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CONDITION ASSESSMENT OF SEWER

NETWORKS

3.1 Introduction

Sewer collection systems are essential elements of municipal services.

To ensure a high level of reliability at the lowest possible cost, asset

management systems and procedures that include effective methods for

condition assessment and rehabilitation must be implemented. Condition

assessment means to evaluate the readiness of a component to perform

its function. Ideally, in any organization with capital assets, a condition

assessment program finds and records the condition of an organization’s

assets. Its tasks are to decide to assess condition for some management

purpose, identify whether pipe segment or system is to be assessed,

locate and access the pipe if required, determine structural or material

condition or perform other tests, and record the information for

management use.

3.2 Monitoring and Assessing Pipe Condition

In the past, utilities decisions were based on operations, maintenance,

and engineering departments. Nowadays organizations are more

integrated; the same decisions require more information about condition.

Pipe condition (or status) is often used to describe the overall

serviceability and structural of pipes at a point of time in their lifetime.

Because deterioration of Sewer pipes leading to pipe failures, the

deterioration of sewer pipes can be divided into structural deterioration

and serviceability deterioration (Tran 2007). The structural deterioration

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is a continuing process that reduces the load bearing capacity and can be

observed through the structural defects such as (WRC 1994):

Fracture (F)

Crack (C)

Deformation (D)

Surface damage (H)

Sag (S)

The serviceability deterioration is also a continuing process that reduces

the discharge capacity of the pipe and can be observed through a

reduction of cross-sectional area and an increase in pipe roughness due

to (WRC 1994):

Roots (R)

Debris (DE)

Encrustation (E) (or scale deposit)

Infiltration (I)

The task of monitoring and assessing the changes of pipe condition over

time becomes extremely important as part of proactive management

strategies. In the current management practice of sewers, this task

consists of three steps (Tran 2007) .As following:

(1) Selection of monitoring frequency,

(2) Selection of inspection techniques and

(3) Grading of pipe conditions.

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3.2.1 Monitoring frequency: regular versus snapshot

Bridges and pavements are subjected to a regular (or repeated)

inspection program to identify structural defects during their lifetime. In

particular, every bridge is legally required to be inspected once every

two years (Madanat et al. 1995).Unfortunately, sewers pipes were not

subjected to such regular inspection programs. Instead, their inspection

programs were of snapshot type, that is, a sample of pipes was inspected

for only once (Kathula 2001).

3.2.2 Inspection Techniques: CCTV versus others

Inspection techniques can be grouped into three levels of assessment

according to the capability of the inspection techniques and the required

information by asset managers (Ratliff 2003). The three levels are:

Field reconnaissance (level 1),

Internal inspection (level 2) and

External inspection (level 3).

Field reconnaissance (level 1) aims to collect data of manholes, pits

and pipelines and assesses the structure of manholes for accessibility of

inspection equipment and even inspectors (Tran 2007). Some of

available inspection techniques for this level are:

• Manhole survey

• Global positioning system (GPS)

This basic step often associates with reviewing as-built drawings and

existing information in order to form the backbone of any management

database. This task continues throughout service lifetime of the pipe

systems whenever new information such as pipe replacement or repair

occurs.

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Internal inspection (level 2) focuses on assessing the internal condition

of pipes so that appropriate action can be taken for pipes in poor

condition before the pipe collapse or occurrence of complete blockage of

pipe (Tran 2007). Some of available inspection techniques for this level

are:

• Man-walk through

• Close circuit television (CCTV)

• Sonar (or ultrasonic)

• Focused electrode leak location (FELL)

• Sewer scanner and evaluation technology (SSET)

• Laser-based scanning system

• Multi-sensor pipeline inspection system

External inspection (level 3) concerns with the soil structure that

supports pipes. Any voids or loss of soil support is potentially leading to

pipe collapse (Ratliff 2003).Some of available inspection techniques for

this level are:

• Infrared thermographs

• Ground penetrating radar

• Micro deflection

• Impact echo wave impedance probe (WIP)

Although as described above, there are several inspection techniques

available for each level of assessment, there are no guidelines for

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selecting these techniques for a particular application. In order to reduce

time and effort in selecting the appropriate inspection technique, a

number of researchers have provided comprehensive reviews of

inspection techniques that were applied in many infrastructure facilities.

