SASTT Trenchless Technology

Customers demand effective service delivery at the lowest cost. Johann Wessels, consulting engineer and former SASTT president, believes it is important for municipalities to understand the capabilities of the available trenchless techniques for pipeline and sewer condition assessment and rehabilitation in order to achieve this. By Danielle Petterson

A look inside your pipes

South Africa sometimes finds itself faced with overflowing sewers due to a lack of capacity and stormwater ingress. However, the overall situation is made worse by the poor physical condition of the pipe network, fat build-up, tree root infiltration, including extreme axial root intrusion, which can cause blockages and cavities.

This makes condition assessment vital to ensure that pipelines and sewer networks are maintained and rehabilitated as needed to ensure optimal service delivery.

Pipe network inspections

CCTV inspections aim to provide visual coverage of 100% of the pipe network under inspection. Wessels believes that, with repetitions and contract incentives, coverage of >90% is

achievable. Flow analysis, maintenance and stormwater transgression data, as well as any complaints for the public or ward councillors must also be considered alongside CCTV inspections.

CCTV footage should be used to grade any incidents (problems or defects) detected in the pipe. This includes the incident value, severity value and structural graded value. These incidents and values should all be plotted on a grading plan (as shown in Figure 1).

Pipes with a grading weight of over 600 – that means more than 10 grading points per metre over an average of 60 m – should be prioritised for rehabilitation or replacement.

A sewer rehabilitation summary plan can be compiled for the overall control of the rehabilitation of the sewer network.

FIGURE 1

Example of a grading plan with six different cases Figure 2 shows such a plan, where the red indicates where CCTV and cleaning has been done and green means rehabilitation has been conducted. The bars on each suburb indicate GIS length (black), CCTV done (blue), replacement/rehabilitation still to be done (yellow), and replacement/rehabilitation completed (white).

At the end of the contract period, blockages before, during and after the contract should be plotted in order to control the effectiveness of the selection and the quality of the contractor’s work.

Outfall sewer assessment

Wessels highlights that the cost of outfall sewer failure is huge, presenting a very high risk. “Condition assessment and prioritisation on outfall sewers is even more important than on networks, but the technology is different,” he explains.

Here, multisensor inspections using laser above the water line and sonar below the water line, augmented by a digital camera, are conducted. Most of the time, a floater containing all three is used, but numerous equipment options are available depending on the diameter and condition of the sewer during inspection.

For example, if the pipe is close to or completely surcharged at the time of inspection, submersible floaters containing only the sonar equipment can be used. If the water levels are low, a trawler with only the laser and digital camera can be used.

The laser, sonar and digital camera take continuous readings. Together, this equipment provides a 3D view of the pipeline, making it easier to identify abnormalities such as protrusions and corrosion.

FIGURE 2 Example of a sewer rehabilitation summary plan

Having digital CCTV to augment the sonar and laser observations offers many benefits. Certain incidents can be hard to identify with sonar or laser but can be very accurately determined with digital CCTV, which can be manipulated from various angles to present a recognisable picture.

“Importantly, the remaining useful life of the sewer should be monitored using the inspection data and the as-built drawings, so that rehabilitation can be planned in good time. The budget for major rehabilitation can therefore be determined and scheduled,” explains Wessels.

Trenchless pipe rehabilitation

Wessels advocates for the use of trenchless technology for pipeline and sewer rehabilitation and replacement because of the many benefits it offers. “Studies have found that open excavation can cost up to seven times as much as trenchless replacement. And that does not even include the cost of blockage removal and the disruption open trenching causes,” he points out. There are several trenchless rehabilitation methods available, including: - pipe bursting - cured-in-place (CIPP) relining - segmented slip lining - close-fit slip lining - spirally wound lining - localised point repair - flood grouting. The choice of method will depend on a variety of factors.

However, Wessels cautions that there is a window of opportunity for replacing pipelines and sewers using trenchless methods. If left too long, the pipeline might collapse, forcing the use of open excavation, likely at far greater cost.

Examples of sonar/laser equipment for outfall sewer assessment