Low Impact Development Manual for Michigan - OSEH - University ...

and installation of structures. Adjacent strips of
geotextile should overlap a minimum of 18 inches,
and should also be secured at least four feet outside
of the bed to prevent any runoff or sediment from
entering the storage bed. This edge strip should
remain in place until storage media is placed in the
bed.
• Clean-washed, uniformly-graded aggregate should
be placed in the bed in maximum eight-inch lifts.
Each layer should be lightly compacted, with
construction equipment kept off the bed bottom as
much as possible.
• Once bed aggregate has been installed, geotextile
can be folded over the top of the aggregate bed.
Additional geotextile should be placed as needed to
provide a minimum overlap of 18 inches between
adjacent geotextile strips.
• Place approved engineered soil media over
infiltration bed in maximum six-inch lifts.
• Seed and stabilize topsoil.
Additional Construction Guidelines for
Infiltration Trenches
• Excavate infiltration trench bottom to a uniform,
level uncompacted subgrade free from rocks and
debris. Do NOT compact subgrade.
• Place nonwoven geotextile along bottom and sides
of trench. Nonwoven geotextile rolls should overlap
by a minimum of 16 inches within the trench. Fold
back and secure excess geotextile during stone
placement.
• Install upstream and downstream control structures,
cleanouts, observation wells, etc.
• Place uniformly graded, clean-washed aggregate in
8-inch lifts, lightly compacting between lifts.
• Install continuously perforated pipe as indicated
on plans. Backfill with uniformly graded, cleanwashed
aggregate in 8-inch lifts, lightly compacting
between lifts.
• Fold and secure nonwoven geotextile over
infiltration trench, with minimum overlap of
16-inches.
• If vegetated, place a minimum six-inch lift of
approved topsoil over infiltration trench, as
indicated on plans.
• Seed and stabilize topsoil.
Causes of Infiltration BMP Failure
With respect to stormwater infiltration BMPs, the
result of “failure” is a reduction in the volume of
runoff anticipated or the discharge of stormwater with
excessive levels of some pollutants. Where the system
includes built structures, such as porous pavements,
failure may include loss of structural integrity for the
wearing surface, whereas the infiltration function may
continue uncompromised. For infiltration systems with
vegetated surfaces, such as play fields or rain gardens,
failure may include the inability to support surface
vegetation, caused by too much or too little water.
The primary causes of reduced performance are:
• Poor construction techniques, especially soil compaction/smearing,
which results in significantly
reduced infiltration rates.
• A lack of site soil stabilization prior to the BMP
receiving runoff, which greatly increases the potential
for sediment clogging from contiguous land
surfaces.
• Inadequate pretreatment, especially of sedimentladen
runoff, which can cause a gradual reduction of
infiltration rates.
• Lack of proper maintenance (erosion repair, revegetation,
removal of detritus, catch basin cleaning,
vacuuming of pervious pavement, etc.), which can
reduce the longevity of infiltration BMPs.
• Inadequate design.
• Inappropriate use of geotextile.
Infiltration systems should always be designed such
that failure of the infiltration component does not completely
eliminate the peak rate attenuation capability
of the BMP. Because infiltration BMPs are designed
to infiltrate small, frequent storms, the loss or reduction
of this capability may not significantly impact the
storage and peak rate mitigation of the BMP during
extreme events.
LID Manual for Michigan – Chapter 7 Page 213