In 2009, the City of Hamilton undertook a pilot project using a PPR inspection tool to locate voids in large diameter sewers that are greater than 1,500 mm in diameter.

The City of Hamilton (City) owns, operates and maintains over 400 km of large diameter storm, sanitary and combined sewer systems. Approximately 50 per cent of the City’s sewer systems are over 100 years old, while the average age ranges around 60 years.

Due to the varying terrain across the City, the majority of the main sanitary interceptor sewer systems are located very deep, running under existing residential areas and traverse their way through the City’s historical Steel Town and through existing environmentally sensitive areas (valleys) to the Woodward Avenue Wastewater Treatment Plant.

Several large diameter storm sewers convey storm flows through Hamilton’s mountain, stepping their way down through waterfalls into plunge pools and eventually outlet into Lake Ontario through numerous outfalls along the Hamilton harbour.

The City considered over 50 per cent of sewers as ‘critical assets’ due to their size, depth, location and amount of flow they convey. The City has placed a zero tolerance for failure on these critical assets. To manage this responsibility, the City carries out a large diameter inspection program using various technologies, including CCTV, combined CCTV and sonar, person-entry inspection where possible, and occasionally multi-sensor inspections.

The above inspection technologies are designed to locate defects only within the inside of the sewers, including cracks and active infiltration. However, these techniques cannot locate voids that are present or forming behind pipe walls.

The detection of voids behind pipe walls in sewers has been an ongoing challenge for municipalities, as there is no current technology available to locate voids on the outside of the pipe wall. Proper management of critical assets and the ability to scan and determine what is occurring behind a pipe wall, would allow a municipality the time to properly plan and manage the repair, rather than the alternative ‘wait-and-see’ approach and hope that the sewer does not deteriorate to an unacceptable structural level.

Background

Determining the need to locate voids behind pipe walls became apparent to the City during an inspection of the Western Sanitary Interceptor, a 1,524 mm diameter sanitary sewer. Several active infiltrations were observed in one 270 m long segment, based on the CCTV camera video review. It was anticipated that voids caused by the water infiltration were forming behind the pipe walls.

A follow-up person-entry inspection was ordered by the City, which is not commonly carried out for large diameter sanitary sewers due to the confined space entry and danger of working in high sewage flows.

The main purpose of the inspection was to determine if using GPR technology could locate voids behind the pipe walls. GPR technology is generally used to locate voids under pavement, voids in concrete and underground utilities from the surface. GPR works by sending a radio signal into the ground. The return signals provide information about changing ground characteristics with depth. The radar measures depth in the terms of the time it takes for a signal to return after emission.

Hamilton’s Asset Management Group manages and maintains the entire sewer system and explores new inspection technologies that could assist them to better understand the condition of their sewers. They identified the need for more accurate and reliable data from their large diameter sewer inspections, which would allow them to make more informed decisions regarding maintenance and rehabilitation planning. To approach this challenge, the City retained R.V. Anderson Associates Limited (RVA) to assist in developing a pilot project to determine if GPR could be effectively used to locate voids behind pipe walls.

Developing a pilot project

To undertake this pilot GPR inspection project, RVA and the contractor, PipeFlo Contracting Corp., employed Sensors and Software Inc., manufacturers of GPR equipment, as a part of the inspection team.

During the planning stage of the GPR trial, the team determined several limitations and challenges that must be looked at when working with GPR in a sewer rather than traditional ground application, including:

• Surface GPR units are large, hand-operated, generally pushed or towed by the technician, along the surface to search for the underground anomaly or utility. Sewers are tightly constrained and person-entry is complex and risky.
• GPR surface units utilise a small on-board computer which is required to collect and store data allowing the technician to observe possible anomalies through a portable computer screen during the scanning operation.
• GPR units have several different size antennas that operate at different frequencies for various applications.

Choosing one system to work effectively within a large diameter sewer required careful consideration. Because of the limited size of the 1,524 mm diameter and harsh conditions, any GPR system must be hand-held, waterproofed and portable.

