Inspecting Large Sewers

Trenchless International

Inspecting large sewers

Canada’s City of Hamilton has a vast network of critical, typically deep and high flowing large diameter sewers that convey flow to sewage treatment plants. A recent pilot project tested a new multiple sensor robot technology in a large diameter sewer.

The City of Hamilton owns and operates more than 85 km of large diameter trunk sanitary sewers that traverse the underground across the far reaches of the city, conveying wastewater to the Woodward Avenue WWTP, located near Lake Ontario.

Hamilton’s large interceptors range in size from 900 mm to 3,200 mm in diameter and are located at various depths ranging from 10 m to 30 m. The majority of the interceptors run under heavily populated areas and/or cross under or run adjacent to highly sensitive, environmentally protected river valleys, such as the Red Hill Creek.

Accurate condition assessment of large diameter, high flowing interceptors remain one of the most significant challenges facing Hamilton’s Asset Management Group, who have taken a leading role in managing the city’s large diameter sewer system.

In order to effectively manage these critical sewers with zero tolerance of failure, detailed and accurate sewer inspections are essential. Currently, Hamilton, through its large diameter inspection program, enlists with local CCTV contractors who use specialised camera equipment that is specifically designed to work in large diameter sewers.

However, due to the constant high velocity flows commonly encountered, CCTV cameras tend to move around too much, or the flow is too high for cameras to operate effectively. In addition to the high flows, sometimes CCTV cameras get bogged down or stuck due to debris.

Hamilton’s Asset Management Group has identified the need for more accurate and reliable data from their large diameter sewer inspections, which in turn will allow the company to make more informed decisions regarding maintenance and rehabilitation planning. To approach this challenge, the City of Hamilton retained R.V. Anderson Associates Limited (RVA) to develop a pilot project to determine the effectiveness of using a multiple sensor inspection (MSI) robot to carry out future inspections for Hamilton’s large diameter sewers.

Developing a pilot project

The Asset Management Group identified the Fennell Avenue Sanitary Interceptor for this pilot project. This 90-year old sewer has never been formally inspected.
The sewer was selcted for several reasons, including:

  • Its diameter varies from 1,500 mm up to 2,853 x 2,700 mm and the shape changes from circular to horseshoe.
  • The sewer is deep with access structure access often greater than 15 m ±.
  • The sewer has a constant high velocity flow.

The city was looking for a sewer inspection technology that is capable of performing detailed and accurate inspections in large diameter sewers with various flow conditions, while also being able to locate any serious defects (such as cracks) that may not be identified during a traditional inspection. Traditional inspection technology may not detect deficiencies due to the insufficient lighting found in large diameter sewers and/or the fact that the flows inside these sewers are too high for the conventional large diameter sewer CCTV camera systems to function properly.

In consultation with the team, it was agreed to use a relatively new robotic inspection system to carry out a MSI in a 1,000 m test section of the Fennell Avenue sanitary sewer.

A sewer inspection company out of Pittsburg Pennsylvania, was brought in under license by D.M. Robichaud Associates Ltd to perform the multiple sensor inspection for this pilot project.

Robotic research

Staff from RVA conducted a desktop review and contacted references regarding the technology and capabilities of the robotics. Part of this review involved assessing what type of multiple sensor inspections were completed, including the length, diameter and flow depth.

Of particular interest were:

  • Potential access issues for the robot;
  • Clarity of the CCTV images during the inspection;
  • How the robot worked around existing obstacles and debris;
  • How effective the overall MSI technology was; and
  • How municipalities were utilising this technology for future inspections.

Case Study

Two separate test locations along the Fennell Avenue Interceptor were selected for the pilot project in order to test this technology in various flow conditions and differing pipe diameters and shapes. These sections were chosen based on information gathered through the background review of the record drawings.

The two selected test sections were:

1. 400 m long – 1,500 mm diameter concrete circular pipe; and
2. 600 m long – 2,700 x 2,850 mm to 3,050 x 3,200 mm horseshoe shaped concrete sewer.

As part of the pilot project, the operational flexibility of the robot was to be assessed. One key reason that the city was very interested in the system was for its ‘one set up’ capability, since the robot can inspect lengths of up to 2 km from one location. This feature is highly beneficial for areas with remote or difficult access, thereby reducing the number of setups required, saving time and money, as well as minimising disruptions in
congested areas.

Test area 1

A 1,500 mm diameter portion of the Fennell Avenue sewer was chosen for this inspection. This section has a circular reinforced concrete sanitary sewer with an average depth of ±6.2 m. The length at the test section was 400 m.

The inspection was carried out during daylight hours to allow for a short demonstration to city staff on how the robot functioned. Staff and other invited guests were given a brief demonstration and overview of the MSI robot, then witnessed the deployment operation. The robotics team used its own truck mounted derrick to lift the 300 kg robot through the recently upsized access structure down to the flowing invert of 6.2 m.

