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Sewer condition classification PRESENTATION BY:


Session outline :

Session outline Sewer Performance Requirements; Performance Assessment; Rehabilitation Prioritization; Pipeline Condition Assessment; Structural Defects; Pipeline Failure Mechanism; Collapse Stages;

Sewer Performance Requirements:

Sewer Performance Requirements The pipeline operates without blocking; Pipeline command area is not flooded; Pipeline is able to provide environmental protection to the command area by efficient drainage; Pipeline does not generate excessive exfiltration or infiltration or CSO/SWD; Structural integrity of the pipeline is maintained throughout the design life; Pipelines do not generate toxicity and odour on their own; Access for repairs and maintenance are available; Pipeline Operators and maintenance personnel are adequately protected and the necessary repairs are effected efficiently; Adjacent structures are not endangered by pipeline

Sewer Performance :

Sewer Performance Hydraulic Performance; Environmental Performance; Structural Performance; Operational Performance

Pipeline Failure :

Pipeline Failure A pipeline unable to perform under a particular duty conditions is termed as a failed pipeline.   Inability in performance of gravity flow pipes like sewers could be known by noticing either of the following: Loss of fluid during passage Exfiltration Gain of fluid during passage Infiltration Overflows or discharges Combined Sewer Overflow (CSO) or Surface Water Discharge (SWD)

Structural Defects :

Structural Defects Circumferential cracking Longitudinal cracking  Bell splitting Corrosion pitting and blow-out holes Bell shearing Spiral cracking

Circumferential cracking :

Circumferential cracking Circumferential cracking is typically caused by bending forces applied to the pipe. Bending stress is often the result of soil movement, thermal contraction or third party interference. Circumferential cracking is the most common failure mode for smaller diameter cast iron pipes.

Longitudinal cracking:

Longitudinal cracking   Longitudinal cracking is caused by a number of different types of loading, such as internal water pressure and ring stress created by the soil cover load, external load or thermal changes. An initial small crack can expand along the length of the pipeline. In some cases, two cracks on opposite sides of the pipe are initiated, resulting in a complete detachment of the section of the pipe that may be as long as the pipe segment itself.

Bell splitting :

Bell splitting Bell splitting is most common in small diameter cast iron pipes. The main reason for bell splitting is the different thermal expansion coefficient of the joint sealants.

Corrosion pitting and blow-out holes :

Corrosion pitting and blow-out holes Corrosion is an important factor that plays a major role in the pipe failure process. A corrosion pit (Figure a) reduces the thickness and mechanical resistance of the pipe wall. When the wall is thinned to a certain point, the internal pressure blows out a hole (Figure b). The size of the hole depends on the distribution of corrosion and the pressure in the pipe.

Bell shearing :

Bell shearing Large diameter gray cast iron pipes fail by having a section of a bell shear off. The simple compressive loading is likely to cause a longitudinal crack that propagates along the length of the pipeline. Bending, however, often results in the occurrence of bell shearing.

Spiral cracking :

Spiral cracking Spiral cracking is a rather unique failure mode that sometimes occurs in medium diameter pipes. The initially circumferential crack propagates along the length of the pipe in a spiral fashion. Historically, this type of failure is associated with pressure surges, but it can also be related to a combination of bending force and internal pressure.

Mechanism of Deterioration and Collapse :

Mechanism of Deterioration and Collapse “Rate of deterioration” of sewer depends upon many factors, including the structural condition Risk or likelihood of collapse related to structural performance grades.

Major Deterioration Factors :

Major Deterioration Factors Soil surrounding the pipe Position of groundwater table Hydraulic regime within the sewer (Surcharging) Load on the sewer Methods and materials of sewer construction 3rd party action Roots, grease, debris causing surcharging and necessitating more frequent cleaning

Stages of Collapse :

Stages of Collapse STAGE 1 -an initial defect enables deterioration process to commence STAGE 2 - deterioration process continues in and/or behind the sewer wall STAGE 3 – Collapse occurs due to weakened wall.

PowerPoint Presentation:

Deformation of Cracked pipes STAGE 1 Pipe cracking is caused by poor construction or subsequent overloading or disturbance. The sewer remains supported and held in position by the surrounding soil. Visible defects : Cracks at crown, invert and sidewall. Infiltration may also be visible. STAGE 2 Infiltration of groundwater or infiltration / exfiltration caused by surcharging of the sewer washes in soil particles. Side support is lost allowing further deformation so that cracks develop into fractures. Side support may also be insufficient to prevent deformation if the original backfill was either poorly compacted or of an unsuitable material. Visible defects : Fractures, slight deformation. Infiltration may or may not be visible. STAGE 3 Loss of side support allows side of pipe to move further outwards and the crown to drop. Once deformation exceeds 10%, the pipe becomes increasingly likely to collapse. Visible defects : Fractures and deformation, possibly broken.

