Maintaining Water Pipe Integrity 101
Service, maintenance, and operational requirements
Most of us don’t think about water pipes until they fail. As a hidden utility and one that has come to be taken for granted, water supply pipelines require considerable effort in design, serving, and maintenance to achieve their desired level of operational anonymity. So what can go wrong with a water main? What problems are we trying to avoid? Water mains fail for any number of reasons. First is physical deterioration. Pipes can rust or corrode as a result of bacteriological or electrochemical reactions in the adjacent soil and groundwater. Second, weak or damaged joints where fixtures and appurtenances such as bends, valves, tees, and other connections are located or where one pipe length is connected to another are inherently less strong than the pipe main itself. These weak spots can be broken by vibration and thrust movements created by the pressure of the water flow itself. Third, internal pressures are not the only forces acting on water main integrity. Earth movements caused by simple settlement of surrounding soils or radical seismic events can directly break a pipeline or severely weaken its structural support. Last, and most common in northern tier states, are the effects of freezing. Water expands when it freezes, and unless sufficient cover or backup heat is supplied to the pipe it could burst.
Pipes and Fixtures
The most common materials for the manufacture of water main pipes and fittings are vinyl plastics, metal (cast iron, ductile iron, steel, and copper), asbestos cement, and reinforced concrete. The most common pipe diameter for water mains is 8 inches, with 6, 10, 12, and 16 inches also being used. Branch lines providing service to individual homes, offices, buildings, and businesses vary in size from 6 inches to 0.5 inch in diameter. Pipe wall thickness (a key element in determining a pipe’s inherent structural integrity) is measured differently by different types of materials.
Vinyl pipelines are usually manufactured from polyvinyl chloride (PVC) and represent the single largest use of this material. Experience has shown PVC to be both durable and cost-effective in water main applications, being both flexible and corrosion resistant. PVC is inert and will not react with the most chemically aggressive water or groundwater. Flexibility (compared to metal or concrete pipes) makes it less vulnerable to earth movements. Its light weight (which reduces shipping costs), ease of assembly (which reduces labor installation costs), and durability (which reduces maintenance costs) makes PVC a cost-effective option.
Little used in new installations today, cast-iron pipe was the norm for water main installation from 1916 to 1976. While easy to manufacture and install using current technology, cast-iron pipe is relatively brittle and easily broken or cracked by applied pressures (especially external bending moments applied by soil movements). To overcome these structural flaws, cast-iron pipe has been mostly replaced with more flexible ductile iron pipe. Ductile iron pipe is both stronger and less brittle than cast-iron pipe. While largely replaced by ductile iron pipe, cast-iron pipe remains in extensive use and still provides water service in old pipeline often found in urban areas.
As mentioned above, ductile is relatively flexible and can handle shocks and vibrations better than brittle cast iron. However, like cast iron, ductile iron has poor resistance against corrosion (rust). To prevent internal corrosion, rusting, and sedimentation buildup, the internal walls of ductile iron pipes are typically lined with a thin layer of cement mortar that prevents contact between the iron and the water it carries. But its inherent material strength allows for its use in areas where proper pipe bedding may be unavailable and with special joints can be used to bridge areas of weak support and avoid differential settlement.
Though typically costing more than ductile iron pipe, steel pipe is lighter, less brittle, and more corrosion resistant. Steel pipe segments can also be welded together to form long, continuous, strong pipe spans. Since steel has a relatively higher coefficient of thermal expansion, care has to be taken to account for potential expansion and contraction of the pipe segments due to temperature changes. This can also be a problem compared to less heat-sensitive pipe insulation or lining layers.
Direct service lines to homes and businesses are usually made of Type K copper tubing. Besides its ease of installation and joint welding, copper lines can be thawed by electrical resistance (the heat resulting from running a mild electrical current through the copper). Non-metal pipes cannot be thawed electrically.
