Package treatment plant infiltration depth?

[flanagaj]

So I'm slightly confused on this.  I've just read Part H regs and only see reference to a minimum depth "distribution pipes should be laid at a minimum
depth of 500mm below the surface", but I've also seen mention of it being a maximum depth of 900mm from other sources, but Part H makes no mention of a maximum depth?

 

The reason I ask is that if we stick with the regs of having the PTP at 7m from the house and site the drainage field where we want it (15m from house), the total run from the first stack at the house is 32m.  If I go 1/80 fall for soil pipe to PTP and then 1/200 for PTP to drainage, it basically equates to 304mm.  Which might make the soil pipe depth at the house ~500mm. 

[nod]

In my experience BC won’t come along with a tape measure Put it in the best pace possible 

[Russell griffiths]

The pipes exiting my house are as shallow as I could get them, literally 100mm below ground, concrete cover over them and then a concrete path over the top, pedestrian traffic only. 
it’s the only way to get the fall, my first toilet is 25m from the plant, if you stuck to the regs the time I got to the plant it would be a metre lower than it is. 

[saveasteading]

The depths are guidelines that avoid analysis and are conservative.

To the best of my understanding, the logic is thus:

Q

The minimum is to keep below trouble, be lower than any wheel loads will damage, and away from frost risk. None of these may apply in your situation.

 

The max depth is to avoid the weight of trench fill crushing the pipe. If you protect the pipe and backfill very thoroughly then a bit more won't matter.

 

Shallower is better of course, for economy and safety.

 

Re the frost risk. What chance is there of sewage freezing? It flows briefly down the pipe and away. Workmanship is important at slow gradients to avoid lips and dips.

 

I think at the steading, where the winter can be chilly , from -300mm cover for both foul and surface water. It wasn't questioned.

Was that realism from the bco or because it was a formal design, I don't  know. 

 

Another thing you can do is have open gullies and slopes, so if ever the pipe blocks, the water runs away harmlessly.

[ProDave]

Also too deep and you might be below the water table in winter.

[saveasteading]

For any  CE nerds.  

I once had to design a surface water drain, about 300dia and 2.5m deep. It was in danger of collapsing under gte weight of thd backfill ( by digging and refilling, the interlock and load spread is lost). We filled half the depth with straw bales to reduce the dead weight and it could also float a bit.   

 

[flanagaj]

I thought the drainage field depth was to do with the waste being in the aerobic zone of the soil, hence why they state no more than 900mm deep.  I am sure Graf make mention of this depth, but I could be wrong.

 

[saveasteading]

18 minutes ago, flanagaj said:

waste being in the aerobic zone of the soil, hence why they state no more than 900mm deep.  

That's not the same as building regs for pipe depth.

There's nothing magic about 900mm either and I'd think the aerobics that far down are ineffectual.

I haven't seen any science on the subject, just don't know how oxygen could replenish at depth.

 

I suspect 900mm is an "out of harms way" statement that is preferable to silence on the subject, and tends to be practical.