Structural Defects

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Infiltration is defined as the inflow of water into pipes and can be attributed to bad joint connections, holes, breaks and physical damages. The infiltration types can be in the form of Dripping, Gushing, Running and Seeping based on flow intensity. Infiltration is considered a type of leakage, which is the intrusion of groundwater through a defect. Leakage may also be an “exfiltration,” which is the seeping of sewer flow out of the pipe through a certain defect.

Table 2.2: Description of Operational Defects in Sewer Pipelines

Defect Category Defect Type Description

Roots Fine Roots that lead to a reduced flow through blocking the pipe’s area

Single A single root in which its thickness is more than 10 mm which would damage the pipe.

Dense Combined roots that might block the whole pipe’s cross section

Infiltration Seeping(1) _{A defect is said to be seeping if it is intruding in a} slow pattern

Dripping(2) _{A defect is said to be dripping if water is dripping,} but not continuously

Running(3) _{A defect is said to be running if water is intruding} in a continuous manner

Gushing(4) _{A defect is said to be gushing if water is intruding} quickly, as though under pressure

Deposits Settled Deposits The settling of deposits on the pipe surface that could reduce the flow capacity

Encrustation Encrustation is formed by the effect of

evaporating infiltrated water throughout defects along the pipe

Foul Attached deposits which are remains of foul sewage

Grease Attached grease above the flow on the sewer walls

Soil intrusion It is the intrusion of surrounding soil into the pipe through certain structural defects

Intruding Services Some pipe materials that would intrude the pipe causing a reduction in its capacity 1. Seeping: When water intrusion flow is slow, it is said to be seeping.

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2. Debris: Attached deposits which are remains of foul sewage. 3. Effluent: Attached grease above the flow on the sewer walls.

The research carried out addressing sewer pipeline defects can be divided into two main parts. The first part is concerned with automatic detection of defects, while the second part is research addressing conditions of sewer pipelines considering defects. While researching a way to detect and classify defects in sewer pipelines, Moselhi and Shehab-Eldeen (2000) developed an automated tool that detects and classifies cracks in sewer pipelines using neural networks technique. In another study, the authors used artificial intelligence and image recognition techniques to develop a tool for measuring infiltration in sewer pipelines (Moselhi and Shehab- Eldeen, 2005).

In essence, several studies have addressed automatic detection for defects in sewer pipelines to avoid errors resulting from human judgments when identifying defects through traditional CCTV methods or other methods requiring a human element in the designation of defects (Halfawy and Hengmeechai, 2013, 2014a and 2014 b). As for research addressing condition assessment, and how defects affect the health of pipelines, deMonsabert et al. (1999) studied the infiltration and inflow in sewer pipelines and how a defect might affect a decision regarding rehabilitation. This research offered a planning tool to determine which pipelines would require rehabilitation. In a similar study, a planning tool was provided which used infiltration and inflow defects to determine which pipelines would require rehabilitation or replacement using fast messy genetic algorithm.

2.5. Rehabilitation Techniques

To extend the lifetime of sewer pipelines, proper maintenance shall be carried out as deemed necessary. One of the interventions that can be divided into several activities is the rehabilitation

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of deteriorated sewer pipelines. Figure 2.3 shows the different sewer rehabilitation methods that include repair, renovation and replacement (WEF, 2009).

Figure 2.3: Sewer Pipelines Rehabilitation Techniques 2.5.1. Repair

Repairs are concerned with maintaining sewer pipelines by cleaning or fixing leakages. Usually the structural condition of the pipeline is examined to ensure that there are no severe structural defects which could threaten the soundness of the pipeline. Jetting or hydro-mechanics are methods used in cleaning the pipes, while chemical grouting or sleeves are used to stop leakages.

2.5.2. Renovation

If the structural integrity of the pipelines does not meet the standard required, renovation or renewal of the affected pipeline must be carried out. In pipeline renovation, either coating or a lining can be applied to the deteriorated pipeline. In this procedure the cross sectional area of the pipeline is reduced. One of the renovation methods is inserting while pushing and pulling prefabricated pipes through the existing pipeline. Another renovation technique is creating a spiral polyvinyl chloride (PVC) sleeve on site and introducing it to the affected segment. A third technique is to insert a liner in the pipeline and apply hot air or pressure to that lining to treat the defective area.

