Browse Forums Building Standards; Getting It Right! Re: M-class Slab on P class soil 11May 27, 2018 9:44 pm Geo1 As you say the end result is a "M".So giving a figure even within 5mm in my opinion is unrealistic. Usually one samples is taken from a site to do Lab testing on.We have found when we have taken numerous samples at different depths from one site the results can vary significantly.This has been the case even with undisturbed tubes samples taken next to each other at the same depth.We have seen this with testing done in collaboration with a University.On this particular test site it was shown that the shrink swell test results varied depending on the initial starting moisture conditions when the samples were taken. The end classification varied "H1" in summer and "H2" in winter.This is a problem with the test which is a common test to determine a soil reactivity and ultimately characteristic ground movement. When you think about it, making even a approximate figure within 5mm you are allowing for: *Differences in the mineralogy of the clay over different depths across the whole site. *Layer depth variation that didn't show on limited borehole tests. *Changes in the clay material including, sand layers, carbonates, floaters, localised rock layers etc. Let alone variation in different Lab technician methods and accuracy. When we test a block that has dimensions 35m by 15m and 2.3m deep (climate zone 3) and we then only do 3 holes to 1.8m deep and each hole only 90mm in diameter.You can do the calculations on how much of the soil we actually look at let alone doing any Lab test. Compare this to the actual amount of soil effecting the slab. From the case you have quoted and what I have seen in similar cases before, lawyers love it when precise figures is given. Re: M-class Slab on P class soil 12May 30, 2018 7:12 am insider bentleigh_boy insider The report says that the site is a class"P" but because the fill is controlled your design engineer can use a "M" type slab with some modifications. The fill has been compacted and is said to comply with the earthworks specifications therefore deemed ok to put a modified "M" type slab. The standards allow you to re classify a site that has properly compacted fill. Giving a range of reactivity is common eg 20-40mm.To give a site a specific ground movement number eg 36mm is unrealistic. Anyone who clams they can give a precise ground movement figure for any site even with all the available site and Laboratory testing is kidding themselves.Lawyers love it when it is done though. Sorry I have absolutely no idea about these things.. So do you think i should do another independent soil test? Or do you find their assessment reasonable? How would i know if the M-slab is modified? I'm completely stumped here Thanks You will have to ask the design engineer about the modifications if any.If everybody has done their job correctly then it is reasonable to expect what they say is correct.How can we judge otherwise if we haven't done a soil test on the block ourselves or just looking at part of a report Yes they did bore 5 holes. Sorry I couldn't upload the whole report coz its huge in size. I guess my concern is whether its possible to have a M slab foundation based on their assessment of the site soil. I have attached the bore logs (it doesn't make sense to me! its like reading a foreign language I can't understand!) Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Re: M-class Slab on P class soil 13May 30, 2018 7:28 am Interesting. Will need opinion of some of the other forum members who are professional geological engineers to comment. One thing I did notice is that they never hit rock. It’s wet clay all the way and they stopped at 1.8m depth. I also got M class soil but I hit rock at 1.2m and kept hitting rock until they stopped at 1.6m so it seems a bit suspicious that you can arrive at the same M rating without hitting bedrock. Anyway will be interested in what Geo1 and insider think. Re: M-class Slab on P class soil 14May 30, 2018 10:18 am bentleigh_boy You could have hired your own Geotech/Civil Engineer to specify footing and he would have explained everything to you. Unfortunately, builders don't give you access to their engineers..for obvious reasons I suggest you turn your focus onto what is a reasonable cost for your m-Type footings Slab for garage - is engineer over engineering it? Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: M-class Slab on P class soil 15May 30, 2018 10:35 am bentleigh_boy, I PMed you the geotech engineer my builder used. They seem to be pretty good. Re: M-class Slab on P class soil 16May 30, 2018 10:45 am The Issue isn't with the Engineers report, kirashogun Once you have handed over a deposit to the builder and signed, the builder can charge whatever he wants for big ticket items It's up to bentleigh_boy now to scrutinise the cost if he thinks it's going to be too high, then his option is to pull out and lose his deposit Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: M-class Slab on P class soil 17May 30, 2018 11:21 am StructuralBIMGuy The Issue isn't with the Engineers report, kirashogun Once you have handed over a deposit to the builder and signed, the builder can charge whatever he wants for big ticket items It's up to bentleigh_boy now to scrutinise the cost if he thinks it's going to be too high, then his option is to pull out and lose his deposit Thanks for your comments, The cost is not an issue here as I have fixed site cost in the contract. I'm only worried coz everyone I know questions me "why do you have M-slab on P-class site?" and honestly I have no answer to that. Everyone is suggesting to look into it and so I'm looking for some feedback here. I asked the builder the same question and got a pretty standard expected reply " Thats what the engineers have suggested and you don't have to worry about such things as you have a 25yr guarantee" I just want be sure its safe and that the builder isn't cutting corners. Re: M-class Slab on P class soil 18May 30, 2018 11:24 am kirashogun bentleigh_boy, I PMed you the geotech engineer my builder used. They seem to be pretty good. Thank you kirashogun, much appreciated. Will talk to them for sure. Can I ask which builder you used and where did you build? M-class Slab on P class soil 19May 30, 2018 11:26 am bentleigh_boy StructuralBIMGuy The Issue isn't with the Engineers report, kirashogun Once you have handed over a deposit to the builder and signed, the builder can charge whatever he wants for big ticket items It's up to bentleigh_boy now to scrutinise the cost if he thinks it's going to be too high, then his option is to pull out and lose his deposit Thanks for your comments, The cost is not an issue here as I have fixed site cost in the contract. I'm only worried coz everyone I know questions me "why do you have M-slab on P-class site?" and honestly I have no answer to that. Everyone is suggesting to look into it and so I'm looking for some feedback here. I asked the builder the same question and got a pretty standard expected reply " Thats what the engineers have suggested and you don't have to worry about such things as you have a 25yr guarantee" I just want be sure its safe and that the builder isn't cutting corners. The more important question is your builder engaging in structural engineer to custom design your slab. A class P requires a custom design regardless of underlying soil conditions. I find it interesting that they never hit rock but till can call the soil grade M without producing a Ys value within 5mm accuracy as Geo1 alluded to. Regardless you should receive a custom slab design done by qualified structure engineer. PMed you with my build details. Re: M-class Slab on P class soil 20May 30, 2018 11:32 am kirashogun bentleigh_boy StructuralBIMGuy The Issue isn't with the Engineers report, kirashogun Once you have handed over a deposit to the builder and signed, the builder can charge whatever he wants for big ticket items It's up to bentleigh_boy now to scrutinise the cost if he thinks it's going to be too high, then his option is to pull out and lose his deposit Thanks for your comments, The cost is not an issue here as I have fixed site cost in the contract. I'm only worried coz everyone I know questions me "why do you have M-slab on P-class site?" and honestly I have no answer to that. Everyone is suggesting to look into it and so I'm looking for some feedback here. I asked the builder the same question and got a pretty standard expected reply " Thats what the engineers have suggested and you don't have to worry about such things as you have a 25yr guarantee" I just want be sure its safe and that the builder isn't cutting corners. The more important question is your builder engaging in structural engineer to custom design your slab. A class P requires a custom design regardless of underlying soil conditions. I find it interesting that they never hit rock but till can call the soil grade M without producing a Ys value within 5mm accuracy as Geo1 alluded to. Regardless you should receive a custom slab design done by qualified structure engineer. So what exactly is a custom design slab? do they go deeper for piers? I'm waiting for them to send me the final foundation slab plan. But here is a preliminary siting plan they gave at contract signing. They have however removed the concrete to the front portico in the final drawings. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ The Soil classification has little to do with piers. The purpose of the classing of the soil is to identify the clay content and the "average expected range of movement… 2 10354 I don’t think so as the floor area over 300 square meters then it is class 3…. 12 18329 DIY, Home Maintenance & Repair I'm in WA and our sandy soils make drainage a bit easier but this is what I'd be doing. Dig down to your footings and let the wall dry out. Clean it all well by brushing… 1 6163 |