Aging Pipe Proves Expensive for Municipalities
A 42-in. cast iron pipe, installed 103 years ago, broke last summer in Boston, causing disastrous flooding which ruined some books at the Boston Public Library.
Thin-walled "victory pipe" installed in Niagara Falls, N.Y., during World Water II constitutes only 3 percent of the total pipe in the city, but represents one-quarter of the citys costs for water main repair and replacement.
Steel pipe installed in the late 1800s in Butte, Mt., during the mining boom days is one reason the city has a four-person "leak gang" working six days a week.
Such stories help to illustrate the challenges municipalities face in keeping the aging water and sewer infrastructure of North America in good condition.
Across the United States, municipal operators face the problems of an aging pipe network: leaks, infiltration, low pressure, tuburculation and even collapse. This is made even more challenging by the fact that older pipe often runs under crowded, well-developed, sometimes historic neighborhoods, necessitating careful planning for repairs and replacement.
The U.S. Environmental Protection Agency recently began collecting information for its second Drinking Water Infrastructure Needs Survey, as required by the Safe Drinking Water Act. During the first survey, the single largest category of infrastructure need was for the installation and rehabilitation of transmission and distribution systems. The survey found that municipalities expected to spend some $77.2 billion over the next 20 years to satisfy that need.
In a similar survey conducted on the wastewater side of the industry, the Clean Water Needs Survey found that over the next 20 years cities need to spend $10 billion on upgrading existing wastewater collection systems, nearly $22 billion for new sewer construction and $45 billion for controlling combined sewer overflows. Another $7 billion is needed to control municipal stormwater.
Small communities have a large need in proportion to their size, according to the survey. New collector sewers account for only 6 percent of the total Clean Water Needs for larger communities, but represent 29 percent for small communities. This reflects, in part, the continuing effort to extend wastewater collection and treatment to the smaller communities.
According to EPAs surveys, corrosion is one of the major culprits in pipe failure, causing some materials to fail in as little as 10 years. An EPA survey of 89 cities showed that 32 of them had reported sewer collapses, most from hydrogen sulfide corrosion.
Site visits from the EPA revealed that corrosion problems are not limited to warm climates. Severe corrosion was observed in Seattle, Wash.; Milwaukee, Wis.; Boise, Idaho; Casper, Wyo.; Albuquerque, N.M.; Baton Rouge, La.; Fort Worth, Texas; Los Angeles County, Calif.; and Tampa, Fla.
Pipe Characteristics
The average life span of pipe depends on a wide variety of factors including the type of pipe, soil and air characteristics and installation. Network designers often use 50 years as the average life expectancy for most pipe types. That estimate may be too conservative, depending on the materials and techniques used.
Pipe Characteristics
Corrosion caused by hydrogen sulfide gas or other sources of acid is the most frequent cause for pipe deterioration. Production of H2S can be hastened by low velocities in a sewer line, longer detention times, and higher temperatures within the pipe. Other factors involved in corrosion include stray electrical currents in the surrounding soil, the presence of toxic materials (metals can reduce bacterial activity), acidity of the sewage, and turbulence.
Pipe Characteristics
Soil movement, improper installation, construction activities and water hammer also can lead to pipe failure. Organic growths and inorganic chemical deposits also can affect performance of piping systems.
Pipe Characteristics
The following is a discussion of pipe technologies on the market today and some of the challenges they face:
Concrete Pipe
Precast, gravity-flow concrete pipe is manufactured in a number of different forms: precast round concrete pipe, elliptical pipe, arch pipe (has arch on top), and precast concrete box culverts (square or rectangular configurations). Pressure pipe is a smaller classification of concrete pipe, often used for drinking water. The majority of concrete pipe is reinforced with steel mesh or cages.
Concrete Pipe
Concrete pipe lasts between 50 and 75 years, said Mike Saubert, Director of Marketing for the American Concrete Pipe Association. In many cases, concrete pipe can last longer than 100 years. It is considered a rigid structure, which means it can serve as a conveyance device and a structure.
Concrete Pipe
"Concrete pipe does not rely upon the soil to impart any type of strength and durability to it," Saubert said.
Concrete Pipe
Concrete pipe can be designed to specific installations, soil requirements, pressures, joint requirements, etc. It is considered to be cost-effective when the overall cost of the project is taken into consideration, because of requirements for proper installation of other pipe materials.
