Growing cities have placed a heavy burden on our aging subsurface infrastructure, which is screaming for attention. As per the 2017 Infrastructure Report Card, about 240 million Americans depend on 14,748 treatment plants for sanitation needs. An additional 56 million people are expected to connect to these centralized treatment plants rather than to private septic tanks by 2032.

With over 800,000 miles of public sewers and 500,000 miles of lateral sewers from private properties, the sewer system is prone to blockages, overflows and even structural failure. According to the Environmental Protection Agency (EPA), 23,000 to 75,000 instances of sewer overflows occur every year in the United States. To meet the growing demand, new sewage treatment facilities and sewer pipelines will have to be installed to manage the increased flow and take the load off of the older sewer systems. (Also read "The Complex World of Sewer Networks.")

Trenchless to the Rescue

Many of the old sewer pipes are beyond repair and need to be replaced. However, trenching and digging up roads for this purpose can lead to traffic congestion and loss of businesses to those operating in that area. (See "Are Construction Traffic Diversions Necessary for Trenchless Projects?")

Trenchless rehabilitation has taken care of this problem by providing techniques to repair as well as replace these pipes without the need to dig. These methods can be carried out from manholes and flow can be restored rather quickly compared to conventional methods. One of these methods is called thermoformed pipe.

Thermoformed Pipe

Thermoform means to shape a plastic material such as high-density polyethylene (HDPE) or polyvinyl chloride (PVC) using heat and pressure. For the purpose of pipeline rehabilitation, a custom PVC compound with the ability to allow quicker heat transfer was specifically developed.

This specific characteristic allows the material to be used for pipelines with larger diameters (36”) compared to other PVC lining products in the market. Thermoformed pipe liners also have the superior ability to conform to the contours of the pipeline including irregularities if any. The method is quick and can accommodate reasonable pipeline diameter variation.

Types of Thermoformed Pipe Liners

Thermoformed pipe liners used for sewer rehabilitation come in two types namely, deformed and reformed (D&R) pipe liners and fold and form (F&F) pipeliners. Both types depend upon temporary alteration of its cross-section into an H, C or U shape for the purpose of insertion into the sewer pipeline.

Two access points, usually manholes, are required because the liner is pulled into the pipe using a winch cable.

Deformed and Reformed Pipe Liners

Deformed and reformed liners are manufactured from polyethylene (PE) and high-density polyethylene (HDPE) in a factory as a round pipe and are later deformed into a reduced cross-section.

The final diameter of the liner is slightly smaller than the host pipe to limit the stretching and to ensure a tight fit. The deformed pipe is first inserted and positioned correctly and later reformed to give a tight fit within the host pipe.

To prevent movement of the liner after installation, the contractor has to perform a precise reforming procedure that takes into account installation stresses and stretch as well as post-installation temperature shrinkage stresses during the cooling down process of the liner. Service connections are later reconnected robotically.

Fold and Form Pipe Liners

Fold and form liners are manufactured from polyvinyl chloride (PVC) compounds in a factory as round pipes and then folded and formed into flat, H, C or U shapes. The folded liners are coiled in a long length to transport to the installation site and are available in sizes varying from 4” to 30”.

Before insertion, the PVC liner is heated in a pipe warmer and then inserted into the host pipe from a manhole. The pipe is then formed into the host pipe using steam and pressure. The service connections are restored by reopening the connections robotically.

The Science of Thermoforming

A liner material has to be sufficiently flexible to prevent damage and allow easy installation. HDPE, plasticized PVC and PVC have a characteristic known as glass transition temperature (Tg). When a plastic is above its Tg, it will be soft and pliable, but when it is below its Tg, the plastic will be in a hard and brittle state.

The defined Tg of HDPE is between -190°F to -210°F and is, therefore, pliable at ambient temperatures. This is the reason that a folded HDPE can be installed without the application of heat at ambient temperatures.

After installation, the HDPE is heated to about 250°F to form it into the contours of the host pipe. The internal pressure is released only after the plastic cools back to its ambient temperature.

The Tg of PVC maybe between 140°F to 170°F, which is much higher than ambient temperatures. Folded PVC liners must, therefore, be heated to above its Tg to make it pliable for the purpose of insertion. Once installed, the pressure is applied to expand the liner while maintaining the temperature above its Tg. The pressure is released only after the liner has cooled to a temperature below its Tg.

Thermoformed Pipe for Culvert Repair

Thermoformed pipe is used extensively in the sewer rehabilitation industry, the industrial pipe rehabilitation market and for repairing storm culverts. A culvert has to be tested for structural integrity before deciding whether it needs to be repaired or replaced.

Since culverts are also subject to vehicular load, a structurally unsound culvert that cannot sustain the designed load should be completely replaced. In some cases, the culverts are deformed, yet it is possible to repair it. However, the diameter of the new pipe liner will have to be smaller than the original diameter of the culvert. Thermoformed pipe using the D&R or F&F methods can be used to repair culverts.

Advantages of Thermoformed Pipes

Thermoformed pipe has been found to be a better option for sewer repairs compared to other options in the market. Design properties such as wall thickness, modulus of elasticity and corrosion resistance are controlled in a factory setting prior to manufacture. These liners retain their structural integrity, wall thickness and modulus even when installed in steep slopes, bends, offsets, diameter variation and also in pipes with running groundwater.

Thermoformed pipe liners are also low cost compared to other lining methods but are high in quality. Since the manufacturing is handled completely at the site, there is no harmful water-soluble chemical brought to the installation site. The forming process is a physical change rather than chemical because there is no involvement of a chemical reaction during the installation process. This makes thermoformed pipe lining method highly desirable, especially in environmentally sensitive areas.