Evaluation models for the assessment of the structural and operational condition of drain and sewer systems – Part I

The first part deals with the evaluation models for assessing the structural and operational states of the drainage systems. The second part is about the advanced evaluation model STATUS_Sewer. The final part contains the results of the analysis and the summary of all survey events.

Evaluation models can be distinguished between:

• Standard evaluation models and
• Advanced evaluation models (Section 5).

1 Sewer Condition classification

The condition classification is defined as “the categorisation of inspection results by means of a comparison with the specified requirements” [1]. When categorising each individual defect (subsequently also called individual condition), the object’s master file data (sewer section, manhole, special structure) are linked with the condition/defect descriptions and at least the type of defect and its extent are evaluated.

The type of defect is described by the main code and the possible characterisations (Ch1, Ch2) according to DIN EN 13508-2 [2]; whereas its extent is quantified in either per cent or as a unit of length (numeric addition) and positioning.

In the process of condition classification, each condition description is enhanced by an assessment of the defect relevance with regard to three essential requirements (subsequently also called performance objectives) to be met by drain and sewer systems:

• Leaktightness (L),
• (Structural) Stability (S) and
• Operational safety (OS).

The term operational safety includes the “Maintenance of the operational flow and compliance with safety regulations for the operating personnel” [1].

The more severe the assessment of an individual defect, the worse the corresponding condition class (in other evaluation models also called defect class) or the higher the rehabilitation urgency. Normally, it is distinguished among five condition classes. Subject to the individual evaluation model, these condition classes are defined differently.

The individual condition is classified in two different ways:

Direct condition classification: The condition class results only from the main code of the respective defect description.

Direct condition classification with numeric assessment: The condition class results from the combination of the main code and the assessed numeric level of the respective defect.

2 Condition assessment

In the context of condition assessment, the results of the condition classification are combined with essential factors of influence or rather performancerelated conditions in the immediate vicinity of the individual defect.

The consideration of these factors not only allows for a better differentiation of the condition classification result in relation to its effects in terms of the performance objectives, but particularly allows for a better assessment of the defect’s hazard potential. Table 1 illustrates an example by listing essential ancillary influencing parameters and, additionally, their relevance for the performance objectives. Thus, the final condition/defect class results from a combination of the main code and the numeric assessment of the individual defect as well as from the relevance of the ancillary parameters.

In adjustment with the method that is used for classification, the ancillary parameters can either be included comprehensively or limited to individual requirements. The data linking can be realised by mathematical models (Section 5).

Table 1: Relevance of essential ancillary influencing parameters for the performance objectives with reference to DWA-M 149-3 [1]

The result of the condition assessment of each condition description is the allocation of items, key figures, weightings or priority indicators separately for the three performance objectives.

3 Condition evaluation

In the course of condition evaluation, the results of the condition classification and assessment are categorised according to the individual need for action that is derived from specified requirements and essential factors of influence. The results are summarised in relation to the individual object (sewer section, manhole, special structure) taking into account the following criteria [1]:

• Most severe defect in an object,
• Frequency and extent of further defects,
• Linear extent of individual defects.

Depending on the evaluation model, the condition class of an object corresponds to either the worst condition class of all individual defects or the maximum final individual defect class.

The results of the condition evaluation are the designation and categorisation of the objects’ rehabilitation needs from a structural/operational point of view in the form of rehabilitation needs list (priority list). This list must be testable for plausibility, i.e. other factors of influence also have to be transparent.

The rehabilitation needs list serves the purpose of allocating the financial resources for rehabilitation and provides the basis for requirement planning in the course of the integral sewer management system “Development of a plan”. According to DWA-A 143-1 [3], requirement planning is understood to be the “methodical determination of the rehabilitation need followed by a determination of corrective measures”. It includes:

• The determination and assessment of the initial situation
• The development and determination of rehabilitation strategies and rehabilitation targets in the form of a rehabilitation concept
• The development of possible solutions
• The determination of precise rehabilitation measures (repair, renovation, replacement) in the form of a rehabilitation plan.

In order to increase the efficiency of precise rehabilitation needs planning that is composed of a rehabilitation concept and a rehabilitation strategy, DWA-M 149-3 [1] suggests considering further aspects, e.g.:

• Construction measures of other service providers, private investors and road maintenance authorities,
• Hydraulic or environmental irregularities,
• Possibility to address several differently categorised objects in one corrective measure,
• traffic conditions,
• Structural improvements in the sewer network, e.g. a change of system (from a combined to a separate system),
• Development measures, etc.

