Vitrified clay pipe (VCP) has been in use since the era of ancient Babylon, around approximately 4,000 B.C.E. It rose to popularity during the 1800s when cities used this type of plumbing as their central sewage system material. However, as VCP began to age, municipalities opted to replace lines with polyvinyl chloride (PVC) pipe instead. Today. vitrified clay pipes are rebounding in popularity thanks to newer technology,

During the age of ancient civilizations, people were limited to resources found within the area. Clay is one component located across the globe, which has been used to make everything from water and wine storage vessels to food preparation and cooking ware. The Babylonians discovered a way to also use clay to transport water and waste. Vitrified clay pipes are a blend of clay and shale subjected to high temperatures. Exposing the newly created pipes to heat helps harden it into a glass-like substance. This process is known as vitrification. These pipes were also used in civilizations across in Africa such as Petra in Jordan.

Vitrified pipes are resistant to a variety of acids naturally occurring within the ground. It is also resistant to most acidic compounds used in everyday life. While this acidic waste will eat away at cast iron plumbing, it will not damage clay. Commercial production of this material is environmentally friendly, as the pipe components are naturally occurring. Mostly, the lines are made of clay with a few additional ingredients in the mixture.

Vitrified clay pipes have a high compressive strength which makes it hard to crush. Newer lines are tested to withstand up to 2,000 pounds of water pressure, and this tested pressure exceeds normal operating conditions.

Problems With Vitrified Clay Pipes

Despite having a high compressive strength, unsupported pipes tend to crack over time. Professionals suggest that vitrified clay pipes have a concrete sheath to prevent cracking. Additionally, clay pipes have been known to crack under extreme pressure. This pressure can come in the form of tree roots infiltrating the line. Failure to immediately remove tree roots cause the pipe to crumble. This issue is expensive to remedy.

Before the 1980s, vitrified clay pipes used oakum and cement mortar for joint creation. These joints were not flexible, which caused cracks and leaks as earth settled around them, requiring the need for constant settlement control. Since the 1980s, newer joints are more flexible. However, it was this inflexibility that led to the development and use of other pipe materials.

A final problem with vitrified clay pipes is that they are heavy. The installation often requires a forklift or crane to move. In the event a line needs to be cut in the field, it is difficult to do, causing unnecessary delays. The weight of clay plumbing forces companies to use flatbed hauling trucks to deliver this material.

Vitrified Clay Pipe and Trenchless Applications

Despite past issues with vitrified clay pipe, this material is making a comeback, as it is still one of the best options for sewer lines. With the advent of trenchless technology, it is simpler and cost efficient to use clay as a replacement pipe for waste systems.

Static Pipe Bursting

When replacing city sewer lines with the clay pipe, static pipe bursting is an optimal method. As pipe bursting requires no large trench, cities enjoy reduced the cost to complete repairs. It also limits disruption to traffic and accommodates repairs in densely populated areas. (Learn more in "An Introduction to Pipe Bursting.")

Static pipe bursting utilizes a pipe bursting mandrel followed by the new product pipeline; in many cases, the replacement plumbing material of choice is vitrified clay pipe. Clay is optimal due to its abrasion-resistant outer wall. It eliminates the need for a carrier pipe.

Pipe bursting techniques allow cities to increase line diameter. An enlarged diameter helps municipalities accommodate growing demands on their sewer system.

Slurry Microtunneling

Another trenchless method in which vitrified clay pipe plays a role is slurry micro-tunneling. This trenchless method uses tunneling equipment which is sized to match the outside diameter and length of the line. As with pipe bursting methods, VCP is ideal for micro-tunneling due to its abrasion resistant outer wall. Its compressive strength of up to 18,000 psi helps the clay pipe withstand the jacking force.

Slurry micro-tunneling creates small diameter tunnels generally between two and four feet in diameter. An operator controls the Jack from an area on the surface using a computer console, which receives feedback from the machine concerning location, hydraulic devices and orientation. The engine pushes the new pipe behind the jacking tool through an entry point.

Guided systems eliminate the need for steel casings and grade adjusted carrier pipes. However, workers using an unguided bore system require the use of a steel casing.

Properly maintained and supported vitrified clay pipe can last for decades. Thanks to its acid-resistant nature and high compressive strength it is ideal for gravity sewer lines. With trenchless technology gaining in popularity, VCP is making a comeback in cities across the United States.