Potholing 101
Drilling through underground utilities can disrupt essential services, costs hundreds-of-thousands of dollars in damage, and results in serious injuries and even loss of life. As utility easements become more crowded, preventing damage to underground utilities has become increasingly challenging. Below ground, our cities have become a spider web of utility lines, including electric, gas, telephone, fiber optic, cable TV, water mains, and sewers. In addition, at some locations there may be national defense communication lines, mass transit tunnels, railroad tunnels, and major oil and high-pressure gas pipelines.
Obviously, excavators should call for locates and wait until the utility marks are made. The next step is "potholing",
but excavators often skip this step. Potholing physically uncovers an underground utility to confirm its location and
is the surest method of determining the exact location. Potholing is becoming a standard practice on a growing number of
projects where the planned drilling location is in close proximity of an existing utility. Many public agencies are adopting
regulations that require potholing and some contractors are establishing their own policies specifying potholing before
drilling begins. Potholes are usually dug with a backhoe, hand shovels or water excavation machines to minimize the risk of
hitting and damaging utilities.In the, potholes were typically dug with backhoes. Digging potholes with a backhoe is a risky endeavor compared to other methods of potholing due to its potentially destructive nature. The backhoe method is inexact and cumbersome; even skilled backhoe operators run the risk of hitting and damaging the very utility they were trying to locate and protect. If a backhoe is utilized, it is essential that a "spotter" is present for the entire excavation. A spotter is a person that observes the excavation and communicates to the backhoe operator when a buried facility is sighted.
Hand digging a test hole is the method of digging a pothole by manual means with hand held equipment such as a shovel. This method is labor intensive and time consuming. The advantage to hand digging is that it does not require expensive equipment and is relatively safe for locating most facilities. As with any excavation, extreme caution should be practiced if digging near hazardous utilities such as electric cable. Using a rod to probe for a cable or duct is often used in conjunction with hand digging and can help reduce labor time by identifying where the utility line is.
Vacuum excavation is the preferred method for non-destructive exposure of buried utilities. Vacuum excavation utilizes either
air or water pressure to break up the soil and a vacuum device to collect the spoil. Of the two methods, air vacuum excavation
is generally preferred, though specific site or soil conditions (e.g. heavy clay) may lead to a decision to use water vacuum
excavation. Air vacuum excavators typically utilize a high velocity air stream to penetrate, expand, and break-up the soil. The
loosened particles of soil and rock shall be removed from the excavation through the use of a vacuum. Water vacuum excavation
systems excavate the pothole using high-pressure water to reduce and loosen the soil. The wet soil and mud slurry shall be removed
to a spoil tank using a vacuum. The resulting slurry of water and debris goes to a debris tank, which typically is hundreds of
gallons in capacity.Regardless of the method used, potholing should be the standard procedure on horizontal directional drilling projects when the path of a planned pilot bore either crosses or is in close proximity to buried lines. The same is true in congested areas having several facilities in close proximity and/or criss-crossing each other. In this scenario, locates have greater potential to be less accurate and potholing should be used for excavations in these congested utility areas.