Ditchmen • NUCA of Florida Ditchmen • May 2018 | Page 11

For concrete pipe,
structural backfill above
the pipe springline
contributes negligibly to
the structural support of
the pipe. The ASTM and
AASHTO specifications
above have material and
compaction requirements
for the bedding and
haunch zones, which are
important for providing
reactionary support
of concrete pipe. For
the overfill zone, which
begins at the concrete
pipe springline, ASTM
and AASHTO allow
for the use of higher
plasticity soils, silts and
clays (MH, CH, ML, CL,
A-5, or A-7). In contrast,
typical specifications do
not differentiate concrete
pipe compaction at the
springline nor allow
A-7 material for trench
backfilling until at least
12 inches above the pipe,
if at all. Unless needed
to provide minimum
bearing support capacity,
such as for roadways
or surface structures,
it may be possible to
specify alternative backfill
materials and compaction
limits – in line with ASTM
and AASHTO - above the
springline in concrete pipe
installations. limits that take advantage
of the inherent – and
plant-verified – strength
class of concrete pipe. For
instance, a Type 2 Standard
Installation allows 85%
to 90% standard proctor
density (SPD) compaction
of clean, coarse-grained
soils (e.g., A-1, A-3) and
even allows fine-grained
CL, ML, or A-6 soils
compacted to 95% SPD in
the haunch area. While
this may not be desirable
in a shallow-cover highway
embankment, it could
be appropriate for other
locations.
ASTM and AASHTO
standard installations allow
a wider range of backfill
materials and compaction If you come across a
specification that you
would like to review or
possibly amend, please
feel free to call me. Or,
feel free to call Angel
(County Materials),
Kenneth (Forterra), or
Sarah (Rinker Materials).
Our contact information
may be found at www.
floridaconcretepipe.com.
We want concrete pipe to
be used efficiently, and we
want you to be successful.
• • •
When specifying any type
of pipe based on a cover
height table, engineers
should be aware of the
table’s design assumptions.
For instance, the FDOT
Drainage Manual Cover
Height Tables assume that
the pipe will be in service
at or above the natural
groundwater elevation.
Contractors have the
right to ask an engineer
to certify that his or her
design takes groundwater
impacts into account,
but it’s worth noting that
concrete pipe structural
performance is negligibly
impacted by natural
groundwater.
MAY 2018 • DITCHMEN
11