Asbestos cement pipe has many advantages (it’s cheap and lightweight, it never corrodes or rusts, and it can be easily manufactured with a smooth interior). However, the presence of asbestos in the pipe material makes it suspect both to health and environment. While asbestos is a known carcinogen when inhaled, when ingested as a particle in drinking water the health data are less certain and the use of asbestos in building products remains highly controversial. Its physical performance is limited by its brittle nature, which makes it unsuited for use in areas where significant earth movements due to settlement or frost heave can be expected.
Far more commonly used is reinforced concrete pipe. However, it is difficult to mold reinforced concrete in small-diameter and/or thin-walled pipes. So reinforced concrete pipe is usually reserved for very large water transmission lines such as main connectors between a water supply source or reservoir and the main distribution point in an urban area. It is a relatively brittle material, though the steel reinforcing overcomes this to a large degree.
Under Pressure
Force mains get their names from the applied force of the pressure that drives the water through the pipeline. In most communities, the pressure is provided by an elevation differential between the water tower containing the community’s reservoir of water and the homes and businesses that use that water. The water is pumped up to the top of the water tower tank from a water supply facility and then fed to the surrounding users via gravity feed. The resultant pressure is measured in feet of head as calculated by the elevation differential between tank and user. The driving head has to overcome minor head losses that occur when the pipeline changes (bends, tees, etc.) or when the pipeline includes fixtures and appurtenances (valves, meters, etc.). Further head losses to be overcome include head resulting from flow velocities. The flow velocity is a function of the overall volume flow rate divided by the cross-sectional area of the pipe, which depends on the pipe’s diameter. Lastly, there are frictional losses that occur in significant quantities over long pipe lengths that are caused by roughness in the interior wall of the pipe and the smoothness of the water flow (whether it is laminar or turbulent). All the joints, fixtures, and the pipe wall itself must be designed and installed so as to resist and contain the internal pressures of the force main flow.
Water hammer can also cause serious pipeline damage. This is a pressure surge or shock wave that occurs when water flow is suddenly stopped (by a valve closing) or to a lesser extent when water is forced to abruptly change direction. To minimize the effects of water hammer, most pipe sizing is chosen so that the resultant flow velocity is less than 5 feet per second. If the resultant pressure from the shock wave is too high, pipes can break or even explode. Conversely, the water that made it past the shut-off valve will continue to flow, causing a drop in internal pressure as a vacuum develops. This can cause the pipe to collapse or violently implode. Air traps, stand pipes, air release valves, vacuum relief valves, and water hammer arrestors are usually installed along a water main to prevent or minimize water hammer.
So what about normal operating pressures and their effects on joints and bends in the pipeline? Movement of these locations is prevented by the use of thrust blocks. A thrust block is a large piece of concrete (typically with a compressive strength of 3,000 psi) placed around a water line to prevent movement. They can be prefabricated or cast in place. The size of a thrust block is measured by its bearing area, or contact area of the block and the adjacent soil that the thrust block is separating the pipe from. This is measured in square feet and increases with increased pipe diameter and increased bend angle. For example, a 6-inch pipe with a 45-degree bend may require a thrusting block with a contact area of 4 square feet to prevent movement of the bend. Meanwhile a thrust block protecting a 16-inch water main with a 90-degree bend may require over 40 square feet of contact area. To prevent differential pipe movement caused by internal pressures, thrust blocks should be used at all fixtures and appurtenances (tees, wyes, bends, caps, blind flanges, plugs, reducers, valves, and hydrants).
Mechanical joint restraints can also be used in lieu of thrust blocks for ductile iron pipe. They are incorporated into the fixture of bend that needs restrain and include a mechanism that (when activated) restrains the pipe or fixture by the action of multiple wedges forced against the pipe. Mechanical restraints consist of a pair of rings that encircle the pipe at the bell and spigot joint and are connected by a half-dozen tie bolts. The one ring is wedged against the sloped exterior wall of the bell end of the pipe, while the other is kept in contact with the spigot end with individually activated gripping wedges. Mechanical restraints should not be used on pipe that is corroded or graphitized. Any corroded pipe segment or fixture should be replaced prior to installation of a mechanical restraint. If this is not possible thrust blocking combined with anchored tie rods is usually acceptable.
