Asking about a soil test report

Hi there, I had a report from a soil tester. Material taken from site for laboratory testing is of a H2. He changed the classification to P by the reason “due to the trees on site which may cause abnormal moisture conditions”.

The point is, there were 2 average size palm trees on site. Is this reasonable? Can soil compaction be another alternative choice? So, the slab can be classified H2 instead of P. Should I have another soil test to confirm?

I would appreciate for some advice. Thanks.

H means: Highly reactive clay sites. Can experience high ground movement from moisture changes.

The presence of trees can make the moisture changes more dramatic so more compaction isn't going to make any difference.

Thanks Bashworth. I have visited your webpage. You have plenty of details for "a new house"..

Not a bad idea getting another soil test done.

The fact they moved you from H2 to P means you are in "Difficult" site category which will impact the design of your slab and footings. I would imagine for a P soil classification your engineer will propose making your slab stiffer with deeper footings, meaning higher cost for you.

The "P" is not a soil type it is a site classification. It simply advises there is a "Problem" that the engineers need to consider when designing for the reactivity of the soils. A 'P' generally relates to Trees as you have said, or quite often uncontrolled fill, sometimes even bearing capacity.
Getting another test may or may not give you a different classification to a H2. When the geo's take a sample of the bore holes they are taking approximately half a coke can worth of soil, the test is to see how much clay content is in the soil and when wet, how much it will expand.
They generally do two bore logs to obtain an average view of the site.

The New test may not draw up or take the same type of soils.

When they take the sample back for their lab they conduct two tests,
(1) shrink, to find how much the soil will shrink in dry conditions. This is done literally by placing a sample of the soil in an oven and measuring before & after.
(2) Swell, a sample of soil is thrown into a container of water and left until it stops expanding, the sample is measured before and after to calculate its expansion.
This provides a range the soil is expected to expand as the clays take up moisture and how much it will shrink in dry conditions.

The engineer now has a range that the site in normal condition will move. Trees cause the "Average Moisture" to be abnormal to a localised area. The tree draws water out of the soil. Think of the root system as a dart board. Close to the bulleye (truck/root ball) the tree takes 80% water out of the soil, 1m ring out from the root ball the roots take 75% of the water in the soil, +1m out 65% water taken, +1m 55% water taken etc etc. This means the clay (soil) will expand / shrink at deferent levels. So the abnormal moisture is due to the tree's root system drawing the water out of the soil.

If you have a H2 and you go shopping for a Lower classification, you are risking your home cracking for $4-$5k, and yes YOU are risking. Your contract has a duty of disclosure if you have a valid and correctly undertaken soil classification, and you get another as H1 and you don't tell the builder you may have a problem!

first time ive heard of somone wanting a less engineered slab.

seriously, you are not the expert in these matters. Let the geotech engineers report the conditions and structural engineers design the slab to the conditions. This is to your benefit.

Thanks everyone for your suggestion. I have just had a quote for 200 m2 slab for $60k. Does this sound OK?

i can get a car for 50k, does that sound ok?

You are always best to over engineer your slab.

For our business, the 6 year warranty we give is for structure and waterproofing.

We never ever want to have to come back and and fix either of those items.

My business partner has a structural engineering background so takes the view that everyone of our slabs should be designed as though it is P rated site.

We always do 450-500mm diameter piers to rock, and chuck as much steel in as we can. Better to be safe than sorry.

Find other areas to cut costs in, like your tiles or appliances or something else.

Better to be conservative and spend more on your slab and footings than be sorry later.

I'm currently building on a H2 site and my slab engineering is designed using steel screw piers - more than 50 of em. Cost for steel piers was $6.5k and H2 slab upgrade was 5.5k. My base site works cost was more than 50K.

I do appreciate for all comments.