One question we get frequently from our clients is: how accurate is underground utility locating? The alternate question is: what's the safe distance to drill or excavate away from marked underground utilities? Although there is no precise answer to this question, there are still some good practices to follow.
Some states require you keep certain distance away from marked underground utilities if powered tools are to be used. For example, in new jersey, you can only hand dig within two feet of a line marked by utility companies. In general, two feet is the minimal distance. but most companies doing drilling or excavation routinely have formed their own policies and 3, 5, or even 10 feet distance has been used. If you have to do it such as taking soil samples within the close range of a marked underground utility line, hand dig or vacuum certain feet down and then take soil samples with Geoprobe or other powered tools.
Why do we need to keep so far away from marked underground utilities? Besides apparent reasons such as utilities with large diameters, the main reason is still that the marks could be inaccurate. It's not uncommon that marks, especially for fiber-optic and PVC utilities, are made by utility companies solely based on old maps or plans, which may or may not be accurate.
Although in general the lines marked by a private utility locating company utilizing the latest technology such as ground penetrating radar (GPR) are supposed to be more accurate, cautions still have to been taken. Although GPR can be really accurate (within inches) locating underground utilities, it doesn't always work well, such as in clay soils. In areas where GPR has limited penetrating depth, an underground utility locator has to rely on other technologies such as precision locating (e.g., using a Radiodetection unit) or electromagnetic induction (e.g., using a Geonics EM61 unit). Both technologies can also be very accurate, but not always. For example, if there are two underground metallic pipes running parallel and close to each other, and one is deeper and with smaller diameter compared to the other one, it may not be properly located due to "signal jumping" -- its signals may get masked by the signals from the other pipe. In situations where there is a congestion of underground utilities with a lot of them running parallel and crossing each other, things can be more complicated, especially there are limited access points, survey spaces or prior knowledge about them. It's a good practice to take caution that there might be other underground utilities running close to the marked ones.
We always keep close communications with our clients for underground utility locating projects and recommend them to keep as far away as possible from marked underground utilities. If it has to be done within a close proximity of a marked utility line, we also provide air knife/vacuum excavation services assisting hand clearing when needed.