Examples can be found in Wirahadikusumah et al. (1998), Morrison and

Thomson (2003).

In current management practice of sewers pipes, the GPS locator and the

CCTV inspection were still the most used techniques for locating

pipelines (i.e. level 1) and assessing internal condition of pipes (i.e. level

2) (Wirahadikusumah et al. 2001; Morrison and Thomson 2003; Baik et

al. 2006). With hundreds of kilometers of pipelines, inspection costs

become a critical factor for asset managers’ decisions, not to mention

productivity.

3.2.3 Grading of pipe condition

The Water Research Center (WRC) in UK devised the world first

condition grading scheme that provided protocols and guidelines for

assessing current condition of individual pipes using the CCTV

inspection technique (WRC 1983). Based on the original scheme of

WRC (1983), several condition grading schemes were later developed in

Canada (McDonald and Zhao 2001) and Australia (WSAA 2002).

Although the structural and serviceability deterioration of pipes are a

continuous process (as explained in Section 3.2), ordinal grading

systems were used in these schemes for mapping the pipe deterioration

into pipe conditions at the time of inspection. For example: The Sewer

Inspection Reporting Code (SIRC) by the Water Service Association of

Australia (WSAA 2002) was developed for assessing conditions of rigid

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sewers (concrete and vitrified clay pipes) using CCTV inspection data as

shown in Table 3.1.

Table 3.1: Description of condition states used in WSAA (2006)

Condition

Grading

Structural condition Serviceability Condition

1 Insignificant deterioration

of the sewer has occurred.

Appears to be in good

condition

No or insignificant loss of

hydraulic performance has

occurred. Appears to be in

good condition

2 Minor deterioration of the

sewer has occurred.

Minor defects are present

causing minor loss of

hydraulic performance

3 Moderate deterioration has

occurred but defects do not

affect short term structural

integrity

Developed defects are

present causing moderate

loss of hydraulic

performance

4 Serious deterioration of the

sewer has occurred and

affected structural integrity

Significant defects are

present causing serious loss

of hydraulic performance

5 Failure of the sewer has

occurred or is imminent

Failure of the sewer has

occurred or is imminent

Figure 3.1 shows the grading process based on the SIRC for a pipe

segment from CCTV data. A pipe segment is defined between two

manholes or pits. As shown in this figure, the CCTV robot was sent to

the pipe of interest. During its movement along the pipe, the CCTV-

recoded images were sent to a monitor where the operator can recognize

any structural or serviceability defects. He or she then coded the defects

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with the aid of computer as shown in Table 3.2 and Table 3.3, each

coded defect was automatically or manually assigned a score according

to the guidelines of weights. For example a structural defect like crack

has a score of 5. All defect scores were aggregated for two condition

measures, namely, peak score and mean score. The peak and mean score

were then compared against pre-defined thresholds for grading the pipe

into either condition one, two or three. In general, structural defects that

receive high scores are surface damage, breaking and deformation, while

debris, roots and obstruction get high scores for serviceability defects.

The peak score indicates the largest score from one defect or multiple

defects in one location (often within a meter length) in the pipe segment.

The mean score is the sum of all defect scores divided by the segment

length (Tran 2007).

Fig. 3.1: Grading process for a pipe from CCTV data

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Table 3.2: Structural defects and weights for pipes (WRC 1994)

Defect type Unit of measure Weight

Light long. Fracture (<10 mm wide) meter 5

Moderate long. Fracture (10 to 25 mm wide) meter 10

Severe Long. Fracture (>25 mm wide) meter 15

light Circum. Fracture (<10 mm wide) meter 5

Moderate Circum. Fracture (10 to 25 mm

wide)

meter 10

Severe Circum. Fracture (>25 mm wide) meter 15

light Diagonal Fracture (<10 mm wide) meter 5

Moderate Diagonal Fracture (10 to 25 mm

wide)

meter 10

Severe Diagonal Fracture (>25 mm wide) meter 15

Fracture Multiple meter 20

Light Long. Crack (up to 3 cracks, no

leakage)

meter 3

Moderate Long. Crack (>3 cracks, leakage) meter 5

Light Circum. Crack (up to 3 cracks, no

leakage)

meter 3

Moderate Circum. Crack (>3 cracks,

leakage)

