No one wants to be standing knee-deep in whatever is coming out of a broken sewer pipe, and broken water lines flood the space they are in until the valve is shut off. These are hazards of digging around water and sewer lines. Still, excavation must sometimes take place near these lines.
Excavating Around Underground Sewer and Water Lines can be a Tricky Business.
Locating underground sewer and water pipes is tricky, too. Since there are so many sizes and pipe materials, there are a variety of ways to locate a sewer or water line, and almost no one locating technique will be the perfect fit for locating every one of these lines.
Sewer pipes can run from inches to feet in diameter, and can be made of steel, concrete, various types of plastic, cast iron, clay, and even wrought iron in the case of very old construction. There are even pipes made of wood, sealed with pitch or tar! With all of these disparate sizes and materials, there are many ways to go about locating them.
Interior Water and Sewer Lines
Most interior sewer and water lines – lines that are inside buildings, either within or under a concrete slab or floor – are made of metal, but even these can be less than straightforward to locate. Metal water lines can sometimes be located using radio detection, although if there is rebar or post-tension cable in the concrete slab on top of the water lines, this can lead to bleed-off, where the signal from the radio transmitter is carried by the metal post-tension cable or rebar instead of the target water line. Sewer lines have a different problem: there are usually non-metal flanges in between the pipe sections of a sewer line which block the signal completely, so direct-connect radio locating has much less chance of working on them. Even at very high frequencies or currents, it can be difficult to trace a typical cast-iron sewer line. It is sometimes possible to use nonmetal-to-metal induction, where the transmitter creates a magnetic field around it that passes through the concrete or ground to create a signal in the target pipe, but this has its drawbacks, too.
Usually the best way to locate both of these types of lines is to perform underground scanning using Ground-Penetrating Radar (GPR). A 1600 MHz antenna can be used to scan concrete up to a depth of eighteen inches. The ground scanning radar will pick up anything metal, including the rebar and/or post tension, but since rebar and post-tension tend to be in a regular pattern, these reinforcements can usually be distinguished from the larger water or sewer pipes beneath them.
If the pipes are deeper than 18 inches, a 400 MHz antenna will scan up to 6 feet deep, although it is much more difficult to use in tighter spaces.
Other methods must sometimes be considered. One method is to find a clean out and push a conductive line down the sewer pipe, so that the conductive line can be located using a standard utility locate. If no clean out is present, the sewer line locator may utilize a floor drain or toilet drain to access the sewer line in question.
Exterior Water and Sewer Lines
The best thing about exterior sewer and water lines – lines found outside of buildings – is that there is almost always a place to access them – a clean out, a manhole, or a valve – some place where the subsurface world is indicated and accessible to a utility worker.
The worst thing about these access points is that often they do not really help in locating the exact position of the lines. For instance, even the tallest person in the world cannot reach his or her arm down a four-foot deep, six-inch diameter hole to direct connect a radio transmitter to a water valve at the bottom of the hole.
Similarly, it might seem great to have direct access to a 24-inch concrete sewer pipe, but do you want to crawl down that pipe holding a sonde (a device that, independently of an external power source, broadcasts a magnetic field of a known frequency)?
Luckily, There are Other Ways for the Water Line Locator to Mark These Lines Accurately.
Some pipes have tracer wire, a metallic wire buried along with the non-metallic pipe that can easily be detected using radio locating methods. Unfortunately, not every non-metallic pipe has a tracer wire.
For sewer laterals that run from the interior of a house or business to the sewer main or septic tank, there are usually cleanouts near the exterior walls of the house. You might have seen a pipe snake operator access one of these cleanouts to unclog a blocked pipe. These cleanouts can also be accessed by a locator and located using radio detection on a conductive line, as outlined above.
Water pipes also tend to be metal from the house to the main or wellhead, and these can be located by direct connecting to the standpipe (often a hose bib) closest to the street or wellhead. More and more, however, plastic lines are taking the place of other materials in both exterior and interior sewer and water lines. Plastic doesn’t conduct electricity, so standard radio locating is useless, unless it is a sewer pipe that has access through a cleanout.
This is where, once again, GPR underground sonar comes in handy. Though the readings can be affected by pipe material, size, and the type of soil in which the pipes rest, this is one of the best methods we have of finding underground plastic pipes. The GPR bounces a radar signal off of the underground pipe (and often, in the case of a water pipe, the water contained within the pipe) and presents a reading to the technician, who can then mark the exact spot of the pipe and a fairly accurate depth. This can work on concrete, clay, plastic, and various other types of pipe when other methods fail.
“The Larger the Pipe, the Deeper it’s Buried.”
This is a good general rule of thumb for utilities. Sometimes a pipe is unable to be located using radio detection or ground-penetrating radar alone, either because it’s too deep, or it simply won’t present a readable signal. In addition to performing standard radio locating and GPR, there are always solutions for creative locators.
Remember When We Asked if You Would Like to be the One to Crawl Down a Sewer Pipe With a Sonde in Hand?
Of course a person should not attempt this, but a crawler camera can. A crawler camera is a specialized piece of equipment, half camera, half RC car, that is built to take on jobs like this. A crawler has a built-in sonde, as well as a camera that can inspect the pipe from the inside without having to do any excavation, and a technician can operate it from the safety of the surface with a simple control panel. While there is a depth limit, there are very few pipes that are deeper than this limit that do not have access to walk upright inside them.
The crawler is not the only solution for large pipes. In one case, there was a four-foot diameter storm water culvert containing a constant stream of water that was approximately two feet deep. This enclosed culvert ran under some residential and business properties, and a client needed to know the position and direction of this pipe. If we sent the crawler down, it would be risky, as the entire thing would be underwater for the duration of the trip.
So we got creative.
We purchased rigid plastic conduit that would accommodate a conductive line and a sonde, and assembled a 120-foot conduit onsite. Once the conduit was assembled the technician pushed it into the culvert, and then pushed the sonde down the conduit. It worked beautifully, even at a depth of about 12 feet.
While the measurement wasn’t as precise as we usually like it to be (a sonde the size of a shotgun shell somewhere within a four-foot pipe required room for error), it allowed the client to proceed with their development sure that they were not in conflict with this culvert.
In need of private utility locating services for your next project?
Encompass Inspections has a track record of finding a way to locate utilities that are hard to locate, and we pride ourselves on being as accurate as possible. If we can’t find it, there’s a good chance that no one can.