Underground Construction - August 2020 - 32
PACP™ AND REMAINING USEFUL LIFE (RUL) PART 2:
UNDERSTANDING THE GROUND
AROUND THE PIPE
By NASSCO TAC Member Christopher Macey, P.Eng.
A primary objective of Condition Assessment (CA) is to gain a better understanding of the remaining
useful life (RUL) of the asset, some sense of what the deterioration process will look like and when
would be the best time to intervene to rehabilitate before failure occurs. As noted in our first article in
this series, PACP provides valuable insight into the answers to these questions, but you need to dig
a little further to get a clearer picture of how the severity of those defects can impact the deterioration
process. For the purposes of this and subsequent articles in this series we'll focus on structural
deterioration of sewers which will eventually lead to collapse of the pipe structure as opposed to
operation and maintenance defects, which tend to reduce the service function of the sewer more
than having a direct impact on sewer collapse.
One of the most comprehensive reviews of the sewer deterioration process dates to the 1980s in
supporting the development of WRc's Sewerage Rehabilitation Manual (SRM) . These studies were
a significant contributing factor in what became the core root structural codes in PACP. Based on
these studies, the process of sewer collapse can be divided into three stages:
STAGE 1 Formation
of an initial defect
during or subsequent
to sewer construction
STAGE 2
Deterioration
arising from
the defect
STAGE 3
Collapse of
the weakened
structure
PACP classifies the nature of defects and their severity. To put these observations into the context
of deterioration we must classify the nature of the deterioration. While some deterioration processes
involve a breakdown of the sewer fabric itself, such as the breakdown of concrete by hydrogen
sulfide (H2S) corrosion; most of the processes are governed simply by the loss of ground around
the sewer. For the latter, the deterioration process is caused by the flow of water in and out of the
pipe which leads to ground loss in the adjacent soil. The ground loss contributes to the eventual
development of unbalanced loading on the pipe. A common example of this is the classic "4 point"
cracking we see in a rigid pipe.
In a pipe with longitudinal fractures it is important to understand that the sewer can sustain a vertical
load far in excess of the load required to cause the initial cracks and fractures. As a crack progresses
to a fracture (a fracture, in theory, is a crack that extends through the entire pipe wall), two important
things change in the pipe. Firstly, the fractures allow the pipe to deform slightly which greatly reduces
Underground Construction - August 2020
Table of Contents for the Digital Edition of Underground Construction - August 2020
Contents
Underground Construction - August 2020 - FC
Underground Construction - August 2020 - IFC
Underground Construction - August 2020 - Contents
Underground Construction - August 2020 - 4
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