Just like a multi-million dollar America’s cup sailboat, composite materials have been used to make the “lowly” manhole cover lighter, safer and essentially a “high tech” version of its former self while still retaining the capacity to bear the wheel weight of a fully loaded city bus. Typically, the “lowly” manhole cover one sees installed in roads, sidewalks and parking lots is made of cast iron. Ductile cast iron (cast iron with added magnesium) has been used for manhole covers and frames since the mid 20th century due to its durability and high compressive strength. The engineering community, and in particular engineers designing underground infrastructure, rarely considered any reason to use an alternative material than cast iron for manhole covers or related products.
This view began to change in the 1980s as companies began to be concerned about the high costs of occupational injuries. In addition to their social costs, workplace injuries have a major impact on an employer's bottom line. It has been estimated that employers pay over $1 billion per week for direct workers' compensation costs alone. As manhole covers and trench panels are used to access underground systems and piping, they are a key component in the infrastructure of many municipalities and numerous industries including electrical utilities, telecommunications, energy companies, wastewater treatment and water distribution to name but a few. Given their prevalence in underground infrastructure, it is obvious that access covers are handled frequently by individuals involved in maintaining such systems.
While cast iron manhole covers provide for a highly durable, strong product capable of standing up to heavy loads and severe impact, the weight of a cast iron manhole cover varies but is typically around 450 pounds per cubic foot. As a result, even a small 32-inch diameter cast iron manhole cover can weigh as much as 250 lbs. When a manhole cover weighing that much is removed using traditional tools such as a crowbar or pick, it has the potential to cause a multitude of injuries to the individual or individuals attempting to remove it. The types of injuries caused by heavy municipal castings are numerous and can be quite severe: lacerations, amputations of fingers and toes, broken bones, repeated muscle strains and sprain, wrenched backs and repetitive trauma caused by improper lifting techniques.
One of the first industries to explore alternatives to cast iron manhole covers was major oil companies operating gasoline service stations. In an effort to reduce employee injuries, these companies became interested in replacing their cast iron covers installed above underground gasoline storage tanks with lighter composite covers. The covers used at a typical gasoline station or fueling facility are removed often in order to fill the tanks with fuel and for inspection and maintenance. Due to the frequency of removal and replacement, employees and contractors at the gasoline stations experienced a significant amount of occupational injuries caused by handling the heavy steel covers. These injuries typically included hand and foot trauma, back pain and muscle and ligament strains.
Fibrelite Composites (Skipton, North Yorkshire, U.K. and Pawcatuck, Conn.) was one of the first companies to respond to the needs of the retail petroleum industry with a lightweight composite cover designed to be removed and replaced with a lifting handle that allowed the cover to be pulled out of the frame. Working with structural and composites engineers from one of the leading universities in the United Kingdom, Fibrelite designed a fiberglass cover that could support the same wheel loads as a 32-inch cast iron manhole cover while weighing one third of the weight or less. The lightweight cover and ergonomic method of removal and replacement eliminated the possibility of injury to the employees or individuals handling the covers. As Fibrelite’s composite covers offered a superior technical design that provided the best strength to weight ratio in the industry, Fibrelite quickly became the leading provider of composite manhole covers to oil companies and gasoline service stations.
The limitations of cast iron extend well beyond the heavy weight of a manhole cover. One of the primary issues challenging engineers regarding cast iron is its ability to conduct electricity and heat. In cities, stray voltage issues and hot manhole covers have become a significant concern for utilities. In 2004, a woman was electrocuted to death after stepping on an electrified metal manhole cover while walking her dog in New York City. There are also numerous incidents of pedestrians being burned by hot manhole covers, including a well-publicized “branding” in which a large utility company’s logo was allegedly burned onto the victim’s back. Fiberglass has a conductivity approximately 150 times less than cast iron and is categorized as an insulator (no ability to conduct heat or electricity). Any heat or electrical voltage coming into contact with a composite manhole cover or frame will simply not be transmitted to a pedestrian or company employee touching the product. A well-designed composite fiberglass cover capable of handling wheel loads is therefore a perfect antidote to the problems of hot or electrified manhole covers.
Composite covers are also extremely resistant to corrosion caused by salts, oils, water and steam. This is extremely important for municipalities and utilities in colder climates where road salt is routinely applied during the winter months. It is also critical for industries that are transmitting or storing corrosive liquids such as wastewater, steam or even sulfuric or hydrochloric acid. Certain types of composite materials are highly resistant to these types of corrosive liquids and can therefore extend the working life of a manhole or access cover used in such environments. There are also a number of “high tech” applications requiring composite covers that are being driven by the need to transmit data or energy through the surface of a cover. In addition to being far less conductive than metal or cast iron covers, composite manhole covers also allow electromagnetic (EMI) or radio frequency (RF) signals to pass directly through them. This allows engineers to install sensors and other devices inside a manhole that can then communicate to a receiver or network located aboveground. Composite covers are also being used in wireless electric vehicle charging systems that are designed to be installed underground in parking lots. These systems will transmit power to the car battery parked overhead via an electric coil placed inside a composite manhole cover – this method of transmitting power allows for much faster vehicle charging than traditional cable charging as there is no power lost due to the resistance of the cable.
A last consideration is that cast iron and steel manhole covers and drainage grates are routinely stolen for the scrap value of the metal cover or grate. This problem is widespread around the globe and generally worsens when the price of scrap metal is high. Last year, over 900 manhole covers disappeared in 9 months in the city of Birmingham in the United Kingdom by people brazenly posing as city workers. In the Indian city of Calcutta, the problem of manhole covers disappearing became so severe at one point that the government started making the covers from concrete, according to a report in the Telegraph India. But thieves took them anyway, and cashed in on the steel rebar embedded inside. Even in New York City, thieves are willing to risk injury pulling heavy cast iron manhole covers out of the street in order to convert them into cash at the smelter. Based on current commodity prices for iron, a stolen cast iron manhole cover might fetch as much as $30-$40 at a recycler. But it costs a utility at least $300 to replace each stolen cover and that doesn’t even include the cost of labor used to replace the cover. And while the financial cost of replacement is quite high, the thefts also lead to open manholes which create significant hazards in the roadway for drivers and on the sidewalks for pedestrians. Composite covers can solve this problem as the fiberglass or composite material used in composite covers has no inherent scrap value. As thieves cannot convert composite covers into quick cash at the recycling yard, they will have no desire to steal such covers.
While not every application for manhole covers currently requires a composite alternative, the technical challenges of a safer and “higher tech” world is causing the designers of underground infrastructure to begin to consider their use. Companies like Fibrelite are continuing to develop new composite materials that will make manhole covers lighter, stronger and safer while end users continue to find new reasons to move away from the traditional cast iron products. As composite manhole covers become more accepted in the engineering community, they may one day be as commonplace in the sidewalk as a fiberglass sailboat is on the water.