Rehabilitation of lateral sewer pipe has some unique challenges, but trenchless construction methods offer viable solutions.
Sewer systems consist of mains pipes running through an area with lateral connections branching off to individual houses or buildings. Mains pipes have large diameters (18” or more) to cater for peak sewerage flows from an entire area. Laterals have small diameters (about 2”) because they only carry sewerage from a single building. (Read: The Complex World of Sewer Networks.)
Mains pipes consist mainly of long straight lengths, but laterals can have multiple bends of up to 90 degrees as they enter a building.
Construction standards and materials have changed over the years, bringing improvements to sewerage pipe life expectancy. From the 1950s to 1970s, tar paper pipe was the standard. This technology was evolved during the Second World War to conserve iron and metal for the war efforts. However, tar paper pipe is not very durable. It tends to collapse under the weight of soil and the ingress of roots. Most tar paper laterals have either failed or are failing as they reach the end of their life.
The Challenges of Rehabilitating Laterals
Because sewer laterals connect buildings to the sewer mains, they run in congested and built-up areas. They often pass under roads or paved areas. In business or restaurant buildings, there may be minimal space between the building and public space for construction crews to work.
Another hazard of working on sewer laterals is the presence of other underground utilities in the same area. Electric cables, gas, and water supply lines all travel through the small patch of ground around the building. Some of these systems may be old and not well documented. Digging into the ground to expose a lateral could result in damage to other pipes or cables. (Read: The Science of Getting it Right: Locating Underground Utilities.)
Trenchless Lateral Rehabilitation Options
The challenges of working on sewer laterals make them an ideal candidate for trenchless rehabilitation. Opening a small entrance pit and an exit pit is far less invasive than digging up the entire pipe.
Crews dig one opening at the lateral connection to the main and the other at the lateral entrance to the building. Trenchless rehabilitation can be executed in very tight spaces with minimum impact on the public.
The internal inspection of the damaged pipe is the first step in executing a trenchless repair. (Read: Pipe Inspection: Where it Goes Wrong and How it SHOULD be Done.) Robotic cameras travel the length of the line as directed by operators in a control center nearby. The camera can be introduced into the lateral using a lateral launch system. Camera operators can view the condition of the pipe on screens in the control center.
There are two main trenchless methods suitable for sewer laterals.
As the name suggests, pipelining is a process of laying a sheath to line the inside the old pipe. The pipe linertakes up a portion of the internal diameter, so the repaired lateral will have a lower capacity than the original. This method is also known as cured-in-place-pipe (CIPP.) The process consists of 3 steps:
- Damaged pipes must be thoroughly cleaned before executing a CIPP repair. Either high-pressure water or mechanical cutters will do the job. Clean surfaces allow the resin to bond to the inner pipe wall.
- Once the pipe is ready, the liner is soaked in resin before insertion. The resin-soaked liner extends from one end of the lateral to the other inside the existing lateral.
- The final step is to use a rubber bladder to push the liner against the existing pipe. The bladder is kept in place until the resin has set before being deflated and then removed.
The result of this exercise is a sealed pipe that is corrosion resistant and able to withstand all the normal operating conditions of the sewer. (Read: Is a CIPP Liner the Right Fit for Your Trenchless Rehabilitation Project?)
Pipe bursting is a method used for replacing a damaged pipe completely. High-density polyethylene (HDPE), which has an expected life of about 100 years, is a typical replacement material. This pipe is far superior in terms of strength and durability than older generation materials.
Pipe bursting uses the same diameter pipe for replacement. Therefore this rehabilitation method will not limit the capacity of the lateral line. There are three steps to a pipe bursting process:
- First, the replacement HDPE pipe lengths are welded together into a single continuous pipe of the required length. Fusing the pipe-lengths creates a strong bond and allows the new lateral to be pulled through the ground without obstruction.
- A cable is run through the existing lateral from the entrance pit to the exit pit. This cable pulls the bursting head back through the old pipe.
- The bursting head is attached to the new HDPE pipe on one side and the cable on the other. As the head breaks through the old lateral, it drags the new HDPE pipe behind it.
The result of this exercise is a new HDPE pipe with a 100 year expected life that replaces the old pipe completely. (Read: INFOGRAPHIC: The Lifespan of Steel, Clay, HDPE and PVC Pipes.)
Lateral pipe rehabilitation is a common requirement due to aging sewer infrastructure and the inferiority of old materials. Pipelining and pipe bursting are the primary trenchless rehabilitation methods used for this application.
Due to the smaller diameter of pipes involved, plumbers and smaller contractors can execute this work. The rehabilitation equipment required is also smaller and does not require as steep a capital investment.
Sewer lateral repairs create an opportunity for specialized trenchless rehabilitation methods to become more easily accessible to the public.