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zResearch Trends and Results

Management of Sewers

MATSUMIYA Yosuke, HeadYOSHIDA Toshiaki, Senior Researcher

FUKUDA Yasuo, Researcher

Wastewater System Division, Water Quality Control Department(Keywords) Sewers, asset management, micro-management, failure risk

Research Trends & Results

1. Introduction
Sewers constructed rapidly since the late 1960s havedeteriorated in recent years, demanding appropriate
maintenance and large rehabilitation works. It is
predicted that population decrease makes it difficult tosecure satisfactory budgets for sewer management.
From now on, measures must be taken to minimize thefailure risk such as road sinkholes and to reduce orlevel expenses.
This report introduces a study of micro-management,
which is a part of sewer asset management.
2. Micro-management of sewers
The core of asset management consists of medium- tolong-term rehabilitation cost predictions based on
statistical analysis of survey results (macro-management) and prioritizing of rehabilitation or
maintenance works in individual spans (sewer lines
between manholes) (micro-management).
In FY 2009, risk assessments were studied on
micro-management. Risk assessments are quantitativeevaluations performed by combining “probability ofsewer failure” and “consequence which failed sewers
mightcause.”
Prioritizing
rehabilitationormaintenance to minimize the risk such as roadsinkholes is studied by this method.
3. Studying risk assessments
Quantifying the probability of sewer failure isconducted by analysis based on the relationship ofsewer survey results (e.g., corrosion or infiltration asin Photo 1) with sewerage registry items (e.g., type ofmaterial, age, diameter). Since sewers are buried undera variety of urban environments and ground conditions,registry items cannot cover all factors which influence
the occurrence of failure.
This study combines two approaches: logistic
regression analysis applied for registry items, and theAHP method for factors other than registry items.
Logistic regression analysis is a study approach using
sewer survey results and registry items obtained fromsampled cities. The AHP method is conducted througha questionnaire survey of engineers of major citieswho have much experience in sewer maintenance.

Photo 1. Examples of failed sewers

Equation 1 is a probability equation for failure
occurrence with registry items as the independent
variables and sewer survey results as the dependent
variable, while Equation 2 is an assessment equationby the AHP method. The quantification of theconsequence of failure is studied similarly using theabove AHP method.

Pr (Y = 1) =1

1 + exp[− (β 0 + β1 X 1 + β 2 X 2 L + β r X r )]Equation 1( β i : coefficients, Xi: independent variables (registryitems), r: number of independent variables)
Assessment of probability of failure occurrence= (Weight of large vehicle passage) × (Score of largevehicle passage) + (Weight of tree planted zones) ×(Score of tree planted zones) + ----- + (Weight ofground conditions) × (Score of ground conditions)
Equation 2
4. Conclusion

This assessment is a simple method which can beadopted in medium and small cities where expertise ofcity engineers is limited, contributing greatly to theintroduction of nation-wide asset management.
ReferencesMatsumiya, Y. et. al.: Projection of Sewer
Rehabilitation Needs and Evaluation of Sewer Failure
Risk, Civil Engineering Journal, No.11, Vol.51, Public
Works Research Center, November 2009http://www.nilim.go.jp/lab/ebg/ (Wastewater SystemDivision)