As many municipalities strive to repair their aging infrastructures, agencies must evaluate a variety of methods for their ability to extend service life in reliable and cost-effective ways. One widely used alternative to renew pipelines is segmental sliplining. There are many advantages to rehabilitating pipelines by this method.  Renewing older infrastructure can restore the structural strength, often maintain or increase hydraulic capacity, prevent further corrosion and substantially decrease or eliminate infiltration and inflow in the sliplined area. 

“Sliplining is a well-established, proven and cost-effective rehabilitation method, which has been used in North America for over 60 years,” says Erez Allouche, Ph.D., P.Eng., executive director of the Trenchless Technology Center at Louisiana Tech University. 

Typically, installing gasket-sealed pipe segments into the sewer under “live” conditions eliminates the cost and risks of bypass pumping. Pipes can be quickly and easily inserted through small access shafts, and laterals can be reinstated via small point excavations. Grouting may require only minor or no further excavations. Grouting the liner sections will prevent migration of soil and water into the annulus, effectively transfer loads to the liner, and may stabilize bedding voids in close proximity to the host.

Preparation is essential

As with any type of construction, especially trenchless installation, preparation is essential. The more thoroughly the existing conditions are evaluated, the higher the likelihood of success. In sliplining, it is necessary to consider verifying true line size, grade and alignment. Planning a sliplining project involves researching existing offset joints, unforeseen or uncharted angles, laterals or even location of manholes, etc. Proper cleaning is necessary for easy and smooth insertion of the liner pipe. A pre-insertion video can be a tremendous asset in locating potential obstacles such as roots, incrustations and protruding laterals.  

The possible length of an individual sliplined reach will depend on many things, including the line orientation and the chosen liner pipe material. It’s well worth the effort to use test sections or mandrels prior to inserting the liner to verify fit. In good conditions, with proper fit, straight alignment and optimal flow conditions, pipes can be pushed very long distances.

There are a variety of reasons for undertaking sliplining projects, including returning the pipe to a structurally safe state, preventing leakage and providing a corrosion-resistant line for the long term.

Structural considerations for sliplining are both short- and long-term. The structural integrity of the host must be established, and must be stable, at least temporarily, during the sliplining. Post-lining structural considerations include the ability to resist the external loading conditions in the long term, but sometimes the more critical capability to resist the grouting pressures during installation must be calculated. Although the grouting pressure is very often a short-term loading condition, it is often more critical than the long-term loading conditions such as overburden and live loading from traffic. Depending on the liner pipe material chosen, heat of hydration of the grout may also be a consideration. “Fiberglass pipes, being made of thermosetting resins, have the necessary temperature resistance and the high-stiffness designs allow for increased factors of safety during grouting,” says Rene Garcia, EIT, engineering supervisor with HOBAS Pipe.

The pipe’s ability to resist buckling induced by the grouting pressure is a function of the pipe stiffness and to some degree the relative sizes of the host and liner.  The relative sizes determine the degree of constraint offered to the liner pipe during grouting. Research is ongoing in this area to determine the support offered to the liner; however, several formulas currently exist to provide estimations (e.g. ASTM F1216-93). Liner pipe manufacturers can provide buoyancy calculations and maximum grouting pressures for each particular application. In larger-diameter pipes, depending on flow depth, they may recommend a multi-phase grouting process to prevent buoyancy.

Leading to collapse

Leaking lines create many problems for a municipality. Aside from excessive treatment of sewage from infiltration and the need for facilities to handle this increased and possibly excessive flow, there is the potential damage to streets, buildings or other structures the lines pass below. 

Engineers attributed a collapse of a sewer in Houston, Texas, to years of soil migration into the joints and cracks in an existing line. Paul Wallick, P.E., with IDS Engineering Group, explained that years of groundwater infiltration had carried fine soils through small cracks in the MCIP pipe, and this had weakened and compromised the native soil to a point of failure. As the embedment worsened, additional cracks developed, causing more infiltration and continuing the vicious cycle. “The cyclical failure process deteriorates bedding strength,” Wallick says. 

Leaking lines can also potentially cause blockage of sewer lines due to buildup of the soil or other materials that were “carried” in with the leakage. The handling of wet-weather overflow can create environmental concerns for the project, compromising safety, not to mention provoking fines. In order for sliplining to be technically viable, the existing embedment must be deemed adequate and stable or be restored.

Products with inherent corrosion resistance and favorable hydraulic characteristics can safeguard against many of the very concerns and reasons involved in sliplining.

Improving flow capacity

If, after evaluating the existing line, sliplining seems a reasonable choice for rehabilitation, the next qualifying factor usually relates to hydraulics. Can a smaller-diameter pipe actually maintain or increase the flow capacity? The answer is “yes.” In many cases involving larger-diameter pipe, this is entirely possible. Sliplining does decrease diameter, but this is usually offset by the much-improved hydraulics of the new liner pipe relative to the deteriorated existing pipe.  Especially in larger diameters, it is not only possible, but also typical, to achieve higher flow capacity once the line has been rehabilitated. Maximizing the new inside diameter increases hydraulic capacity. 

“On one installation that began midday and ended in the late afternoon when the flow was theoretically higher, a marked decrease in flow depth occurred. It was obvious that the decreased flow depth from the start of the push until completion was directly related to the increased hydraulic capacity of the liner pipe,” says David Ellett, project manager with BRH-Garver of Houston, Texas.

When comparing different materials for trenchless installation such as sliplining, it is important to consider the total installed “life cycle” cost of the project. A true cost comparison must also consider the costs incurred or avoided throughout the design life of the sewer. The total cost includes expenses accumulated over the study period to operate the system, maintain it, repair it (if necessary), and ultimately replace or rehab it, not just to purchase and install it.

With thorough evaluation of a system’s requirements, repair alternatives and the cost of each, and their long- and short-term benefits, many future repair or replacement costs can be deferred or even avoided to the satisfaction of a municipality’s Public Works budget.

About the Author

Kimberly Paggioli, P.E., is the vice president of marketing and quality control for HOBAS Pipe USA.