The trenchless sewer rehabilitation industry is well established in Australia and New Zealand and has grown from its origins in pipeline rehabilitation to now include additional parts of the network such as house service lines, lateral connection junctions, and access chambers.
In recent years the importance of renewing and sealing the entire sewer network to prevent leaks, infiltration and collapses has been widely recognised by many sewer asset owners. One aspect that is becoming increasingly important is renewal of access chambers (also referred to as manholes, maintenance holes, or utility holes).
Access chambers represent a significant proportion of the total surface area of a sewer network. The table below puts this into perspective.
A program that leaves the access chamber untreated, leaves up to 30 per cent of the network untreated. Additionally, if the main pipeline is relined, the level of hydrogen sulphide present in the sewer atmosphere will typically increase because the gas sink present in the host pipe walls has been removed. This may result in an accelerated rate of gas attack at the access chambers.Article continues below…
Given that it is accepted that access chambers do require a rehabilitation strategy the questions then become:
1. When do they need to be rehabilitated? 2. How do we determine the condition of the asset? 3. What are the most appropriate products to use?
These are essentially the same questions that apply to the pipelines but the dynamics with access chambers are quite different.
When do access chambers need to be rehabilitated?
When answering this question as it applies to pipelines, asset owners will typically take an approach that considers the condition of the asset and the criticality of asset (i.e. the consequence of failure). Based on ranking each asset according to its criticality and condition, a priority matrix can be created that identifies which pipelines should be done and when.
The same approach seems reasonable for access chambers but there are significant differences in two areas:
a. The cost dynamics
b. The method of condition assessment.
It is important to understand the difference in cost dynamics that apply to pipeline rehabilitation versus access chamber rehabilitation. When a pipe is renewed a liner is placed inside the host pipe and it is typically assumed that the existing pipe is fully deteriorated. In other words, no attempt is made to determine the condition of the existing pipe and to offer a solution that replaces its lost strength. It is simply too difficult to assess the remaining strength of pipe so the industry takes a conservative approach and assumes its strength is zero. As a consequence, from an asset management point of view there is no sliding scale of renewal costs as the asset deteriorates. Whether the asset has lost all of its strength or only five per cent of its strength the relining cost will be the same. Therefore the optimum renewal time for the pipe (from a lifecycle cost point of view) will be just before it fails.
This is not the case with access chamber rehabilitation. There are several types of products that can be applied to access chambers each with very different cost dynamics:
A protective coating can be applied to a structure that has not undergone significant structural loss. For all intents the structure remaining is fully intact. Under this scenario a thin protective layer can be applied. The coating would not need to offer any structural characteristic. It would be capable of withstanding the gas attack and therefore extending the asset life. This would be considered to be taking a preventative maintenance approach. The costs for this type of coating would be relatively low compared to the other types of coatings. The types of products would include thin polymeric coatings such as polyurea, polyurethane and epoxy and cementitious products such as calcium aluminate cement.
A rebuild coating would be needed where the structural integrity of the existing access chamber has been compromised. In this case it is appropriate to reinstate the lost structure with a material that has some structural strength and it would need to be applied in a thickness commensurate with the degree of deterioration. Typically costs for this type of treatment would be up to double that of the protective coating and gradually increase as the thickness of product increases (i.e. the amount required to restore the structural strength).
A reconstruct is needed when the existing access chamber has lost nearly all of its structure. In this case some form of reconstruction technique is needed. This can include reboring a new access chamber, slip lining the existing chamber or internally reconstructing. Naturally this situation is very expensive and would expect to cost at least four times more than the protective coating.
From the foregoing discussion it can be appreciated that the optimum time to renew an access chamber from an economic point of view is not necessarily obvious. The graphs above left express the difference in cost dynamics between access chambers and pipelines, graphically. The optimum economic time for a pipeline is towards the end of its life. For an access chamber it could be at points 1, 2 or 3.
How do we determine the condition of an access chamber?
Determining the condition of a pipeline is a relatively difficult task that has developed significantly and is now able to be performed in a consistent way by skilled operators that follow techniques and guidelines set out by industry standards. The tool of choice in all cases is CCTV footage that is subsequently interpreted by a skilled operator.
An access chamber has one significant benefit over a pipeline in that it is accessible. As such we are not limited to assessing the condition by using images. We can actually take core samples directly from the access chamber. This is particularly helpful in circumstances where the access chamber was cast in-situ, because it tells us exactly how thick the structure is, not just how much concrete has been lost.
There are significant benefits in taking core samples from manholes. Appearances can be very deceiving. The following sequence of images, showing three access chambers from the same suburb at the same time, highlights this. If we were to only look at the images, the condition of each appears equivalent, i.e. one would probably conclude that the chamber is structurally sound and requires only a protective coating. However after taking a core sample, as shown in the image, a very different conclusion was drawn.
Given the costs of each solution are potentially very different, and that the risk profile of each is very different, the consequences of using only a visual assessment to determine the condition of the access chamber can be quite serious.
What are the most appropriate products to use to rehabilitate access chambers?
In access chambers the two broad families of materials that tend to be used around the world are polymeric coatings and hydrogen sulphide resistant cementitious products such as calcium aluminate cement (CAC). The two families of materials have benefits in different areas. Broadly speaking CACs will be very cost-effective and can be used both as a protective coating (thickness of approximately 12 mm) and as a rebuild coating by applying the product at a thickness equivalent to the amount of concrete lost.
The relative cost effectiveness of CACs has seen an increased interest in recent times in the Australian and New Zealand market. CACs can be applied in relatively high thicknesses and will typically have compressive strengths greater than the existing concrete structure. This means that a ‘best of both worlds’ result can be achieved. But where does this product sit in the overall product choice equation?
The limitation of CACs are where the pH of the substrate is expected to be lower (more acidic) than two. In this circumstance a polymeric coating would be required on top of a CAC.
In summary, at low pHs polymeric coatings must be used (either alone or one top of a CAC). At higher (less acidic) pHs, CACs or a polymer coating can be used.
This in turn leads to the question – how do we determine the pH of the substrate? Direct measurement in practice is not feasible. The most simplistic and accurate method of determining the pH of an access chamber is to determine the rate of attack of the existing concrete surface. If the pH is less than two, one would expect to see a rapid rate of gas attack – in the order of five mm per year. Therefore if we know the age of the access chamber and the depth of gas attack (from the core sample) we can determine how aggressive the conditions inside the access chamber are and whether a polymer coating is required.
The table on the opposite page attempts to summarise the product choices available according to the condition of the access chamber and hence identify where each product fits in the market. It does not make any attempt to rank the suitable products by cost-effectiveness.
In order to obtain the optimum solution for access chamber rehabilitation there are many factors to consider and the approach may need to be a little different to that taken for pipelines. The cost dynamics of the rehabilitation solutions are very different meaning the optimum time in the lifecycle to perform the work may not be immediately obvious. Furthermore, the condition of the access chamber can be determined quite accurately by coring, and as such a specific solution can be determined based on the results. If we overlay the final factor, which is the rate of gas attack, we can further refine the product choice.
If these factors are taken into account the asset owner has the opportunity to receive the optimum product choice and thus minimise rehabilitation costs and maximise value for money.
An experienced pipeline renewal company such as Interflow can offer clients an objective condition assessment method and offer a full range of products including both polymeric coatings and CACs. As such the most cost-effective product can be applied to the structure based on its condition.