Some believe that relining a pipe with a smaller one will decrease its capacity. However, this is not always the case, and often flow capacity can be increased by slipline rehabilitation. More often than not, the minimal diameter decrease is offset by the gains made in flow coefficients.
Municipalities striving to repair their aging infrastructure with limited resources evaluate a variety of methods to extend the service life in reliable and cost-effective ways. One widely used alternative to renew pipelines that has many advantages is segmental sliplining. 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.
AUI of Albuquerque, N.M., performs trenchless work including sliplining. “AUI has been rehabilitating pipes by sliplining for over 25 years,” said Mike Rocco, AUI Trenchless Division manager. “Our first project was in 1991. It is a well-established, proven, and cost-effective rehabilitation method. We have utilized HOBAS pipes on many projects, including one in Littleton, Colo., [in] which a 66-inch reinforced concrete pipe was sliplined with 54-inch pipes. The material performed well pushing around curves, and HOBAS even produced custom fittings to fit the existing directional changes in the line.”
Sewers remain in service
Typically, gasket-sealed pipe segments are installed into the sewer under “live” conditions, eliminating the cost and risks of bypass pumping. This process can be accomplished quickly and easily by inserting the pipes through small access shafts and later reinstating the laterals via small point excavations. Grouting may require only minor or no further excavations. Grouting of the liner sections prevents migration of soil and water into the annulus, transfers loads to the liner, and may stabilize bedding voids in close proximity to the host. As with any type of construction, especially trenchless installations, preparation is essential.
The more thoroughly existing conditions are evaluated, the higher the likelihood of success. In sliplining, verifying true line size, grade, and alignment are essential. Proper cleaning is also necessary 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. Some installers simply pull a mandrel or test section of pipe to ensure passage of the liner.
The possible length of an individual sliplined reach will depend on many factors, including the liner pipe material. Centrifugally cast fiberglass reinforced polymer mortar (CCFRPM) pipes have been pushed as far as a 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, thus limiting surface disruption. Design factors such as the potential for existing offset joints, unforeseen or uncharted angles, laterals, or even location of manholes must be considered.
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 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 consideration is the line’s capability to resist the grouting pressures during installation. Although the grouting pressure is a short-term loading condition, it is often more critical than the long-term loading conditions such as overburden and live loading from traffic. Therefore, grout pressure and uplift created by grouting should be considered.
Leaking lines and sinkholes
Leaking lines create many problems for a municipality. Aside from excessive treatment of sewerage 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 beneath. Engineers attributed a collapse of a sewer in Houston to years of soil migration into the joints and cracks in an existing line. Even though the line was 40 feet below the ground surface and 30 feet below the groundwater table, evidence of the problem was reaching the surface and in November 2002 a sinkhole appeared.
Years of groundwater infiltration had carried fine soils through small cracks in the monolithically cast-in-place pipe, weakening and compromising the native soil to a point of failure. As the embedment worsened, additional cracks developed, causing more infiltration and continuing the vicious cycle. This same phenomenon is not isolated to Houston. Similar cases have been found recently near Detroit, Tucson, New York, and many other cities.
Leaking lines can also cause potential blockage of sewer lines due to buildup of the soil or other materials that were “carried” in with the leakage. Environmental concerns related to the handling of wet weather overflow can compromise safety, not to mention provoke fines. For sliplining to be technically viable, the existing embedment must be deemed adequate and stable, or must be restored. Products with inherent corrosion resistance, favorable hydraulic characteristics, and leak-free joints can safeguard against many of the concerns and reasons for sliplining.
After performing an evaluation of the existing line and having begun to consider sliplining, the next question usually relates to hydraulics. Can a smaller-diameter pipe maintain or increase the flow capacity? 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.
David Ellett, project manager with BRH-Garver of Houston, witnessed the results of hydraulic improvements. “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 (insertions of slipline pipe) until completion was directly related to the increased hydraulic capacity of the liner pipe.”
Investing in the future
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 experienced over the study period to operate, maintain, repair (if necessary), and ultimately replace or rehabilitate it, not just to purchase and install it.
Taking the time to evaluate the requirements for a pipeline system, alternatives for repair, overall cost, and long- and short-term benefits will result in superior long-term pipe performance and avoid or defer many future costs.
Kimberly Paggioli, P.E., is vice president, Marketing, with HOBAS Pipe USA (www.hobaspipe.com).