Low Invert Levels and backflow - Passive slab

[SuperJohnG]

I'm rapidly approaching starting work on site. Civils mate who is doing groundworks called yesterday to say he could start next week if I wanted, but I haven;'t yet engineered the life out my drainage and levels and hence I am not quite prepared.

 

I've a large flat plot, and using Treatment plant and a direct discharge into a burn (SEPA approved). I'd deliberately set the house 'into the' ground rather than on it, so that it looked natural rather than sitting on top of the ground, and also to avoid excessive extra infill material if I raised it up. However this leavesme limited amount if space from FFL to the normal water level in the burn, approx 1.25m. 

 

I am following the popular Kore foundation system route and expecting that I need to drop all drainage out through the bottom, rather than put sewage pipes out walls. 

 

The issue I have is that I am struggling to get the fall required and also protect against backflow from the burn. Normally the burn is only 50mm deep, but when there is heavy rainfall and storms it can rise 1m. 

 

The distance from the drainage through the slab to the TP is 45m, and this is where i am struggling to achieve the minimum 80:1 fall on the sewage outlet. See diagram below, some buildup notes 

 

Depth of Kore system - approx 400mm

Hadcore and sand blinding: 250mm

 

From here, I have guessed that I can leave the drainage pipe 50mm below the underside of the hardcore. Now using a GRAF one 2clean (It'll be that or equivalent, biopure,  etc). From where the septic tank is to be sited, I drop it 675 below surface (535 + 140), I f i assume that the drainage pipe top is 50mm clear below the bottom of hardcore, and it's a 110mm pipe. Then that means the fall height is 465mm (625-160) which is over 45m giving a ratio of 96:1, which is a good shallower than it needs to be. 

 

Second issue being, even with the TP being as high as possible here, during storms I will backflow the tank. I think this can easily be dealt with using a backflow device (picture below) the water usually comes down within a few hours - however I suppose it could stay up for a day if we had a prolonged period of rain (I'm on the west coast of Scotland - so highly likely).  

 

Now possible solution is to drop the tank down further 200mm - which would give  a much more reasonable (465+200= 665 fall height) 67:1 over the 45m. However now I'm very close to the water level (205mm) constantly and regularly it would come up above the outfall. 

 

I was trying to make sense of the best route here by asking - but in writing this thread I think I have answered my own question. It would seem that the best plan is to drop the Tank down the extra 200mm and then just changed it from a gravity type to a pumped solution, then this all goes away? or do I do somethign different and pop the waster out the wall - which won't seem to be an elegant solution? 

 

In either case I'll need a backflow device, regardless for peace of mind. I've attached the site plan and foundation plan for reference. Apologies it might be a little disjointed and hard to follow...I've been rattling this around in my dreams all last night and needed to put it on paper to work it through.

 

 

 

 

 

724-11 Site Plan.pdf 724-13 Foundation Plan.pdf

 

Edited September 27, 2020 by SuperJohnG
removed extra image

[Russell griffiths]

Why is the TP so far from the house, why not get it closer and have a decent fall to the tank and a shallower fall to the burn. 

[SuperJohnG]

16 minutes ago, Russell griffiths said:

Why is the TP so far from the house, why not get it closer and have a decent fall to the tank and a shallower fall to the burn. 

I'd originally had it at the back of the house. But moved it to the front as it made sense  as it was downstream. 

I can getbit closer but was trying to move it out the way as such from the front view and also I have my borehole wellhead between the current position and on the drainage route there. 

 

Maybe I'd just be better to move it back to the rear. Shorter run for power and drainage and deals with the fall issue straight away and reduces costs  for groundworks.

[SuperJohnG]

I've also now reread my OP... I should really focus on fixing my spelling and grammar before posting. Jesus. 

[Oz07]

21 minutes ago, SuperJohnG said:

I'd originally had it at the back of the house. But moved it to the front as it made sense  as it was downstream. 

I can getbit closer but was trying to move it out the way as such from the front view and also I have my borehole wellhead between the current position and on the drainage route there. 

 

Maybe I'd just be better to move it back to the rear. Shorter run for power and drainage and deals with the fall issue straight away and reduces costs  for groundworks.

sometimes the simplest solution is the best

[Mr Punter]

Can you fit a non return valve on the pipe if it is liable to surcharge?

[dpmiller]

get a pumped outlet version and you can have the unit at whatever level suits the house

[SuperJohnG]

3 hours ago, Oz07 said:

sometimes the simplest solution is the best

 I think so, leads me right back there doesn't it. 

 

1 hour ago, Mr Punter said:

Can you fit a non return valve on the pipe if it is liable to surcharge?

 

Yes. That was my planning either case, I.meant to add a picture of them. Can just go downstream of outlet and protect when required. Fairly simple and cheap

25 minutes ago, dpmiller said:

get a pumped outlet version and you can have the unit at whatever level suits the house

 

I think that's definitely going to be a better option too, gives me the freedom and less worry about being able to get it within these tight tolerances.