Utility Locates

Here is a fire hydrant. You may notice that there's a wire wrapped around the base.



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The nozzle section of this hydrant has one pumper nozzle (the larger one) and two hose nozzles. As shown in the datasheet here, this is a dry barrel hydrant, where the valve is in the base (at the very bottom – so normally, the water isn't in the hydrant or lower barrel). The main advantage of this is that the water won't freeze in the winter, since it is all below grade, where the water main is. In fact, there's usually a small hole in the base of the barrel, so if the valve does leak, the barrel won't fill with water, freeze in the winter, and therefore possibly not gush water when needed. The other advantage of a dry hydrant is that if a car knocks the whole thing over, you won't get a geyser of water, since the shut-off valve is 6' below grade. In fact, the flange at the top of the lower barrel is specifically designed to break (before the rest of the hydrant), so the $2,000, 500-pound hydrant (hopefully) isn't damaged. The datasheet for the $150 flange repair kit is here.



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You can see that the spigot end of the pipe has been bevelled, and the bell end has a larger diameter, with a black rubber “O” sealing ring in it. The bevelled end is pushed into the O-ring end (up to the black mark).



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The lid of the concrete valve chamber has a round opening, above which a cast-iron man hole will be installed, the top surface of it will be flush with the pavement.



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Municipalities do many things to prolong the life of iron water pipes. Sometimes they are lined with a polymer to reduce corrosion, but here is a new plastic section of water main spliced into an existing (old) cast iron water pipe. Let's talk about that the big cardboard tube with the blue wire spiral (which will soon be welded to the iron pipe).

While most metals corrode (that is, they rust), some like to corrode more than others, and this is due to the potential (voltage) formed when metals are in contact with an electrolyte (which allows ions to migrate from one metal to the other). The galvanic series (or electropotential series) below shows for galvanic corrosion, which metal will corrode (the one farther down the list becomes the anode, which loses ions) to protect the other (which therefore benefits from cathodic protection).

• Platinum
• Gold
• Silver
• Stainless steel (the exact position in this list depends on its composition)
• Brass
• Chrome
• Copper
• Cast iron
• Steel
• Lead
• Tin
• Aluminum
• Zinc
• Magnesium

Note that compared to all the other metals listed, magnesium is corroded, and the other metal is therefore protected. A block of magnesium therefore becomes a sacrificial anode when it is electrically attached to a metal lower in the series (such as a water pipe, which you really don't want to rust).

The data sheet here shows that there is a 32 pound block of magnesium in this cardboard tube. The rest of this tube is filled with powdered Wyoming Bentonite and other great things to keep the magnesium block in good electrical contact with the soil. The blue wire is electrically connected to the magnesium block inside the cardboard tube, and the wire is attached to the water pipe, which then receives the cathodic protection.

While as-built drawings and other documentation should indicate approximately where everything is below grade, the most reliable way to determine what is buried is to use an electronic device to detect exactly where everything is (though there are credible reports of divining rods and sticks being sucessfully used).

Since it is important that utility locates be done quickly (since construction needs to be scheduled in advance), and since all utilities (gas, water, sewer, electricity, cable TV, telephone, and possibly others) need to locate their services, the utilities in many areas support a central service that performs all the locates for all of them, at the same time. Examples are Ontario One Call, Miss Utility, Dig Safe. Typically, these organizations are somewhat like call centres, and subcontract out the actual locating, to an organization such as this.

When a locate is done, coloured spray paint is used for the markings, which are either:

• A line with arrows, indicating the path of the buried cable or pipe.
• Two parallel lines, indicating the width of the trench in which the cables, pipe, or duct (generally white or grey 3" or 4" diameter plastic pipe, which may be encased in concrete) are installed.

