Soil properties and percolation

[aaron252]

Hi,

I am currently working on a project which involves a septic tank and will discharge to a raised mound soakaway system.

As part of the filter process the mound will be predominantly sand (percolation = 12 - 72 s/mm) however i also want to specify a 300mm thick layer of imported soil at the base of the mound due to the underlying soil conditions.

This soil layer will slow the absorption into the ground and i have indicative percolation properties of 2.4 - 288 s/mm. Unfortunately i have no idea what kind of soil has these properties and have no idea where to look so i thought i'd ask you guys.

Anyone got any suggestions?

 

[BigH]

Question - if you don't know what kind of soils you want to use - how do you come up with a three significant digit rate of 288 (versus 300?) or 2.4 (rather than 2 or 3). Seems pretty precise.
I've seen some references in the past on perc rates and estimates of the coefficient of permeability. I'll try to look for it; perhaps others will have it handy. BUT, I am quite interested in the 'accuracy' of the rates you want to specify - and how you come up with them!

 

[eric1037]

There are some correlations between grain size (D10) and the coefficient of permeability that are in the NAVFAC DM-7.01 on page 7.1-139.

However, these are only approximations. You could potentially set up a test program of different gradations and run lab perms on them.

Still, you won't really know until the soil is placed.

It is also difficult to specify a gradation if it's not readily available in the area where the work will be performed. You may want to compare some of the standard DOT gradations in your area to the chart I mentioned.

 

[BigH]

Off the internet from following source:
Washington County, Oregon

Texture Permeability Percolation
(inch/hour) (minute/inch)

Sand > 6.0 < 10

Sandy loams
/porous silty loams/
silty clay loams 0.2 to 6.0 10-45

Clays, compact
/silt loams/silty
clay loams < 0.2 > 45

 

[aaron252]

The figures are based on a design manual for the raised mound hence the 'accuracy'. I understand it is quite difficult to specify an exact soil however i thought it might be worth a try.

Cheers

 

[BigH]

Ma'af but I think it is totally ludicrous for any source to specify some value such as 288 (units?) given the fact that if you are within 30% of the 'real' value on your estimate, you've had a good day. Determining coefficient of permeability and percolation tests are fraught with errors - both operator and physcial. Just my 2000 Rupiah worth!

 

[aaron252]

The units are s/mm as in seconds per millimetre for water to percolate into the soil. The British Standard specifies how to carry out the test and which results are unacceptable for a soakaway system, i.e the permeability is too low.

Close this thread i dont think it is going to get any where.

Thanks