Once the robot was set into the invert, a member of the team was also lowered to the bottom of the access structure to install the sensor mast assembly. The MSI robot used all sensors for this inspection. The technicians then ran through a detailed functionality checklist to confirm that the robot and all sensors were
working properly.

Test section 2

Test section 2 was a 2,700 mm x 2,858mm diameter, ‘horseshoe shape’ concrete section of the Fennell Avenue sanitary sewer, located beneath a busy arterial road in a congested residential area.

Therefore, staff requested that the MSI inspection be completed at night to minimise disturbance to traffic and surrounding residences. A traffic management plan was prepared by the contractor to temporarily divert traffic, as the inspection access structure was situated near the centre of the road.

The depth of the access structure from the road was ±16 m. Upsizing and ladder rung/safety grate removal were carried out prior to the inspection.

Once the robot began moving down the sewer, it was apparent that there was inadequate lighting to illuminate the horseshoe shaped sewer tunnel, which resulted in dark images from the V-360 panoramic camera.

Prior to this inspection, the crew installed a new version of the camera, which had previously only been tested in smaller diameter sewers. Additionally, some of the external lights had not been installed. The team made adjustments to the lighting by increasing the power and further adjustments to the aperture on the panoramic camera.

Due to the low flow, the sonar sensor did not function effectively, as most the inspection was above the waterline. Initially, this was thought to be a concern. However, it was pointed out that the lower flows allowed for a better 3D laser scan across the entire pipe cross-section, down to a longitudinal construction joint, which – during a high flow – would not have been visible.

Effective uses of data

Soon after the MSI inspection had been completed, the robotics crew carried out a comprehensive review and examination of the raw data from all sensors. Reports were then completed and the large amounts of data were delivered to the owner via a portable hard drive.

One of the interesting challenges that faces the City of Hamilton, or any other municipality, is how to effectively integrate and manage this data into their sewer management framework. It should be noted that the amount of data is not necessarily important; it is the combination of different data from each sensor and accuracy of the collected data that will assist the city to formulate more comprehensive decisions in the future.

For example, the combined data from multiple sensors allow for accurate determination of debris locations. This, in turn, would allow the city to direct sewer cleaning operations to those specific locations as opposed to cleaning the entire sewer, saving significant time and money.

Benefits

It was determined that the benefits out-weighed the limitations in using this MSI robot to carry out condition assessment inspections for large diameter sewers.

  • The robot is capable of inspecting deep, remote, high flowing sewers that could have never been inspected in the past without a by-pass, which is expensive.
  • The sewer videos were very stable with clear images, which allow the reviewer to make better assessments of the deficiencies within the sewer.
  • The robot has the ability to perform continuous, long duration inspections up to 2 km in length from one access structure. This feature is particularly useful for remote locations (such as deep valleys) where it is very difficult to bring in trucks and equipment, especially if the areas are environmentally protected.
  • The multiple sensor platform allows for more data to be captured during an inspection. This eliminates the need for multiple inspections (reducing future cost and effort) and allows municipalities to make better decisions since they will have more diverse and accurate data from different technologies.

Limitations

Due to the size of the robot, existing access structure frames and covers may need to be upsized. Ladder rungs and landings may also need to be removed prior to carrying out an inspection. This can be costly if access structures are located in roads with concrete bases.

The cost is generally higher than a traditional large diameter sewer inspection. However, this cost is offset by eliminating the need for multiple inspections.

Sensors are very sensitive and can break down during an inspection. If so, they may need to be sent in from the USA, which can cause delays during inspections. Currently, there are only two MSI robots in existence, so availability is limited. Careful planning is therefore essential.

Conclusion

The pilot project demonstrated that using an MSI robot to simultaneously collect multiple types of data from within a large diameter sewer is an effective inspection tool. This is due to its ability to navigate through all types of sewer flows in a stable manner, using multiple sensors to gather data.

The valuable sewer data that is gathered can assist asset managers in establishing accurate baselines for future inspections, and identify potential problems to a municipality, thereby reducing risk of failure.

This article is an edited version of a paper entitled “Determining the effectiveness of using a multiple sensor (MSI) robot for large diameter sewer inspections,” by David Crowder and Gerald Bauer; RV Anderson Associates and Donald Young; City of Hamilton, Ontario, presented at No-Dig Toronto 2009. Please refer to the paper for more detailed information, references and acknowledgments.

Multiple Sensor Inspection

The MSI robot carries several data collection tools, including:

  • CCTV pan/tilt/zoom camera
  • V-360 camera
  • Sonar
  • 3D Laser
  • H2S Gas sensor
  • Inertial Measurement Unit (IMU)

 

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