Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 367

Design Criteria for Granular Mineral Envelopes

For gravel envelope, Vlotman et al. ( 2000 ) suggested the following criteria for the
coarse boundary envelop material: for filter retention, D 15c <7 d 85f ; for gradation
curve guide, D 60c = 5 D 15c ; segregation criteria, D 100 <9.5 mm. For the fine boundary

envelope materials, the criterions are the following: for hydraulic, D 15f = >4 d 15c ;

D 15c /5 (based on C u ≤ for gradation curve guide (bandwidth), D 15f = 6 and band-

≤

width ratio 5); for hydraulic criterion, D 5 > 0.074 mm; for gradation curve guide

(bandwidth), D 60f =

D 60c /5; for retention criteria (bridging), D 85 =

D 60c .
In the above criterion, symbols “ D ” and “ d ” refer to particle size distribution of
the gravel and base soil, respectively, where number gives the percentage with a
smaller diameter. For example, D 50 is the diameter of the sieve where 50% passes.
The “ C u ” means uniformity coefficient and the superscript “b” means base materi-
als. The indices c and f in the subscripts refer to the upper and lower particle size
diameters of the base-soil and the gravel.

Design Criteria by SCS (1971)
The SCS has combined other researcher's results into a specification. Their criteri-
ons are

D 50f
D 50b =

12 to 50

D 15f
D 15b =

12 to 40

Other criterions: filter materials should pass 1.5 in. sieve, 90% should pass the
0.75 in. sieve, and no more than 10% should pass the 0.01 in. sieve.
If the filter and base materials are nearly uniform, a filter stability ratio of less
than 5 is generally safe, i.e., D 15f
D 85b

5.
The uniformity co-efficient, C u ,( D 60 / D 10 ) was also considered as a design factor.
A low uniformity co-efficient indicates a uniform material. Kruse ( 1962 ) considered
envelope material with a C u of 1.78 to be uniform.

<

9.7.4.3 Envelope Thickness

As the amount of envelope material requirement increases with the square of the
diameter, it is important to judicious selection of envelope thickness. The US Bureau
of Reclamation recommended a minimum thickness of 10 cm around the pipe. The
SCS ( 1971 ) recommends 8 cm minimum thickness.

9.7.5 Use of Particle Size Distribution Curve in Designing
Envelop Material

Particle size distribution curve shows the relationship between grain size and per-
cent larger (or finer) at that size. Particle size distribution curves of the soil and