Bearing a Heavy Load
Internal pressures aren’t the only forces acting on a pipe’s structural integrity; earth movement and overburden add loads that can cause pipes to break. Earth movement comes in three varieties: differential settlement, frost heave, and shocks delivered by seismic or construction activities. When such earth movements result in permanent ground deformation of the soil beneath the pipe, serious structural damage to the pipeline can result.
Digging the water main trench is only the first step. The bottom soil of the trench needs to be compacted in place by a trench compactor to consolidate the soil and minimize future settlement. Many trench compactors are of the vibratory wheel design with shaker boxes transferring the vibratory force to the soil. Once the bottom of the trench has been stabilized, the water pipeline requires sufficient granular soil bedding for foundation support. Typically, a utility pipe has up to 3 inches of granular or aggregate soil bedding under the pipe, surrounding the sides of the pipe, filling in the space between the pipe and the trench sidewalls, and extending up and over the pipe to a height of at least 6 inches above the top of the pipe. The soil excavated out of the trench is then backfilled and compacted in place in controlled loose lifts until the backfill extends above the surface. Usually the backfill extends higher than the adjacent surface in anticipation of settlement of the backfill. In areas of exceptional soft or organic soils, little can be done to ensure the stability of the trench itself as the underlying soils lack sufficient bearing capacity to resist the applied loads from the utility.
Frost heave causes soil that has both a high affinity for water and the ability to conduct significant amounts of water to swell and expand. While clays have a high affinity for water and sand has a high permeability, silts are more susceptible to frost heave as they have both characteristics in moderate proportions. The initial expansion causes vertical displacement of the pipe resting on this heaved soil. Heaved soils can draw in water from surrounding non-frozen areas and form ice lenses that displace significant amounts of soil. When temperatures fall, the ice melts and the soil settles, causing the pipeline to displace downward vertically, often below its original alignment. Pipeline breakage can result from the slow-motion whipsaw of the swell and settlement cycles. These movements can also displace the alignment of the pipeline in relation to their fixtures and appurtenances leading to ruptures. Frost heave can be minimized by the extensive use of aggregates in expanded pipe beddings. The aggregate materials lack an affinity for water and prevent the formation of ice lenses.
Post-earthquake studies have shown that the greatest amount of earth movement occurs in areas of manmade fill materials. To counter these movements, water pipelines can be equipped with double-spigot pipe connections utilizing cast sleeves. Though more expensive than standard joints, this allows for greater bending at the joints and minimizes the potential for rupture.
Icy Cold Grip
In addition to frost heave in the surrounding soils, falling temperatures can cause the water inside the pipe to freeze. Every schoolchild knows that when water freezes it expands. What most people don’t know is how easy it is to avoid this problem in water pipes and how little a temperature drop is required for pipe damage. Freezing isn’t even necessary for pipe damage; a 10-degree change in surrounding air and water temperatures can result in temperature strain on a pipe. At only 40 degrees, well above freezing, pipes can become more brittle. The actual water main break and the freezing that causes it can occur days after the air temperature has fallen due to the lag time between a drop in the surrounding air temperature and the loss of ambient heat from the water.
Cast-iron pipe is the most susceptible to breaks due to freezing, especially if it has suffered damage due to corrosion. Other factors that make a pipeline vulnerable to breakage due to freezing is a previous pipeline break (unlike lightning, water main breaks tend to strike in the same places over and over again), damage or disturbance caused by adjacent construction activities, and erosion or settlement of the soil surrounding the pipe. Smaller-diameter pipes have smaller circumferences than larger-diameter pipes and require proportionally smaller radial tensile loads from expanding ice to suffer a rupture. Older pipes of all types, not just those made with materials no longer in widespread use, also tend to break more often.
