Pipe Cleaning: The Challenges of Trunk Sewer Cleaning

D.M. Robichaud and Associates is well accustomed to cleaning sewer pipes of all sizes and has become an expert in assessing large diameter conditions and developing site specific cleaning plans for its clients.  

Before a manhole lid is even lifted, large diameter pipe lines offer unique challenges. Surface land use, accessibility into the sewer line, volume of flow, worker safety, pipe material, duty (storm, combined or sanitary), pipe size, characteristics of the debris and risks of the work, are all things that need to be taken into consideration when attempting a project on the scale of large diameter trunk sewer cleaning.

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Typical cleaning methods D.M. Robichaud uses include: high-pressure water, robotic cutting, reaming/grinding and man entry. It’s not until all the site-specific conditions and challenges are fully assessed that a method of cleaning that suits the conditions can be employed successfully. Added to the mix is that the nature of the debris and amount of debris are often not well defined at the onset of the work. D.M. Robichaud’s experience in Ontario reveals that the conditions in most trunk sewers are essentially unknown until the job is almost completed — this obviously makes tendering work for owners and bidding for contractors a very difficult and risky business.


Large diameter lines and trunk sewers often flow beneath roads in high traffic areas causing the need for extensive traffic control. Work hours are scheduled, taking into account peak flow periods as well as peak traffic hours, and many times night work is the only viable option. If not under major roadways, trunk sewers in Ontario  and other areas often cut deeply through river valley easements making equipment accessibility a major factor.  Makeshift roadways are often constructed to accommodate the needed service vehicle traffic. D.M. Robichaud often has to bring in large equipment weighing more than loaded concrete trucks into sensitive conservation lands. This requires extensive planning and regulatory authority approvals.

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However, once things are organized above the surface, accessibility is still a factor to consider below.  The sheer depth of some large diameter sewers (more than 100 ft), makes cleaning them all the more difficult. Trenchless equipment is limited by the finite length of hose or cable technicians have at their disposal and sometimes manholes are over a mile apart. Lift distances are sometimes so great that special hydraulic pumps become necessary. Up and downstream structures like pumping stations can have a huge impact on flow conditions as well.

Volume of Flow

Determining the depth, velocity and volume of flow is extremely important as cleaning equipment like robotic cutters and reaming/grinding attachments are only effective in a narrow range of conditions. Robotic cutting and reaming are methods that use a CCTV system to aid technicians in their work, and high flow decreases visibility thereby rendering the technician blind. Reducing the flows can be accomplished by various dewatering systems including above ground flow diversion procedures. With careful planning and preparation, sometimes work can also be scheduled during off-peak hours where flows are naturally reduced.  

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Pipe Size and Material

Pipe size and material can help or hinder the technician’s ability to effectively clean debris, root growth and calcite. Pipes over about 36 in. call for specialized equipment. For example, a robotic cutter needs enhanced stability to counterbalance its extended cutter head when working in large diameter pipe.  Moreover, pipe construction material can also play an important role, as certain types of reaming equipment such as flailers are too abrasive for pipes constructed with vitrified clay. The power of the metal heads as it flails through the pipe is perfect for breaking up calcite. However, the composition of clay pipe is not strong enough to withstand the rigors of this method.  

The type and size of debris dictates what method can be used for its removal. As the pipe size increases, the range of debris and actual size/amount of debris increases as well. One could say, the bigger the pipe, the bigger the problems!

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High Pressure Water Cleaning

Municipalities and contractors alike use standard combination flusher and vacuum units to do the bulk of their sewer cleaning. A typical unit uses high-pressure water pumps and hoses combined with various flushing head attachments to clean debris.  Standard equipment has a water output of 200 gpm from a 1-in. hose with the jet head producing upward of 2,000 psi. This allows technicians to “pressure wash” the walls as the hose is pulled through the pipe. The water pulls the suspended debris back to the manhole where it can be vacuumed or removed by hand. The water at the downstream manhole may be pumped into a special filtering system to remove the suspended debris from the water, with the filtered water pumped back into the sewer. The resulting “dry” debris usually passes sump and leachate extraction tests and is much less costly to dispose for our clients. For large diameter pipe, the process is the same only magnified.  

