Underground Utility Mapping & Design Surveys

Identification of Public and Private utilities: Field mark utilities in a specified area for the purpose of preliminary engineering and planned development.

 

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Utility Survey Mapping

IMAGE CREDIT: VINCI LLC

Underground Utility Mapping uses the principles of geophysics to detect, locate, identify and investigate buried utilities.

Utility Design Surveys provide underground utility mapping to update utility plans to ensure that our clients can carry out safe intrusive work. We employ the techniques of Radiodetection (electromagnetics) and ground penetrating radar to locate and mark utilities. The depth of investigation will depend on the conditions of the site. In general, Radiodetection and ground penetrating radar techniques will detect services up to 10-15’ below the ground surface. The depth of penetration of radar can be increased but often at the expense of resolution.

Radiodetection uses the principles of electromagnetics to detect underground utilities. This principle works on the basis that a current flowing along a conductor creates a magnetic field which extends around the conductor in concentric circles. A receiver coil can be used to detect the amplitude of this magnetic field. The amplitude varies depending on the position and orientation of the receiver within the field. The amplitude is at a maximum when the receiver is in line with the field and directly above the conductor. Moving the receiver from side to side it is possible to follow the maximum signal response and, therefore, the line of the buried service.
Both active and passive methods are used to ensure all possible detectable services are located.

Active Methods

An active signal is a signal that has been artificially generated by an external source. Active radiodetection methods can allow different services such as telephone and electric to be individually identified. Active methods involve the use of a transmitter. The transmitter can either be used for direct connection or for induction. Direct connection involves the application of an active signal to a conductor using a clip or clamp. Induction involves a signal being radiated from the internal antenna of the transmitter which is induced to any conductors in the vicinity and re-radiated. Generally 8, 33, 65 or 83KHz frequencies are used. It is often possible to determine the depth of the service in these modes.

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Passive Methods

A passive signal is a signal that occurs ‘naturally’ on a buried conductor. Passive radio detection methods will provide an indication only that services are present underground. Passive methods use the electro-magnetic fields already present around the underground utility to locate them. Two modes are generally used:

  • Power – detects 50-60Hz energy present on most buried conductors
  • Radio – detects re-radiated radio energy often present on conductors – it is not possible to determine depth in this mode.

Surveying in the traced services

As the survey progresses the positions of the detected services are marked unobtrusively onto the ground surface. Utility marking is done using chalk or spray paint. The coordinates of these points can be surveyed using a total station allowing quick and accurate transfer into AutoCAD.

Ground Probing Radar (GPR)

image007Radiodetection techniques are unable to detect non-metallic buried services including plastic water and gas pipes and clay drainage pipes. To overcome this potentially hazardous situation, we can often combine the use of Radiodetection with Ground Probing Radar (GPR). GPR is able to detect non metallic as well as metallic pipes. It will also give an indication of the location and depth of buried services but will not identify them.

GPR works by emitting a short pulse of energy into the ground. Echoes are returned from the different interfaces between different materials in the ground. As the antennae emit a “cone” shaped pulse of energy an offset target showing a perpendicular face to the radar wave will be “seen” before the antenna passes over it. Resultant characteristic diffraction pattern is thus built up in the shape of a hyperbola. A classic target generating such a diffraction is a pipeline when the antenna is traveling across the line of the pipe. However it should be pointed out that if the interface between the target and its surroundings does not result in a marked change in velocity then only a weak hyperbola will be seen, if at all.

WARNING – AS NOT ALL SERVICES ARE DETECTABLE, EXTREME CAUTION
SHOULD BE USED WHEN EXCAVATIONS ARE UNDERTAKEN.

Levels of Survey Typical Application Limitations
Level 1
Desk top survey to include a search of existing utility records for consolidation onto one plan in CAD.
Low cost assessment of site to demonstrate extent of services. Can be carried out by client. Low level of accuracy may totally omit some types of utilities and recent additions/alterations.
Level 2
Radiodetection survey in passive and active mode with located services being marked onto ground surface.
A quick low cost site assessment mainly useful for locating cables and ferrous pipes. Useful for sighting trial trenches or boreholes. Will not detect non-metallic services. No record retained.
Level 3
Levels 1 & 2. Desk top survey to include a search of existing utility records for consolidation onto one plan in CAD. Radiodetection survey in passive and active mode with located services being marked onto ground surface. Depth assessed in active mode.
A quick low cost site assessment mainly useful for locating cables and ferrous pipes. Useful for sighting trial trenches or boreholes. Will not detect non-metallic services. No record retained.
Level 4
Level 3 & lifting of all manhole and inspection chamber, gully and valve covers. Rodding Sonde or flexitrace through gravity pipes & unoccupied ducts to assess depth. Mark line of service onto ground surface.
A “value added” Radiodetection survey particularly useful in understanding drainage and telecom ducts etc. Will not detect plastic gas & water mains and fiber optic cabling etc. No record retained.
Level 5
Level 4 & GPR survey at 3’ orthogonal grid (or optional 18″) over targeted sections of site identified from Level 1 survey. Collect data, process, analyze and interpret results and mark line & depth onto site plans.
A “lower cost” GPR survey using this additional technique to understand complex areas. Some areas still not covered.

Level 6
Level 4 & GPR survey at 3’ orthogonal grid (or optional 18”) over whole survey area
The most comprehensive level of survey. Highest expectation of picking up all services and other underground obstructions.

Level 7
Levels 4, 5 & 6 & Selected trial holes over targeted salient services to assess accuracy of plan and depth.
A verification of the quality of the report. Used for excavating in various site and ground conditions.

Part of the results of a typical survey of underground utilities. This shows a particularly complex section beneath a road.

For more information and details on how to put CNI Locates utility design survey skills to work for you, call (877) 826-1177

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