Slab Heave Cause?

Long story short. We hired a structural engineer. He identified in the main effected area we have a 62 mm heave in our raft slab. We have excellent drainage.

What we've tested:

- Stormwater system which failed, but only during pressure testing and the failures were on top of the pipes (cracks). Not what we believe is the cause/source of water coming in as we fixed the item and are still having issues.
- Tube tested on neighbours side fence next to his pool. No water came in.
- Dye tested neighbours stormwater system next to problem area of house. No joy.
- Sewerage and hot water service tested all fine.
- When we dug boreholes along our neighbours fence the clay soil was saturated at a depth of about 300mm. The boreholes right against our slab were moist but not saturated. At a depth of 700 against the house we hit virgin dirt which is dry.
- Some further things to make life difficult, our house and our 4 neighbours along have the same issues. There is a water bore hole about 100 metres from our house. Not sure if it's been decommissioned.
- One of the neighbours approx. 4 months ago had a water main burst in there front yard.

Structural engineer can't figure it out.... I'm out of ideas. Can anyone please help?

Sorry for the short story/lack of details I'm just exhausted... Happy to provide any further info.

What is your soil classification and when was handover?

FirstHomeBuilder25
Class M
How old is the Estate?
Is it a commercial bore?
Check old geological records for strata types?

Handover was on the 4th of December, 2015, and the site classification is P and a H2 slab.

It’s the Eynesbury estate, I’d have to look into it

If I could pose a question. If during the build the builder didn’t have site drainage, Flexi tubing or install the stormwater system until after the roof and cladding was on, could the water that may of caused the slab to heave during construction still be causing it to progressively get worse 3 years down the track... or am I still looking for a source of water somewhere causing the issue?

Thanks, I would explore the bore or similar, possibly a natural fault which is bringing moisture up,
also check the chemical composition of the water.hth

The cause of worsening heave can only be determined by an expert onsite investigation but it is strange that the neighbours are having similar issues.

AS 2870 was revised in 2011 but many builders and their contractors did not adhere to new regulations for highly reactive sites for many years and some may still not. One area often ignored was the requirement to clay plug pipe trenches under the slab to prevent the trench being a conduit. This and other 'unseens' can only be determined by examination.

Heave can also cause the drain (sewer) pipe fittings and sometimes the pipes themselves under the slab to crack. Were these also investigated?

Your landscaping could also be at fault but we don't know without being on site. Do you have a sloped impervious perimeter?

insider is the highly valued resident forum expert and he will know your area.

My understanding is it’s just the sewerage that runs under the house. These pipes have all been inspected/pressure tested and have passed.

I wouldn’t say the landscaping is impervious but the structural engineer who conducted the first assessment on our house said the site drainage which we did was excellent.

Good. Landscaping is the first thing that builders look at as a get out of jail free card.

When did the 'heave' start? Melbourne had a few La Nina years after the Millennium drought broke at the end of 2010 but the last few years and this year have seen well below average rainfall. 'Heave' can also be caused by subsidence on one side, were slab levels taken at different areas?

There’s no trees or anything that is watered/retaining water near our property. It started one month after moving in in December 2015. Yes slab leveled were taken throughout the house we had some spikes in other areas but about 10-20mm whereas the problem area is about 60mm

Hi FirstHomeBuilder25,

Can you indicate why the site was originally classified as Class "P"? This maybe something that others have overlooked?

You have a relatively very stiff slab, so I am interested to understand whats the problem (i.e. you have indicated areas of cracking, degree of cracking etc).

For example, if you had a site that prior to build or at a minimum 6months prior to build with trees on it, and then the builder/developer/yourself removed all the vegetation then adding a slab can cause significant amounts of heave (i.e. 50-80% above your estimated "normal" ys value or H2).

Same principle behind others mentioning moving from a dry period to wet period. However your site was classified as Class "P" and an indication of "why" may help people on this forum either rule in or rule out different scenarios.

Regards

Geo1

Hi Geo1,

Here is the soil report pages which answer your questions.

Hi firsthomwbuyer25

You have a class "p" due to the depth of clay fill on site.
Your house was built during a drought which was the second halve of 2015 followed by a mini wet period.The soil around that estate has highly reactive soil.You are very close to several old volcanoes.The fill used in your area is from highly reactive clay and going by your logs shows it was on the dry side.If the builder didn't use temporary downpipes and grade the soil away from the footings then you have all the factors that can lead to slab heave.
Eynesbury and the Melton area have had their fair share of slab heave

Highly reactive fill can swell more that similarly reactive soil in its natural state.
Once it has happened what to do about it is a difficult question.

You mentioned sewer drains, sewer trenches can be irrigation system under slab if they have not been plugged off.

I’m starting to think that there were some issues around the construction phase. Here’s some photos that I’ve got. There is a fair bit of spill from the slab being poured.

The cladding on and no stormwater system was even installed as you can see it under the back deck and the spouting ran straight to alongside the slab (house was built in wet period) soil was not graded away.
I’m not even sure they scraped the block from the start

I think you are past the stage of trying to figure this out yourself, also your engineer does not have the answers (why am I not surprised?).
Your house will not fix itself and the builder won't fix it unless there is compelling evidence. What is your builder saying?
Your best bet is someone with extensive experience beyond engineering for forensic examination of all the evidence and to build a credible argument for the builder to fix your home (if indeed the evidence falls that way) otherwise it's your loss.

Builder is blaming the council and developer for the land although he has done no investigation and has no interest. He has said he won’t accept any responsibility. At one stage he offered to buy the house and then backtracked and said he wouldn’t. He previously has made promises to fix everything but he’s all talk and has never done anything. Everything is writing. Just on a separate note from the building inspectors I’ve dealt with and the engineer I don’t think the house can be fixed.

your call

Did you take any photos during the construction's 'wet period' that show the poor drainage and no temporary downpipes etc?