Universitā degli studi di Roma “Tor Vergata” - ART - TORVERGATA ...

Kleiner and Rajani (2001A and 2001B) wrote a comprehensive review about the models
used for quantifying the structural deterioration of water mains by analysing historical
performance data. Such models are divided in two groups: physical and statistical.
The physical models are scientifically more robust because consider the real loads
supported by the pipe and its mechanical resistance, but because they needed the huge amount of
data, this approach is applied when the important consequences of pipe failures can justify the
accumulation of such data.
The statistical models with various levels of input data may be useful for minor water
mains for which there are few data available or for which the cost of failure does not justify the
amount and the quality of the required data. The statistical models assume that the historical
patters can continue in the future, and the prediction can be done by:
• deterministic models, that are either time-exponential models or time-linear
models based on two or three parameters. They are applied on an homogeneous
group of pipes that have to be selected carefully;
• probabilistic multi-variate models consider many covariates (defined with
awareness by an expert), that influence the pipe breakage patterns, and the
selection of homogeneous groups it is not necessary. This kind of model can be
useful for define a priority for rehabilitation;
• probabilistic single-variate group-processing models, that include models that use
probabilistic processes on grouped data to derive: probabilities of pipe life
expectancy (useful for future financial needs), probability of breakage and
probabilistic analysis of break clustering phenomenon (useful for short-term
planning of water main rehabilitation and renewal).
For example, the EN 752-5 formulates: “the investigation of the construction may
comprise either a complete examination of the drainage system or a selective method”. Mueller
(2003) argued out the benefit of random selection because it is convenient in terms of time and
costs. Then the sewer network has to be grouped in quite homogeneous set according to the
relevant characteristics to be considered.
The number of the selected groups depend on heterogeneity of the urban area and the aim
of the selective inspection is to infer from the distribution of the classes of condition resulting
from a representative random sample the distribution of the classes of condition of specific
groups of reaches (Mueller, 2003). These groups have to be small enough to be quite uniform but
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