Hand held in-sewer GPR trial

The in-sewer GPR trial was carried out in a segment of the Western Sanitary Interceptor. This 1,524 mm diameter sewer was built in a tunnel by hand mining techniques in the 1960s. The pipe wall design thickness was 375 mm and the average depth of the sewer test segment was 28 m.

Due to the extreme depth of this sewer and associated hazards working in a high flowing sanitary sewer, the crews set-up in-sewer teams with live communications, including a CCTV robot from the downstream manhole to provide a full-time camera view of the technicians.

During the in-sewer GPR trial, Sensors and Software modified one of its portable GPR units, allowing for two antennas, 500 MHz and 1,000 MHz, to determine which worked better to locate voids. Three separate lines were established longitudinally down the sewer at the 9:00, 12:00and 3:00 o’clock positions.

The 500 MHz antenna was used first and was followed by the 1,000 MHz antenna. Both sensors followed the same clock positions for post inspection comparison and analysis of the data captured. The length of the test lines were 10 m.

Due to the size limitation of the 1,524 mm sewer diameter, the project team on the surface paid particular attention to the technicians and their ability to operate the modified GPR equipment inside the confined space of the sanitary sewer.

When the technicians moved down the sewer, existing slime on the pipe walls caused the sensor antennas to slip or shudder. The other noticeable problem was keeping accurate control of the distance from start to the end of the profile to ensure any existing cracks or active infiltrations in the sewer were identified. This would become more apparent during the post-inspection analysis.

Results of the in-sewer GPR trial

Overall, the test was successful and both antennas managed to see the back of the existing concrete sewer.

The higher frequency (1,000 MHz antenna) clearly showed better resolution, allowing for more detail to be seen in the data but did not provide penetration into deeper layers. The lower frequency (500 MHz antenna) had better penetration resulting in higher amplitude signals, received, making features deeper behind the sewer walls, potentially detectable.

Interestingly enough, it was found that both antennas showed evidence of inconsistency at the same location. Further analysis concluded there was a possible existence of voids within the pipe wall.

The other concern identified during the post-inspection data review was that the depth of the concrete pipe wall significantly varied from the designed thickness of 375 mm. The data indicated that the concrete pipe wall thickness varies from 100–800 mm. The possibility of a sewer collapse is greater if wall thickness is considerably less than 375 mm at the depth of 28 m.

An additional depth verification inspection was undertaken to confirm the actual pipe wall thickness. This inspection was carried out by PipeFlo technicians who drilled through the walls in seven pre-determined locations based on the data analysis gathered during the GPR trial. The depth verification inspection found that the concrete depth close to the outside of the manhole shaft was 800 mm in depth. This exceeded the expected designed depth of 375 mm.

The most interesting finding revealed the existence of voids within the actual pipe wall. As the technicians began drilling into the pipe wall, the drill bit slipped easily through the open cavity, then hit concrete again on the other side of the void. Careful measurements were taken to confirm the actual size of the inside cavity, which were compared to the depth determined during data analysis. Following the depth verification inspection, all located voids, including voids located behind the pipe wall were filled with chemical grout.

Development of a PPR inspection tool

Based on the data gathered from the in-sewer GPR trial and the confirming results from the post inspection data analysis, the project team made a decision to go forward to design a full size Pipe Penetrating Radar (PPR) inspection tool.

To avoid the risk of technicians working in deep, high flowing sewers and to eliminate the risk of not being able to accurately correlate the location of possible voids within a pipe, the project team gathered in the Spring of 2009 to develop the design criteria to build a full size PPR inspection tool that has the ability to scan the walls in the pipe from a 9:00 o’clock to 3:00 o’clock range, while meeting the following specific design requirements:

• The inspection tool must be able to fit through a standard manhole opening and/or be able to be assembled in the invert of a manhole similar to other inspection tools.
• This inspection tool must be designed to work in 1,500 mm diameter and larger sewers.
• All sensors, antennas and associated electronic hardware must be fully waterproof.
• The inspection tool must be designed to be able to be transported on a stable platform to work in flow to eliminate the need and additional cost for sewer bypass.
• The design must have a tethered connection in order for data to be viewed at all times during an inspection.
• This inspection tool must be designed to accommodate several sensor antennas (minimum of three, maximum of nine) for very large pipes.

