Building near a septic tank

[Steve_W]

Bit of background, we've planning permission for an oak-framed double garage with room over - so a non-habitable building.

What I can't pin down is how close we can build to our existing old brick double-chamber septic tank?

Building Control Officer himself isn't sure and thought it was 5 metres, but can't find anything in Approved Doc Part H to back that. There's the 7 metres from habitable parts but, as he says, that doesn't apply to a garage.

I found this tank manufacturer's website that says 4 metres for England & Wales, but when I rang to ask they couldn't say where they got that distance from other than "probably Part H" https://tricel.co.uk/sewage-treatment/septic-tank-...

BCO's about to go on a fortnight's holiday so I'm trying to gather facts before he's back.

His suggestion was to get a structural engineer involved to discuss a raft foundation and see what they can suggest. Since it's a garage it shouldn't be as "heavy" as a brick & block house, for example, so maybe the raft could be nearer the tank. Anyone know a good SE near Reading?

[bobtail4x4]

what`s the "room above"?

I would always work on 5m as in the British standard

[Steve_W]

Just a single room the size of the footprint, likely to be used for storing parts etc. Not intending to be used as a home office or anything like that, but will have veluxes and a window in the gable opposite to the door. Stairs up to it are outside.

Ah, 5 metres would be a PITA as it puts one wall practically next to a bloody great willow tree - doubt its roots would like that, hence trying to get as close to the tank as possible.

It would be great if I could place the garage 1 or 2 metres away from the tank as that avoids the willow. Do you have a link to the 5 metres rule at all please?

[satans worm]

Im no expert..... but you need to be careful of the drainage field from the tank (not sure if they all have this though)

This can be a large area down hill of the septic there water flows out into the ground.

Nothing can be build on this otherwise you can damage the drain field

Hopefully an expert will be along soon ......

[Steve_W]

Yes, the tank serves both us and next door and is still in use.

Luckily the outfall pipe and leach field is downslope of the tank, away from the garage site - the other side of the willow (which no doubt has its feet in the "goodness!"

[Steve_W]

Luckily my neighbour's a great guy - helped me fix the incoming pipe when the original salt glaze one failed recently.

Also, the BCO is definitely trying to be helpful, just he's unable to pin down any chapter and verse on distances. That might work in my favour though since he's suggested getting a structural engineer to look at it and think about a raft foundation. HIs words were along the line of "if the SE comes up with a good design that moves it closer, I can't see a problem with it in principle"

Have to find a good SE near Reading now. Have a groundworks guy coming out tomorrow so will ask him if he knows a SE, amongst everything else.

[bobtail4x4]

by the time you have paid a struct engineer, You could dig deep strip founds to the depth of the tank, and mass filled,

[KTMsm]

bobtail4x4 said:

by the time you have paid a struct engineer, You could dig deep strip founds to the depth of the tank, and mass filled,

I'm not an expert but the above would be my initial thoughts.

The distance requirement is due to load, if you take the footing down to the same depth you negate the issue (I believe)

I'd empty the tank if you're digging out near it but I see no reason you couldn't be against it with the right construction

[Steve_W]

By mass fill I guess you mean concrete? (Hands up, I'm not in the trade) as opposed to trench and blockwork?

From memory the BCO has to inspect the trench before a concrete pour? Might be a bit of a juggling act to get tank empty, trench dug, BCO out, and concrete poured all in short order but not impossible. Next door are good folks but can't see them chuckling at "don't flush the bog for a few days"

Hadn't thought about the separation distance being about the impact of sideways loads, makes sense.

Oh and the tank's 5 foot deep. I made sure I held the right end when pulling the measuring stick out too!

I do appreciate the help and suggestions folks.

[Magooagain]

Most modern septic tanks get emptied of the waste but are refilled by the tanker driver with water/dirty water as its not prudent to leave modern tanks empty for any time period.
It's still worth bearing this in mind with an older brick two chamber system as if it's empty when mass filled concrete is poured then it may just be possible of unequal pressure.

It wIll all depend on how close foundations really are going to be and what a professional on site visit will turn up.

[Imasurv]

Willow tree you say? Clay sub soil…..?

Would suggest that as the garage isn’t habitable, then the distance to the garage should depend on drainage outfall, maintaining access to the tank and structural considerations. The 7m given in AD H presumably only applying to habitable parts due to nuisance from potential smells and could be less.

Edited by Imasurv on Friday 18th June 20:45

[Steve_W]

Imasurv said:

Willow tree you say? Clay sub soil…..?

It is clay, yes, can you tell that just from the presence of the willow? Impressive tree/soil knowledge!

Mind you, you have to dig through a foot of so of made up ground from years of dumping stuff it seems, before you get to the proper soil then the clay below.

[Imasurv]

It’s more from the point of view that Willow trees, clay soil and buildings don’t mix due to high potential for heave. Willows are high water demand trees. Did you say the founds are going to be a raft? Has the bco agreed this to mitigate the clay and willow? I would expect traditional strip footings will be a complete no no as they would need to be over 3m deep if the willow is as close as you say…

[Murph7355]

Sounds like a raft might help with several of your considerations. I'd get an SE to look as the BCO has suggested. Might give you much more flexibility all round.