What data says about service defects in sewer pipes (and how to prevent them)

Back in October, we published a blog outlining the most common structural defect types within sewer pipe infrastructure, according to a data set of around 30 km which we’d borrowed from footage in Australia, NZ and the UK.

Now, we’re rounding things out by providing the same information for service defects within sewer pipe infrastructure, using the same data set to provide unique, data-led insights about the most common service defects in sewer pipes, and how to prevent them.

About Service Defects

By way of background, 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. 

Compared to structural defects, service defects will not involve the structure of the pipe itself. Commonly observed service defects include displaced joints, debris or root intrusions.

A Quick Summary of Our Data Set

If you’re after a more comprehensive overview of the dataset we used, I’d definitely recommend checking out our previous blog on structural defects (that shared the same dataset) to get a complete picture. In this instance, I’ll just cover the key points to avoid bogging things down.

The data set we used consisted of 605 pieces of wastewater pipe inspection footage, representing 26.45 km in combined network length from Australia, NZ and the UK.

Concrete and vitrified clay were the most commonly observed pipe construction materials, representing 45.25% and 42.02% of the dataset respectively, flexible plastics represented 11.79%, and a miscellaneous ‘other’ covered a tiny 0.95% of materials.

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).

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 represented 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.

Defects – An Overview

Immediately, there’s a few clear key trends observable in the data set; occurrences of displaced joints and root intrusions are by far the two most common defect categories, with instances of lesser severity significantly more common than those of greater severity. In fact, instances of displaced joints were so common that they formed three of the top four most common service defects we observed.

A full breakdown of the defects we discovered can be seen in the treemap diagram below:

With the overarching trends spelt out, let’s dive a little deeper into the two most common classifications of defects that we’ve just discussed.

Displaced Joints

Firstly, it’s important to note that since WSA 05 2020 (Australia’s version of a conduit assessment code) was released, displaced joints are now classified as both structural and service defects. If you’re keen to learn about the changes that WSA 05 2020 brought, I’d highly recommend reading another blog we released in September which outlines the most important considerations on this topic.

Looking into the data we collected, it’s clear that among service defects within sewer pipes, instances of displaced joints are king (perhaps instead, they should be labelled a royal pain!). Diving in a little deeper, instances of joint offset (sometimes referred to as ‘radial’ displacement) occur around 2.5 times as frequently as their joint separation (can also be referred to as ‘open’ joint) counterparts.

The prevalence of displaced joints within pipe infrastructure is most common in vitrified clay pipes, particularly older designs without rubber rings in joints. With our dataset including a high proportion of vitrified clay (42.02%, to be precise), there is a pretty clear correlation found in the defects we observed compared to the materials used in their pipes.

Roots

The other main service defect culprit which we identified within the data set was that of fine root intrusion, which was the second-most observed defect category.

Similar to displaced joints, the prevalence of root intrusions won’t be any great surprise to anyone who deals with sewer pipes day-to-day; they’re a constant thorn in our side!

From our data set, we observed that the majority of fine roots intruding into sewer pipes entered the pipe through the joints. This suggests that implementing better pipe sealing could act as a remedy to regular incursions of fine roots.

A study undertaken by University of Melbourne 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.”

The study also investigates the efficacy of both chemical and physical treatments to prevent root intrusion. The physical treatments included compaction and cement slurry, both of which showed quite promising results in inhibiting root growth compared to their chemical counterparts – which I found particularly interesting. The study attributes the effectiveness of the cement treatment to “increasing soil strength above the force that the roots were able to exert”.

What can be done to prevent these defects?

Joint Displacement

Although there’s no silver bullet solution to prevent joint displacement, there are a few ways that asset owners can limit the prevalence of displaced joints in their sewer pipe infrastructure.

The best time to take action to prevent displaced joints is prior to, or during the installation process. Adding spigot and socket (or similar) pipe joints and a flexible joint material into engineering specifications will help to maintain the water-tightness of the pipe even with some degree of inevitable joint articulation.

Additionally, during installation, ensuring that pipe bedding material has undergone a thorough level of compaction will help to prevent settlement or movements in these pipes over time.

Finally, when installing sewer pipes that have spigot and socket joints, drainlayers should ensure that pipes are pushed precisely to the witness mark. By implementing a careful approach during installation, asset owners will notice fewer cases of damaged pipe ends and rubber seals.

In instances where joint displacement has already been identified, more extensive works will generally be needed to address the issue. Depending on the severity of the defect, pipe relining may be required. In particularly severe instances, excavation and replacement of offending pipe sections may be the only solution.

Roots

Firstly, I’d like to point out that one of the most effective ways to minimise root intrusion into pipes is by minimising joint displacement in pipe networks. So, if you’ve implemented the advice directly above – you’re already halfway there!

The application of herbicidal foam in sewers is one of the leading solutions for reducing regrowth of roots that enter sewer pipes. I’d highly recommend checking out this video if you’ve never experienced how this process works.

There’s a trend which is seeing asset owners move away from root cutting as a solution to root-related defects, and with good reason. A great article from Newman Plumbing explains it brilliantly; “each time roots are cut they respond by regrowing thicker and faster, similar to pruning a hedge. The aggressive nature of the cutting process will also damage the condition of the pipe and will inevitably result in costly rehabilitation or replacement.” Many asset owners seem to be in agreement, and are moving towards other means such as herbicides so as to minimize the impact of repairs on their pipe infrastructure. 

For particularly problematic sections of sewer pipe, relining the complete asset may be necessary so as to remove the opportunity for the roots to reach water via the joints.

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That’s all, folks! If you haven’t already seen them, check out our other blogs on structural defects in sewer pipes, and defects in stormwater pipes.