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Sliplining away

Sliplining away

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    The existing pipe alignment and diameter are major considerations determining the distance between access shafts. Pushes of 1,000 to 1,500 feet are common. Photos: Hobas Pipe USA

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    Custom fittings can be manufactured with the correct miter and couplings so that they are easier to join. This makes jobs convenient for the contractor.

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    Sliplining with CCFRPM pipe can rehabilitate an old pipeline with minimum interruption to the flow.

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Kimberly Paggioli, PE

In these days of limited public works budgets, sliplining a new liner pipe into a deteriorated host pipe offers a way to maximize the value of an existing resource. The process is semi-trenchless and requires only minimal excavation. Generally, access pits are required at liner pipe insertion locations, and smaller shafts or point excavations are needed to reinstate laterals.

Typically, the new factory-made pipe is pushed into place, adding one segment at a time to the “train,” with a system of hydraulic jacks. In some cases, the new pipe liner can be pulled into the host pipe. A major benefit of sliplining is that it can usually be done “live” while the existing line is working; bypass pumping or a diversion is usually not necessary.

When considering sliplining, it's of vital importance to examine — with some kind of survey — exactly what the existing host pipe looks like. The engineer must measure the diameter of the existing pipe as well as its bends or curves, and assess its condition. Such surveys can be carried out with video cameras and closed-circuit television systems or other methods.

There are several manufacturers of sliplining pipe. Several advantages are offered, though, by centrifugally cast fiberglass reinforced polymer mortar (CCFRPM) pipe. Simply stated, it is essentially a fiberglass pipe manufactured by a centrifugal process. CCFRPM pipe are typically installed in 10- or 20-foot segments, so only small access shafts are required, and are gasket-sealed. Installation and assembly go quickly and can be done under live flow.

KEY QUESTIONS

Three of the most common questions that need to be addressed are:

  • “What is an appropriate liner size?”
  • “Can flow capacity be maintained?”
  • “How far can the liner be pushed?”
  • Regarding liner size, the general rule of thumb for relatively straight sections is that the liner pipe's outside diameter (OD) should be 5% smaller than the host pipe's inside diameter (ID) with an absolute minimum of 1-inch difference on radius between the liner OD and host ID (or a 2-inch difference in diameter).

    For example, consider a 26-inch-diameter CCFRPM pipe with a 28-inch outside diameter that is being sliplined into a 30-inch host pipe. The difference between 28 and 30 inches is 7%. The difference on radius should be at least one 1 inch, or two 2 inches in diameter.

    One of the tightest fits ever recorded was in Los Angeles, where a 30-inch nominal pipe with 32-inch OD was inserted into a 33-inch clay sewer. That allowed only a ½-inch radial clearance. To add to the complexity of the project, not only was the diameter tight, but the existing joint lengths were quite short. The CCFRPM liner pipe, installed in 10-foot joints, could potentially bridge across three existing 4-foot clay pipe joints.

    Often it is not the tightness of fit but the offset joints, locations of undersized host pipes, and directional changes that create challenges. In many cases, the existing line is not straight. It could be a monolithically poured curve, have angularly deflected pipe segments, or even a series of mitered fittings. To determine if the sliplined segments will pass, the public works agency needs an accurate survey.

    For assurance, pull a test section of liner pipe as “proof.” The proof pipe should be the same length as the segments the engineer is planning to use. For existing non-straight pipe sections, segmental sliplining often resembles running straight railroad cars (pipe segments) around a curve in the tracks (host pipe).

    Ensure that the joints will seal once the liner has found its final resting position. Navigation of the curve during sliplining is only one aspect to be considered. The designer needs to determine if the curve is uniform in radius. Also, it's important to know the required angle at each joint location. Whatever the existing alignment is, the designer will need to make sure that the liner joint capability is within the limits imposed by the host pipe.

    For sliplining around true curves, it is often possible to install shorter segments of pipe in the curves so that the joints will deflect slightly and the seals are maintained. Generally, depending on the manufacturer, liners can make 1- to 2-degree bends at each joint and still maintain integrity of the joints. The more accurate the survey of the existing pipe and conditions, the higher the success rate in sliplining.

