Before the commencement of excavation for the construction of any underground structure or utility, existing utilities must first be located and exposed. Utility location is a crucial step that is necessary to ensure that there is no potential or resulting conflict during construction.
Utility exposure is also imperative when undertaking routine maintenance checks on facilities to evaluate any damage to pipelines and other structures.
If there is not proper care while exposing utilities, there could be severe consequences that affect construction workers, end-users, utility companies and contractors. For example: a horizontal directional drilling (HDD) exercise in Kansas City resulted in an explosion and fire due to damage to an underground natural gas line.
A lack of thorough utility location investigations caused this event.
The unfortunate accident resulted in damage to nearby buildings, severe injuries to workers and bystanders, as well as the loss of life to one individual.
What is Potholing?
Potholing, sometimes referred to a daylighting, is one of the primary investigative construction techniques used to expose the horizontal and vertical location of utility lines. The purpose of potholing is to gain onsite confirmation of the location of utilities and any other existing structures that are in the proposed construction zone.
During the potholing process, exploratory holes are drilled from the surface to required depths to identify any hidden utility lines. Potholing allows for tracking devices to be used during HDD operations to avoid hitting any utility lines in the drill area, through the emission of an electromagnetic field that detects the drill head location.
The position of the utility must be appropriately defined, once found, through potholing. Survey benchmarks or other features located above grade are means of tying in the utility location. GPS readings, measurement of horizontal distances to permanent features, and the use of survey coordinates also verify the position. It is also essential to assess vertical distances from the surface to below-grade.
Which Potholing Method Is Right for Your Project?
Potholing can be carried out in various ways depending on your project requirements, budget and level of risk associated with the intended works. There are three main methods of potholing that are implemented in the construction industry today.
Hand Digging
This method employs the use of hand tools, such as shovels and digging bars, to manually uncover and remove excavated material. Though this method is generally the least costly as expensive equipment is not warranted, it is incredibly time-consuming and labor-intensive. Hand digging is suitable for applications where working space is limited and access restricted.
Hand digging is generally considered a safe method of potholing, as the equipment used is not likely to rupture an underground pipe. However, caution should be exercised when digging near to electrical lines and fiber optics.
Backhoes
The digging of potholes with backhoes was once the most popular method of potholing. However, due to the risk involved with this method, its popularity has waned. Backhoes are used in applications where precision is not significant, such as new housing developments, where the probability of utility interference is low. Due to its size and bulky nature, backhoes are not suitable for applications where there are space limitations.
Excavating with a backhoe is an inexact method; therefore, operators run a very high risk of damaging the utilities that they were tasked to expose. If backhoes are used in potholing exercises, a spotter must be used for the duration of the process. The spotter is tasked with observing operations and communicating with the backhoe operator once he sees any evidence of utilities.
Vacuum Excavation
Vacuum excavation is a non-destructive method of potholing and is the preferred method by contractors. Aside from preventing damage to utilities, vacuum excavation is efficient, precise, provides a safer working environment, and is cost-effective.
This type of excavation is carried out by the use of either air or water to break up the soil. A vacuum device then collects the soil.
Air Excavation
In air excavation, the ground is disturbed through the use of a high-velocity air stream, which produces kinetic energy. Rocks and soil that have been displaced are then removed from the hole using a vacuum device to reveal the existing utility.
The depth of excavation required and the limitations of the vacuum determine the size and shape of the hole warranted by the air excavation process. Typically test holes range from 6 to 12 inches in diameter and 4 to 6 feet in depth and one-foot square; however, depths of 20 feet can be reached if necessary.
Air excavation is a dry method and boasts some advantages over water excavation. The requirement for disposal of mud is eliminated through air excavation since the process is quick, and the dry soil can be used as backfill immediately. It is also safer than the water method but is not as useful for use in soils that are wet and have heavy clay content.
Hydro Excavation
High-pressure water is applied to loosen and dig the land in this method. This creates a mud slurry that is housed in a spoil tank through a vacuum. Holes created are typically in the range of that of the air excavation process.
Hydro excavation is suitable for potholing use throughout the year as heated water can be used in frozen ground conditions. Caution must be taken when implementing water vacuum excavation as the high pressure can result in damage to cables and pipelines.
Final Thoughts
Potholing is one of the most effective methods of utility location as compared to other methods such as reading utility maps and charts that can be out of date. The exact position of utilities can shift over time due to factors such as settlement. Potholing negates the risk of improper location, once the proper technique is used, as it is an onsite operation that can be controlled and tracked.
Of all the methods of potholing, vacuum excavation tends to be the most popular procedure as it is considered safer than hand-digging or using backhoe machinery, due to its non-invasive nature. The use of vacuum methods bodes well for all parties involved as the likelihood of occurrence of threats such as loss of life, project delays, service outages and negative cost implications are reduced.
Potholing has proven to be a versatile excavation method, as it can be implemented other applications such as the placement of utility poles, sign bases and poles as well as installation of fences.