The simplest solution to the threat of freezing is to bury the water mains to a depth at least 4 feet below grade, beyond the reach of frost penetration in most areas. Some areas, such as southern tier states, have much shallower frost penetration depths. Exposed service lines, connections, fixtures, and appurtenances that will not be used during cold spells can be drained and stopped to prevent ice formation. In addition to proper burial depths, or in areas where placing the pipes at sufficient depth is not possible, the pipe can be equipped with an exterior sleeve of insulating material. A typical insulating material is high-density urethane foam provided as a factory-bonded cover to the exterior of the pipe. This insulating layer is further covered by either a thin sheath of steel or a 40-mil high-density polyethylene jacket to provide protection against puncture and groundwater contact.
When these passive methods fail, there is always heat tracing. As mentioned above, metal pipes can be heated by the resistance to the flow of an electrical current induced into the pipe itself. This initial capital cost of installing a heat trace system and the energy costs to operate it can be substantial. Constant monitoring by sensing bulbs and continuous adjustment by controlling thermostats is required to ensure that the right amount of electricity is used to heat the pipe. Too little, and the pipe remains frozen. Too much, and the system will cost considerably more than it should. Heat tracing is by constant watt applied power per foot of pipe with a typical wattage being 2.5 watts per foot for service lines and 4.0 watts per foot for water mains.
Chemical Warfare—Acidity and Corrosion
Two of the most common types of pipe, ductile iron and cast iron, are also the most vulnerable to weakening by corrosion. Corrosion is almost always a necessary precursor to other types of failure modes. Corrosion has many causes. Dissimilar soils adjacent to the pipe with unequal moisture, differing levels of pH, etc., can lead to exterior corrosion. Soil bacteria can also eat away at pipe. Surface scratches that lead to stress points and strain patterns at the microscopic level can be the starting places for corrosion. Inside the pipe, lack of a proper interior lining to protect the iron from the water it carries leads to rust.
Iron pipe is especially prone to a kind of corrosion called “graphitization.” This type of corrosion is most prevalent in highly acidic soils, soils high in sulfates, or soil containing sulfate-producing bacteria. When iron is dissolved and becomes rust, what is left behind was the carbon matrix that was formed during the casting process. Without the solid iron, the remaining carbon becomes a weak porous structure resembling soft graphite. The remaining graphite gives the pipe a solid appearance, which is dispelled as soon as it crumbles to the touch. It can even hold water until it is disturbed, but it’s a leakage failure waiting to happen. Leaks are no small matters. A single pipe leaking only 1 gallon per minute can result in the loss of over half a million gallons per year.
The simplest means of protecting metal pipe from corrosion is cathodic protection. This type of protection utilizes a sacrificial bar made out of zinc, which attracts the corrosive forces and thereby spares the pipe from damage. Impressed current cathodic protection is typically used. In this operation, a direct electrical current is impressed between the buried pipeline and the anode. Depending on the size of the pipe, soil characteristics, etc., the amount of current required can vary considerably with 10 to 30 amps being typical. The power is either supplied by the local grid (and rectified into direct current) or in remote areas by photovoltaic power arrays. Being a less noble metal than steel or iron, zinc protects the pipeline by giving up its electrons as it is being consumed. Eventually the zinc is used up and has to be replaced, but the pipeline is protected.
That Giant Flushing Sound
Most people don’t think that water pipes—not having any moving parts—require regular preventative maintenance. But sediment, rust, iron modules, and bacteria can accumulate in water mains. To mitigate these problems many communities flush their water lines on an annual basis. Prior to flushing, the pipe segment or loop is isolated by closing appropriate valves. Hydrants located on the line are then opened and the water velocity is used to flush materials out of the pipeline. The required flow velocity will depend on the pipeline’s diameter, the number of hydrants being opened at any one time, and the overall pressure of the water supply system. Visual observation usually determines that a degree of clarity has been reached so that there are no longer any significant quantities of sediment or other impurities in the water. The flushed water is usually dechlorinated and directed via temporary PVC pipelines or pressure hoses into the adjacent storm sewer or sanitary sewer, ponds, or streams.
Author's Bio: Daniel P. Duffy, P.E. is an environmental engineer for URS Corp. in Akron, OH.
