Assessing and understanding sewer pipeline deterioration
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UCT International Conference and Exhibition Track III-A. Assessing and Understanding Sewer Pipeline Deterioration. Rod Thornhill, PE White Rock Consultants Dallas, Texas. Proactive Renovation and Replacement. The pavement

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Assessing and Understanding Sewer Pipeline Deterioration

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UCT International Conference and Exhibition

Track III-A

Assessing and Understanding Sewer Pipeline Deterioration

Rod Thornhill, PE

White Rock Consultants

Dallas, Texas

Proactive Renovation and Replacement

The pavement

Management industry has used a history of condition assessment to justify cost-effective proactive renovation

Sewer Failure

History of Sewer Pipe Installation

Miles of Pipe

Total Approx 520,000 Miles

EPA Gap Analysis

Average Age of Sewer Pipes

Age in Years

EPA Gap Analysis

Pipeline Deterioration Analysis

  • A “before” and “after” assessment of a pipeline

  • Provides a quantitative understanding of rate of deterioration progression

  • Should also include pipe condition and other factors such as soils, surcharging, groundwater, roots, age, etc.

  • PACP standards provide the ability to share with other utilities nation-wide

Deficiencies in Condition Assessment to Date

  • Majority of sewers in place today were only first televised years after construction

  • Many of the defects in pipes were created during construction

  • Up until now, the US had no ability to quantitatively measure change in pipe condition

Steps to Understanding Pipe Condition Change

  • Thorough assessment of current condition of pipe

  • Identification and quantification of factors affecting each individual pipe

  • Understand era and circumstances of original construction

  • Understand Maintenance and Repair History of Pipe

  • Use retro-assessment of previous inspection to detect and evaluate change

  • Apply knowledge gained to plan the future of each pipe

Step 1 Thorough Assessment/Benchmarking of Current Condition of Pipe

  • Adopt and implement a standard code set and procedures for logging pipe conditions

  • Require use of standard condition assessment by all in-house personnel and outside firms.

  • Develop a software and data management strategy that assures the longevity of the condition assessment information

  • Maintain an on-going coding quality control program

Hydrogen Sulfide Attack


Inverted Syphon / Force Main

Under anaerobic (septic) conditions, sulfate present in the wastewater is converted to sulfides within the slime layer inside the pipe. Force mains generally flow full with little aeration therefore likely points for sulfide production. Dissolved oxygen levels must be near zero in order for sulfide production to occur.


Discharge manhole/gravity sewer

Sulfides in the wastewater are released by turbulent conditions at discharge point into the sewer atmosphere and form hydrogen sulfide gas (H2S).The H2S condenses on the pipe surfaces and is converted by bacteria into a weak sulfuric acid. The sulfuric acid attacks concrete and metal surfaces.


Gravity sewer

Under anaerobic (septic) conditions sulfate present in the wastewater is converted into sulfides within the slime layer. Sewers with laminar flow therefore little aeration are most susceptible to low dissolved oxygen levels


Turbulence releases dissolved sulfides into the sewer atmosphere in the form of hydrogen sulfide (H2S). The H2S then condenses on sewer surfaces in the form of sulfuric acid. The sulfuric acid attacks cement based materials and metals.

PACP Concrete Pipe/H2S Damage Descriptors

  • Roughness increased (SRI)

  • Aggregate visible (SAV)

  • Aggregate projecting (SAP)

  • Aggregate missing (SAM)

  • Reinforcement Visible (SRV)

  • Reinforcement Projecting (SRP)

  • Reinforcement Corroded (SRC)

  • Missing Wall (SMW)

PACP Reinforcement Projecting (SRP)

Step 2 Identification and Quantification of Factors Affecting Each Individual Pipe

  • Root Growth

    • Characteristics and extent

  • Surcharging

    • frequency and depth

  • Presence of groundwater or mineral encrustation

  • Maintenance and repair history of pipe segment

Root Induced Deterioration

  • Roots intrude through existing pipe defects

  • Root growth expands existing pipe defects and creates new defects

  • Root growth can result in blockages and overflows

  • Surcharging caused by root growth will accelerate structural deterioration



50 Years of Root Growth

Aug 2006

Step 3 Understand Era and Circumstances of Original Construction

  • Many, perhaps most of defects in sewers today were created during construction

  • Most sewers were not first internally inspected until decades after construction

  • The need for watertight joints was not established until the late 1950s.