meter 5

Light Diagonal Crack (up to 3 cracks, no

leakage)

meter 3

Moderate Diagonal Crack (>3 cracks,

leakage)

meter 5

Severe (multiple cracks, leakage) meter 10

Light Deformation (<5% change in

diameter)

meter 5

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Moderate Deformation (5 to 10 % change in

diameter)

meter 10

Severe Deformation (11 to 25% change in

diameter)

meter 15

Broken pipe (>100 mm diameter or > 100 x

100 mm area or equivalent)

each 15

light Joint Displacement (1/4 pipe wall

thickness)

each 3

moderate Joint Displacement (1/4-1/2 pipe

wall thickness)

each 10

severe Joint Displacement (>1/2 pipe wall

thickness)

each 15

light Joint Opening (<10 mm, gasket in

place)

each 3

moderate Joint Opening (10 to 50 mm,

gasket off, leakage)

each 10

severe Joint Opening (>50 mm, soil visible,

leakage)

each 15

light Surface Damage(<5 mm wall thickness

lost, slight spalling or wear, pitting on metal

pipe)

meter 3

moderate Surface Damage (5 to 10 mm wall

thickness lost, exposed reinforcement or

aggregates, extended corrosion in metal

pipe)

meter 10

severe Surface Damage (>10 mm pipe wall

thickness lost, corroded reinforcement,

corroded through metal pipe)

meter 15

light Sag (< 50 mm) meter 4

moderate Sag (50 to 100 mm) meter 10

severe Sag (> 100 mm) meter 15

Table 3.3: Serviceability defects and weights for pipes (WRC 1994)

Table3.2:Continue

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Defect type Unit of

measure

Weight

Light roots (fine roots, reduction in diameter

<10%) Each 2

Moderate roots (reduction in diameter 10% to

25%) Each 8

Severe roots (reduction in diameter >25%) Each 10

Light debris (fine roots, reduction in diameter

<10%)

meter 5

Moderate debris (reduction in diameter 10% to

25%)

meter 8

Severe debris (reduction in diameter >25%) meter 10

Light encrustation (fine roots, reduction in

diameter <10%)

meter or

each

5

Moderate encrustation (reduction in diameter

10% to 25%)

meter or

each

10

Severe encrustation (reduction in diameter >25%) meter or

each

15

Light protruding (fine roots, reduction in diameter

<10%)

each 2

Moderate protruding (reduction in diameter 10%

to 25%)

each 8

Severe protruding (reduction in diameter >25%) each 10

light infiltration (seeping, dripping) each 2

moderate infiltration (running, trickling) each 5

severe infiltration (gushing or spurting) each 10

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We notes that values recorded in Table 3.2 and Table 3.3 are suitable for

WRC. However an extensive research is necessary to adjust these values

for sewer utilities in Egypt.

3.2.4Physical Condition of sewer pipe

While inspection is a process of information gathering, condition

assessment can be regarded as a process of data analysis and decision-

making based on the information gathered. The physical condition is

first step in worldwide sewer rehabilitation industry as shown in Table

3.5. It depends on structural and serviceability rates by peak scores as

shown in Tables 3.4 and 3.5 based on the worst defect (ASCE 1994).

Table 3.4: Physical Condition and recommended action (WRC 1994)

Physical condition Implication Rehabilitation priority

5 Failed or failure

imminent

Immediate

4 poor condition

Immediate

3 Good condition

Medium

2 Very Good condition Low

1-0 Excellent condition Not required

Table 3.5: Structural condition rating for sewer pipes (assessment length

= 1 m) (ASCE 1994)

Peak score range Structural condition rating, Rstr

0 0

1-4 1

5-9 2

10-14 3

15-19 4

20 5

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Table 3.6: Serviceability condition rating for sewer pipes (assessment

length = 1 m) (ASCE 1994)

Peak score range Serviceability condition rating. Rser

0 0

1-2 1

3-4 2

5-6 3

7-8 4

9-10 5

(Chughtai and Zayed 2008) expressed the physical grade of the sewer

pipeline by combining the effects of structural and serviceability rates as

shown:

Pipe Condition Grade (Physical Condition) =

0.541+0.273 (Structural Condition Rate)2 +0.37(Operational Condition

Rate)2

(3.1)

3.3 Implementation

The condition assessment module in a spreadsheet using Microsoft

Excel program as shown in Fig. 3.2, illustrate main screen of condition

assessment of pipelines. An example of a pipeline used for

determination of current grad and condition index. The example consists

of Structural defects, in addition to Serviceability and a level of each

defect is required.