March-April 2007
Maintaining Water Pipe Integrity 101
Service, maintenance, and operational requirements
Most of us don’t think about water pipes until they fail. As a hidden utility and one that has come to be taken for granted, water supply pipelines require considerable effort in design, serving, and maintenance to achieve their desired level of operational anonymity. So what can go wrong with a water main? What problems are we trying to avoid? Water mains fail for any number of reasons. First is physical deterioration. Pipes can rust or corrode as a result of bacteriological or electrochemical reactions in the adjacent soil and groundwater. Second, weak or damaged joints where fixtures and appurtenances such as bends, valves, tees, and other connections are located or where one pipe length is connected to another are inherently less strong than the pipe main itself. These weak spots can be broken by vibration and thrust movements created by the pressure of the water flow itself. Third, internal pressures are not the only forces acting on water main integrity. Earth movements caused by simple settlement of surrounding soils or radical seismic events can directly break a pipeline or severely weaken its structural support. Last, and most common in northern tier states, are the effects of freezing. Water expands when it freezes, and unless sufficient cover or backup heat is supplied to the pipe it could burst.
Pipes and Fixtures
The most common materials for the manufacture of water main pipes and fittings are vinyl plastics, metal (cast iron, ductile iron, steel, and copper), asbestos cement, and reinforced concrete. The most common pipe diameter for water mains is 8 inches, with 6, 10, 12, and 16 inches also being used. Branch lines providing service to individual homes, offices, buildings, and businesses vary in size from 6 inches to 0.5 inch in diameter. Pipe wall thickness (a key element in determining a pipe’s inherent structural integrity) is measured differently by different types of materials.
Vinyl pipelines are usually manufactured from polyvinyl chloride (PVC) and represent the single largest use of this material. Experience has shown PVC to be both durable and cost-effective in water main applications, being both flexible and corrosion resistant. PVC is inert and will not react with the most chemically aggressive water or groundwater. Flexibility (compared to metal or concrete pipes) makes it less vulnerable to earth movements. Its light weight (which reduces shipping costs), ease of assembly (which reduces labor installation costs), and durability (which reduces maintenance costs) makes PVC a cost-effective option.
Little used in new installations today, cast-iron pipe was the norm for water main installation from 1916 to 1976. While easy to manufacture and install using current technology, cast-iron pipe is relatively brittle and easily broken or cracked by applied pressures (especially external bending moments applied by soil movements). To overcome these structural flaws, cast-iron pipe has been mostly replaced with more flexible ductile iron pipe. Ductile iron pipe is both stronger and less brittle than cast-iron pipe. While largely replaced by ductile iron pipe, cast-iron pipe remains in extensive use and still provides water service in old pipeline often found in urban areas.
As mentioned above, ductile is relatively flexible and can handle shocks and vibrations better than brittle cast iron. However, like cast iron, ductile iron has poor resistance against corrosion (rust). To prevent internal corrosion, rusting, and sedimentation buildup, the internal walls of ductile iron pipes are typically lined with a thin layer of cement mortar that prevents contact between the iron and the water it carries. But its inherent material strength allows for its use in areas where proper pipe bedding may be unavailable and with special joints can be used to bridge areas of weak support and avoid differential settlement.
Though typically costing more than ductile iron pipe, steel pipe is lighter, less brittle, and more corrosion resistant. Steel pipe segments can also be welded together to form long, continuous, strong pipe spans. Since steel has a relatively higher coefficient of thermal expansion, care has to be taken to account for potential expansion and contraction of the pipe segments due to temperature changes. This can also be a problem compared to less heat-sensitive pipe insulation or lining layers.
Direct service lines to homes and businesses are usually made of Type K copper tubing. Besides its ease of installation and joint welding, copper lines can be thawed by electrical resistance (the heat resulting from running a mild electrical current through the copper). Non-metal pipes cannot be thawed electrically.