As described here, the colour code used for the spray painted markings are usually as follows:

• Red: Power lines. Many areas currently use 27,000 volt power distribution to the transformers which are pole-mounted or below grade in transformer vaults. So accidentally digging into one of these would be a very bad thing. Generally an “H” is spray painted alongside the red line, to indicate Hydro (as in hydro-electric power – which is how most electrical power was generated before nuclear power became more common).
• Yellow: Usually indicates natural gas, such as that which heats most people's homes and water, but can also indicate an oil or steam line. A “GM” indicates the gas main running down a street, and a “GS” indicates the gas service running from the gas main to a house or business.
• Orange: Communication cables, such as telephone lines and trunks, cable TV coaxial cable, and fibre optic cables. “BT” indicates telephone (as in Bell Telephone), TV indicates a cable television coaxial cable or fibre optic cable and a “FO” indicates a fibre optic cable. Traffic signal cables are also often marked with orange paint, sometimes with the letter “T”.
• Blue: Indicates water, most often the water supply to people's homes (often called potable water). A “WM” indicates a water main running down a street (often 6" or 10" diameter), and a “WS” or “S” indicates the water supply line (often a ¾" or 1" copper pipe) running from the water main to a house or business.
• Green: Indicates a sanitary sewer (that usually eventually goes to a water treatment plant) or storm sewer (that typically goes to the nearest stream or river). These will typically be marked by an “SS” or “S” (which doesn't quite differentiate the two).
• Pink: Survey markings, for example, to either indicate property boundaries, or to match markings on drawings for road or sidewalk construction. The marking is often an “X”, where the centre indicates the exact survey point (often along with a narrow and shallow saw cut).
• White: Proposed excavation markings, such as where saw cuts are to be made in the asphalt for a road repair or to dig up a pipe.

So, in the picture, there's a buried water main underground (note the “WM” is a 180° rotational ambigram of itself).

Finally, the reason for the wire. Except for fancy ground penetrating radar devices (such as this), subsurface locating systems rely on the buried cable or pipe being metalic, for example to:

• Disturb an electric field generated by the locating device.
• Conduct a signal injected by a transmitter. This transmitter is usually part of the locating system, and may be:
  • Electrically attached to the cable or pipe (another wire from the transmitter will be grounded, usually through a stake driven into the earth). By attaching the transmitter to the pipe, one can be sure that the desired pipe is being followed.
  • Inductively coupled to it either by placing the transmitter parallel (and as close as possible, for example, directly over the expected location) to the cable or pipe, or by clamping a coil around the conductor or pipe.
• Carry the power for which the cable was installed. That is, the locating device senses the 60 Hz frequency from a power line.
• Carry a signal associated with the cable. Amazingly (read about it here), the 31.5 kHz second harmonic of the NTSC television horizontal scan frequency is coupled onto the cable TV coaxial cable by the yoke coils of an operating television receiver, and this can be used to locate buried coaxial cable.

Brochures on locating devices are here and here (the whole manual for that one is here).

So, the wire at the base of recently-installed fire hydrants is a tracer wire, and contractors are required to lay these in the trench, above plastic water mains, solely for the purpose of later locating the exact path of the buried water main.

Finally finally, there is the issue of how deep is the buried pipe.

• While the signal strength from the transmitter is a rough indication, locators typically will open man-holes and valve chambers and measure the distance down to the pipe using a tape measure. By doing this at each end of the pipe, the depth at points between these can be estimated through interpolation.
• Some locating systems have a device (which may be called a beacon or a sond ) which can be pushed into the pipe using a fibreglass rod, and the locating device measures the received signal at its receiving antenna close to the ground and another antenna farther up the locating device, closer to the operator. The distance to the beacon can then be determined by the time difference of the received signals.

Often this will be done when a house is purchased (the mortgage company generally requests a survey of the property, and if one is not available, a surveyor must be requested to locate the property boundaries).



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Sometimes the markings will indicate an offset from what is really of interest. For example, the distance to the crown (centre) of a road to be rebuilt will be indicated – you can't put the markings directly on the road, since they would be covered-up by the road work, so a point (for example) exactly 3 meters away will be marked, with a designation such as “+3.00” along with an arrow.



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