PowerPoint Presentation:

Subsidence of Sewer STAGE 1: Gap in sewer at joint or a poor lateral connection. Visible defects: Offset joint, badly made connection. Infiltration. STAGE 2: Infiltration of groundwater or infiltration/exfiltration caused by surcharging of the sewer washes in soil particles. Loss of soil support around the sewer allows pipe to move, opening joints and increasing the in wash of soil. Visible defects : Open and displaced joints, loss of line and level. Infiltration. NOTE: Care must be exercised when viewing video tape recordings as displaced or slightly displaced joints can be overcompensated by the camera’s lighting system. STAGE 3: Uneven loading of pipes due to joint displacement causes cracking of pipes. Process then accelerates and cracked pipes may also deform. Visible Defects : Open and displaced joints, cracked and fractured pipes, loss of line and level. NOTE: The camera may be submerged due to loss of gradient. Development of Zones of Loose ground or voids caused By loss of ground into sewer

PowerPoint Presentation:

Hydrogen Sulfide Attack STAGE 1 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. STAGE 2 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. STAGE 1 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 STAGE 2 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.

PowerPoint Presentation:

Loss of Side Support STAGE 1 The sewer has poor mortar joints from original construction method and/or mortar loss as the mortar deteriorates. Visible defects : Mortar loss, infiltration, open joints. STAGE 2 Infiltration of groundwater or infiltration/exfiltration caused by surcharging of the sewer or variations in depth of flow washes in soil particles. Zones of loosened or softened ground formed outside sewer walls allow the sewer to “spread” at sidewalls, causing the crown to crack and drop. Visible defects : Deformation, crack at crown, possible closing up of brickwork joints in the haunches and also possible loss of line and level in brick courses. STAGE 3 Mechanism proceeds and fracture forms at crown as the sidewalls continue to spread. Sewer develops “heart” shape and crown eventually collapses. Visible defects : Deformation, hearting, fracture at crown, closing up of brickwork joints in the haunches and loss of line and level in brick courses .

PowerPoint Presentation:

Loss of Bricks in Crown STAGE 1 Mortar is eroded or attacked chemically. Visible defects: Mortar loss. STAGE 2 Loss of mortar between bricks allows joints to close causing inner ring of crown to "squat” and separate from outer ring. Visible defects: Total mortar loss, deformation of crown, displaced bricks. STAGE 3 Loss of compressive load on inner ring of bricks in crown allows bricks to be dislodged and fall causing progressive collapse of inner ring. Visible defects: Missing bricks, bricks in invert. Note: this mechanism may occur in single ring sewers where the ground “arches” above the sewer, as well as in multi-ring sewers.

PowerPoint Presentation:

Dropped Invert STAGE 1 Deterioration of mortar allows infiltration. Visible defects: Mortar loss. Infiltration is rarely visible in the invert. STAGE 2 Infiltration rate rises and brings in fine soil particles causing voids to form around sewer invert. Visible defects: Mortar loss, sand in sewer, longitudinal crack may be visible near water level. STAGE 3 Deterioration of mortar and loss of support beneath sewer allows invert section to drop into void. Sewer loses structural integrity and side walls may drop or may be held by mortar or friction. Visible defects: As above, plus fracture around water line, dropped invert, displaced bricks in walls, deformation, loss of level.

Root Induced Deterioration :

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

Grease Induced Pipe Deterioration :

Grease Induced Pipe Deterioration Accumulated grease can create blockages and overflows Periodic sloughing of grease can create blockages and surcharging downstream Surcharging caused by grease will accelerate structural deterioration

Miscellaneous Deterioration Mechanisms :

Miscellaneous Deterioration Mechanisms Protruding and otherwise defective junctions/ connections Improper pipe repairs Third party damage Obstructions and debris Poor access to manholes for maintenance

Concluding Remarks:

Concluding Remarks Sewer Performance requirements are location and demand specific; Requirements can be categorized into four groups; Pipeline Condition assessment evaluates the pipeline performances; Composite Failures can have three stages; Reasons of failures need to be identified and appropriate remedial solutions need to be identified; Rehabilitation at the initial stages can be less expensive.

PowerPoint Presentation:

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