Trade associations, general media, and governmental agencies have documented the tremendous financial implications of the aging US water infrastructure extensively. Initially disregarded as alarmist, US EPA estimates of the nearly $500 billion required to rehabilitate water and wastewater systems in the coming decades are increasingly believable in light of rapidly increasing water main failures and “boil orders”(UniBell PVC Pipe Association).
The so-called “demographic echo” of installed pipe will inexorably lead to a steep acceleration in water and sewer line repair expenses for utilities over the next 10 to 20 years, as the existing inventory of underground pipe reaches the end of its useful life.
In order to meet this impending wave of rehabilitation expense and activity, utilities, contractors, and manufacturers have been steadily pioneering new installation modes and new technologies that can offer life cycle savings, upfront “hard” cost savings, long-term performance efficiencies, and better overall sustainability. While some pipeline materials utilized today have not changed appreciably in the last 50 years (ductile iron, clay, and concrete), the widespread adoption of thermoplastic piping systems has brought a degree of longevity and economy that traditional materials cannot match. Consequently, the vast majority (~77%) of newly installed water and wastewater pipes are plastic (either PVC or HDPE, Global Water Intelligence 2005).
In North America, buried PVC pipe has been used for over 50 years in 12- to 20-foot lengths with push-on, bell-and-spigot joints that rely on an elastomeric gasket material to create a watertight compression seal. The obvious advantages of being lightweight (relative to ductile iron and concrete pipe), stiff (high-modulus), crack-resistant (high-strain capacity), and corrosion-free (unrivaled durability), have made PVC pipe the market share leader in the US. Compression-seal PVC joints generally work well, but are susceptible to problems associated with joint over-insertion, excess joint deflection, and foreign material in the joint–all creating the potential for leaks and compromised system reliability.
The more recent introduction of thermally “butt-fused” polyethylene (HDPE) pipe ushered in a new era of expanded installation techniques and overall sustainability that has further improved the nature of pipeline installation and rehabilitation. Thermally “butt-fused”–or field-welded joints–result in a monolithic pipeline that completely eliminates joint leakage.
Coincident with the advent of thermally butt-fused HDPE pipe, utilities accelerated the adoption of “trenchless” methods, to eliminate “open-cutting” and its associated disruptions and environmental consequences. Over 60% of respondents in a recent survey of water utilities report that trenchless techniques had a major or moderate impact on their operations (Underground Construction Magazine, 2007).
Trenchless methods offer cost-efficient options for replacement or rehabilitation of pressurized sewer and water lines. The success of “cured-in-place pipe” (CIPP) trenchless rehabilitation of gravity (non-pressure) sewer lines paved the way for acceptance of a host of new trenchless techniques for pressure pipe rehabilitation. A no-profile, thermally fused HDPE pipe can be pulled inside of a compromised or failing “host” pipe (slip lining) to offer a brand new pressure line (although smaller in diameter–to fit inside the “host”) without digging up and removing the entire host line. HDPE’s no-profile, thermally fused joints also enhanced installation efficiency for horizontal directional drilling–using a specialized drill rig to drill a horizontal borehole (HDD)–over pipes joined using traditional larger profile joints or couplers. The absence of a joint profile reduces the size of the borehole needed and the associated volume of disposal material. Finally, pipe bursting (fragmenting an existing conduit and pulling a new pipe into its place) was enabled by the tough, flexible nature of fused-joint HDPE pipe (Ariaratnam 2008).
It is important to note, however, that some of the material characteristics of HDPE that make it useful in trenchless applications, such as flexibility (lower strength and stiffness), can also be costly for owners and their contractors. Standard, commonly stocked reconnection fittings and repair hardware, which are used on stiffer PVC and ductile iron pipes, cannot be used on HDPE pipe. HDPE pipe requires more expensive fused-on fittings or specially fabricated fittings that have additional stiffening components to prevent leakage or pullout during service. Furthermore, independent evaluations have revealed that HDPE water pipes are highly susceptible to both hydrocarbon permeation (associated with gasoline and similarly common liquids) and degradation from oxidative attack by common water disinfectants (chlorine dioxide, chlorine, and chloramines; AWWA Research Foundation 2008; Castagnetti 2007; Lundback 2005).
In 2003, a thermally butt-fused PVC pipe product was launched commercially for use in trenchless pipeline projects. Fusible PVC pipe, introduced by Underground Solutions, effectively addresses the shortcomings of HDPE pipe: It can be connected and repaired by municipal pipeline workers with the same appurtenances and fittings that are routinely used for the current inventory of ductile iron pipe and bell-and-spigot PVC pipe. The higher-tensile strength of PVC enables Fusible PVC pipe to be used in longer individual “pull-ins” for sliplining and HDD, as evidenced by the record-setting 5,120-foot HDD installation of Fusible PVC pipe in Beaufort, SC in 2007 (Trenchless Technology Magazine 2008).
Moreover, PVC’s much higher hydrostatic design basis (tensile strength capacity) versus that of HDPE means that significantly less material (and reduced wall thickness) is required for a given pressure class; resulting in less plastic material for any given flow rate and pressure. The larger flow cross section reduces energy loss (pumping energy and cost), lending further economy to projects. Alternatively, a smaller diameter PVC pipe can be used to reduce the size of the borehole and the associated volume of disposal material. Conversely, PVC’s higher modulus–or stiffness–versus HDPE precludes its use in highly constrained installation environments where the tighter allowable “bend radius” of HDPE pipe makes it the better choice.
Overall, water system owners who can attain higher levels of savings have welcomed the acceptance of Fusible PVC pipe. As Dave Dalsoglio, Superintendent of Public Works for the City and County of Broomfield, CO, comments, “We try to serve customers with the least impact on their daily lives…. Fusible PVC pipe is one of the best advancements I’ve seen. We saved $50,000 on our very first project.”
With over 1,000 projects installed in the US, Canada, and Mexico, Fusible PVC products are the latest generations of thermally fused plastic pipes helping municipalities meet their daunting replacement needs with a keen eye toward cost-savings, longer life, leak-free sustainability, and energy efficiency.
