Identifying and Managing Dangers Associated with Subsurface Utilities
Subsurface utilities, if not properly identified, can pose a substantial threat to trenchless projects.
Subsurface utilities have made our lives comfortable, clean, safe and healthy – four things that are prime human necessities. Utility lines can be found buried under roads, sidewalks, buildings and properties, crisscrossing each other and running for miles under our feet. Every now and then a new utility line is installed or an old one is upgraded or renewed. "Road closed" signs are pretty common everywhere, but that is now changing, thanks to trenchless technology. However, even that is fraught with risk unless adequate precautions are taken and sufficient data is available.
The risk associated with digging blindly can be dangerous or even fatal. Knowing the location of different utility lines is very important prior to undertaking any new installation or renewal project to ensure the safety and health of workers and the community. (For more on underground utilities, see The Science of Getting it Right: Locating Underground Utilities.)
Subsurface Utility Services
There are different types of utility lines that are buried underground, such as electricity lines, oil and natural gas pipelines, water and sewage pipelines, and telecommunication ducts. Though there is the option of leaving these on the ground surface or stringing them overhead on poles, there are certain advantages to burying utility lines:
- It's aesthetically pleasing and less likely to be vandalized.
- The fact that it's buried makes it less likely to be damaged by human error such as accidents.
- The pipes, cables and conduits are protected from temperature fluctuations, especially during freezing temperatures and high winds.
- It has low cost of maintenance and failure.
- Considering the natural support provided by soil underground, it is slightly cheaper than having to provide support and protection for exposed pipelines and conduits.
In spite of these advantages, there are dangers that need to be identified and managed to prevent severe or fatal injury and environmental damage. In the United States, the American Public Works Association (APWA) has come up with uniform color codes for temporary marking of underground utilities.
- Red – Electric power lines, cables, conduit and lighting cables.
- Orange – Telecommunication, alarm or signal lines, cables or conduits.
- Yellow – Natural gas, oil, steam, petroleum, other gaseous or flammable material.
- Green – Sewers and drain lines.
- Blue – Drinking water.
- Purple – Reclaimed water, irrigation and slurry lines.
- Pink – Temporary survey markings, unknown/unidentified utilities.
- White – proposed excavation limits or route.
Dangers Associated with Underground Services
When trenchless installation or rehabilitation is undertaken, there is always a chance of striking other utility lines in the vicinity. Below are some of the utility line types and the dangers associated with striking them. (To learn more about safety, check out Trenchless Operations Safety Do’s and Don’ts.)
Damage to water pipes is not very likely to cause an injury unless a main line is damaged, emitting a jet of water at high pressure. Workers near the pipe can be hurt by the jet or by pieces of stone and debris picked up by the exiting water.
It can also cause flooding and erosion, leading to loss of support from adjacent structures. Cross-boring of gas pipelines can happen through water pipes unless the location and size of the pipe is known in advance.
Most sewer pipes are gravity fed, but some are pumped at pressure, especially when main sewer lines are at an elevation. Damage to sewer lines can also happen due to cross-boring during trenchless installations such as horizontal directional drilling (HDD), pipe jacking, etc.
A leaking sewer can become an environmental problem, risking the health and safety of workers exposed to raw, untreated sewage. It can also cause contamination of damaged water lines in the vicinity.
When a live cable is penetrated by a sharp object such as a tool, arcing current can cause an explosion, causing a fire. Sometimes old or damaged live terminating cables or connections are left unreported and can be very dangerous, especially when plastic gas pipes are in the vicinity.
Injuries associated with electric cables range from very severe to fatal, causing severe burns to the face, hands and body.
Damage to gas pipes can be of two types: damage that causes an immediate leak, such as when the pipe is installed, or damage that causes a slow leak due to poor joint connections or inadequate support and is only detected at a later stage. Fire or explosion usually results from gas leaks and the risk is greater for liquefied petroleum gas (LPG) than natural gas.
LPG is a heavy gas and some leaks may allow it to travel quite a distance before it accumulates in low-lying areas such as basements and cellars, manholes and ducts, risking the lives of workers.
Safe Working System
As per the Occupational Safety and Health Administration (OSHA), employers have to ensure that workers are provided with all information and equipment required to work safely. To ensure this, the contractor should plan the work, detect, identify and mark all underground utilities in the region, and use safe digging practices.
When carrying out any trenchless operation, the following points should be kept in mind:
- All relevant utility owners should be contacted and information should be obtained regarding location and depth of utilities. Contacting Dig Safe is an option, but in-depth details such as depth of installation and unrecorded old utility lines may not be available with them.
- A thorough geotechnical investigation report using a geographic information system (GIS) and ground penetrating radar (GPR) grid survey should be undertaken with respect to the project to locate all utilities passing through the proposed location.
- Once location and depth are obtained, the utility lines should be exposed at certain intervals to confirm the exact position and depth.
- All utility lines passing through the area should be treated as live unless proved otherwise.
- Correct installation techniques should be used. Some installations may have to pass through constrained spaces with tight tolerances requiring sophisticated detectors installed on drill heads that constantly relay information to the operator.
- Excavations done to expose utility lines should be left open until the new pipeline or conduit has been successfully installed.
Trenchless installation techniques have proved to be environmentally friendly and cost effective in the long run. But as mentioned before, there are risks associated with them that can cause fatalities and substantial financial losses.
The above points are in no way exhaustive, and contractors should follow all safe working practices outlined by OSHA and undertake surveys and site investigations before starting a project and choosing the right equipment.