Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 365

Writing the above formula and drain spacing formula in Excel worksheet,
and solving for a series of values (trial values) for the variable “S” and “de,”
we can find that
when S = 9.8 m, then d e = 1.1 m, and new value of S = 9.98, which is close to

9.8.
Thus, taking d e = 1.1m,

S = 9.98 m (Ans.)

9.7 Envelope Materials

9.7.1 Need of Using Envelop Material Around Subsurface Drain

Water moves to the drain due to hydraulic gradient. The hydraulic gradient that
develops at the drain openings is often high enough to cause piping of soil material
through the openings. For this reason an adequate amount of protective materials
(termed as “envelope material”) is needed around subsurface drain.
The functions of placing envelop material around subsurface drain conduits are
the following:

(i) Prevent the movement of soil particles into the drains which may settle and
close the drains (Barrier function)
(ii) Provide material in the immediate vicinity of the drain openings that is more
permeable than the surrounding soils, thus increasing effective diameter of the
drain (Hydraulic function)
(iii) Provide suitable bedding for the drain (Bed material function)

9.7.2 Need of Proper Designing of Envelop Material

The envelope material should be selected such that the material or any combina-
tion of materials gives adequate protection against siltation, yet providing relatively
unrestricted water movement from the soil to the drain. To fulfill the objective of
using envelope material, proper selection of envelope material and design is a must.
Good design aims to assure an optimum combination of performance, long survive,
and reasonable cost.

9.7.3 Materials for Envelope

Materials used for subsurface drain have included almost all permeable porous
materials that are available economically in large quantities. Granular mineral mate-
rials such as coarse sand, fine gravel, and crushed stone have been used for decades
in arid and semi-arid countries as a drainpipe envelope.