Asbestos cement pipe has many advantages (it’s cheap and lightweight, it never corrodes or rusts, and it can be easily manufactured with a smooth interior). However, the presence of asbestos in the pipe material makes it suspect both to health and environment. While asbestos is a known carcinogen when inhaled, when ingested as a particle in drinking water the health data are less certain and the use of asbestos in building products remains highly controversial. Its physical performance is limited by its brittle nature, which makes it unsuited for use in areas where significant earth movements due to settlement or frost heave can be expected.
Far more commonly used is reinforced concrete pipe. However, it is difficult to mold reinforced concrete in small-diameter and/or thin-walled pipes. So reinforced concrete pipe is usually reserved for very large water transmission lines such as main connectors between a water supply source or reservoir and the main distribution point in an urban area. It is a relatively brittle material, though the steel reinforcing overcomes this to a large degree.
Under Pressure
Force mains get their names from the applied force of the pressure that drives the water through the pipeline. In most communities, the pressure is provided by an elevation differential between the water tower containing the community’s reservoir of water and the homes and businesses that use that water. The water is pumped up to the top of the water tower tank from a water supply facility and then fed to the surrounding users via gravity feed. The resultant pressure is measured in feet of head as calculated by the elevation differential between tank and user. The driving head has to overcome minor head losses that occur when the pipeline changes (bends, tees, etc.) or when the pipeline includes fixtures and appurtenances (valves, meters, etc.). Further head losses to be overcome include head resulting from flow velocities. The flow velocity is a function of the overall volume flow rate divided by the cross-sectional area of the pipe, which depends on the pipe’s diameter. Lastly, there are frictional losses that occur in significant quantities over long pipe lengths that are caused by roughness in the interior wall of the pipe and the smoothness of the water flow (whether it is laminar or turbulent). All the joints, fixtures, and the pipe wall itself must be designed and installed so as to resist and contain the internal pressures of the force main flow.
Water hammer can also cause serious pipeline damage. This is a pressure surge or shock wave that occurs when water flow is suddenly stopped (by a valve closing) or to a lesser extent when water is forced to abruptly change direction. To minimize the effects of water hammer, most pipe sizing is chosen so that the resultant flow velocity is less than 5 feet per second. If the resultant pressure from the shock wave is too high, pipes can break or even explode. Conversely, the water that made it past the shut-off valve will continue to flow, causing a drop in internal pressure as a vacuum develops. This can cause the pipe to collapse or violently implode. Air traps, stand pipes, air release valves, vacuum relief valves, and water hammer arrestors are usually installed along a water main to prevent or minimize water hammer.
So what about normal operating pressures and their effects on joints and bends in the pipeline? Movement of these locations is prevented by the use of thrust blocks. A thrust block is a large piece of concrete (typically with a compressive strength of 3,000 psi) placed around a water line to prevent movement. They can be prefabricated or cast in place. The size of a thrust block is measured by its bearing area, or contact area of the block and the adjacent soil that the thrust block is separating the pipe from. This is measured in square feet and increases with increased pipe diameter and increased bend angle. For example, a 6-inch pipe with a 45-degree bend may require a thrusting block with a contact area of 4 square feet to prevent movement of the bend. Meanwhile a thrust block protecting a 16-inch water main with a 90-degree bend may require over 40 square feet of contact area. To prevent differential pipe movement caused by internal pressures, thrust blocks should be used at all fixtures and appurtenances (tees, wyes, bends, caps, blind flanges, plugs, reducers, valves, and hydrants).
Mechanical joint restraints can also be used in lieu of thrust blocks for ductile iron pipe. They are incorporated into the fixture of bend that needs restrain and include a mechanism that (when activated) restrains the pipe or fixture by the action of multiple wedges forced against the pipe. Mechanical restraints consist of a pair of rings that encircle the pipe at the bell and spigot joint and are connected by a half-dozen tie bolts. The one ring is wedged against the sloped exterior wall of the bell end of the pipe, while the other is kept in contact with the spigot end with individually activated gripping wedges. Mechanical restraints should not be used on pipe that is corroded or graphitized. Any corroded pipe segment or fixture should be replaced prior to installation of a mechanical restraint. If this is not possible thrust blocking combined with anchored tie rods is usually acceptable.