Concrete Pipe
In 1997 the Concrete Pressure Pipe Association conducted a survey of 25 states and provinces in the U.S. and Canada to estimate average costs per mile of pipe over a five-year period, including capital and maintenance costs.
Concrete Pipe
The survey found that Prestressed Concrete Cylinder Pipe, AWWA C301, had the lowest average O&M cost per mile ($117.80); followed closely by Concrete Pressure Pipe overall ($158.60); Bar-Wrapped Concrete Cylinder Pipe, AWWA C303 ($162.60) and Reinforced Concrete Cylinder Pipe, AWWA C300 ($167.00). Ductile iron was the next closest, with an average cost of $326. Cast iron and steel posted the highest average costs, each above $602.00 per mile.
Concrete Pipe
One of the biggest problems with concrete pipe is that it is vulnerable to H2S attack, Saubert said. In an extremely acidic environment, it can fail in as little as 15 years. However, steps can be taken in the preparation of concrete to minimize H2S formation.
Concrete Pipe
The U.S. Army Corps of Engineers regulations, based on a least cost analysis of various pipe materials, list the life expectancy of concrete pipe at 75 to 100 years.
Ductile Iron Pipe and Steel Pipe
A recent study determined that 531 utilities in the United States and Canada have cast-iron pipe that has been in service more than 100 years. Thirteen of those utilities have cast iron pipe still serving after 150 years, according to Larry Dunn of U.S. Pipe & Foundry Co.
Ductile Iron Pipe and Steel Pipe
When the 42-in. cast iron pipe broke in Boston (mentioned earlier in this article), an analysis determined there was nothing actually wrong with the pipe, said John Sullivan, Bostons water and sewer chief engineer.
Ductile Iron Pipe and Steel Pipe
"The cause is of an unknown nature, but were confident an external force was involved because of all the construction in that area." Sullivan said. "We have now rehabbed that 42-inch pipe, and we expect another 50 to 100 years out of it."
Ductile Iron Pipe and Steel Pipe
A report on water main breaks in the city of Boston from 1987 to 1997 showed their most common type of pipe, pit cast iron, lasted an average of 83 years before breaking. Even when construction-related breaks were disregarded, that average was still the same.
Ductile Iron Pipe and Steel Pipe
"Ductile iron is generally stable in most environments," said Grant Whittle, Vice President of Ultraliner, which installs pipe lining systems.
Ductile Iron Pipe and Steel Pipe
A report from U.S. Army Corps of Engineers "Conduits, Culverts and Pipe," states that ductile iron pipe can be used under levees and for water mains and other installations where fluids are carried under pressure. It is also suitable for pressure sewers and for gravity sewers where watertightness is essential. It can resist relatively high internal and external pressures and corrosion in most soils.
Ductile Iron Pipe and Steel Pipe
However, it is subject to corrosion caused by acids, highly septic sewage, and acid soils. Iron pipes also suffer from scaling, which causes cleaning problems.
Clay Pipe
If clay pipe is installed well and the soil is stable it seems to last 100 years or more. With its corrosion-proof properties, it was the material of choice in the 1950s for sanitary sewers.
Clay Pipe
When it is not installed properly, there can be stress concentrations on the pipe. Poorly installed clay pipe will fail within a few years of installation. Fifty years ago, when workers were putting in a lot of clay pipe, proper bedding wasnt widely understood and much of the clay pipe in the nation has been cracked since shortly after it was installed. The rigid nature of clay makes it very brittle, increasing the need for proper bedding.
Clay Pipe
Further complicating the assessment of the lifespan of clay pipe is the natural shifting of the soil. This is illustrated by an attempt to put in a new dig-in-place line going through a graveyard.
Clay Pipe
"Supposedly, everything was mapped out, and there were no caskets going through the area, but they started digging through and every time theyd go 10 or 20 feet, theyd have to call a Catholic priest out, because they would run into another grave that had shifted with the soil," Whittle said.
Clay Pipe
As soil drifts, it puts stresses on the pipe wall, and brittle pipes tend to crack and split under those stresses. Once a sewer pipe starts leaking, the surrounding soil enters the pipe with any inflow, creating voids and uneven loads on the pipe. This can cause collapse.
