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What the data says about the cause of defects in wastewater networks

VAPAR digs into the data on its pipe assessment platform which processes and organises inspection footage from inside wastewater pipes.

Background

Pipe inspections and condition assessments are an important part of the operations of water utilities and city councils who manage wastewater networks.

The collection and analysis of this information has traditionally been managed by each organisation individually, making generalised insights difficult to come by.

VAPAR has developed a pipe assessment platform where pipe asset owners and CCTV companies can process and organise video footage of the inside of wastewater pipes and either manually or automatically detect and classify the defects inside these pipes.

The footage and information that has been collected as part of this process from various locations and organisations has yielded some interesting insights which are summarised within this article, with the full version accessible online.

Stormwater vs. sewer pipes

Diameters: in urban areas, properties need to be connected to the sewers and the pipes they join will generally start at 150 mm in diameter – a relatively small diameter.

Conversely, stormwater pipes are installed where overland flow depths in kerb profiles are insufficient.

Stormwater pipes need to carry a larger capacity from the upstream point which generally start at 300 mm in diameter.

Material: due to the sheer number of sewer pipes that need to be installed and mix of greywater, blackwater and trade waste, price and accessibility of materials plays a large factor, meaning many more sewer pipes will be made of PVC or vitrified clay than stormwater pipes.

Since stormwater pipes are typically of larger diameter (and therefore demand greater reinforcement), more expensive materials such as concrete will be used for installation more frequently.

Environmental factors: sewer pipes will generally contain less debris (street litter, leaves, branches) than their stormwater counterparts.

However, because sewer pipes are typically smaller in diameter, always flowing, and have more lateral connection/junction points, there’s a greater comparative risk of blockage.

Sewer pipes are also more susceptible to pipe material degradation due to constant flows and chemical byproducts of wastewater e.g. hydrogen sulphide causing pipe corrosion. 

Stormwater lengths are slightly longer on average than their sewer counterparts since they have fewer connection points and changes in direction, thus needing less maintenance pits on average.

Datasets for analysis

The footage used for analysis had been uploaded to VAPAR’s cloud platform for automated analysis during late 2020, and was sourced from clients located in Australia, New Zealand and the UK.

Structural defects in sewer pipes

Concrete: surface damage is the most common structural defect that was found in concrete sewer pipes, typically caused by flow turbulence and the chemical composition of the sewer matter.

Once concrete aggregate has been exposed from initial surface damage, corrosion accelerates with concrete cover reducing, steel reinforcements become exposed to air and moisture, causing them to corrode.

Small cracks were the second highest frequency defects found.

Due to reinforcement, large cracks are less prevalent in concrete – once the cracks become large they often become breaks.

Vitrified clay (VC): unlike concrete, vitrified clay pipes are not reinforced, so longitudinal, circumferential and multiple cracking are very common defects for this material type.

This was the case for the data analysed, with the majority of the cracking characterised as longitudinal. Almost 80 per cent of the defects reported in VC were cracks.

The second most common defects were breaks, likely due to cracking.

Flexible Plastics: Due to the “flexible” nature of flexible plastics, it’s no surprise that small magnitude deformation was the highest frequency defect found for this material type.

Different characterisations of cracking were also observed.

Structural defects in stormwater pipes

Reinforced concrete: A surprising amount of circumferential cracking, spalling and minor infiltration can be introduced through lifting holes (also referred to as a lifting eye, found at 12 o’clock inside the pipe to aid handling and installation).

Similarly to concrete in sewers, concrete stormwater pipes are also subject to cracking; however, surface damage was less common, since stormwater is typically less gaseous and corrosive than sewage.

PVC: small, medium and large magnitude deformation defects were more than 80 per cent of the defects found in stormwater PVC pipes.

Most PVC pipes will deform over their design life. The degree of deformation, and the time span over which this deformation is observed are the important factors for asset owners to understand when making maintenance decisions for these pipes.

Service defects in sewer pipes

Displaced joints: minor radial displacements were the most common defect type, occurring 2.5x as often as longitudinal displacements (also known as “open joints”).

The prevalence of displaced joints within pipe infrastructure is most common in vitrified clay pipes, particularly older designs without rubber rings in joints.

Roots: roots were the second most observed category. Roots often make their way into the pipes through joints so it’s interesting to note that displaced joints are the most common defect type, and there may be some cause and effect.

A study undertaken by the University of Melbourne1 outlined the most common factors causing greater frequency of root invasions, those being tree proximity, tree maturity, tree type, soil type, and temperature/evaporation rates.

The study found that “blockages occurred most frequently when temperatures and evaporation were at their lowest, i.e., August to October”.

Service defects in stormwater pipes

Debris: small amounts of debris (up to 20 per cent) in stormwater pipes were the most common defect found.

Apart from the reduction in cross sectional area, small amounts of debris and deposits start to become an issue when the presence of that debris/deposit increases the risk of additional debris accumulation over time.

An additional challenge is that debris tends to collect in the maintenance shafts and entry points of stormwater drains, making a total blockage more likely in these areas.

Infiltration: infiltration/inflow in underground pipes is another common observation in the stormwater CCTV sample dataset, and in stormwater CCTV inspection footage in general.

Groundwater ingress and other water source interactions with stormwater pipes are responsible for this defect.

Infiltration/inflow is not as consequential in stormwater pipes, though something to note during CCTV inspections as they may indicate poor joint tightness or other defects that are less visible.

Defect prevention

Structural defects: structural defects often stem from initially minor imperfections that degrade over time.

The rate of degradation is important to note in order to undertake the right remedial actions at the right time to optimise expenditure.

Additionally, there are a number of details, that if addressed at the time of installation, can extend the structural lifetime of a pipe.

Examples include, correct bedding material and compaction, sufficient cover, and suitable hydraulic grading of the pipe to specification.  

Service defects: as for structural defects, displaced joints and root intrusion defects can be minimised with the right installation and using spigot and socket (or similar) pipe joints with sealing rings.

The application of herbicidal foam in sewers is one of the leading solutions for reducing regrowth of roots that enter sewer pipes.

This tends to perform better rather than root cutting, as roots tend to grow thicker and faster when cut.

In stormwater, ideally debris would be collected in gross pollutant traps, or litter traps, at key points in the network, and pipes should be laid with sufficient fall to allow debris to flow more smoothly through the network. 

Summary

There is a wide range of factors that contribute to the structural and service defects that impact wastewater networks.

Understanding the trends in various materials can assist when making maintenance decisions to extend the lifetime of a pipe asset and to optimise maintenance expenditure.

For more information visit the VAPAR website.

This article was featured in the June 2021 edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet or mobile device, click here.

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