2.5.3. Replacement

Rehabilitation Techniques

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Replacing sewers is considered the conventional technique for sewer rehabilitation, and although it is cheaper than renewing sewer pipelines, it requires more time and work. As such, innovative trenchless technologies such as pipe bursting, directional drilling, micro-tunneling, and Horizontal Directional Drilling, were introduced to replace sewers without the need for excavation work. Trenchless technologies have the advantage of saving cost because no traffic or pavement disruption takes place without affecting businesses or the environment. Table 2.3 shows the different trenchless techniques and the major limitations for using each technique. It can be observed that the sewer diameter plays an important role when determining which technique should be used. It is worth noting that decisions regarding which technique to use when replacing sewers should involve a cost analysis to determine which option is best.

2.6. Criticality of Infrastructures

Critical infrastructures are defined as services for which an interruption or failure in them would have potentially adverse impacts on the social, economic, and environmental wellbeing of the public. Critical infrastructures, also known as critical assets, are usually identified using a risk based assessment methodology as promulgated by the American Electric Reliability Corporation (North American Electric Reliability Corporation, 2006). Risk analysis methodologies help in assessing critical infrastructures, the results of which may form the basis of a proper assessment management plan that can be carried out to protect these infrastructures.

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Table 2.3: Different Pipe Replacement Techniques and Main Advantages and Limitations for Each Technique

Technique Description Advantages Limitations

Pipe Bursting

It is a trenchless replacement method in which a bursting tool is inserted inside a pipe and mechanical forces are applied until the pipe is broken (Griffin, 2012). An insertion pit upstream the pipe is installed where a cable attached to a new pipe of the same diameter or larger is pulled from another end until the pipe coincides with the burst location.

- Can be applied to various pipe diameters, materials and soil conditions.

- The pipe diameters to be replaced ranges between 50mm and 900 mm.

Bursting length in this technique doesn’t exceed 300 meters.

Horizontal Directional Drilling

In this method, a pilot alignment is drilled along the path of the pipe to be installed. A reamer then starts enlarging the pilot alignment to the required diameter. The pipe is then installed by pulling it in the reamed path.

- Doesn’t require large excavation pits.

- Doesn’t interfere with traffic. - Can be used for large spans and

pipe diameters (Najafi, 2005). - The pipe diameters to be

replaced ranges between 50mm and 1200 mm.

Drilling length in this technique doesn’t exceed 300 meters.

Pipe Jacking / Micro- tunneling

The concepts behind pipe jacking and micro-tunneling are almost the same. The two techniques depend mainly on controlling and guiding remotely by applying mechanical or hydro mechanical pressures. In applying the method, shafts are installed at both ends from which the boring machine followed by the pipe segments are jacked from one end and received from the other end. In pipe jacking, the same process is applied; however pipe segments are jacked one after another.

- Doesn’t have a size limitation, usually pipes of diameter between 600 mm and 2300 mm)

- Can be applied in difficult ground conditions.

- Not recommended for sizes less than 900 mm and more than 2800 mm.

- Requires a person on the operating hatch.

Open cut replacement

It is the most widely used method in replacing pipes, however due to the presence of trenchless technologies, this method has been used less over the past two decades. As the name implies, equipment is used to cut open the location of the pipe to be replaced, then pipe is installed and buried under ground.

To determine the cost effectiveness a cost benefit analysis should be carried out.

Sometimes environmental

constraints and socioeconomic costs lead to use other methods due to high indirect costs for open cut method.

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These are usually carried out by performing a “what-if” scenario for a possible failure event. This hypothetical outcome is then usually assessed in a qualitative or quantitative manner (Emergency Management Australia, 2003). There are three aspects to assess the impact of failure of infrastructure. These are: the geographic area affected by the failure, how long it would take for this failed infrastructure to disrupt the surrounding environment, and the intensity of that failure (European Commission, 2006 and Ministry of the Interior and Kingdom Relations, 2008). Usually the first two aspects are analyzed using qualitative or semi-qualitative criteria whereas the intensity is usually analyzed using detailed qualitative and quantitative criteria.