As many municipalities strive to repair aging infrastructure, city leaders must evaluate a variety of methods available 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, including restoring the structural strength, maintaining or increasing hydraulic capacity, preventing further corrosion, and substantially decreasing or eliminating 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,” said Erez Allouche, Ph.D., P.E., associate director of the Trenchless Technology Center at Louisiana Tech University.
Typically, you can install gasket-sealed pipe segments into the sewer under “live” conditions, eliminating the cost and risks of bypass pumping. You can quickly and easily insert the pipes through small access shafts and reinstate the laterals via small point excavations. Grouting may require only minor excavation or none at all. Grouting of the liner sections will prevent migration of soil and water into the annulus, transfer loads to the liner, and may stabilize bedding voids in close proximity to the host. As with any type of construction, especially trenchless installation, preparation is essential.
The more thoroughly you evaluate the conditions, the higher the likelihood of success. In sliplining, it is necessary to consider verifying true line size, grade, and alignment. Proper cleaning is required to allow for ease of 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 liner pipe material. It's well worth the effort to use test sections or mandrels prior to inserting the liner. Centrifugally cast fiberglass reinforced polymer mortar (CCFRPM) pipes have been pushed as far as 1 mile in one direction from an installation shaft. Although this distance is rare, with a clean straight sewer, enough flow to maintain proper ballast, and a properly chosen liner pipe, installers can achieve amazing results. When you plan a sliplining project, you must stay aware of considerations such as the potential for existing offset joints, unforeseen or uncharted angles, laterals, and even the location of manholes.
WHY CHOOSE SLIPLINING?
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. Structural integrity of the host must be established, and it must be stable at least temporarily during the sliplining. Post-lining structural considerations can include the ability to resist the external loading conditions in the long term. Although the grouting pressure during installation is 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.
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 exist to provide estimations (for example, ASTM F 1216-93). Liner pipe manufacturers can provide buoyancy calculations and maximum grouting pressures for a particular application. In large-diameter pipes, depending on flow depth, they may recommend a multi-phase grouting process to prevent buoyancy.
Leaking lines create many problems for municipalities. Aside from excessive treatment of sewage from infiltration and the need for facilities to handle this excessive flow, there is the potential damage to streets, buildings, or other structures that the lines pass under. Engineers attributed a recent collapse of a sewer in Houston to years of soil migration into the joints and cracks in an existing line. J.E. Pate Jr., principal at Pate Engineers in Houston, explained that years of ground-water infiltration had carried fine soils through small cracks in the monolithically cast-in-place pipe; this weakened and compromised the native soil to the 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,” said Pate.
Leaking lines also can cause potential blockage of sewer lines due to buildup of the soil or other materials that were “carried” in with the leakage. Blockages can cause environmental issues related to the handling of wet-weather overflow. This can compromise safety, not to mention provoke fines.
After you have evaluated the existing line and considered sliplining, the next question usually relates to hydraulics. Can a smaller-diameter pipe actually maintain or increase the flow capacity? Yes; in many cases involving large-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 with large 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,” said David Ellett, project manager with BRH-Garver of Houston. “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.”
When comparing different materials for trenchless installation such as sliplining, it's 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 you experience over the study period to operate it, maintain it, repair it (if necessary), and ultimately replace or rehab it—not just to purchase and install it.
If you take the time to evaluate the requirements for your system, alternates for repair, the cost, and long- and short-term benefits, you can achieve superior long-term pipe performance and avoid or defer many future costs.
— Kimberly Paggioli, P.E., is marketing manager with HOBAS Pipe USA, Houston.
Maintaining flow capacity by sliplining
The relining myth: “I can't use sliplining because it will decrease the flow.”
Don't assume that by relining the current pipe with a smaller one that the flow will be compromised. More often than not, the small decrease in diameter is offset by the gains made in flow coefficients.
To illustrate this, make the following simple calculation using Manning's equation for open channel flow.
Q: Flow (ft3/s)
n: Manning flow coefficient
A: Flow area (ft2)
S: Sewer slope, (ft/ft)
R: Hydraulic radius = A / wetted perimeter (ft)
Begin by using two simultaneous equations, the first prior to relining the sewer pipe and the second post-rehabilitation. By reducing the equation for two different diameter sewer pipes with different coefficients installed on the same slope, we can compare relative flow capacities. This will compute the relative capacity increase or decrease of various liner pipes (D1) and existing pipelines (D2) with their respective Manning's coefficients (n1 and n2).
A new simpler equation results:
So, how much flow recovery can you expect? Let's run through a common size—a 36-inch-diameter, CCFRPM liner pipe placed into a 42-inch deteriorated, concrete pipe. Let's suppose that the existing Manning's value was assumed or shown to be 0.015 and the new fiberglass liner was assumed to be 0.009. The inside diameter of the fiberglass is 36.5 inches, and the inside diameter of the existing concrete is 42 inches. If we plug these variables into the above equation, we derive a Q1/Q2 value of 1.15. This means that the flow after sliplining (Q1) is 15% greater than the flow before lining (Q2).
You can use this equation to make comparisons. Many agencies have been pleasantly surprised when the depth of flow after relining is lower than that prior to relining. The old adage is true—don't make assumptions about a given technology until you have evaluated it to its potential.