Why do sewer pipes break? Here’s what the data actually says

In May, VAPAR completed a data-dive to check out the most common defects found in stormwater pipe infrastructure. More importantly, we provided recommendations based on this data to help asset owners to reduce the risk of defects developing.

Last time, we noted that our stormwater CCTV footage inspections unearthed patterns of defects according to the material used to construct the pipe. This was a great development, since an element of predictability gives asset owners the chance to take informed action during their design and construction process to mitigate the risk of defects occurring down the line.

At the time of publication, I’d promised to deliver a similar piece relating to sewer pipe infrastructure. This time around, I’ll be breaking my analysis down into two parts – one with data and recommendations around structural defects (you’re reading it now!) and one around service defects (which will be released soon).

Defects in Sewer Pipes vs Stormwater Pipes

Before we get stuck in, I’ll just clarify a few points for any non-engineers who might be reading – there’s some differences between wastewater and stormwater pipes.

Sewer pipes and stormwater pipes mainly differ due to:

Diameters

In urban areas, all properties need wastewater connections, which will generally (on average) start at 150 mm (6 inch) in diameter. 

Conversely, stormwater pipes are only used when overland flow depths in kerb profiles exceed the allowable, in which case a pipe should be installed. These pipes need to carry a larger capacity from the upstream point, generally (on average) starting at 300 mm (12 inch) in diameter.

Pipe Material

Because sewer pipes are ubiquitous, reasonably priced and readily-accessible pipe materials are most commonly sought for installation. This means that wastewater pipes will more frequently be made of materials such as PVC or vitrified clay than stormwater pipes.

On the other hand, 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.

Other

It’s also commonly noted that sewer pipes will generally contain less debris (street litter, leaves, branches) than their stormwater counterparts. However, because wastewater pipes are typically smaller in diameter, always flowing, and have more lateral connection/junction points, there’s a greater comparative risk of blockage.

In most cases, wastewater network odours are managed through controlled venting of the system. This can reduce the rate of pipe degradation, however the chemical and biological composition of wastewater still means that design lives of between stormwater and wastewater can still greatly differ.

Snapshot – Our Dataset

We used 605 pieces of sewer pipe inspection footage, representing 26.45 km in combined network length as our dataset for our investigation. 

The footage had been uploaded to VAPAR’s cloud platform by VAPAR clients over the last few months for automated analysis, and was sourced from clients located in Australia, New Zealand and the UK.

A Breakdown of our Dataset

Material Types

From the footage which we used, concrete and vitrified clay were by far the most commonly identified construction materials, representing 45.25% and 42.02% of the dataset, respectively.

The third most common material present was flexible plastic (11.79%), whilst a miscellaneous the ‘other’ category encompassed just 0.95% of materials.

Vitrified clay is a very common material type for pipes of smaller diameters. As mentioned earlier, the ubiquitous nature of sewer pipes make it an ideal material for installation – vitrified clay is a natural material, and is readily available in many countries. 

Moving into the larger wastewater pipe diameters, concrete will more commonly be used as a construction material due to the increased reinforcement demands larger diameters impose. For the purpose of this blog, I have defined concrete construction to encompass steel reinforced (SR), fibre reinforced (FR) and asbestos cement – although strictly speaking these don’t have the same structural attributes as each other.

Plastic pipe construction is the ‘up and comer’ amongst wastewater materials. Plastic construction is becoming more and more prevalent as the material behaviour is becoming better understood by asset owners.

Pipe Diameters

A variety of pipe diameters were also identified; 150mm (6 inch) diameter was the most commonly found within the footage, followed by 225mm (9 inch) and 300mm (12 inch).

From a logical perspective, the spread of pipe diameters in our dataset makes a lot of sense, and absolutely aligns with my expectations of a typical wastewater network. Pipe of smaller diameters connect to properties everywhere, and are the most common diameter; these in turn feed into increasingly larger pipe diameters deeper into the network. As we move deeper and deeper into a wastewater network, we can expect the pipe diameters to increase, with their frequency to decrease accordingly.

Chainage

Within the dataset, shorter pipe chainages were more frequently observed than longer ones; the ‘0-20m’ chainage category forming just under a third of total results – combined with the ‘20-40m’ category, these two formed around 54% of our total data set.

Chainages of over 100m were infrequently observed in the data set; with any chainage over 100m forming just 5% of the total observed chainages.

Again, the distribution of this data makes perfect sense given the context of a sewer network. Because sewer pipes are so ubiquitous, there are large numbers of pipes connecting properties to mains, meaning that networks of mains pipes change direction frequently to facilitate connectivity.

Structural vs Service Defects – What’s the Difference?

Before we dive in, I’ll quickly clear up the difference between structural defects compared to service defects.

A structural defect is one which has an impact on the structural integrity of the pipe itself. Over time, structural defects may worsen significantly to the point that the pipe requires significant repair work, or even replacement. Common structural defects could include cracking, breaking, or surface damage.

