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Underdrain Backfill Material
———— ee Material

Underdrains are commonly backfilled with pea gravel (3/8" minus) or coarser
drain rock (1/2" to 1 1/2"). Either is acceptiwle as long as it is washed to remove

fine materials,

Filter Fabric

There is reluctance to instail filter fabric around the drains for fear of the filter
fabric plugging and restricting flow. Some older fields (8 to 10 years) have
experienced filter fabric plugging. No similar evidence has been found in newer

fields.

Dr. Goss referred to two fields that had failed due to blocked drainage and the
blame laid to the use of both cases the underdrains consisted of
"Big-O' fabric pulled on like a tube sock (i.e.
fitting ally suspect that the failures may not
have b ; fabric, but possibly due to the filter
i rainage slots in the pipe. Dr. Goss has
eated into the pea gravel pipe surround
ees that placement of filter fabric over

the pea gravel for ph of the sand would be safer and he could
see no problems. ere is no flow of water, only seepage, no

filter fabric is necessary. Various typical trench sections are shown on Figure 3.

Underdrain Spacing

When underdrained fields were first installed underdrains wer commonly spaced
about 30 feet apart. This has often proven to be unsatisfactory and most fields
now being installed have drains 15 to 20 feet apart. It is recognized that 20 feet
Should be the maximum separation and 15 feet preferred if sufficient money is

available.

where no filter fabric

Crown
town

Fields constructed with a surface mixture of sand and topsoil relied to a large
extent on surface runoff and fields were usually crowned (assuming generally
level construction). Fields on sloping ground are usually graded to a similar
crossfall. Sand base fields however rely more on vertical drainage and it is now

common to design fields with no crown although there is-no disadvantage with a
crowned surface. A crowned surface will Provide for surface runoff if the sand
is saturated, frozen or impermeable due to thatch build-up. It is virtually
impossible to construct a level field, and there is less probability of low areas or

on Pe)