The user needs to follow two steps to use the template and, or adapt it, to

model a given pipe line.

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Fig. 3.2: Main screen of condition assessment of sewer pipelines

Step 1:

User starts by opening the main screen and clicking to the inspection

button (first module), then the structural defects spread sheet will appear

as shown in Figure 3.3

Fig. 3.3: Spread sheet of structural defects

Step 2:

Now user will select the real rate (None, Light, Moderate or Severe) of

each structural defect (Fractures, cracks, deformation, joint

displacement, joint opening and sag) as shown in Figure 3.4

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Fig. 3.4: Selection of structural defects rates

After a complete selection of each defect rate the weights, peak score

and structural condition rate determined automatically. Then user can

move forward to serviceability defects spread sheet by clicking on

forward arrow, the spread sheet will appear as shown in figure 3.5

Fig. 3.5: Spread sheet of structural defects

As previous, user select the real rate (None, Light, Moderate or Severe)

of each serviceability defect (Roots, Debris, Encrustation and

infiltration). Then weights, peak score and serviceability condition rate

determined automatically as shown in Figure 3.6

Finally user can get the current grade and condition index by clicking on

forward button, the overall network condition index will be the average

of pipelines as shown in Figure 3.7

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Fig. 3.6: Selection of serviceability defects rates

Fig. 3.7: Condition index and grade for pipelines and network

REFRENCES

1. ASCE, (1994), Existing Sewer Evaluation and Rehabilitation,

ASCE Manuals and Reports on Engineering Practice No. 62.

American Society of Civil Engineers.

2. Baik, H. S., Jeong, H. S. & Abraham, D. M., (2006), "Estimating

Transition Probabilities in Markov Chain-Based Deterioration

Models for Management of Wastewater Systems", Journal of

Water Resources Planning and Management, ASCE, Vol. 132,

No. 1, pp. 15-24.

3. Chughtai, F., and Zayed, T. (2008). “Infrastructure condition

prediction models for sustainable sewer pipelines.”Journal of

Performance of Constructed Facilities, Vol. 17, No. 3, pp.130-135.

4. Kathula, V. S., (2001), Structural Distress Condition Modeling for

Sanitary Sewers, PhD Thesis, Civil of Engineering, Louisiana

Tech University, pp.293.

5. McDonald, S. E. & Zhao, J. Q., (2001), "Condition Assessment

and Rehabilitation of Large Sewers", International Conference on

Undreground Infrastructure Research, Waterloo, Canada, pp.361-

369.

6. Morrison, R. S. & Thomson, J. C., (2003), "Innovative Inspection

Methodologies for Wastewater Systems", Pipelines 2003, ASCE,

pp.176-182.

7. Ratliff, A., (2003), "An Overview of Current and Developing

Technologies for Pipe Condition Assessment", Pipelines 2003,

ASCE, pp.16-22.

8. Tran, D., H., (2007), "Investigation of Deterioration Models for

Sewer Pipe systems", PhD. Thesis, Victoria University.

9. Wirahadikusumah, R., Abraham, D., and Iseley, T. (2001).

“Challenging issues in modeling deterioration of combined

sewers.” Journal of Infrastructure Systems, Vol. 7, No. 2, pp. 77-

84.

10. Wirahadikusumah, R., Abraham, M. D., Iseley, T. &Prasanth, R.

K., (1998), "Assessment Technologies for Sewer System

Rehabilitation", Automation in Construction, Vol. 7, pp. 259-270.

11. WRC, (1983), Sewerage Rehabilitation Manual, Water Research

Center, UK, London.

12. WRC. (1994) Sewerage Rehabilitation Manual, Second Edition,

Water Research Centre, U.K.

13. WSAA, (2002), Sewer Inspection Reporting Code of Australia,

Water Service Association of Australia (WSAA), Melbourne.

14. WSAA, (2006), Conduit Inspection Reporting Code of Australia,

Water Service Association of Australia (WSAA), Melbourne.