Evaluating Large Diameter PVC Versus Fiberglass for Sewer Projects

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Evaluating Large-Diameter PVC versus Fiberglass
for Sewer Projects
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Pipe Material Selection Selection of the most
appropriate pipe material is critical for
maximizing cost-effectiveness and ensuring long-
term reliability of sewer systems. This document
compares solid-wall polyvinyl chloride (PVC) and
fiberglass (FRP) pipes, the two most commonly
installed pipe materials for large- diameter
gravity-sewer applications. Some engineers still
consider 18-inch as large-diameter, but the
36-inch size was added to the ASTM F679 standard
in 1986 (about 40 years ago). Comparisons in this
paper are limited to 24- through 60-inch sizes
which are commonly available for both products.
The Primary Difference Deflection As will be
shown, the primary difference between PVC and
FRP pipe is how each pipe is able to respond to
ring deflection when buried. FRP can withstand
only small deflections before the material fails
while PVC pipe can endure significant deflections
with no material failure. Thus, engineers and
owners should consider controlling the deflection
of FRP for their projects. Additionally, flexible
pipe design demonstrates there is never a need to
specify a higher stiffness for PVC than for FRP.
In most cases, FRP should be specified at a
higher stiffnesses than PVC.
48-inch PVC pipe
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EXTERNAL LOAD EXTERNAL LOAD EXTERNAL LOAD EXTERNAL LOAD EXTERNAL LOAD EXTERNAL LOAD




Pipe Stiffness Importance of Direct
Comparison For flexible pipe, pipe stiffness is
the ability to resist ring deflection from
external load. A common misconception is that PVC
pipes need higher pipe stiffness to match the
performance of FRP. ASTM D2412 is the test method
used to determine pipe stiffness for both
materials, making a direct comparison
appropriate. There can be confusion due to the
different terminology used, but PVCs pipe
stiffness (PS) is the same as FRPs stiffness
number (SN). Both pipes are widely available in
46 psi stiffness, but both have higher-stiffness
products if required by project parameters. For
this document, 46 psi is the basis for
comparison. This stiffness is suitable for most
PVC installations, but not necessarily for FRP
pipelines. For some projects with high loads or
poor soils, FRP pipe may require a higher
stiffness (i.e., SN 90 or SN 120) to keep
deflections within allowable limits. Conversely,
PVC PS 46 is acceptable for almost all projects,
because anticipated deflection is less than its
allowable deflection limit.
Pipe stiffness and soil stiffness resist
deflection for all flexible pipes.
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• Pipe Deflection Not All Materials are Equal
• Both PVC and FRP are considered flexible pipes
  which are designed based on limiting pipe
  deflection. However, there is a noticeable
  difference in deflection limits for the two
  materials
• PVC pipe 7.5 (per ASTM D3034)
• FRP pipe 5.0 (per ASTM D3262)
• This means that PVC pipe is better suited for a
  wide range of installation conditions. FRP pipe,
  with a lower deflection limit, is more prone to
  failure from deflection. Unlike FRP, PVC pipe
  does not experience material failure at any level
  of deflection.
• Another indicator of the difference in deflection
  capability is the quality-control flattening test
  required by both pipe materials
• ASTM D3034 PVC pipe requires flattening of 60 of
  the original diameter without damage.
• At 30 deflection, reverse curvature occurs (no
  material damage), which is considered the design
  limit for PVC. Allowable long-term PVC deflection
  is 7.5, providing a safety factor of 4.0 against
  the design limit (no material failure).
• ASTM D3262 FRP pipe also requires a flattening
  test (called ring-deflection testing) in which
  the pipe must reach a deflection limit before
  material damage (i.e., surface cracking). For a
  46-psi stiffness fiberglass pipe, this limit is
  10. At a long-term FRP

Above, a 48-inch PVC pipe is deflected at 40,
twice as much as the 36-inch FRP at 20. The PVC
pipe has suffered no damage, while the FRP pipe
has failed.
deflection of 5, this equates to a safety factor
of 2.0 for material failure.

• Site Conditions that Reduce Soil Stiffness
• Based on the previous sections, FRP pipe requires
  more stringent deflection control. There are many
  conditions where the soil embedment may not
  provide support for the pipe as designed
• Large-diameter sewer pipes are frequently
  installed in areas of low soil strength such as
  near rivers, lakes, or coastal areas, which may
  reduce trench wall support.
• High water tables may significantly affect soil
  strength due to soil migration.
• Inadequate backfill compaction can have a
  significant effect on soil support of the pipe.
• These conditions will likely reduce the soil
  stiffness assumed for the design.

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• Design Example Pipe Deflection
• The following design example demonstrates the
  importance of the pipe-soil structure to control
  deflection and shows the long-term deflections
  that result from these conditions
• PS/SN 46 psi is used for both pipes. FRP SN 72
  and SN 120 are included for comparison.
• The Modified Iowa Equation is used to calculate
  long-term deflection for both materials under the
  same conditions.
• Depth of bury 22 ft
• Composite modulus of soil reaction, E 400 psi
  Calculated deflections are shown in the table
  below.

