Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 366

Envelope materials can be categorized into three groups:

(a) Organic envelopes : organic materials that are used as envelope materials.
Frequently used materials are sawdust, chaff, cereal straw, flax straw, cedar, leaf,
bamboo, corncobs, wood chips, reeds, heathers, bushes, grass sod, and coconut
fiber.
(b) In-organic/mineral envelopes : the most common and most widely used enve-
lope materials are naturally graded coarse sands and fine gravels.
(c) Man-made/synthetic envelopes : this type includes fiberglass, geo-textile sheets
(<0.5 mm), successors of glass fiber sheets. Synthetic materials are rel-
atively inexpensive and can be manufactured at large quantities of exact

specification.

9.7.4 Design of Drain Envelope

9.7.4.1 Steps
The steps for designing drain envelope materials are the following:

• Make a mechanical analysis of both the soil and the proposed envelope material

• Prepare particle size distribution curve for each material

• Decide by some set of criteria whether the envelope material is satisfactory or not.

9.7.4.2 Criteria for Selecting Envelope Material
Many researchers have done work on specification of envelope materials, and dif-
ferent ratios were recommended. Terzaghi ( 1922 ) , Karpoff ( 1955 ) , Juusela ( 1958 ) ,
Kruse ( 1962 ) , Soil Conservation Service of USDA (SCS, 1971 , 1988 , 1994 ) , the
United States Army Corps of Engineers (US Army Corps of Engineers, 1941 , 1978 ) ,
the United States Bureau of Reclamation (USBR, 1973, 1978 , 1993), Stuyt ( 1992 )
sequentially improved and suggested criteria for envelop materials. More recent
work was carried out by Vlotman et al. ( 1994 , 1995 , 2000 ) and Stuyt et al. ( 2005 ) .

Design Criteria for Synthetic Fiber Envelope

Dierickx (1993) and Vlotman et al. ( 2000 ) made detail review of various proposed
retention criteria, primarily for geotextiles. The ratio of O 90 / d 90 is frequently used,
with O 90 and d 90 the envelope pore size and the soil particle size, respectively, for
which 90% of the pores or particles are smaller. Vlotman et al. ( 2000 ) propose that
the retention criteria O 90 / d 90 will range from 2.5 to 5 for envelope thickness ranging
from 1 to 5 mm.
Stuyt et al. ( 2005 ) suggested the following criteria for synthetic fiber enve-
lope: for thickness ≥ 5 mm, O 90 / d 90 ≤ 5; for thickness between 3 and 5 mm,

O 90 / d 90 ≤ 4; for thickness between 1 and 3 mm, O 90 / d 90 ≤ 3; and for thickness

≤ 1 mm, O 90 / d 90 ≤ 2.5. They noted that O 90 / d 90 ≥ 1 minimized the risk of mineral

clogging.