Introduction

Trenchless construction and rehabilitation methods for sewerage and water pipelines are gaining popularity due to the fact that they cause little to no inconvenience or surface disruption. However; while surface disruption is minimized, trenchless methods sometimes cause damage to existing underground utilities that have not been recorded and are therefore not available for reference in municipal utility records. As populations in cities have grown, the need for an effective sanitary disposal system and an adequate water supply has also increased. A city’s underbelly not only carries millions of gallons of potable water and untreated sewage in miles of pipelines, but also caters to miles of cables supplying electricity, phone connections, cable connections, pipelines carrying gas and other chemicals.

Trenchless technology brings with it the challenge of laying pipeline while avoiding existing utilities. Many fatal accidents have taken place when instruments have struck and penetrated electric cables, gas pipelines and the like. Damaged pipelines carrying sewage can have an extensive impact on the environment in neighboring areas if left undetected. Previously, municipal utility records were the only means of knowing what lay underneath the surface. With advances in science and technology, it is now possible to map existing underground cables and utilities without digging and probing. Methods of detection such as ground penetrating radar and highly specific pipe penetrating radar provide details of subsurface utilities and underground conditions which allow for efficient and safe trenchless construction efforts.

Methods of detecting underground utilities

1. Geographical Information System (GIS)

  • The geographical information system is a computer application capable of capturing, storing, manipulating and displaying information that has been geographically referenced to include surface features and information on specific attributes. This information is stored in the system, and is integral to complex analyses such as information retrieval, overlay and data output. Using simple commands, specific details can be retrieved from the database. The application has the capacity to determine the relationship between different aspects of the immediate area, including roads, rail, administrative boundaries, buildings and more, and correlate them for a comprehensive picture. The data output system enables planners to make decisions regarding a particular locality.

2. Remote sensing

  • Remote sensing uses data from aerial and satellite imagery to detect objects on earth. Data received from aerial surveys, using various satellites in different resolutions, is widely used by planners. This data can be gained from the national agencies overseeing remote sensing technology. High resolution data from satellites can give highly accurate mapping information, including small features like ponds, road dividers, parks and buildings. This technology helps gather information regarding the geological features of the earth’s terrain including natural and artificial landmarks. Usually, ground surveys accompany GIS and remote sensing data to update invisible or indistinguishable features like manholes, electric and telephone poles, transformers and buildings (learn more at "Why a Detailed Geotechnical Report Means Success for Your Trenchless Project"). Remote sensing is used by geologists to study the formation of mineral deposits, sedimentary rocks, oil fields and the effect of the water table. Many trenchless construction projects like tunnels, pipelines and bridges run for long distances and may pass through very different geographical and soil conditions. With the help of data gathered by remote sensing, it is possible to create a map of the area through which the project will pass.

3. Induction utility locators

  • This system operates by locating a signal that has been introduced by a transmitter into the utility pipeline. The utility line acts like a radio antenna and transmits electromagnetic signals which are picked up by a receiver. A signal can also be introduced into a utility line indirectly through a transmitter placed above the line. An induction clamp that induces signals on specific utilities can be used to generate a direct induced signal. Since it forms a closed loop signal, interference is minimized; making this particular process the preferred method for non-metallic conduits. The receiver is moved horizontally across the approximate location of the utility pipeline until the strongest signal is received which indicates the approximate horizontal location. For determining the approximate orientation, the receiver is rotated until the weakest signal is detected. The depth of the utility, however, cannot be accurately ascertained by this method.

4. Electromagnetic and Magnetic locators

  • To locate subsurface metallic utilities like metal pipes, conduits and buried structural steel, electromagnetic survey instruments are used. This method involves sending electromagnetic pulses through the ground and recording the transmission returned from contact with metal utilities. Magnetic locators detect objects with a high content of ferrous metal content such as manhole covers and storage tanks.

5. Ground penetrating radar (GPR)

  • Ground penetrating radar is the one of the favored methods of locating underground utilities and is capable of distinguishing the interface between two materials with different dielectric constants. The system consists of an antenna with a receiver and transmitter. Short duration electromagnetic pulses are radiated into the earth from the antenna moving along the ground surface. Depending on the signal received, the profile recorder converts the data into a graphic display. Changes such as voids, the composition of sediments, interface between bedding and cement, backfill, debris, pipelines and tanks are recorded and displayed as a continuous cross section along the path of the antenna.

6. Pipe penetrating radar (PPR)

  • Pipe penetrating radar is an improvement over ground penetrating radar as it is related to trenchless technology. It is used to detect damages and leaks within and without the pipe diameter. This method can be used to detect damage in underground pipes of different materials including PVC, concrete, vitrified clay, and HDPE. Unlike ground penetrating radar, PPR can pass through non-ferrous pipe walls, the bedding of the pipe and through conductive soil and, to a certain extent, can map the outside of pipes that are filled with air or water. The robot carrying the PPR is allowed to traverse through the pipe under inspection. The pulse emanating from the PPR travels through the material of the pipe. Depending on any sharp change in the property of the material, like at the interface of pipe and water, this pulse is reflected or refracted. The greater the difference in the properties of the materials, the greater amount of energy that is reflected back. A receiving antenna detects these reflected waves and is recorded as a single trace. As the pulse travels, a profile of the reflected waves is created. In sewer rehabilitation, this method can be used to detect liner bonding and voids outside pipe walls by detecting variations in bedding conditions.

7. Radio frequency identification (RFID)

  • Radio frequency identification is being widely used for the purpose of tagging or marking new services. The tag contains information such as the kind of service for which the pipeline is being used and its depth, which can easily be read by detecting devices. This identification system is very new, and needs compatible devices programmed to read them, but will be a great benefit for trenchless technology.

Conclusion

With most utility services going underground to prevent damage from falling trees, hurricanes and storms, below surface areas have become a network of cables, wires and pipes. Excavations would inevitably strike one line or the other without knowing the precise locations of these substructure elements. Now that systems are available to locate and map utilities, it should be the responsibility of every contractor and owner to carry out a geotechnical survey (read more about this in "Planning a Microtunneling Project: What You Need to Know Before You Begin") to properly plan the project before drills are allowed to penetrate the ground.