Service defects are those that have an impact on the operational capacity of a pipe, impairing the pipes effectiveness to convey wastewater through the pipe network. Common service defects include displaced joints, debris or root intrusions.

In this edition, we’ll be covering structural defects encountered in wastewater pipe infrastructure. We’ll release the results for service defects in the next edition of our blog.

Structural Defects

Combined Results

Overall, minor instances of longitudinal cracking was the most commonly observed defect from the dataset, representing one of the three most common defect classifications across all material types. 

Similarly, minor instances of circumferential cracking was also highly represented, also present in the top three defect categories across all pipe materials.

Surface damage (aggregate exposed) and minor instances of multiple cracks were the other main defect culprits our platform recognized.

A full breakdown of combined structural defects across all pipe types is below:

Small cracks and exposed aggregate formed the most common defect types within our data set, which makes logical sense. Both of these defect classifications are most common amongst concrete pipes, which were the most prevalent in our data set, making up 45.25% of the total material observed. 

Over time, and with changes in the pipe environment, these types of defects would continue to degrade, eventually progressing to more severe defects which could have meaningful interruptions on wastewater service to the areas they service.

Since it’s difficult to make informed recommendations from this data without specific contextual parameters, we decided to separate our defect analysis by pipe material type to identify clearer patterns and trends.

Defects by Pipe Material

Concrete

Surface damage (aggregate exposed) was the most frequently discovered defect found within pipes of concrete construction. Minor instances of circumferential cracking and longitudinal cracking were also present in concrete pipes, whilst surface damage caused by corrosion was also prevalent in the dataset.

A full breakdown of defects found in concrete pipe is below:

The results gleaned from our concrete pipes are extremely interesting, and supports the typical engineering hypothesis of failure modes from surface related defects. 

Once concrete aggregate has been exposed from initial surface damage, corrosion becomes more prevalent – with concrete cover reducing, steel reinforcements become exposed to air and moisture, causing them to corrode. Corrosion products will then leach out of any cracks or fissures in the concrete cover, resulting in spalling of the concrete cover.

Although cracking is prevalent in the dataset, it is on aggregate less prevalent than what we would see in other rigid pipe materials (i.e. vitrified clay) due to the structural reinforcement. 

Circumferential cracking may suggest angular movement of the pipe (relative to the pipe central axis), potentially caused by ground movement over time or poor bedding compaction. 

Longitudinal cracking (especially at the 12, 3, 6 and 9 o’clock positions) can suggest that design loads have been exceeded. 

I will note that this data includes other types of concrete/cement materials, which may impact the results.

Vitrified Clay

It’s not an exaggeration to say that if you’ve got a defect within a vitrified clay pipe, far more often than not, there’s going to be cracking involved.

Minor longitudinal cracks, minor circumferential cracks, and minor multiple cracks were the most common defect that the VAPAR platform picked up in pipes of this material. More severe longitudinal cracking, multiple cracking and circumferential cracking were the next most common defects observed.

Again, a full breakdown of defects within vitrified clay pipes is below:

Unlike concrete, vitrified clay pipes are not reinforced, so longitudinal, circumferential and multiple cracking are perfectly normal (and expected) defects for this material type.

Some progression in the severity of cracks is evident; whilst smaller cracks are the most evident defects for vitrified clay pipes, there is still a sizable (albeit smaller) representation of large cracks too.

In case you were interested in finding out a little more about the vitrified clay vs earthenware, I would definitely recommend this excellent piece provided by Jonathan Morris of Opus International Consultants (Wellington).

Flexible Plastics

Cracks again dominated the most frequently observed defects within pipes made with flexible plastics, taking out five of the six most frequent defect causes.

However, they didn’t take out the top spot, which went to minor instances of deformation (severity less than 5%) – no great surprise given the flexible nature of plastic construction.

Below are the defect results for flexible plastic pipes:

There’s a growing sentiment that using plastics for pipe construction is the way forward for asset owners. 

A study conducted on 4 different pipe materials (concrete, polyvinyl chloride (PVC), vitrified clay, and ductile iron) suggested that “that PVC pipe is the most sustainable option from both environmental and economic viewpoints”. I’m personally interested to see how this unfolds across the industry.

How can good planning and design help avoid impactful structural defects?

A common theme that we’ve observed is that the majority of more impactful defects (i.e., those that are more severe) generally stem from degradation of initially minor issues.

Asset owners should log instances of minor defects, and assess other factors that could potentially affect the degradation of the asset. This will allow for an informed, effective reinspection plan to be established in order to monitor the progression of defects, and prevent major, impactful structural issues to infrastructure.

Asset owners should also make an effort to observe the technical specification of their pipe construction materials. By doing this, they will be able to make informed, data-driven decisions when evaluating the contents of their asset management plans, ensuring that pipe infrastructure is allocated an appropriate amount of resources and attention.

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That’s all, folks! We’ll be releasing similar information relating to service defects soon in an upcoming blog, so stay tuned.