Will depend if your footings are on piers or not and the soil profile

Yes get a soil test done first

Have you got any cracking or signs of slab movement ?
What part of Mickelham roughly ?
One thing I would do is get floor levels done which should let you know…

So when was the house built ?

Thats frustrating hopefully your drainage plan will work.
Another thing to look at is where the sewer pipe goes under the slab that the trench is seal off to stop water…

That is most likely not from erosion but an original poor slab pour.
The big question is :
Is the water flowing under the slab from poor drainage during wet periods and…

Geotechnical engineers : R I Brown, DM lawrance, VHC engineers, Smolders geotechnical, USL group, Macfarlane partners, Tim Gibney engineers.

Looks like it is flooding in from your neighbours property. Is the neighbours concrete path sloping towards your fence and dumping water onto your side ? Another issue I…

Stormwater has to be connected to legal point of discharge sub surface drainage doesn't but where it discharges may cause problems so ideally it should be connected to stormwater usually…

They very rarely grade the soil away. I would use builders plastic and make it a double layer you can use it under the deck as well.

So it has been confirmed there are no plumbing leaks so it could be poor drainage ponding water under or against your slab.
From your videos you have pebble landscaping…

When you say gushing out do you mean directly under the edge of the slab beam where the plastic is or down below the depth of the slab at the…

Get in contact with a company called slabsense who will set up equipment to measure slab movement on a daily basis. Also contact a geotechnical engineering company that does forensic…

Is it that wet each time you go to site or was that photo taken after a wet few days ?

Does look like you may have drainage issue with the retaining wall from the photo.

That is the wrong design, never have a strip drain up against your house. The concrete should slope away from the wall into a strip drain or inlet drain that…

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Go to google earth pro and use the history button at the top of the screen to go back through satellite images from the past and see for yourself if…

It's not the roots themselves it's the effect they have on soil moisture. You should have bored piers or screw piles to stop slab settlement. If the trees are removed…

Floor levels , footing probes and boreholes with moisture or suction samples.

I agree with Althom but there also may be a problem with the terra cotta stormwater pipe discharging water in that areas as well. You can just see the top…

Did they check the hydro valve ?

You can use a better version of a French drain called a cut off drain along your fence line and run a plastic membrane over the ground between the house…

Yes I agree and best to have porches, front columns etc all part of the slab less chance of seperate movement.
This is not a waffle slab it's a conventional…

The soil profile looks like fill which is common in Mickelham. What happens is water permeates through fill and then ponds on the natural soil layer so when you dig…

It may be the trench for the pipe under there that wasn't properly backfilled and compacted. How wide is the settlement area ? The builder will shake it off. Slabs…

Are they stumps or screw piles ? How much fill is there, is the site a class "P" ? A "H2" site is not supposed to have stumps it is…

Looks like the fill is settling may be localised hopefully. Do you have the original soil test? One thing I noticed in your photos is you have a tap without…