Mechanical analysis

Soil tested against a type 'modified P' sand.

Shaded areas indicate the particle size range for this soil type.

Comments:
The D85 of this sand is 0.52 mm.

Any gravel to match this sand must have a D15 < 5 x the D85, i.e. > 2.58 mm.

The grading of the gravel using AS sieves is:

% retained
> 6.7 mm : 0%
4.75 - 6.7 mm : 14.4%
4.0 - 4.75 mm : 20.6%
2.36 - 4.0 mm : 47.3%
1.18 - 2.36 mm : 14.7%
0.60 - 1.18 mm : 1.9%
0.30 - 0.60 mm : 0.3%
0.15 - 0.30 mm : 0.1%
0.075 - 0.15 mm : 0.1%
<0.075mm : 0.6

The D15 of this gravel is 2.2 mm and any sand to match it must have a D85 > 0.44 mm to prevent migration. This matches the sand above and provides some margin for the grading of the gravel to change slightly (as invariably happens).

The D90 of the gravel is 5.0 mm. Uniformity must be less than 2.5 and is the D90/D15, in this case is 2.3 – also OK.

Some general points to note:

• Any sand used in a perched water table construction must be matched to the gravel to prevent potential migration problems.

• A sand that is too coarse will probably not hold enough water and will be droughty, and a sand with too many fine particles is likely to have slow drainage and a high water holding capacity.

• Organic matter, in the form of peat moss, cocopeat, pine bark fines etc., should be added to increase both water holding capacity and cation exchange capacity.

• At least 100 mm of gravel is desirable and 300 mm of the particular sand above would be required. Organic matter should only be added to the top 100 mm as organic matter in the perched water table may cause black layer problems as this area is constantly saturated. The aim is to have the perched water table 100 mm below the surface.