How to increase the lifetime of your stormwater pipe assets: analytics on commonly observed defects
At VAPAR, we’ve been doing more and more work with a number of councils and municipalities over the last couple of months and are finding some interesting insights from the CCTV inspection data that are relevant to more than just the pipes in our clients drainage network.
Most of the stormwater CCTV inspection footage that we are processing is producing the same types of defects, and in some cases, there are things that can be done during the design and construction phase that can prevent the occurrence and frequency of common defects. After all, prevention is better than cure.
There are also a handful of ways that these common defects can be addressed if the timing of any intervention is being undertaken a long period after the pipe has been installed.
Based on 1,421 stormwater inspection CCTV videos, representing 35.53 kilometres (or approximately 22 miles) that have been analysed by VAPAR’s web platform in the last couple of months, we dug into the data to look at the most common defects that were present, based on what was identified from the inspection footage.
But first, by way of context, below is a breakdown of the factors that characterise the sample dataset:
By material type
The majority of the sample dataset was from steel reinforced concrete pipe. The next most common material type was polyvinyl chloride (PVC). Other pipe materials were excluded from the sample dataset as there were not enough of them to be representative of ‘common’ defects.
Note that this analysis is focused on Stormwater pipes only; the majority of sewer pipes processed through our system are excluded from this dataset and will be covered in another post.
The below is a tree diagram displaying that the greatest number of CCTV videos in the sample dataset came from 375 mm (15 inch) diameter pipes, followed by 450 mm (18 inch) and 300 mm (12 inch).
The graph below shows the spread of chainages inspected in each of the CCTV videos from the sample dataset. As you will notice, the majority of the CCTV videos are less than 50 linear metres (or 164 liner feet) in length.
Steel reinforced concrete pipes
When explaining the defects observed in these types of pipes to our non-engineering team members, I found this particular explainer video of ‘how concrete pipes are made’, including showing the wrapping of reo-bar and casting of concrete pipes, to be particularly helpful and engaging.
Below is a treemap diagram of the most common defects identified in the most common defects that were observed in the sample dataset. From this you will see that longitudinal cracking, damaged lifting holes and circumferential cracking are the most common defects for this pipe material.
There are a number of reasons steel reinforced pipes can crack, many of which are described here. Longitudinal cracking is more notable than circumferential cracking, as circumferential cracking does not have as much of an impact on the load bearing capacity of the pipe.
Due to the weight of the steel reinforcement in each pipe, most steel reinforced concrete pipes have a manufactured lifting hole (also referred to as a lifting eye) on the top of the pipe to aid handling and installation. However, if the wrong concrete pipe lifting device is used during handling, lifting holes can become damaged, whereby there is excessive spalling of surrounding concrete etc. Over time damaged lifting holes can cause other issues such as circumferential cracking and infiltration, which may or may not cause further deterioration to the overall pipe condition, and are therefore defects worth noting during CCTV inspections.
Surface damage showing corrosion products or exposed reinforcement was also a common defect observation. This defect is typically weighted quite severely in condition assessment standards because of the theoretical importance of concrete cover in maintaining the integral strength of the steel reinforcement. However I found this blog post to provide an interesting perspective based on in-ground results of steel reinforced concrete pipes over a 30 year life span. Check it out.
Polyvinyl chloride pipes
The treemap diagram of defects for polyvinyl chloride (PVC) pipe from the sample dataset shows that deformation to varying extents are the top three most common defects identified. Anecdotally, I have experienced deformation to be a commonly observed defect during inspection of PVC pipes, and I was really interested to see this bear out in the data analysis of the sample dataset.
Some degree of deformation in PVC pipes is expected. After all, they are ‘flexible’ pipes. In fact, 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 inspecting the pipes. For example, high degrees of deformation at any age of PVC pipe is a cause for concern. However, for small to moderate degrees of deformation, knowledge of the age of the pipe can help asset owners understand whether the structural integrity of the pipe is impacted or not.
What should I do about these structural defects?
Without a doubt, and as a chorus of literature will attest to, the bedding material, thickness and compaction can go a long way to overcome a number of the structural defects listed above that relate to the load bearing capacity of the pipe. This holds true for both rigid and flexible pipe materials. In fact, for flexible pipes in particular, I would go so far as to say that bedding and bedding compaction is the major factor to the structural performance of pipes. Using experienced drain layers and a well-planned inspection and testing plan during installation can reduce the risk of these common structural defects developing later on.
Additionally, pipes need to be designed for both design loads, as well as the construction loads that are imposed before the allowable depth of cover is achieved. This is quite a common cause of structural defects in newly laid pipes, alongside improper handling. In my opinion, the designer should ensure this is accounted for, as once the construction starts, site engineers are challenged with too many moving parts on site to constantly check weight restrictions for their heavy equipment as and when the pipes are getting laid. Accounting for live construction loading may add to the project's capital cost (from purchasing higher strength pipes), but can save overall costs in quicker acceptance for adoption by the asset owner, and the asset owners operation and maintenance activities down the track.
Pre-adoption CCTV surveys should be a ‘hold point’ in the inspection and test plans before taking on the asset, and are an opportunity for asset owners to request that drain layer contractors remediate any defects identified before adding the asset to their network.
Service (or O&M) defects
Minor levels of debris and deposits were the most commonly observed defects in the stormwater CCTV sample dataset. This comes as no surprise to those of us that are familiar with the operation of stormwater pipes; at times it feels as though they pass just as much leaf litter and street litter as they do surface water. 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.
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 as it is in sewer pipes, though something to note during CCTV inspections as they may indicate poor joint tightness.
On a side note, the current (2013) version of the Australian conduit inspection reporting code has displaced joints as a service defect for stormwater pipes which is why ‘displaced joints’ appear here in the service defects. This ‘service’ categorisation is expected to be updated in the upcoming (2019) version where displaced joints will be classed as ‘structural’ defects.
What should I do about these service defects?
Source control is definitely the name of the game to reduce the instances of debris and deposit build up in pipes. Gross pollutant traps, or litter traps, at key points in the network can reduce the introduction of debris into the system. Designing pipes with enough fall can also aid the pipes 'self-cleaning’ ability. The other common defects that are joint related can be addressed during the design, construction and commissioning phase. Where possible, installed pipes should be installed with a rubber ring joint to facilitate allowable joint articulation whilst keeping the joint as watertight as possible. However, even with rubber ring joints in design drawings, if the drain layer contractor has not pushed the pipes to the witness mark, or even over-inserted, this can cause issues with the joint. A commissioning CCTV survey should pick this defect up for remediation.