Bearing a Heavy Load
Internal pressures aren’t the only forces acting on a pipe’s structural integrity; earth movement and overburden add loads that can cause pipes to break. Earth movement comes in three varieties: differential settlement, frost heave, and shocks delivered by seismic or construction activities. When such earth movements result in permanent ground deformation of the soil beneath the pipe, serious structural damage to the pipeline can result.
Digging the water main trench is only the first step. The bottom soil of the trench needs to be compacted in place by a trench compactor to consolidate the soil and minimize future settlement. Many trench compactors are of the vibratory wheel design with shaker boxes transferring the vibratory force to the soil. Once the bottom of the trench has been stabilized, the water pipeline requires sufficient granular soil bedding for foundation support. Typically, a utility pipe has up to 3 inches of granular or aggregate soil bedding under the pipe, surrounding the sides of the pipe, filling in the space between the pipe and the trench sidewalls, and extending up and over the pipe to a height of at least 6 inches above the top of the pipe. The soil excavated out of the trench is then backfilled and compacted in place in controlled loose lifts until the backfill extends above the surface. Usually the backfill extends higher than the adjacent surface in anticipation of settlement of the backfill. In areas of exceptional soft or organic soils, little can be done to ensure the stability of the trench itself as the underlying soils lack sufficient bearing capacity to resist the applied loads from the utility.
Frost heave causes soil that has both a high affinity for water and the ability to conduct significant amounts of water to swell and expand. While clays have a high affinity for water and sand has a high permeability, silts are more susceptible to frost heave as they have both characteristics in moderate proportions. The initial expansion causes vertical displacement of the pipe resting on this heaved soil. Heaved soils can draw in water from surrounding non-frozen areas and form ice lenses that displace significant amounts of soil. When temperatures fall, the ice melts and the soil settles, causing the pipeline to displace downward vertically, often below its original alignment. Pipeline breakage can result from the slow-motion whipsaw of the swell and settlement cycles. These movements can also displace the alignment of the pipeline in relation to their fixtures and appurtenances leading to ruptures. Frost heave can be minimized by the extensive use of aggregates in expanded pipe beddings. The aggregate materials lack an affinity for water and prevent the formation of ice lenses.
Post-earthquake studies have shown that the greatest amount of earth movement occurs in areas of manmade fill materials. To counter these movements, water pipelines can be equipped with double-spigot pipe connections utilizing cast sleeves. Though more expensive than standard joints, this allows for greater bending at the joints and minimizes the potential for rupture.
Icy Cold Grip
In addition to frost heave in the surrounding soils, falling temperatures can cause the water inside the pipe to freeze. Every schoolchild knows that when water freezes it expands. What most people don’t know is how easy it is to avoid this problem in water pipes and how little a temperature drop is required for pipe damage. Freezing isn’t even necessary for pipe damage; a 10-degree change in surrounding air and water temperatures can result in temperature strain on a pipe. At only 40 degrees, well above freezing, pipes can become more brittle. The actual water main break and the freezing that causes it can occur days after the air temperature has fallen due to the lag time between a drop in the surrounding air temperature and the loss of ambient heat from the water.
Cast-iron pipe is the most susceptible to breaks due to freezing, especially if it has suffered damage due to corrosion. Other factors that make a pipeline vulnerable to breakage due to freezing is a previous pipeline break (unlike lightning, water main breaks tend to strike in the same places over and over again), damage or disturbance caused by adjacent construction activities, and erosion or settlement of the soil surrounding the pipe. Smaller-diameter pipes have smaller circumferences than larger-diameter pipes and require proportionally smaller radial tensile loads from expanding ice to suffer a rupture. Older pipes of all types, not just those made with materials no longer in widespread use, also tend to break more often.