These very specialized combination units are equipped with larger hoses allowing for higher water output and higher pressure producing jet heads that increase the psi. However, as the amount of water used increases, the need for water supply also increases. To be productive, the equipment must have a constant supply of water. Sometimes two dedicated hydrants on larger diameter watermains are required.

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Robotic Cutting, Reaming and Grinding

For root masses and other point specific blockages, a robotic cutter is used to detach the blockage from the wall of the pipe allowing the debris to again be flushed to the manhole and then removed. For hardened stalactite calcite deposits, the same method can often be employed. The versatility and range of movement that robotic cutting provides is invaluable as a pipe cleaning tool.

In conjunction with combination truck technology, specially engineered reaming attachments are propelled by hydraulic motors to spin rapidly with controlled torque, knocking hard deposits and calcite from pipe walls. This method of cleaning consists of a CCTV operator and a combination truck operator working together. Due to the duplicity of equipment needed, the setup requires careful planning as both the upstream and downstream manholes need to be accessed with the CCTV unit at one end and the combination unit at the other.  The CCTV technician is in constant communication with that combination unit operator to provide visual feedback as the operation takes place.

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Many things must come together for this work to be successful — the flailer type must be correctly selected for the task, it must be sized perfectly to the in-situ pipe conditions and must be operated by experienced technicians. All reaming and cutting is monitored in real time via CCTV cameras. Trained field technicians are required to be able to assess the site-specific conditions and modify their equipment to suit.


In large diameter pipe, the occurrence of debris too large to be removed by usual trenchless technology is increased. Large pieces of concrete, boulders or other large foreign objects sometimes need to be removed by trained technicians equipped with hand tools. The diameter of large pipe lends itself to this method, however man-entry procedures are always the last choice.

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Flow diversion, the use of gas detectors, self-contained breathing apparatus, safety harnesses, retrieval systems and confined space entry and rescue procedures are all critical parts of any man-entry sewer cleaning procedure. Rescue procedures for pipeline entry require dedicated trained and certified rescue personnel to be on site, special communication systems, redundant equipment and competent workers who are able to work in tight spaces. Man entry work is always the last choice and every effort is made to avoid this option by using technology or procedures, but sometimes you just have to go in and get it.


For the more difficult large diameter pipe cleaning operations, D.M.Robichaud has joined forces with Redzone Robotics and its multi-sensor technology.  The RedZone Responder Robot is specially engineered to handle the rigors of large diameter trunk sewers. Its 750-lb weight and 8,000-ft tether allow it to go where no other technology can. Its CCTV, sonar, H2S gas, 3D laser, grade and line sensors essentially map the pipe from within. This technology provides a complete picture of the pipe conditions before and after cleaning while providing invaluable information for assessing pipe integrity.  

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Overcoming the Challenges

Pipe cleaning, in all pipe sizes, is an integral part of maintaining the vast network of sewer infrastructure we depend on daily. Cleaning serves to prevent major blockages caused by foreign objects, solidified calcite, root growth and various other forms of debris. Historically trunk sewers have not been cleaned because of the belief that they are self cleaning or they are too difficult to clean. Modern technology and more sophisticated contractors can today provide this important service to the municipalities.

D.M. Robichaud’s experience shows that deployment plans, drawn up during the planning stage of trunk sewer projects, often have to be extensively reworked on site. The available municipal record drawings are often incomplete and inaccurate, and completely checking the conditions and the record data during the bid stage or at the onset of work is usually not practical or even possible.

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D.M. Robichaud has concluded that in Ontario the essentially unknown work conditions of trunk sewer cleaning makes these projects well suited to a request for proposal (RFP) process over the traditional tendering system when looking for market solutions. The right procurement process, skilled technicians, proper equipment and the right plan are all required to overcome the challenges of large diameter trunk sewer cleaning.

Lisa Timms is a technology specialist and Randy Kowal is an engineering technologist with DM Robichaud Associates.For For more information visit www.trenchlesstechnology.com/info

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