During the industry boom a few years back, a large number of conduits were installed using horizontal directional drilling (HDD). Some of these conduits are currently in use, while others are still empty, anticipating future capacity upgrades. As construction projects of various kinds are undertaken in the vicinity of these conduits, the ability to locate them becomes extremely important.
Many of the conduits are HDPE or PE (high density polyethylene) pipe. Poly-vinyl chloride (PVC) is a second type of plastic pipe seeing increased usage in water systems. The primary benefits of these plastic pipes is that they are lightweight, resistant to chemical and bacterial buildup and non-corrosive.
In many cases, the documentation of where these conduits are located is inadequate. Log books or installation records may have been lost or the companies involved are no longer in business. Needless to say, the task of identifying the location of these conduits can be daunting.
In any event, unless the pipes were installed with a tracer wire, these pipes can not be tracked using conventional utility locating methods. These methods rely on a conductive pipe (or tracer wire) and an aboveground transmitter inducing a signal on the pipe in question. The pipe then effectively turns into an antenna and it can be tracked by a locating device. Even if a tracer wire was to be present, many of these conduits have been installed at depths greater than what typical pipe locators can handle and there-in lies the problem.
One solution to the above-mentioned difficulty in tracking deep plastic pipes is to make use of the very systems used to install many of them in the first place. Those systems have a greater depth range than the utility locating system and can quite accurately determine the depth and position of a transmitter underground. By pulling a transmitter through the conduit in question, it can be tracked quite accurately from aboveground.
Conventional HDD transmitters are, however, not designed for this application. They are often larger than needed; they do not have any attachment points and most are designed to shut off after a certain time period without rotation to conserve battery power. Locating system manufacturers have responded by designing transmitters specifically for this purpose. These are both smaller in diameter and shorter with built-in pulling eyes or rodder connections and do not have a sleep mode. Typically they do not send out a clock and pitch signal since this is not required for the pipe tracking operation. Such transmitters can also be used to locate a blockage in a pipe by using air pressure to blow the transmitter through the pipe until it stops at the blockage.
Prior to tracking the pipe, it is recommended that the locator be used to check for active interference, much like you would do prior to a bore. By walking the right of way of the installed pipe, you can determine the amount of interference (background noise) that might affect the pipe tracking operation. The signal coming from the pipe tracking transmitter, at a distance equivalent to the maximum anticipated depth of the pipe, needs to be significantly greater than the interference present to increase the tracking accuracy.
Assuming the interference has been checked and found to be at an acceptable level, the tracking system needs to be calibrated to the transmitter. Once the calibration is complete, the depth readings on the receiver should be checked at a few different distances to verify the calibration.
The transmitter can now be inserted into the pipe to be tracked. There are two main methods of moving the transmitter through the pipe; one is to shoot a line through first and pull it and the second is to use a fiberglass rodder and push it through. Either way, it is recommended that some method be devised to keep track of how far the transmitter has been pushed or pulled in. This can, for example, be done with distance marks on the line or rodder. This will help to independently establish the distance of the transmitter from the insertion point. Depending on the size of the pipe, it may be necessary to use a centering device to ensure that the transmitter’s orientation is that of the pipe’s to increase locating accuracy.
As the transmitter is being moved through the pipe, it is tracked identically to normal drilling operations, where the direction and depth of the transmitter is identified, marked on the ground and entered into the log book. Once the tracking is complete, the information gathered can then be turned into an as-built and conventional surveying or GPS measurements. The markings can be used to permanently fix the position of the below ground pipe.
Siggi Finnsson is product manager at Digital Control Inc. All Drillmaster Reports are reviewed by the Electronic Drillmaster Advisory Board: Finnsson, John Archambeault, McLaughlin Mfg.; John Bieberdorf, Subsite Electronics; and Ed Savage, Vermeer Mfg. Co.