The simplest solution to the threat of freezing is to bury the water mains to a depth at least 4 feet below grade, beyond the reach of frost penetration in most areas. Some areas, such as southern tier states, have much shallower frost penetration depths. Exposed service lines, connections, fixtures, and appurtenances that will not be used during cold spells can be drained and stopped to prevent ice formation. In addition to proper burial depths, or in areas where placing the pipes at sufficient depth is not possible, the pipe can be equipped with an exterior sleeve of insulating material. A typical insulating material is high-density urethane foam provided as a factory-bonded cover to the exterior of the pipe. This insulating layer is further covered by either a thin sheath of steel or a 40-mil high-density polyethylene jacket to provide protection against puncture and groundwater contact.
When these passive methods fail, there is always heat tracing. As mentioned above, metal pipes can be heated by the resistance to the flow of an electrical current induced into the pipe itself. This initial capital cost of installing a heat trace system and the energy costs to operate it can be substantial. Constant monitoring by sensing bulbs and continuous adjustment by controlling thermostats is required to ensure that the right amount of electricity is used to heat the pipe. Too little, and the pipe remains frozen. Too much, and the system will cost considerably more than it should. Heat tracing is by constant watt applied power per foot of pipe with a typical wattage being 2.5 watts per foot for service lines and 4.0 watts per foot for water mains.
Chemical Warfare—Acidity and Corrosion
Two of the most common types of pipe, ductile iron and cast iron, are also the most vulnerable to weakening by corrosion. Corrosion is almost always a necessary precursor to other types of failure modes. Corrosion has many causes. Dissimilar soils adjacent to the pipe with unequal moisture, differing levels of pH, etc., can lead to exterior corrosion. Soil bacteria can also eat away at pipe. Surface scratches that lead to stress points and strain patterns at the microscopic level can be the starting places for corrosion. Inside the pipe, lack of a proper interior lining to protect the iron from the water it carries leads to rust.
Iron pipe is especially prone to a kind of corrosion called “graphitization.” This type of corrosion is most prevalent in highly acidic soils, soils high in sulfates, or soil containing sulfate-producing bacteria. When iron is dissolved and becomes rust, what is left behind was the carbon matrix that was formed during the casting process. Without the solid iron, the remaining carbon becomes a weak porous structure resembling soft graphite. The remaining graphite gives the pipe a solid appearance, which is dispelled as soon as it crumbles to the touch. It can even hold water until it is disturbed, but it’s a leakage failure waiting to happen. Leaks are no small matters. A single pipe leaking only 1 gallon per minute can result in the loss of over half a million gallons per year.
The simplest means of protecting metal pipe from corrosion is cathodic protection. This type of protection utilizes a sacrificial bar made out of zinc, which attracts the corrosive forces and thereby spares the pipe from damage. Impressed current cathodic protection is typically used. In this operation, a direct electrical current is impressed between the buried pipeline and the anode. Depending on the size of the pipe, soil characteristics, etc., the amount of current required can vary considerably with 10 to 30 amps being typical. The power is either supplied by the local grid (and rectified into direct current) or in remote areas by photovoltaic power arrays. Being a less noble metal than steel or iron, zinc protects the pipeline by giving up its electrons as it is being consumed. Eventually the zinc is used up and has to be replaced, but the pipeline is protected.
That Giant Flushing Sound
Most people don’t think that water pipes—not having any moving parts—require regular preventative maintenance. But sediment, rust, iron modules, and bacteria can accumulate in water mains. To mitigate these problems many communities flush their water lines on an annual basis. Prior to flushing, the pipe segment or loop is isolated by closing appropriate valves. Hydrants located on the line are then opened and the water velocity is used to flush materials out of the pipeline. The required flow velocity will depend on the pipeline’s diameter, the number of hydrants being opened at any one time, and the overall pressure of the water supply system. Visual observation usually determines that a degree of clarity has been reached so that there are no longer any significant quantities of sediment or other impurities in the water. The flushed water is usually dechlorinated and directed via temporary PVC pipelines or pressure hoses into the adjacent storm sewer or sanitary sewer, ponds, or streams.