank you Mr T, yes I feel much better because of your explanation. Very kind of you to both take time to explain xox We've had a lot of rain here in ADL. I've have stormwater running into the house. We we're not quick enough to get temporary sw 'ribbon pipes' up and I forgot to ask. Site has a lot of soupy wet clay pooled around base at moment. And I keep coming across 'slab heave' threads Goody keeps digging up - thanks Goody! Thanks mate. Yeah good points! Leaning towards Option 3 to get a bit extra space in the cabinets but not going too crazy high (and expensive). Would require a mini… 13 40248 Thanks for the insights, that makes perfect sense, and yeah, I will be leaning on the experience of the excavator operator entirely. 6 16463 4 5175 |