Thread: Re-stumping on clay/reactive soil foundation

Bottom line is that any kind of support structure founded on reactive soil requires heavier footings that if founded on sand or rock. The options would be to build a suitable platform footing for each stump or to bore to suitable foundation depth to obtain suitable bearing - which in this case, would probably be as expensive as underpinning. If your cracking is being caused by soil drying in the drought, there's not a great deal that can be meaningfully done for the long-term, because if you choose to restump for the current conditions, soil expansion when the rains return will cause the reverse problem to what you're experiencing now. A possible alternative you might want to consider is Uretek (www.uretek.com.au), which is an exanding urethane foam that is pumped under subsiding structures to improve the ground conditions. It doesn't just lift the structure, it permeates the soil, thereby improving bearing capacity.
In short, there isn't a single solution, and you should find out precisely what mechanism is at work befre you consider any remedy. who did the inspection? was it an engineer, or a building surveyor of some kind?

Greetings GSJ.
If you have been advised that the soil is reactive clay then you are more than likely within a basalt area which is not uncommon on the northern/north east areas of melb.
Possibly the main variable in the restumping cost will be the height of the floor & access. The stump depth should be around 500mm which I would advise to have verified by a soil test & report. The report will give the correct founding depth which will help to minimise movement.

Is the cracking in the plasterwork on internal walls ?
Is the home a brick veneer, solid brick or weatherboard ?
If its brick & the cracking is in the bricks or the external walls then restumping will do nothing.

Originally Posted by GSJ

Hi,

What are the implications of re-stumping a c1900 Victorian period house in Northcote (Melbourne), that is in an area with clay/reactive soil type?

Do stumps need to be deeper/longer and have more concrete at the base???

Is it much more expensive to re-stump a house with this soil type?

How effective is it likely to be in preventing cracks?

A building inspection of this property, suggested that re-stumping should be considered...Underpinning, though, was not recommended as it was quite expensive, and may not fix the problem of cracking...

Any comments appreciated...

GSJ

Hi GSJ,

I live next door in Thornbury and if you looked through the archives you would have seen the dramas we have had having had our place restumped twice within 9 months. The first crowd recommended to go 500mm deep (which they did) and within 3 months I was having trouble opening/closing some of our internal/external doors:mad:. After contacting the building commission we had to bite the bullet and got in a second crew who said it was common practice (if you knew what you were doing!!) to put stumps in at least a metre (around the perimeter of an existing house) in such clay based areas. Although it is only 3 months since it was completed I haven't had to adjust a door yet. However, it can all change if we have a hot summer after this dry winter - but hopefully not though.

In regard to underpinning that is only required if you have a concrete base which I assume you have under part of your house?.

Regards
David

Hi,

In reply...

The building inspection was done by an Architect.

The cracking is on internal and external walls and through bricks, most noticeable on the internal walls though.

I believe the house is solid brick.

I guess I am not expecting any major long-term fix here, particularly in regards to cracking, but if the floors are sagging and bouncing (a little bit at present, mainly in kithen/living area), and the property has not been re-stumped in a long time, then I´d still be inclined to re-stump. Not going to underpin, neighbours did this and their cracks got worse.

I am mainly interested in doing what I can to maintain this property for future years. It´s been standing so far for 100 years, so if it´s to make it to the next 100, it certainly needs a bit of help.

A combination of things are being done or planned at present: improving subfloor ventilation with new vents and a mechanical system, removal of large trees and shrubs in rear yard, repairing damaged roof, chimney flashings, and weathered mortar joints.

As part of a larger renovation (ie. including changing the floor plan, new flooring and re-painting), I would do the re-stumping (including soil test, clearing subfloor debris + pest inspection), add a new damp-proof course and put new plasterboard sheeting over internal walls to cover the cracks, and improve the cosmetic appearance.

Basically trying to make the most of the property given it´s structural problems and the soil type it is sitting on.

The Uretek system sounds interesting. If it can in some way help decrease or minimise the movement in this soil type, then I would certainly consider this. Particularly if it is a much cheaper, simpler and faster process than the usual method of underpinning.

Has anyone used Uretek on their properties or know of someone who has, or used a similar system, I think they call it "resin injections"??? Any ideas of the ball-park costs involved, this house is 130m2)???

Thanks,

GSJ

gday,
Firstly, I suggest you have a geotechnical engineer have a look at it, as the cracking could be caused by several mechanisms and each one requires a different approach to rectify. I can recommend Douglas Partners (I used to work for them or Golder Associates, both of which are national firms, but there are any number of firms who could assist.

Uretek is essentially a urethane grout, which has the added benefit that it expands in place, something like bentonite, although it doesn't have the moisture reaction that bentonite is susceptible to. the only thing new about the technology is the material, the technique has been around for years. it would probably only be suitable for some of your stumps, as it is likely that your problems are caused by a combination of ground expansion and shrinkage. Why did the architect recommend improved subfloor ventilation? Is the subfloor damp? Is there adequate drainage around the home? does the home have decent garden beds (at least 1.5m width) around all four sides? are there no large trees within 1.5 times the height of the tree from the house? All these things help to stabilise moisture profiles in the foundation, which is the likely cause of your problems. It sounds as though the outer stumps may have subsided, whilst the inner ones have not, thus causing the house to "break its back". Simply restumping won't stop that recurring unless you do something to stop the large moisture variation under the stumps. bear in mind that a reactive soil could change its level by up to 70-80 mm between seasons, so if there is any variation in the rate of moisture movement in the soil, you can end up with big differences across the house.

