Old Septic Tank

[letiha]

Hi

So its been a few years since I was last on here after building 6 houses in 2 years...we are ready to go and build again.

We have found a block we like and the price is right, however it has been a block that has been split and the old septic tank is on the left hand side of the block towards the back, possibly just within the building envelope.

Someone has suggested that we can fill with concrete and just build over but I am unsure.

Has anyone come across this before and what was the outcome and costs.

 

[Television]

Hi

So its been a few years since I was last on here after building 6 houses in 2 years...we are ready to go and build again.

We have found a block we like and the price is right, however it has been a block that has been split and the old septic tank is on the left hand side of the block towards the back, possibly just within the building envelope.

Someone has suggested that we can fill with concrete and just build over but I am unsure.

Has anyone come across this before and what was the outcome and costs.

Knowing what little I do about council and environmental conditions, I would think they would want it pumped out/cleaned at the very least. After that you could probably try and fill it with concrete, though without seeing the design it might be hard to say how effective this will be.

I'd try and get in contact with council and ask.

 

[Scott No Mates]

Usually, you'd pump it out and fill with dirt, no need to concrete it in. You can always have it smashed up providing it has been disconnected property. Have a chat to your friendly plumber or check with council.

 

[Doovalacky]

If it does turn out to be past the building envelope I have heard of them being turned into large soak wells by drilling a number of holes into them.

Otherwise pump out, drill holes/smash bottom and fill with dirt.
However this might have already been done depending on local rules. My local area you had to provide council an invoice as proof it had been done within x months of septic being connected.

 

[Peter 14.7]

If within envelope, part or full, you have issue.

You cannot load a new building on to this and then onto natural ground, two different bearing pressures.

If outside of area and the angle of repose ( the load of the building line) , so the tank bottom is lower than a line draw 45 degree from bottom of new footing, then reuse it as a water tank. Pump out etc...That assumed tank is not cracked and will actually hold water. If not, fill with dirt and avoid any services, driveways, car parking over.

The type of ground you have influences this.

Peter

 

[Steel Concrete]

Buried structures - differential settlement hazard

As above, the risk of building over a buried object (old pool / tank / stormdrain) is a significant issue, especially if the future house is to be of brittle finishes eg masonry / gyprok; It will generally crack and move around even if the old septic is broken out and backfilled with good compacted engineered fill as the fill further compacts or the different material fills with water, affects nearby reactive clays etc... God forbid there is flooding on the site (eg near a river / creek) as it will accelerate the engineered fill compaction.

Hence, it is strongly recommend carrying out a geotechnical investigation ($800) and then getting a well regarded structural engineer ($3000-5000) to design a house with a very generous stiffened raft footing system for the likely Class P footing in AS2870 ($20,000+??)

If not you / the builder will be liable for repairs / unlikely underpinning for up to 7 years after construction completition. You really dont want to go down with a BSA investigation as the property owner genearlly wears most of the cost even though you think the builder is at fault....

Hope this helps,

 

[letiha]

Thanks for all the advice, i think we will give this one a miss, there are also some issues with having to increase the height due to potential flooding so not what we are looking for.