Based on the above criteria, a concept inspection tool was designed (see Figure 3).

PPR inspection tool

GPR equipment consists of antennas, electronics and a recording device. They are digitally controlled, and data is usually recorded for post survey processing and display. The digital control and display output of a GPR system most commonly consists of a micro-processor, memory and a mass storage medium to store the field measurements. A micro-computer and standard operating system is often utilised to control the measurement process, store data and serve as a user interface (Daniels, 2000). PPR’s primary use is to detect variations in pipe bedding conditions to identify the location and extent of voids outside pipe walls (Najafi, 2010).

The PPR inspection equipment consists of a three-wheeled steel cart that carries adjustable arms that support the sensor antennas directly against the inside face of the pipe.

The sensor arms are designed to be adjustable to fit through a manhole opening and allow for vertical movement (the ability for the sensor to move away from the pipe wall over existing encrustation buildups on the pipe walls and/or offset joints as the cart is being deployed down a sewer). The three arms were set at the 10:00, 12:00 and 2:00 o’clock positions, designed to fit inside a 1,500 mm diameter sewer.

Attached directly to the cart is a waterproof box that houses the electronics needed to potentially support up to ten sensors. A series of network interface cards, a router and a built-in backup power supply unit are also included in this box.

Attached to the box is an Ethernet/power cable which supplies power to the sensors and transfers data directly to the surface where a laptop computer is used to view and store data. A pan/tilt CCTV camera is located on the top of the cart and is used to observe the pipe during a pipe scan to correlate existing deficiencies with the PPR data.

Results of field trials

A field trial of the PPR inspection tool was performed on 19 April 2010, at the Binbrook Road site near Hamilton in a 1,500 mm diameter reinforced concrete storm sewer pipe. The PPR tool was assembled and attached to the three 500 MHz antenna sensors and to the cart. It was inserted into the 1,500mm diameter concrete pipe through the pipes outfall. This test site was chosen due to its easy access.

Sensors and Software staff managed the data gathering process as the cart was deployed down the sewer. The data was captured, recorded and viewed on the screen from the portable toughbook laptop computer. Post-field trial data analysis completed by Sensors and Software identified that there were some potential artifacts (possible interference by the metal cart design).

To confirm the presence of interference from the cart, the Sensors and Software team deployed again to the same test location to perform additional tests on their hand-held equipment. A single 500 MHz sensor antenna located on the cart, used in the original PPR inspection trial, was mounted on a portable handle. The hand-held sensor was deployed down the pipe in the same positions as the original field test. Sensors and Software team analysed and compared data captured from this hand-held trial with the data from the first field trial. The results indicate no interference from the hand-held field test, leading us to believe that some interference during the original field trial was caused by the metal cart.

Based on the above field trials, the team confirmed that the three-channel PPR inspection tool performed to expectations and, with further modifications and testing, can be used effectively to locate voids behind pipe walls.

Conclusions

The City of Hamilton is one of the first municipalities in Canada to allow a trial of this technology to attempt to locate voids behind pipe walls for critical sewers.

Using PPR as an inspection tool can help municipalities determine if voids are present behind pipe walls. The data that is captured from PPR inspections can assist municipalities by giving them advanced notice of possible problems, such as voids that are present and/or are forming behind the pipe walls. In the past, this could never be achieved through traditional inspection techniques.

Through the trials completed in Hamilton it was determined that the PPR inspection tool performed well, but to improve the PPR inspection tool there is additional work required by the trenchless industry.