Selecting Backfill Material
backfill means a longer lasting drainage system
At Varicore, we regularly receive questions about backfill. We sometimes
even encounter the misconception that select backfill is a concept
linked exclusively to Multi-Flow drainage. Your backfill choice will
have no greater and no less effect on the life of a Multi-Flow system
than it will on any other drainage system. Multi-Flow systems, and
all other drainage systems enjoy longer life when quality backfill
It is a well known fact that French
drains frequently block up, sometimes in a remarkably short
amount of time. This blockage typically occurs on the trench liner.
Small particles of clay or silt are carried by moving water until
they are intercepted by the filter, which eventually fills in.
The actual life span of a French drain depends on the soil type
and the rainfall amounts. This same blockage can occur with round
pipe or panel drain wrapped in geotextile. Highway departments
and golf course managers have wrestled with this issue for many
The best solution to this problem is to surround the geotextile
filter with very coarse sand. Sand is an excellent filter of clay
and silt. As the water containing these contaminants moves through
the sand, it slows down and the particulate matter drops out.
An inch or more of sand is a very effective filter.
A sand filter is far more feasible with a Multi-Flow system than
with a traditional French drain. It would be very difficult, if
not impossible, to insert a layer of sand between the trench wall
and the geotextile liner in a French drain. However, it is relatively
easy and affordable to use sand as a backfill medium surrounding
Multi-Flow in a four inch wide trench. With sand as a primary
filter and the 4-ounce needle-punched geotextile as a secondary
filter, a Multi-Flow system will provide long-lasting, effective
properly placed sand filter will dramatically extend the life
of a drainage system.
According to the USDA system of classification, very coarse sand has
an approximate particle size of between 1.0 and 2.0 mm. Some designers
have used this for a sand spec:
When passed over a sieve, very coarse sand will have:
- less than 5% retained on a #10 U S standard sieve
- less than 5% passing a #30 U S standard sieve
- no more than 1 % should pass through a #50 U S standard sieve
good quality sand is not uniformly available. The closer installers
come to this recommendation, the longer their system will last.
Absolute conformity is frequently not practical. On one hand,
sand contaminated with clay or silt will impede the movement of
water as well as accelerate the blinding of the drainage system.
On the other hand, large diameter pea rock and mixed particle
size gravel allow for rapid movement of water in the beginning,
but are susceptible to infiltration by fines. They might not be
aggressive enough in protecting the fabric filter. Buck shot or
washed medium sand are better choices, but will not perform as
well as clean, very coarse sand.
the perfect sand cannot be found, look for an alternative following
these two criteria and in this order:
for sand that is clean
for sand that is coarse
coarse sand makes for the best backfill.