Clay Pipe
When contractors installed laterals in clay pipe decades ago they sometimes used a method called "hammer taps". This involved breaking a hole in the host pipe, pushing the lateral into the hole and filling the gaps with jute or some other sealing material. This not only caused cracks in the main, but the sealing material eventually washes away, allowing groundwater to enter the pipe.
Clay Pipe
Another outdated method that caused some structural problems was restraining a lateral with boards, bricks or rocks to hold them in place while backfilling.
Clay Pipe
"These supports form rigid inclusions that dont move as easily as the soft soil around it. As the pipe differentially settles, and the lateral doesnt want to settle because of the rigid inclusion below it, it creates a shear fracture at the connection between the lateral and the main," Whittle said.
Plastic Pipe
Plastic pipe consists of bell gasketed preformed pipe and pipeliners such as thermoplastics and thermoset processes. Plastic pipes possess the general attributes normally associated with plastics. Thermoplastics like polyvinyl chloride (PVC) and high density polyethylene (HDPE) can serve as stand-alone pipe and are often used as pull-to-form pipeliners.
Plastic Pipe
While they have a long lifetime, plastic pipe can experience molecular flow away from applied loads or stresses, leading to collapse. Rigid inclusions such as rocks can cause too much stress, and if the pipe is the jointed type, joints can still come apart if they werent installed correctly.
Plastic Pipe
How long PVC pipe can last is a controversial issue. Some estimates put the life span of thermoplastic pipe at up to 500 years, while the standard design life is 50 years.
Plastic Pipe
Louisiana Tech Universitys Trenchless Technology Center has conducted studies on the life expectancy of thermoplastic and thermoset pipe lining systems. The centers tests were based on 10,000 hours of use (about 1 1/3 years). Called the Construction Productivity Advancement program, the liner samples were put under different pressures designed to give a series of failures within the 10,000 hours.
Plastic Pipe
"By drawing a line through those tests," said Dr. Ray Sterling, "the researchers could project the pressure that the pipes could withstand over a longer time. Results were too variable to project very far, though. I think the general results of the research were certainly that properly designed liner can be expected to last the 50-year design life, which is the normal design lifetime thats looked at in the industry."
Plastic Pipe
The standard testing method may not be the best for use with thermoplastic pipe, Whittle said, because it does not reflect properly supported PVC pipe. A typical soil situation will properly support thermoplastic pipe.
Plastic Pipe
"With the thermosets, you have a gradual degradation of the thermosetting material that causes a gradual falloff," Whittle said. "This shows that it will degrade and you will have a limited life span. The thermoplastic has indefinite design life because you cant cause it to degrade and you cant cause it to lose structural integrity over time when its supported."
Plastic Pipe
With pipe lining systems some creep caused by stress concentrations during installation are inevitable, Whittle said.
Plastic Pipe
"Youve got to design around the imperfections caused by that, but in most cases you can stabilize it," he said.
Conclusion
The American Society of Civil Engineers (ASCE) and Black & Veatch have begun gathering specific data from municipalities to learn how long certain types of pipe last in different areas of the United States. The objective of this project, entitled "Optimization of Collection System Maintenance Frequencies and System Performance," is to find the best approach for maintenance of collection systems. This effort will result in a decision-making model which can be used by cities and agencies in evaluating the cost of maintenance (as measured by maintenance frequency) and system performance. The project, funded by EPAs Office of Water under its 104b3 Cooperative Agreement Program, is in the final stages. The report will be available through ASCE shortly, possibly by the end of April.
Conclusion
Another project, "Collection Systems: Methods for Evaluating and Improving Performance," by Ken Kerri of California State University and Rick Arbour, is already available. It can be ordered at http://www.owp.csus.edu/CollectionSys.html. The report contains detailed system and operations data from 13 high-performing collection system agencies relating to sewer maintenance, pump station and force main maintenance, emergency response, planning, control of infiltration/inflow and sanitary sewer overflows, safety, and regulatory compliance.
Conclusion
The U.S. water and wastewater infrastructure is on the brink of a restructuring process that will last for decades. EPA and other agencies already are working on the task of evaluating and recommending new procedures for water and sewer pipe design, maintenance and rehabilitation.
Conclusion
With careful planning and investment on the part of communities that will benefit from sound distribution and collection systems, the year 2020 will find the United States with a well-maintained and cost-effective pipe infrastructure.
