Flexible and able to absorb external loading
A good reason for choosing BorECO Polypropylene (PP) for sewage and drainage
systems
Introduction
The continued urbanization throughout Asia and Africa places
increasing demands on infrastructure, including the buried pipe
systems. Failures in the sewer systems can be extremely
distressing to any community as they separate the waste
materials from fresh water which protects us all from many
diseases. It is therefore essential to choose a sewage pipe system
that can be easily installed and yet can withstand the external
loads and the effects of the corrosive effluents for at least 100
years without maintenance.
Current Situation
Figure 1. Typical failure of a rigid sewer
pipe in KRV survey
Much of the sewage network in the world today looks like the
photograph shown in figure 1, where the brittle concrete pipes
have failed due to external loading from a rock. This picture is one
of hundreds taken by the German Plastic Pipes Association (KRV)
when they surveyed nearly 2,000 km of sewers made of different
all materials that were all less than 30 years old
Concrete and clay pipes are brittle and if they are heavily loaded
they will crack – in this case there was a rock impinging on the top
of the pipe and the pipe has cracked and then completely
collapsed leading to a partial blockage of the sewer. In many
cases the pipes were correctly installed initially but then the pipe
and backfill material was disturbed later by other utilities laying
gas or water pipes or telecommunication cables leading to
increased external loading and failures.
Risk Analysis
Details of survey
Video inspection of sewers up to 800 mm in diameter
All sewers less than 30 years old
Concrete, clay and plastics pipes inspected
Results from 1,800 km of sewers inspected in 3 countries namely:
Germany (D), Holland (N) and Sweden (SW)
5
4
3
2
Overall Results
1
The environmental risk potential of flexible pipe systems is
only 15% that of the rigid pipe sytems made from concrete &
clay.
D
NL
SWE
(Source; Sustainable sewers, KRV, 2005)
Plastics
Concrete & Clay
Why do flexible plastic pipes perform so much
better than rigid pipes?
Figure 2.
Flexible pipes deform and shed load – rigid pipes
retain load & crack
For flexible plastic pipes, as the traffic load or ground loading
from buildings and construction are transmitted by the soil to the
pipe the plastic pipe will slowly deform (see figure 2). As it
deforms it transfers the load to the side fill around the pipe.
Eventually the pipe and the soil will be in equilibrium and the load
will have been redistributed. Therefore the stresses that remain
in the pipe wall are much lower and can easily be sustained by
the pipe without any damage or deformation. In a TEPPFA study
published in 1999 three experts in the underground infrastructure
of buried pipes, Frans Alferink, Lars-Eric Janson and Jonathon
Oliff concluded amongst other things that;
“Flexibility is strength, not a disadvantage, as previously
thought”
With a rigid pipe it cannot deform and the stress just builds up
until the pipe cracks and eventually the sewer collapses. This
process will be accelerated in concrete sewers by the erosion
that takes place inside the crown of the pipe due to the build up
of acids in the sewer. This is a particular problem in warm
climates such as in the Middle East and Asia where the acid
build up is much faster. For more information on corrosion
resistance refer to our BorECO Tech Note No.8, Corrosion
Resistant.
Concluding Remarks
References
1.
2.
3.
4.
Flexible Plastic Pipes are especially suitable for laying in ground
in which settlements and other forms of movement make great
demands on the pipe’s ability to transfer and redistribute soil
pressure without rupturing. Furthermore the mechanical
properties of high quality pipe resins, such as BorECO PP-HM,
will not change over time unless the material is exposed to any
media, increased temperatures or UV radiation.
Dreiling, G. “PP Sewage Systems – market and prospective for the Middle East,” Dubai Plast
Pro, Dubai, 2007.Bergstrom, G. et al. “Durability testing for 100 year lifetime for buried non-pressure plastics pipes.” Plastics
Pipes XIII, Washington, 2006.
Janson, L-E, Plastics Pipes for water supply and sewage disposal 4th Edition, 2003
Frans Alferink, Lars-Eric Janson and Jonathon Oliff, Flexible pipes perform better, TEPPA, 1999
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