  • History of sewers is not only interesting, it is essential to the development of a pipeline condition management discipline

Excerpts from Metcalf and Eddy Design of Sewers Volume I, 1914

“American sewerage practice is noteworthy among the branches of engineering for the prepondering influence of experience rather than experiment upon the development of many of it’s features, apart from those concerned with treatment of sewerage”

  • First sentence of Introduction

Excerpts from Metcalf and Eddy Design of Sewers Volume I, 1914

“The amount of capital required to put up a small plant for making cement tile and pipe is so moderate that a large number of these little works have been built. Owing mainly to lack of skill, working capital, or both, much inferior pipe has been produced in these small plants, and this poor product has prejudiced many engineers against all cement pipe”

  • Discussion of early cement pipe

Draining for Profit and Draining For HealthCol. George E. Waring 1867

“Every reported case of failure in drainage which we have investigated, has resolved itself into ignorance, blundering, bad management, or bad execution” Gisborne

  • Quote on title page of book, referring to William Gisborne, Minister of Public Works, New Zealand

Pipe Failure Likely Created During Construction

Step 4 Understand Maintenance and Repair History of Pipe

  • Requirement and frequency for cleaning, if any

  • Root treatment frequency

  • Grease accumulation

  • Point repair or partial replacement history

  • Service requests

  • Backups or SSOs

  • Pending work orders

  • Third party damage

Step 5 Use Retro-Assessment of Previous Inspections to Detect and Evaluate Change

  • Very inexpensive compared to obtaining new data

  • Audio and Video often of excellent quality

  • Can add up quickly to a considerable portion of the system

  • Provides immediate ability to assess rate of change by comparing “old” data to “new”

Step 6 Use Knowledge Gained to Plan the Future of Each Pipe

  • Which defects probably are construction-related

  • Does pipe need to be cleaned

  • What impact does roots and root control have

    • Aggravating existing defects

    • Creating new defects

  • Is the Pipe Material deteriorating (i.e. H2S) and at what rate

  • When is the next inspection needed

Process of Pipeline Condition Management

Fundamentals of Sewer Pipeline Condition Management

Definition:An approach that uses continual condition assessment, preventive maintenance, and renewal to provide an acceptable level of service for all pipelines, in perpetuity

Process of Pipeline

Condition Management











Major Components of Pipeline Condition Management

  • Recognition that some pipelines are more important than others (Criticality, Consequence of Failure)

  • Comprehensive condition assessment, data collection, and data dissemination (PACP)

  • Documentation and understanding factors that influence the rate of sewer pipeline condition decline (Deterioration Mechanisms)

Major Components of Pipeline Condition Management (Cont’d)

  • A work process that continually utilizes new data to assign maintenance activities and intervals, replacement priorities, management reports, and geographical display of information (Proactive Decision Matrix)

  • A long term, big picture approach (Continual Improvement)

What are Critical Sewers?

  • Sewer where the costs associated with the failure of the sewer likely to be high.

  • Fall into three broad bands

    • construction costs associated with repair

    • traffic delay costs

    • strategically important (trunk sewers)

Sample Critical Sewer Matrix

Most Critical

5-10% of System


10-15% of System




All Other Sewers


Life Cycle of Sewer Line









Replacement Costs






Structural Grade


Time = 0


  • Temporal, time-sensitive approach needed to better understand deterioration mechanisms and rates of deterioration

  • Standards for describing and documenting structural and O&M conditions essential for industry

  • Historical documents have a wealth of information

  • Think long term, big picture

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