Deflection Comparisons for PVC and FRP Pipes PVC PS 46 FRP SN 46 FRP SN 72 FRP SN 120
Calculated long-term deflection 5.9 5.9 5.2 4.3
Allowable long-term deflection per ASTM D3034 and D3262 7.5 5.0 5.0 5.0
Within allowable deflection ?? YES ?? NO ?? NO ?? YES
In this example, PVC is below its allowable
long-term deflection limit while FRP has exceeded
its limit for both SN 46 and SN 72. For FRP not
to exceed the deflection limit, SN 120 is
required. If PVC pipe is required to have a
higher stiffness than FRP in a project you are
working on, please contact a PVCPA regional
engineer for technical design support.
FRP More Stringent Embedment Requirements
As shown in the design example, there are
conditions that can lead to failure for FRP,
whereas PVC PS 46 will not only be below its
design limit, it will also not experience
material failure at any deflection. Designers may
need to specify ways to control FRP deflection by
using more stringent embedment requirements (such
as flowable fill with wider trench) and/or using
higher pipe stiffness than what is required for
PVC. Using these methods for decreasing FRPs
deflection allows the designs for both materials
to be equivalent. PVC does not require either of
these methods since it does not fail due to
deflection and has a high safety factor for
design. The adjacent figure shows how embedment
can be specified for FRP and PVC to obtain
similar overall performance.
SPECIFIED BACKFILL OR FLOWABLE FILL TO ABOVE PIPE
SPECIFIED BACKFILL TO SPRING LINE
FRP
PVC
Example of trench detail that provides
equivalency between PVC and FRP pipe.
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Installation As explained, the main difference
between FRP and PVC is that FRP is much more
susceptible to failure when the pipe deflects
more than designed. As a result, FRP requires
much more stringent bedding and backfill
specifications. To ensure that the embedment
around the FRP pipe is properly placed and
compacted, utilities should consider requiring
enhanced inspection and testing during
construction (such as laser-profiling and camera
inspection). Utilities should also consider
long-term monitoring programs for all FRP
pipelines in service.

• Cost The Importance of Long-Term Performance
• PVC and FRP often have differences in initial
  costs. However, during the evaluation process, it
  is important to consider not only bid costs but
  also factors that affect long-term performance,
  such as
• Trench design
• Need for additional inspection
• Failure mode
• Likelihood of failure
• Consequence of failure

FRP Pipe-Failure Case Studies There are an
estimated 3,500 miles of 24-inch and larger PVC
solid-wall sewer pipe installed in North America,
compared to approximately 1,200 miles of FRP.
There is no history of failures of solid-wall PVC
sewer pipe due to deflection. However, there are
numerous well-documented instances of premature,
catastrophic failures of FRP pipe that can be
found from news articles or other publicly
available information. Case 1 Bentonville,
AR Over 47,000 feet of 30- and 36-inch FRP were
installed in 2009 and 2010. Starting in 2016, the
utility began experiencing failures. A
comprehensive assessment of the line showed that
only 1,700 feet of pipe were deflected below the
5 deflection limit for FRP. The remaining 45,000
feet were all over-deflected and required varying
levels of repair or replacement. The estimate to
replace the entire line was 26.7 million. The
utility determined that this was the most
cost-effective solution. In 2020, the utility
replaced the entire pipeline with 30- and 36-inch
PVC pipe. Click here for additional
information. Case 2 Franklin, TN The utility
installed approximately 25,000 feet of 18- to
54-inch FRP pipe in three phases. The project was
completed in 2001. Only 13 years into the
expected 50-year service life, the pipeline began
failing. In the course of three years, three
sections failed, costing the city more than 1.8
million in emergency repairs. Click here and here
for additional information.
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Case 3 Grand Prairie, TX A 104-inch diameter
FRP interceptor was installed in 2008. The
pipeline experienced catastrophic failure in
2020. Emergency repair of the pipeline, which
carried up to 55 million gallons per day, was
completed in two months and required installation
of more than 4,000 feet of replacement pipe.
Click here for additional information. Case 4
Murfreesboro, TN Beginning in February 2019, the
utility had multiple failures of 36-inch FRP.
These failures initially cost the utility over
350,000 in emergency repairs. Later, it was
determined that a 1,400-foot section of the
interceptor needed to be replaced, at an
estimated cost of 3.0 million. Click here and
here for additional information.
Large-Diameter PVC Sewer Installations The map
shows the locations of some large-diameter PVC
sewer pipe installations across the U.S. and
Canada. These utilities evaluated different sewer
pipe materials and selected PVC pipe as the best
option, demonstrating its ability to perform in
all environmental conditions, including areas
with poor soil stiffness or high water table.
For information on any of these installations,
contact a PVCPA regional engineer. Deflection
The Critical Consideration The primary difference
between PVC and FRP pipe is how each pipe
responds to deflection when buried. While both
pipes have the same pipe stiffness and will
deflect the same amount, FRP is more susceptible
to failure. As a result, FRP is much more reliant
on controlling deflection by conservative
embedment design, strict installation
specifications, and continuous installation
inspection. Flexible pipe design demonstrates
there is never a need to specify a higher
stiffness for PVC than for FRP. In most cases,
FRP may need to be specified at a higher
stiffness than PVC. Owners, engineers, and
contractors should consider these factors for
large-diameter sewer projects. PVCPA regional
engineers are available to address all aspects of
this document and assist with project-specific
questions. Please visit www.uni-bell.org/contact
or call [phone].
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UNI-PUB-21-25
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UNI-BELL PVC PIPE ASSOCIATION 201 E. John
Carpenter Freeway Suite 750 Irving, TX 75062