    In Los Angeles, for example, a contractor installed 17 CCFRPM segments, each 2½ feet long, at the beginning of a 3,500-foot push. The entire project included three such curves with a 45-foot radius. In that case, the curves were able to be evaluated, but they were not directly accessible, so push shafts were located at the end of the straight sections. Shorter 2½-foot segments were installed first and pushed through the straight sections to end in the curve. The success of the complex installation was largely due to knowing the host pipe size and orientation.

    In areas where host pipes are not navigable through pushing and where pit locations are convenient, CCFRPM fittings can be manufactured to fit the existing sewer. Those custom pieces can be simple elbows or more complex fittings such as wye branches, crosses, and other nonstandard configurations.

    Alternatively, in sewers where bypass pumping is possible and the sewer alignment prevents sliplining in the traditional way, segments of pipe and fittings can be carried into the sewer and installed.

    MAINTAIN FLOW CAPACITY

    Flow capacity is affected by the relative sizes of pipe and their respective hydraulic characteristics. In many cases, especially for large-diameter sewers, the capacity can be maintained or improved even with a reduction in diameter. The roughness of a pipe wall is expressed as an “n” value that is used in Manning's formula to predict flow. The Manning value of CCFRPM pipe is between 0.009 and 0.011. By contrast, the value for new concrete is somewhat rougher, at 0.013. Old concrete can range from 0.018 to 0.020, or even worse, depending on the level of deterioration.

    One can calculate the reduction in diameter that is possible with the liner pipe in order to maintain flow (see sidebar). By manipulating Manning's formula (the empirical formula for determining channel flow) as a function of flow, the designer can determine the ratio of the liner flow (Q1) to the host flow (Q2). If the ratio of the liner flow to the host flow is 90%, for example, the liner has resulted in a 10% reduction in flow. But if the ratio of Q1 over Q2 is greater than one, flow has improved.

    Depending on the liner pipe chosen, a step down in liner size is typically 4 to 12 inches less than the host pipe. That depends on the strength of the liner pipe, its thickness, and the resulting OD and ID. A liner with a Manning's value of 0.009 can be 13% smaller than the host with a value of 0.013 and maintain equal flow. The more efficient the cross-section of pipe, the greater is the potential for flow recovery.

    PUSHING THE LINER

    Many factors affect the distances that are possible to push the liner pipe. Aside from the obvious alignment issues, buoyancy, equipment, and friction are important considerations.

    Buoyancy provided by the existing flow in the sewer can be measured. Displaced flow will lift up the liner, and the flow in liner plus the pipe weight will press it downward. Buoyancy considerations include the flow depth in the host, the liner pipe weight, and the ability to control the depth in the liner pipe. Means such as a nose cone or weir on the push ring affect the flow depth in the liner during insertion. The more neutrally buoyant the liner is, the less friction is encountered.

    The distance that the liner may be pushed is also a function of equipment. Hydraulic jacking systems are probably the most commonly used equipment for major sliplining projects. But simple winch systems and even ordinary construction equipment may be used as well, usually for shorter runs.

    The force that it takes to install the liner is dependent on the friction. Maximum load that may be placed on the liner pipe and its weight are specifications that are available from the pipe manufacturer. With this information, and an appropriate factor of safety (3.0 is common), the safe push distance can be calculated. Friction factors are installation-specific but are commonly in the 0.3 to 0.5 range. For example, with 164 tons of force, a contractor can jack 48-inch pipe 5,800 feet with a friction factor of 0.4 in the host. That pipe weighs 141 pounds/foot.

    In one project for the Los Angeles County Sanitation District, 51-inch and 57-inch CCFRPM was sliplined into 57-inch and 63-inch reinforced concrete pipe, respectively. Maximum pushing force was about 100 tons on all drives including curves, angles, and offsets. The average friction factor was 0.3, and the longest single push in one direction was 5,600 feet.

    In short, sliplining can provide leak-free service, eliminate corrosion deterioration, and restore structural integrity to old pipes. When properly designed and evaluated, a liner can be installed safely with a minimal amount of downsizing, and capacity can be maintained or even improved. And the potential for pushing thousands of feet means that surface disruption is minimized.

    — Kimberly Paggioli, PE, is vice president of marketing and quality assurance for HOBAS Pipe USA. She has an undergraduate degree in civil engineering and an MBA, and has worked in the pipe industry for 14 years.