'morning GSJ
It is not uncommon for homes of this style & age to have subfloor ventillation problems & increasing the airflow will definitely be a good step but I wouldn't be inclined to go overboard unless you know that the lack of air circulation is definitely causing problems.
Its quite proper for someone doing an inspection to say that there is a lack of subfloor airflow, but that could also be said of possibly the majority of homes of this era.
The cracking in the brickwork will not be rectified without underpinning the strip footings & I agree, the cost may not be warranted espec. if the cracks are cosmetic. The footings for this era are renowned for being 'light on' & if you do decide to go down the road of underpinning, only take the advice of a geotechnical firm after they have done test bores. - there are a lot of firms with heaps of advice & little knowledge.
As far as the restumping goes you mention 'bouncy floors' which leads me to conclude that a simple restumping will remove the majority of your concerns. I would take a guess that the home is probably just due for restumping - whether thats a result of lack of ventillation or ponding of water around the stumps or some other cause you probably won't know until you rip the floor up - one thing I will bet on is that it is not caused by the soil type.
Keep us up to date with the progress.

Hi,

In reply,

The architect said there was evidence of rising damp throughout the house, and there were only 3 old subfloor vents for the whole house.

Could not access the subfloor during the inspection and so couldn´t comment on whether it was damp or not.

The coucil had put new stormwater drains in the area 3 years ago, and I believe the drainage around the house is adequate, apart from some small pooling of water in the alley way after a rainy night.

At the rear of the house is a small courtyard. A large 6-8m or so Ash tree was planted by the previous owners 3m from the house, closest to the rear windows and the right side party wall (this wall has the biggest cracks). This was removed recently.

There is a garden bed on the rear fence and right sided fence, about 1.5 depth. There are a few small shrubs there, and the tenants have a small vegetable garden, about 2m X 1.5m in area - it is about 2m from the house wall.

I was planning at some stage to remove all the plants/garden and just put new paving in the courtyard with a roller door and off-street car park, and maybe some pot plants. It will be lower maintenance, add value to the property, and ?help decrease the moisture variations in the soil to some extent...

Certainly, the services of a geotechnical engineer would be of benefit here, and I think it is something I will follow up. Thanks for the referrals.

At present and in the near future, my plans are to continue to proceed with the smaller maintenance jobs in stages, as I can afford. Then later, get paid professional advice on the major structural work and renovation work that needs to be done.

Will post my progress...

Any further comments appreciated...

GSJ

Are the cracks near where the ash tree was removed larger at the bottom or the top? what about the rest of the home? A sketch plan of where they are and which way they open would be useful when you visit an engineer. Also include the stump locations if possible. is there also an in-ground strip footing? I would be more inclined to put garden beds than concrete around the home, simply because a garden bed allows moisture to be added to the soil around the home, maintaining decent pore pressure and assisting to retain decent bearing capacity. You may even find that you can remedy part of the problem by extended watering over a period, especially if the problem is due to the outer footings subsiding. What you are trying to achieve, ultimately, is a moist soil that has much the same moisture content across the entire foundation of the house.

Hi,

Here´s a picture of the cracks across the right side wall at the back of the house, where the living and kitchen area is. I think this area of the house was renovated many years ago by a previous owner. Makes me cringe whenever I see it...Any thoughts...

GSJ

Originally Posted by GSJ

Hi,

Here´s a picture of the cracks across the right side wall at the back of the house, where the living and kitchen area is. I think this area of the house was renovated many years ago by a previous owner. Makes me cringe whenever I see it...Any thoughts...

GSJ

At first glance, I'd say the foundation has subsided under the footing in the corner near the door. The cracks are larger at that end and almost closed further away. Is this the wall that the ash tree was closest to? As a side note, the mortar used was a bit strong for the bricks too, if you're getting cracks in the bricks, but that's hardly surprising 100 years ago.A sketch plan of the location and nature of the cracks and the location and style of the footing would be much more useful. Have you done any kind of level check across the floor?

Yes, that wall is closest to where the ash tree was...ie. the tree was situated on the left side of the photo shown. Not exactly sure what a level check is...but the floor appears to sag downwards in the middle of this living area.

GSJ

Originally Posted by GSJ

Yes, that wall is closest to where the ash tree was...ie. the tree was situated on the left side of the photo shown. Not exactly sure what a level check is...but the floor appears to sag downwards in the middle of this living area.

GSJ

Are ash trees shallow or deep-rooted? It could wel be that the roots have lifted a footing on the left of that photo, in which case you have a bigger problem that just that of soil movement. Without having more info mate, that's about as much as I can give you. As far as the floor sagging goes, that will be a simple re-stumping or possibly just a matter of chocking, ISTM.