Page 242 - Handbook of Materials Failure Analysis

238 CHAPTER 9 Reliability and failure analysis of wastewater systems

During all the years under investigation, the highest numbers of failures were
recorded in cold and autumn periods. In general, the highest numbers of failures
were recorded in winter, except for 2008, when the highest number of failures
was recorded in autumn. The lowest numbers of recorded failures occurred in
spring and summer. The above-described failure distribution over the four
seasons is clearly related to changes of the air temperature and the correlated
changes of the temperature of the ground and the temperature of water
transported through the network.
In 2012, 15 failures were recorded to have affected arterial conduits. Almost all
of them affected cast iron conduits. 2/3 of failure events involved forcing out
of the sealing material. Arterial conduit failures occurred with similar frequencies
in all four seasons. Only one failure event was related to the impact of the
surrounding ground, where the groundwater level was less than 2 m. The pipeline
in question was constructed in sandy soil.
In the case of distribution conduits, the majority of failures occurs in winter
time. Most often, they affected cast iron conduits that suffered from transversal
and longitudinal cracking. Oftentimes, pipelines suffered failures due to
mechanical damages or sealing damage. Every fifth damage influenced the
surrounding groundwater reaching above the level of network conduit
installation. More than half of the damaged conduits were built on boulder
clays. Apart from that, water supply conduit failures occurred in sandy soils
(25% of all failure events).
Conduits made of asbestos cement were more often affected by failures in autumn
and winter (60% of failure events). They suffered primarily from transversal
and longitudinal cracking. Failures of asbestos cement conduits affected conduits
laid in clays, sands, and glacial lake deposits.
In the case of PE conduits, no seasonal correlation of susceptibility to failure
was observed. These conduits were affected primarily by mechanical damages.
Only one failure event affecting a pipeline located below groundwater level
was recorded.
Two failure events affecting PP conduits were recorded. Both occurred in
autumn and were caused by transversal cracking. They did not affect
groundwater and the damaged pipelines were installed in clays and glacial
lake deposits.
The seasons most probably have no influence on the vulnerability of PVC
conduits. These conduits most often suffered from cracking and 30% of these
failure events could affect groundwater or result in secondary contamination of
water in the water supply network.
Conduits made of steel suffered damages both in the summer and in winter. They
were usually affected by corrosion.
The vulnerability of cast iron conduits was decidedly higher in winter months.
These conduits usually suffered from transversal and longitudinal cracking.
Every sixth failure event affected groundwater, while over a half of the damaged
cast iron conduits was installed in boulder clays.