Irrespective of these aspects, there are structural or operational conditions which require immediate corrective measures based on their urgency. According to DWA-M 149-3 [1], these conditions include e.g.:

• Functional impairments which largely hinder the operational function of the object,
• All structural defects in a designated water protection area where the leaktightness of the drain and sewer system is critical,
• An identification of an actual groundwater impairment due to leaking sewage,
• All situations where a danger of collapse for the object, adjacent structures or the surrounding soil must be expected. Depending on the extent of damage, such situations can include, for example:
  • A groundwater rush with intrusion of soil,
  • An identification of cavity spaces in the sewer area,
  • A breaking of roads in the sewer area,
• Conditions that represent danger to the operating personnel’s lives

A sound condition evaluation should be computer-based for economic reasons and requires a database which is as comprehensive and accurate as possible [1].

4 Standard evaluation models

Since the early 1990s, a number of different models for the evaluation of sewer system conditions have been developed in the Federal Republic of Germany in order to determine the rehabilitation need of objects from a structural as well as operational point of view in the sense of DIN EN 752 [4].

Among these, there are:

•  Working aids “Sewage” (ISYBAU) [5]
• DWA-M 149 Part 3 [1]
• WRc (2001) [6]

They are based on multiplicative or additive point systems with defined values. Where required, relative weightings are carried out subject to the ancillary parameters and/or several defect conditions of the same object (sewer section, manhole, special structure) are overlapped (agglomeration).

In addition to the evaluation models mentioned above, there are other models that have hitherto only been used in a few exceptional cases, such as the evaluation system “KAIN” [7] [8] and the “Pforzheim Model” [7] [9] and which will not be discussed further.

5 Advanced evaluation models

The previously introduced standard evaluation models allow for an assessment and classification of the structural/operational condition of drain and sewer systems within the framework of the whole rehabilitation process with the target to illustrate the rehabilitation needs of the objects (sewer sections, manholes, connections and special structures) in the form of a requirements or priority list.

The decisive criterion for evaluating the rehabilitation priority of an object is its condition class, which is determined with the help of multiplicative or additive scoring systems and, is eventually, defined by the defect class of its most severe individual defect. The defect class is generally determined merely based on the defect type and extent and thus, describes only an object condition of local extent.

Another fundamental problem of standard assessment systems is their way of classifying conditions. They turn out to be too rough, undifferentiated and uncertain which, in particular, results from the use of discrete (rigid) classes. In addition, essential system-related and/or local influencing conditions as well as parameters such as the estimated performance reserve hereinafter also being referred to as fabric decay and the estimated remaining useful life are not taken into account.

Under the precondition that, according to DWA-M 149-3 [1], “the visual inspection and condition assessment are to be implemented within an adequate, possibly close temporal correlation”, the requirements list determined in the above way is sufficient to derive both short term rehabilitation decisions and immediate emergency measures.

For medium- and long-term rehabilitation decisions and as a basis for the development of differentiated rehabilitation and investment strategies for drain and sewer systems, this type of condition classification is not suitable, as it does not consider the drain and sewer system’s overall condition and the types and rates of deterioration that are to be expected in future, i.e. the prognosis for its future performance and longevity referred to as fabric decay.

Planning for rehabilitation requirements that does not take realistic account of the latest structural/operational situation as well as the network condition and future performance trends will necessarily involve a wrong assessment of both the rehabilitation and investment needs.

The advanced evaluation models have the objective to eliminate the abovementioned downsides of the standard evaluation models. They are based on the knowledge about the relationships among the damage symptoms as a whole, the object’s derivable probability of failure and the given reliability of the entire drain and sewer system that can be concluded from the assessment of all objects. Furthermore, they allow for the development of targeted statistical aging models that provide an insight into the future structure-related condition and performance trends of both individual objects and a drain and sewer system as an entire network. Use of these models over time also results in an assessment of their own probable future reliability.

Figure 1: Modular structure of STATUS_Sewer [17]

• Strategy development and analyses model STATUS_Sewer
• Bietigheim Model (DynaStrat) [10] 
• GompSoft [11] [12].

In the following, the strategy development and analyses model STATUS_Sewer that has been used very successfully in numerous German drain and sewer networks since 2004, will be introduced [13] [14] [15] [16].

The network analyses are done in a multistage, modularly designed process (Figure 1):

• Data import and plausibility check
• Individual defect assessment (defect class)
• Object assessment (condition class / fabric decay class)
• Cluster analysis
• Actual analysis based on an aging model
• Strategy development and network development forecast

The following links will take you to the second and third part of the article:

• "Evaluation models for the assessment of the structural and operational condition of drain and sewer systems – Part II"
• "Evaluation models for the assessment of the structural and operational condition of drain and sewer systems – Part III"

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