M-class Slab on P class soil

Looking for some advice regarding our slab which the builder's engineers have recommended to be M class but they have classified the the Soil as 'P due to fill'
I know people get a H class slab on a P class soil. Very confused coz the engineer's report says only the the "controlled compact fill by developer is P class but the soil below is natural M class. the Fill is about 800mm . I've attached a snapshot of the soil test results. Any guidance would be very much appreciated. TIA

Did they even do a soil test or did they infer. Because a Ys value of 20-40mm sounds suspect. My soil report has a single figure rather than a range that is exactly matching soil classification. Sounds like all they did is infer. Not good enough.

P just means your slab needs to be custom designed due to something unusual about the site. It is not strictly a soil condition grade. In your case it’s the fill. All KDR sites are P grade. But a separate soil grade still needs to be given.

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.

Is visual tactile identification combined with local knowledge how they do soil test these days? No drilling required?

Nobody is asking for a precise number but a range of 20-40 is just laziness. I have a Ys value as an approximate in my report. Not a range but then again they drilled till bedrock at several locations to make sure.

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

In the report they have mentioned that they went on site on 23rd oct and did the soil test.
Is a M slab acceptable based on their assessment?

Did they give you the result of the borehole logs because the report you have posted only showed they went on site, had a squeeze of the soil with their hand and that’s it.

The borehole log will tell you what sort of soil you have at what depth.

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

Did they do Lab testing on soil samples or just plucked that figure out of thin air?Not all KDR have to be class "P" sites

Hi Kirashogun,

I do have a research paper at the office, which is only a few years old. In essence, its asked some local soil testers/professors and "locally experienced" engineers to use ""visual tactile" identification of a soil (all were given the same sample), from memory the results were with a variation of up to 70% from the actual laboratory results. Not many were even close and the conclusion of the paper were that the "visual tactile" identification clause within AS2870 should be removed.

I do believe that AS2870 also requires the site classification to provide a ys within 5mm. Therefore technically 20-40mm is not correct, however in some instances I often use 20-40mm where we have deep fills of non reactive material and we are purely trying to give an estimate of long term differential settlement from front to back or side to side etc.

However when you have a laboratory sample, then there should be no reason why you cannot provide a "ys" within 5mm. (i.e. 35-40mm instead of 20-40mm). Not really crucial these days because every Class is "Standard" designed (except for Class P). Therefore there is actually no change in design/costs between a design ys of say 20-25mm then there is to a design ys of 35-40mm. As both get a standard design Class M slab.

I do remember a high profile court case of recent history where in essence;

- Team 1 (home owner) argued the site had a ys of 72mm
- Team 2 (engineer) argued the site was a Class H (old AS2870 version before H1 and H2 existed).

The engineer designed a Class H.
The home owners experts argued the Class H was not correct because the results show a ys of 72mm meaning Class E.

The engineer's team was a little smarter then the owners team as they argued that AS2870 requires the ys to be rounded to the nearest 5mm. Therefore regardless of whether the actual ys was 72mm when rounded to the nearest 5mm the design ys was 70mm = Class H.

The Engineer one and the owner lost.

Regards

Geo1

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.

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!)

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.

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?

bentleigh_boy, I PMed you the geotech engineer my builder used. They seem to be pretty good.

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.

Thank you kirashogun, much appreciated. Will talk to them for sure.
Can I ask which builder you used and where did you build?

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.

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.