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Increasing flow capacity with slipline rehabilitation
Some believe that relining a pipe with a smaller one will decrease its capacity. However, this is not always the case, and slipline rehabilitation often can increase flow capacity. More often than not, the minimal diameter decrease is offset by gains made in flow coefficients.
Municipalities with limited resources striving to repair their aging infrastructure tend to evaluate a variety of methods to extend its service life in reliable and cost-effective ways. Segmental sliplining is one way to renew pipelines, and there are many advantages to rehabilitating pipelines by this method. Renewing older infrastructure can restore 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, trenchless division manager for AUI. “Our first project was in 1991. ... We have utilized Hobas pipes on many projects, including one in Littleton, Colo., which a 66-in. [reinforced concrete pipe] was sliplined with 54-in. pipes. The material performed well pushing around curves, and Hobas even produced custom fittings to fit the existing directional changes in the line.”
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, 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 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 desirable results, thus limiting surface disruption. Design factors such as the potential for existing offset joints, unforeseen or uncharted angles, laterals or even the 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 often is 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 can create many problems for a municipality. 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 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 ft below the ground’s surface and 30 ft 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.
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. Environmental concerns related to the handling of wet weather overflow can compromise safety and lead to fines. In order 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 considering sliplining, the next question usually relates to hydraulics. Can a smaller-diameter pipe actually maintain or increase the flow capacity? In many cases involving larger-diameter pipe, this is entirely possible. Sliplining does decrease diameter, but this is usually offset by the 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 after 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 for BRH-Garver of Houston.” It was obvious that the decreased flow depth from the start of the push (insertion of slipline pipe) until completion was directly related to the increased hydraulic capacity of the liner pipe.”
When comparing different materials for trenchless installation, it is important to consider the project’s total installed lifecycle cost. 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 rehab it—not just to purchase and install it.
Taking the time to evaluate the requirements for your system, alternatives for repair, the overall cost, and long- and short-term benefits will result in long-term pipe performance and avoid or defer many future costs.