Sinking floor

Oh dear. Nearly fainted with the mention of 30-40k. I think it's been gradual, the builder seemed to think it was fairly normal. Will definitely get it checked out asap though.

newtec1, that is what the pile looks like, so I'm hoping you're correct.

A couple of other points you need to consider. Is the house single story and what type of roof. If it is single story and tile roof before trusses we used to strut the roof rafters down to a plate which ran through the centre of the house which transferred the weight down a wall to the piles below .Maybe that is your problem,a load bearing wall above the piles.

# 12 maybe. ?

A couple of things to note:
If the work was major, more than 20% of piles to be replaced, then building consent would probably (almost certainly) be needed. Council may then insist on Geotech survey / engineered pile design. Less than 20%, and it's probably exempt from consent under S1 (but still needs to meet building code).
If you leap in now, and get a full geotech survey, then find out that the job is big enough to trigger the requirement for full building consent, there's some chance that the survey Council will insist on won't be the same as the one you've had done - so you'll have to do it again. Mentioning this, because these Geotech surveys are quite expensive - you don't want to risk paying twice. Your builder should have a good understanding of what's needed, and what may be overkill if it's clearly exempt - or not. Or you might want to get a geotech in for advice and quick site inspection, but without carrying out a full survey in the first instance.
The 20% figure may vary from council to council. It's also possible that the number can't be accurately estimated until propping/leveling removes the load, some piles which seem okay may need to be replaced because the packing required to them is over what's allowed (50mm IIRC). If there's good access to inspect each pile condition, level etc before starting work, then a better guess can be made - but it is still likely to be a bit of a guess. The quote may be for a fixed number of piles to be replaced etc, with extras as required. They're probably not trying to have you on, but getting a fixed price quote to complete the whole job without extras may not be possible. But do get several quotes - prices may vary a lot.

Next is a very important but neglected issue, Most home insurance policies are void if you remove a structural element - even if that's "minor" work and exempt from consent - even if it's just one pile etc. The builder's public liability policy should cover them - so also you if say they made a mistake and your house fell down, or they accidentally set it on fire etc. But it won't cover you if your house falls down or burns to the ground for some reason not related to the work the builder is doing. So you need to advise your insurer what you're doing, and either get an agreement from them that your cover is not voided, or take out separate construction insurance.

This has been really helpful and given me something to think about. Now I don't feel like I'm going in, completely in the dark. Thank you :)

You will be in the dark under there.

Hah.

You didn't tell us what type of house or roof. It could make a big difference as to the design etc.

The house that we are doing at the moment, the council had rubber stamped a BC to replace like with like, new piles for the rotten Totara ones.

A geotech survey revealed that the ground at the front of the dwelling was so poor, that you could push an new earth spike in over 1.5 metres by hand no hammer or banging just a push and not a very hard one at that.

Anyway, as a result, the entire underneath of the house has been excavated out, it went from about 600 to about 150 above ground front to back, we now have around 760 from the bottom of the bearer to ground, so you can sit and shuffle around.

Then strip footings are excavated,boxing installed, steel reinforcing (if you end up using that make sure you get the right grade supplied, we found a load of cages had ordinary steel instead of high grade reinforcing bar). then pile fixings are located, concrete is pumped into the strip footings, and then the piles are fixed into the protruding fixings with SS bolts and all new pile/bearer fixings put on.

It is a plik of a job, we have done it ourselves, with engineer design and overseeing the job before pour as well as council inspector at every stage.

The house will probably at a guess about use 50 or 60 cubes of concrete in the strips which not only run under the bearers, but across the house front rear and two strip in the centre, so it is all tied together.

My advice would be, dont bother with a builder until you have had soil tests done, then go from there.

The worst part of the job has been digging in to obtain head room to work, followed by getting the steel cages in and finishing off tying them all together.

If the ground is proven to be unstable on top but sound underneath at depth, then another option is to have a set of deep piles set around either side of the house & steel beams run underneath.
Make it a bit wider on 1 side then needed & build a deck on the overhang.

Seen this done a few years ago on ex peat land for relatives & it worked well.
Cost wasn't too bad either.

You're right. It's a single storey house with concrete tile roofing.

Jeesh Ken - you will be scaring the cr@p out of the original poster.
Unless it's quite bad (and the OP's builder suggests it is "normal" so perhaps only an inch or two in one area) then hopefully it's consent exempt and such extremes aren't needed.
Even on liquefaction affected ground in Chch, repair / lift of up to 100mm is allowed without foundation rebuild. A bit much IMO, but EQC are not exactly generous.
There are however many thousands of houses being done here as you suggest - or variations of.
The OP's home is 1960s. The perimeter foundation may be ok - there should be some steel in it, unlike pre 1940s houses. Even if it's sunk badly in places, then it might be able to be lifted and underpinned rather than removed and rebuilt.
Cladding type/weight is also an issue.

Builders know nothing about geotechnical issues, I have had that proven to me three times now. One case the builder said 450 mm would be fine, we drove a 3.6 metre pile into the ground its full length without achieving the required level of resistance under engineering supervision so it was all kocher.

A simple soil penatrometer (sp) test is all that is required, takes about 30 minutes at the max, I think its a few hundred dollars.

Once you have that information you start to design your foundations.

If you do not have it done, you are really peeing in the wind.

The problem of foundations is they come back to bite at a later stage.

i saw one job, a new build, where they had a digger excavating down, they had gone down a fair bit and the soil was rough, but the engineer said keep going, with that the digger broke through into the underground waterway that was causing all the issues.

So spend a few hundred bux on the soil penetration test, and then you know.

You can look back at the building file and see if any test was done at the time of building , yeah right, and hope that they did.

i would suggest that in many cases in ChCh that if a proper soil test was done, that many of these houses would require an engineer designed foundation rebuild, I wonder if anyone has ever tried it.

This house we are doing at the moment was actually quite good and level all around, it defies belief when you see what it was built on, and what was left holding it up.

Sorry to the OP to paint this picture, but have seen a foundation job go wrong, and the battle as to whose fault it was took years to sort out. Luckily only an onlooker.

Anyway have fun.

Just a point, there is no way that the Council should issue a permit on a like for like basis, as the like has clearly failed within the lifespan of the dwelling.

So i am guessing it is the centre row of piles that are load bearing for the roof struts.

She only wants a few piles replaced,not the foundations for the sky tower. And yes we have done major underpining before,as well as pile replacment, but this sounds like a simple replacement of bearing piles in the centre of the house. If that is the case just removing the piles and replacing them in a new deeper/bigger hole on a pad of conc should be sufficient.