Any ground workers or experts in soakaways?

[Camdig]

Closed link/loop systems are what I installed for the last 15 years before I retired from ground works. Local Building control would not accept herringbone systems unless all the ends were connected and on a level stone base of at least 200mm depthView attachment 338263View attachment 338264. All connecting pipework need to be level in order to get even distribution of soakaway discharge. All pipework would then be covered by 50mm clean stone/gravel with a Terram/geotex membrane on top.

That's the way my last few have been, even for treatment plants.
The closed loop seems to be the way of engineers here on Scotland, SEPA still accept herring bone but needs to be sitting on gravel like you say, every council district has different rules which for a country the size of Scotland is ridiculous.

 

[Tazz]

you need to create voids for the water to sit in during high flow peaks, then it can get away over a period,
herringbone /fingers as you call them will work better for the same amount of digging/stone,
most new soakaways are build as a void area using plastic cages, known as milkcrates locally,

Soakaways have clearly moved on since “ Dig hole fill with broken bricks” every day is school day

 

[Bell308]

if you want chapter and verse on sokaway design have a look at a document called BRE 365 should be able to help you but even in herring bone systems the space between the fingers is allways likly to become saturated so this will eventually become ineffective as a means if infiltrating water into the sub-soils, you can install a few geotextile wrapped storage crates within the system to give you some additional capacity, the outflow from your septic tank can be calculated from the british water document entitled Flows and Loads etc.

trust this helps.

 

[Norfolk Horn]

To decide on the appropriate design of your soakaway you need to carry out a percolation test and then from this work out the volume of water the land can absorb for a given area, then you need to know the volume of waste water that is produced by your home to calculate the size of the soakaway to do it properly.
Otherwise just stick in as much as you can and hope for the best.
The outlet in your existing soakaway looks quiet deep how is this in relationship to to outlet on you onion as you call it.
Are you sure that no rainwater is entering your foul system so overloading it.